Wheel Corner Modules

Technology and Concept Analysis

Johan Hag

Vehicle Dynamics
Aeronautical and Vehicle Engineering
Royal Institute of Technology

Master Thesis

TRITA-AVE 2011:29
ISSN 1651-7660

Postal address Visiting Address Telephone Telefax Internet

KTH Teknikringen 8 +46 8 790 6000 +46 8 790 9290 www.kth.se
Vehicle Dynamics Stockholm
SE-100 44 Stockholm, Sweden
Abstract
The wheel corner module represents a new technology for controlling the motion of a
vehicle. It is based on a modular design around the geometric boundaries of a conventional
wheel. The typical WCM consists of a wheel containing an electrical in-wheel propulsion
motor, a friction brake, a steering system and a suspension system. Generally, the braking,
steering and suspension systems are controlled by means of electrical actuators. The WCM
is designed to easily, by means of bolted connections and a power connector, attach to
a vehicle platform constructed for the specic purpose. All functions are controlled via
an electrical system, connecting the steering column to the module. A WCM vehicle can
contain two or four wheel corner modules.

The purpose of this thesis is to serve as an introduction to wheel corner module tech-
nology. The technology itself, as well as advantages and disadvantages related to wheel
corner modules are discussed. An analysis of a variety of wheel corner module concepts is
carried out. In addition, simulations are conducted in order to estimate how an increased
unsprung mass aects the ride comfort and handling performance of a vehicle.

Longitudinal translation over two types of road disturbance proles, a curb and a bump,
is simulated. A quarter car model as well as a full car model is utilized. The obtained
results indicate that handling performance is deteriorated in connection to an increased
unsprung mass. The RMS value of the tire force uctuation increases with up to 18%,
when 20 kg is added to each of the rear wheels of the full car model. Ride comfort is
deteriorated or enhanced in connection to an increased unsprung mass, depending on the
disturbance frequency of the road. When subjected to a road disturbance frequency below
the eigenfrequency of the unsprung mass, ride comfort deterioration is indicated. The
RMS vertical acceleration of the sprung mass increases with up to 6%, in terms of the full
car model. When subjected to a road disturbance frequency above the eigenfrequency of
the unsprung mass, decreased RMS vertical acceleration of up to 25% is noted, indicating
a signicantly enhanced ride comfort.

Implementation of wheel corner module technology enables improved handling perfor-

mance, safety and ride comfort compared to conventional vehicle technology. Further
development, e.g. in terms of in-wheel motors and alternative power sources, is how-
ever required. In addition, major investments related to manufacturing equipment and
technology is regarded as a signicant obstacle in terms of serial production.

I
Preface
This master thesis represents my last step towards graduation from the Master of Science
programme at the Vehicle Dynamics Department, Royal Institute of Technology. I would
like to thank Daniel Wanner, who has greatly contributed to this thesis by providing me
with continuous guidance and feedback. I would also like to thank my examiner Lars
Drugge.

In addition, a thank you goes out to my fellow master students Mikael Sjöholm and Igor
Kovacevic, for great company.

Eternal gratitude is dedicated to my father Börje, mother Monica, sister Susanne and
brother Fredrik, for always supporting and strengthening me. Hilda and Edith, thank
you for brightening my life!

II
III
Contents
1 Introduction 1
2 Wheel Corner Modules 2
2.1 X-by-wire . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2.1.1 Fault tolerant x-by-wire systems . . . . . . . . . . . . . . . . . . . . 4

2.1.2 Communication network architecture . . . . . . . . . . . . . . . . . 5

2.2 Longitudinal motion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2.2.1 Propulsion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2.2.2 Brakes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

2.3 Vertical motion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

2.3.1 Passive suspension . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

2.3.2 Semi-active suspension . . . . . . . . . . . . . . . . . . . . . . . . . 15

2.3.3 Active suspension . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

2.4 Lateral motion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

2.4.1 Steer-by-wire . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

2.4.2 Steering device . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

2.5 Power source . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

2.5.1 Chemical batteries . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

3 The eects of wheel corner module technology 24

3.1 Vehicular aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

3.1.1 Handling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

3.1.2 Comfort . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

3.1.3 Safety . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

3.1.4 Unsprung mass . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

3.2 Environmental aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

3.3 Economical aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

4 Concept analysis 31
4.1 Michelin Active Wheel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

4.2 Bridgestone Dynamic-Damping In-Wheel Motor Drive System . . . . . . . 36

4.3 Siemens VDO eCorner . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

4.4 Volvo Autonomous Corner Module . . . . . . . . . . . . . . . . . . . . . . 41

4.5 MIT Media Lab's Robot Wheel 5 . . . . . . . . . . . . . . . . . . . . . . . 44

4.6 General Motors Wheel Module . . . . . . . . . . . . . . . . . . . . . . . . . 46

4.7 Summary of concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

IV
5 Modeling and simulation 50
5.1 Quarter car model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

5.1.1 Simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

5.1.2 Results and analysis . . . . . . . . . . . . . . . . . . . . . . . . . . 52

5.1.3 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

5.2 Full car model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

5.2.1 Simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62

5.2.2 Results and analysis . . . . . . . . . . . . . . . . . . . . . . . . . . 63

5.2.3 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69

6 Conclusions 70

References 71

V
Nomenclature
ABS Anti-lock braking system

ACM Autonomous corner module

AFPM Axial ux permanent magnet

BBW Brake-by-wire

CAN Controller area network

ECU Electronic control unit

EHB Electro-hydraulic brake

EMB Electro-mechanical brake

EWB Electronic wedge brake

FCM Full car model

GM General Motors

ICE Internal combustion engine

MR Magnetorheological

PM Permanent magnet

QCM Quarter car model

RMS Root mean square

SBW Steer-by-wire

TBW Throttle-by-wire

TTCAN Time-triggered controller area network

TTP Time-triggered protocol

WCM Wheel corner module

XBW X-by-wire

Li-ion Lithium-ion

Ni-Cd Nickel-cadmium

Ni-MH Nickel-metal hydride

Pb-acid Lead-acid

ωs Eigenfrequency of sprung mass [rad/s]

VI
ωu Eigenfrequency of unsprung mass [rad/s]

cs Damper coecient [Ns/m]

ct Tire damper coecient [Ns/m]

ks Spring stiness coecient [N/m]

kt Tire spring stiness coecient [N/m]

ms Sprung mass [kg]

mu Unsprung mass [kg]

VII
1 Introduction
Ever since the Ford Model T was introduced in the beginning of the 20th century, road
vehicles have gradually progressed in terms of technology. Safety, comfort and perfor-
mance have gone through vast improvements, yet the basic vehicle layout is similar to
what it was one hundred years ago.

A wheel corner module (WCM), also called an active wheel module, electric corner module
or robot wheel, represents a new way of controlling the motion of a vehicle. It is based on
a modular design around the geometric boundaries of a conventional wheel. The typical
WCM consists of a wheel containing an electrical in-wheel propulsion motor, a friction
brake, a steering system and a suspension system. Generally, the braking, steering and
suspension systems are controlled by means of electrical actuators. The WCM is designed
to easily, by means of bolted connections and a power connector, attach to a vehicle
platform constructed for the specic purpose. All functions are controlled via an electrical
system, connecting the steering column to the module. The steering column may comprise
a conventional steering wheel and pedals, or any other feasible solution, such as e.g. a
joystick.

WCMs represent a fairly new technology, currently being developed by several car man-
ufacturers and subcontractors for future implementation in road vehicles. According to
Frost & Sullivan WCMs are likely to be on rear wheels by 2015 and on all four wheels
after 2020 [1].

The goal with this thesis is to provide an overview of WCM technology and to serve as
an introduction to the subject. Advantages and disadvantages related to the technology
shall be evaluated. In addition, this thesis shall provide a view of the present development
stage.

The rst part of the thesis treats the technology which WCMs are based on. A thorough
literature study of technical reports and conference papers is the foundation of this part.
The main focus is aimed at novel solutions, even though other more conventional solutions
may also be utilized in terms of WCMs.

In the second part, the impact in terms of vehicular, environmental and economical aspects
are discussed.

A concept analysis is presented in the third part of the thesis. Since detailed technical
information is generally infrequent in terms of concept solutions, the descriptions aim
towards principal function rather than details. Most of the information presented in this
part has been attained from patents and journal papers.

One concern regarding the WCM is increased unsprung mass. In the fourth part, modeling
and simulations are conducted in the dynamic modeling program Dymola, in order to
estimate the impact on ride comfort and handing performance, due to this matter.

1
2 Wheel Corner Modules
A wheel corner module, see gure 1, is a novel type of electro-mechanical system related to
vehicular motion control. As the name implies it is based on a modular design. Generally
the module is held within the boundaries of a conventional wheel. The WCM contains
subsystems responsible for longitudinal, lateral and vertical motion respectively. These
systems comprise electro-mechanical actuators and linkages and are operated upon input
from a control system. The corner module is designed to easily, by means of bolted
connections and an electrical connector port, mount to a vehicle body. The vehicle body
together with two or four WCMs form a WCM vehicle. All functions of the WCM are
electrically controlled based upon input from an operational device, in connection with one
or several control units. It represents a pure x-by-wire based vehicle maneuvering system.
Mounted around the vehicle are several sensors that continuously supply the control units
with information regarding the vehicle position and state. The sensors might include
position sensors, velocity sensors, acceleration sensors, force and torque sensors, pressure
sensors, ow meters, temperature sensors, etc. [2]. The information supplied by these
sensors might be yaw rate, lateral acceleration, angular wheel velocity, steering angle and
chassis velocity [2]. The operational device comprises a conventional steering wheel and
pedals, or any other feasible solution such as e.g. a joystick.

WCMs represent a novel technology related to vehicular propulsion systems, not yet avail-
able in any serial-produced vehicle. However, some of the technical solutions utilized in
WCMs are already available in subsystems of current conventional passenger vehicles.
Hence, comparisons to conventional vehicles and conventional vehicle technology are con-
tinuously carried out in the context of this thesis. The following paragraph denes these
two expressions according to how they are used within this thesis.

Conventional vehicle technology refers to standardized technical solutions as found in

most passenger cars, manufactured and sold in large quantities during the last decade.
The conventional technology discussed within this thesis is mainly connected to vehicular
motion control and is of mechanical type. It generally involves an internal combustion
engine (ICE) and friction brakes for means of longitudinal motion control, both connected
to pedals in the cabin. Lateral motion is actuated by operation of a steering wheel
connected to the front wheels through a rack and pinion construction. Vertical motion
is passively controlled by means of a conventional suspension comprising springs and
dampers. Conventional vehicles are referred to as vehicles which are constructed around
conventional technology as the one described. A conventional vehicle may alternatively
include one or several unconventional technical solutions, such as throttle-by-wire (TBW)
or brake-by-wire (BBW), however the main part of the structure shall be of conventional
type.

A WCM vehicle, as referred to throughout this thesis, is a vehicle containing two or four
WCMs. WCM vehicles involve an increased number of actuators compared to conventional
automotive constructions, enabling improved vehicular motion control, see section 3.1.1.
The corner modules may have dierent setups according to the previous description, hence
the number of actuators may vary. Generally however, the number of actuators contained
in a WCM vehicle exceeds the degrees of freedom, thereby forming an overactuated system
[3].

2
 Figure 1: A wheel corner module, the Michelin Active Wheel [4].

2.1 X-by-wire

Traditionally, vehicular motion control have been executed through operation of a steering
wheel and pedals mechanically connected to the wheels and ICE. X-by-wire is a fairly
novel technology which involves pure electronic control of longitudinal, lateral or vertical
vehicular motion. It has been successfully utilized in the aviation industry for decades, in
that sense called y-by-wire [5]. There are a variety of terms for the technology in context
of road vehicles, such as x-by-wire, drive-by-wire or simply by-wire.

The basic principle of an x-by-wire (XBW) system is replacement of all mechanical/hydraulic

linkages with electric ones [5]. In turn, the connection between driver and subsystem is
no longer direct. Instead, the mechanical input supplied by the driver through the opera-
tional device, is interpreted and processed by computer electronics prior to realization of
the demanded action. In accordance with gure 2, the input device contains a mechanical
operational component, including e.g. a mechanical pedal, sensors for registration of the
pedal movement, microelectronics and a haptic feedback device. The latter applies a force
to the pedal to recreate the feeling of a conventional operational device, by use of a spring
and damper device or an electrical actuator. Depending on what subsystem is being op-
erated, it might allow the driver to physically sense the activity occurring in connection
to the controlled sub-system, i.e. transfer haptic information. If the operational device is
a steering wheel, an electrical actuator mounted in connection to the steering wheel may
transfer a torque to the steering wheel. This torque corresponds to the torque transferred
from the front wheels onto the steering wheel via the steering rack and pinion, during
operation of a conventional steering system. Connected to the operational device via a
bus system is the actual control system containing a microcomputer, power electronics,
electrical actuators, mechanical components and sensors [6]. The mechanical components
include eg. steering linkages or friction brakes. The microcomputer is responsible for ac-
tuator control, function control, supervision and management such as fault handling and
optimization [6]. Owing to the non-direct connection structure including computer power,
benecial new handling and safety solutions are enabled, further discussed in subsection
3.1.

3
 Figure 2: Block diagram of an XBW system.

2.1.1 Fault tolerant x-by-wire systems

A fault tolerant system behavior implies a system which can continue to operate properly
even in the event of one or several malfunctions. Concerning vehicular x-by-wire sys-
tems, fault-tolerant behavior is of utmost importance, as failure might lead to hazardous
complications including personal injury.

There are three main XBW subsystems which apply to conventional vehicles, throttle-
by-wire, brake-by-wire and steer-by-wire (SBW). TBW is not part of WCM technology,
however the applications made possible due to the use of TBW, are similar to the ones
enabled by the by-wire controlled in-wheel motors of WCMs. Therefore it will be further
discussed in this thesis, see section 2.2.1. The TBW system is available in passenger
vehicles as of today, and in its present form a pure XBW solution. The other two systems
are currently utilized in a few conventional vehicles, however coupled with mechanical
back-up systems. This is done in order to achieve a more reliable system. Steering and
braking are safety-critical functions, i.e. a failure in such might involve personal injury.
Electronic components have a dierent fault behavior compared to mechanical compo-
nents, therefore fault-tolerant systems have to be incorporated in order to meet the high
safety demands [6]. Until this is achieved and proven safe, mechanical back-up systems
admit use of the XBW systems and their advantages, by oering reliable, fault-tolerant
back-up technology in case of malfunction. Electrical failure is often caused by shortcuts,
loose connections, parameter changes, contact problems or electromagnetic compatibil-
ity problems [6]. However, the potential reliability of well-designed, well-manufactured
electronic systems is extremely high [7]. Typically the proportions that are defective in
any purchased quantity are in the order of less than ten per million in case of complex
components, and even lower for simple components [7].

A design containing a mechanical backup system involves additional mass and manufac-
turing costs as well as an increased structural complexity, therefore fault-tolerant electrical
back-up systems are preferable when possible [6, 8]. A way of improving the safety prole
of an electrical system is to implement redundant electrical components. The redundancy
can either be static or dynamic. Static redundancy means that multiple redundant mod-
ules govern the same function by operating in parallel. If one module fails, the system
might be degraded but still function, since all modules except one still maintain proper
functioning. Concerning dynamic redundancy, two or several modules are available, how-
ever merely one of them is in operation. The other modules are in standby mode, ready
to be utilized in case of malfunction regarding the module currently in operation.

Sucient fault detection performance is of utmost importance in order to maintain safe

4
operating conditions [6]. The number of back-up components may vary according to the
required fault tolerance of the function it governs, a higher number of redundant compo-
nents generally imply a higher level of fault tolerance. An essential benet associated to
electrical back-up systems is the reduced weight compared to mechanical equivalents.

2.1.2 Communication network architecture

Each vehicle containing one or several XBW solutions requires a communication network.
The objective of this network is to handle the data transfer and communication among
components within a subsystem, as well as between dierent subsystems. The compo-
nents might be e.g. maneuvering devices, sensors and actuators. The communication
network is responsible for transmission of control signals managing all functions of the
XBW subsystems. Such a function might be to set a specic steering angle of a wheel
contained in a steer-by-wire system, or to decelerate a vehicle by properly distributing
brake forces among the wheels via a brake-by-wire system. A failure concerning the com-
munication network might lead to maneuvering malfunction, thus fault-tolerant behavior
of the network is of utmost importance.

An important issue is to nd a network protocol which provides fast data transfer as
well as sucient levels of safety and reliability. Lots of industrial and academic work
have been directed to solve this matter, and resulted in several reliable communication
technologies, such as Time-Triggered Protocol (TTP), Time-Triggered Controller Area
Network (TTCAN) and FlexRay [9]. The latter was designed specically for automotive
applications [10]. TTP, TTCAN and FlexRay are all mainly time-triggered architectures.
Common for these protocols is that signicant events, such as tasks and messages, occur
not randomly in time, such as with the traditional event-triggered Controller Area Net-
work (CAN), but according to a pre-determined time-schedule. CAN is currently used in
automotive systems.

There are several reasons why time-triggered protocols are more appropriate for use in
XBW applications, compared to event-triggered protocols. In order to understand this,
one should possess basic knowledge of the structural dierences between these two com-
munication architectures. An XBW system comprises several electronic control units
(ECU), set up as nodes in a network. The nodes are connected via a medium, generally
an assembly of isolated copperwires forming a so called bus-system. They communicate
via the bus by sending messages containing information regarding either the state of the
node or an event that have occurred in the node. A pure time-triggered protocol involves
solely state messages, while an event-triggered protocol involves solely event messages. As
◦
an example, consider a change of the steering angle regarding a SBW system from 9 to
◦
10 . A state message from a wheel angle sensor would include information stating a steer-
◦
ing angle of 10 , as would a corresponding event message state information concerning a
◦
change of the steering angle of 1 , i.e. an event is a change of state. Since event-messages
may be sent at any time, problems arise when two or more event messages are simultane-
ously sent to the same node. Only one message can be received at each instant, therefore
the messages might collide and merely the one with the highest priority reaches the re-
cipient. All event messages contain priority information stating its priority in relation to
other messages. Message collisions are possible to prevent by adding a queue function

5
to the system, but the architecture is still non-deterministic, there is no way to guar-
antee when a message will be successfully transmitted. A time-triggered state message
on the other hand, can only be sent at specic moments, according to a time-schedule.
Thus, provided that the system is properly designed, no collisions arise, and the time at
which a message can be successfully transmitted is guaranteed. The behavior described
makes time-triggered protocols appropriate for XBW applications, where a deterministic
behavior is of utmost importance [10, 11].

One drawback with the time-triggered protocols is due to the time-scheduling. If a node
does not need to send a message during one of its designated time-slots, that specic
slot is left un-used. Thereby, the performance of the system is not fully taken advantage
of. Another problem is that time-triggered systems have to be synchronized and are
complicated to expand. All nodes have to be implemented to the time-schedule from the
start or excessive reconguration might be necessary [10].

TTP is a pure time-triggered protocol, see table 1. Every time-slot is assigned to a specic
node. It supports bitrates up to 2-25 Mbps depending on transfer medium. TTCAN and
FlexRay diers from TTP by implementing the event-triggered function as a lower layer to
the time-triggered structure. Certain time-slots are assigned exclusively to specic nodes,
and others are assigned to several nodes simultaneously in priority order, according to
the event-triggered architecture. TTCAN, in resemblance with CAN, supports bitrates
of up to 1 Mbps. Flexray supports bitrates of up to 10 Mbps on two channels. They
can either be used together to achieve bitrates of up to 20 Mbps, or work redundantly,
thereby implementing fault-tolerance to the system [10].

Table 1: Protocol properties.

Protocol name CAN TTCAN TTP FlexRay

Classification Event-triggered Time-triggered Time-triggered Time-triggered
Message type Event Event/State State Event/State
Bitrate 1 Mbps 1 Mbps 2-25 Mbps 10-20 Mbps

2.2 Longitudinal motion

The longitudinal motion of a WCM vehicle is controlled through operation of in-wheel

motors and friction brakes. The in-wheel motors are able to accelerate as well as decelerate
the vehicle, whilst the friction brakes are utilized when additional brake force is required.

2.2.1 Propulsion
In-wheel motor properties All of the considered WCM concepts, discussed in section
4, utilize electrical in-wheel motors for means of propulsion. One major advantage and
key property related to in-wheel motors is the compact design. In-wheel motors are
constructed to t into the unexploited volume inside the rim of a conventional wheel, see
gure 5, in terms of conventional vehicles generally housing no more than a wheelhub and

6
brake components. By replacing the traditional ICE with in-wheel motors coupled with
batteries, more structural freedom is enabled concerning the layout of the vehicle body
and interiors. Hence, important aspects concerning passive safety, manufacturing costs
and interior design versatility can be improved.
friction, which is a linear function of weight. Therefore, CASE III: PARALLEL HEV
for a certain vehicle size the maximum extended speed
A conventional
ratio ICE
is unique and, in this based
exampledriveline requires
it is (100/19.2) 5.2x. a gearbox mainly for two reasons. Firstly, it
transforms the high angular velocity and low torque In on
parallel
the hybrid either shaft
outgoing the electric
of the motor
ICEorto engine or
12000
even both can propel the vehicle. Therefore, use of
a low angular velocity and high torque on the propelling multi-gearwheels. Secondly,
transmission becomesthe gearbox
necessary in parallel
HEV because of the engine. Depending on the position
provides10000 the ability to compensate for the narrow power spectrum generally related to
M=2000 kg, of the transmission system there can be two parallel
combustion engines, by shifting gear ratio depending
Pm=85kW HEV onarchitectures:
desired vehicular pre-transmission
velocity, and see post-
Force in Newton

8000 transmission hybrids. In pre-transmission hybrid (figure

gure 3a. M=1600 kg,
5.a) the gearbox is on the main drive shaft and before
Pm=69kW pointedthe outtorque
a unique methodology
coupler. Therefore, the of selecting the
gearbox affects the limits the
6000 motors possess quite dierent output
Electrical traction motor basedof
characteristics
performance onboth
the capability
compared
engine and of
tomotor
ICEs.
electricoperation
An In post-
drive. is calcula
M=1000 kg,
Pm=44.5kW in the transmission
extended constant power 5.b),
hybrid (figure regionthe [6]. It was
gearbox is on the
electrical
4000 motor power curve can be divided into two
revealedenginesections,
that the
initial and
shaft constant
acceleration
after the and
torque force
grade region
condition
coupler. Therefore,
and the constant power region, referring to tractive could beonlymetthewith
force minimum
engine
and power
performance
motor power israting if the
affected by power
respectively. the gearbox.
2000
train can
Thebe operated
electric drivemostly in the constant
may function on singlepower
gear speed
The specic rotationalv rm velocity in between these region.sections
Generating
reducer. is called
This theextended
the optimal
demands base speed,
torque-speed often
profile from
speed operation
throughthe the vehicle’s propulsion system is extremely
motor.
expressed 0 in revolutions per minute. Below important, the basebecausespeed itthe tractive
reduces the cost force is at its
by reducing the Here, fr
0 10 20 30 40 50 60 70 80 90 100
system power rating.
maximum rated level, Fmax . At higher rotational velocity, the motor produces a constant maximum
Speed in mph Engine vehicle c
power output, Pm , illustrated in gure 3b [12]. Since tractive force is available from
Clutch base and
Figure 3. Maximum extended speed range of EV/series CASE I: EV AND SERIES HEV
standstill, no clutch is needed to initiate movement. Owing to the constant power delivery
center of
HEV. (2) define
Torque
related to electrical drives, shifting the gear ratio is unnecessary. Dependingcoupler on motor Axle (3) impo
In a general EV or series HEV system, there is only one of tire sli
CASE II: CONVENTIONAL
specications CARS
a xed gear might be necessary between motor and wheel. However, in-
propulsion unit, which is the electric motor. Gearbox
The ideal we get eq
wheel motors are often constructed to produce a high
force-speed profile of an electric motor is shown figure 1.
torque at low angular velocity,
Speed
Battery
This typical drive characteristic can be divided into two
Electric Motor Reducer
Although equations (1), (2) and (5) were obtained for a distinct sections: constant Wheel
thereby enabling direct drive operation. Considering the characteristics torque (ordescribed,
force) regionthe
and
pure electric propulsion system, the conception of constant power region. In constant torque region the
presence
extended of a multi-speed
speed gearbox
operation still holds isgood
generally unnecessary
for electric in combination
motor provides with
its constant rated in-wheel
torque ‘T ’ (or max The qua
conventional internal combustion engine (ICE)-based Fmax ) up to its base speed ‘Nb’ (or vrm). At this speed the
motor
vehicles.drives.
In engine driven vehicles, constant power motor solutions
reaches its rated power limit ‘Pm ’. The operation
operation is attained through a multi-gear transmission beyondFigure 5.a.speed
The pre-transmission architecture. will be im
the base is called constant power region.
The weight
system. of transmission
In figure components
4, we observe a 90 kW are generally high, therefore removal of such might
engine solution
In this region the motor provides rated power up to its
achieving the ideal force-speed profile of a drive system maximum speed. This is obtained by reducing the field the resul
enable weight
using a 5-gear reduction.
transmission. The However, one13.45,
gear ratios are shall have in mind that a WCM vehicle might
Engine
flux of the motor Clutch Gearboxis also known as ‘field-
and, therefore,
7.57, 5.01,
include 3.77 components
other and 2.84. The carrying
engine is modeled by weakening
signicant mass, such as Figure
region’. dense1battery
denotes apackages.
‘3.3x’ type motor,
linearly scaling the torque axis of a 102 kW spark ignition where the constant power region extends beyond the
Dodge Caravan engine [5]. constant torque region by a factor 3.3 (vmax / vrm).
Torque
Axle
coupler
11000 7000 90
Engine force Constant Tractive force
10000 1st gear Ideal curve Battery SpeedMotor power
Force, FmaxElectric Motor 80
6000 Reducer Wheel Point A
9000
Tractive force in Newton

Maximum allowable 70
8000 force, Fmax 5000 Wr
Force in Newton

60
Power in kW

7000
Constant
6000 2nd gear Figure 5.b. The post-transmission architecture.
4000 50
Power, Pm Figure 2
5000 40
3rd gear 3000
4000
4th gear Post transmission HEV For the w 30
3000 5th gear 2000
case wa 20
2000
1000
In a post-transmission hybrid system the extended drag and
1000 speed ratio depends on the ‘hybridization10factor’ (HF). form as s
This is the ratio V rm betweenV rvmotor power and the total
0 0
propulsion v
0 10 20 30 40 50 60 70 80 90 100 0 10 20power 30 40 ‘PTotal
50’, 60
and 70 is expressed
80 90 100in percentage rm
Speed in mph (HF=100%×P m /P Total
Vehicle ). Performance
speed in mph of the parallel HEV
depends on HF. Increased HF means increased motor 0 F − m
∫
(a) Conventional vehicle equipped with ICE attain-(b) Ideal 1.force curve by electric
use ofcharacteristics.
an idealis electrical motor.
max
power. Since the motor naturally more efficient
Figure than Anthe
ideal motor
ICE, drive
increased HF will increase the overall
ing desired
Figure ideal force curve
4. Conventional by use
vehicle of a 5-speed
achieving gear-
the optimal
system efficiency. However, the weight of the power
box force-speed profile with 5-gear transmission train increases Here, ρ i
system. To calculate the total with increased
traction power hybridization.
of a vehicle, We the carried and A is
out commonly
constraint a detailedimposed analysisonon thethe performance
propulsion unit isof pre- by nume f

Figure 3: Ideal force curve illustrative

transmission
the initial comparison
acceleration. andThepost-transmission hybrid system with
[12].objective is to meet the
basic
differentperformance
acceleration HFs [8]. We with concluded minimumthat a power.
50% HF An gives the get the m
(3) and (
analytical expression relating traction power ‘Pm ’ with J1711 from num
best fuel economy and also passes the SAE
partial charge
initial acceleration (0 totest (PCT).
vrv mph in tfAseconds)
parallel isHEV
givenover
in 50% speed ra
hybridization failed to meet the charge sustainability
7 equation (1) where aerodynamic resistance and friction passenge
is neglected for the time being [6]. initial acc
m 2 (vrv =26.8
Pm = ( vrm + v rv )
2
(1) and, ma
2t f Necessa
m 2 presente
or, Fmax ⋅ v rm = ( v rm + vrv )
2
(2)
The degree of eciency related to in-wheel motor operation is often very high. Theoreti-
cally, the eciency can be as high as 96 % [13]. In comparison to other electrical drives
like e.g. a single electrical motor setup, in-wheel motor applications benet from the
fact that they generally do not contain any transmission. A concept vehicle called IZA,
presented at an IEEE conference in 1997, had a maximum total drive system eciency of
91 %, including losses in the power electronics. This vehicle used four permanent magnet
synchronous in-wheel motors with a maximum output of 25 kW and 417 Nm each [14].

In-wheel motors are presently not used in any serial-produced vehicle. Large amounts of
time and eort are spent for development and evaluation of this technology. The exposed
mounting position inside the wheel rim makes the motor subject for vast amounts of dirt
and dust. As part of the unsprung mass, it is also compelled to withstand a large amount
of vibrations. High voltage cables connected to the hub are exposed to constant friction
challenges due to the wheel's movement in relation to the chassis [15]. Before in-wheel
motor drives can be implemented into serial-produced vehicles, such issues need to be
sorted out, since reliability is a key property related to modern vehicles.

In 2008, Frost & Sullivan estimated that around 150,000 vehicles in North America and
120,000 vehicles in the European Union will be equipped with in-wheel motor technology
by 2015 [16].

Accelerate-By-Wire The rst XBW solution to be introduced in passenger cars was

the throttle-by-wire system. A TBW system admits the following function. In conven-
tional vehicles the driver regulates the amount of air included in the combustion process
by adjusting the accelerator pedal. A cable connects the accelerator pedal to the throttle-
plate. The driver controls the amount of air passing through the inlet manifold in a
direct manner, by setting the angular position of the throttle-plate. Contrarily, in terms
of TBW, an electrical actuator sets the throttle-plate angular position based on electri-
cal signals transmitted from a control unit. The latter is inuenced by, but independent
from, the driver input. Thereby, the control unit may set the throttle-plate angle not only
according to pedal position, but also by taking into account surrounding factors, hence
enabling optimized throttle-plate regulation. Driver assisting applications such as adap-
tive cruise control and collision avoidance functions are made possible, due to the fact that
the control unit can override the driver input. TBW technology is already implemented
in many modern vehicles.

Regarding the motor control of WCMs, throttle-by-wire would be a misleading expression,

since it refers to the throttle which regulates air infusion of an ICE. WCMs are not
powered by ICEs, consequently they do not include any throttle components. In terms
of WCMs, the corresponding function to TBW is the by-wire controlled angular velocity
and acceleration of the propelling motors. Hence, a more suitable expression, would be
accelerate-by-wire.

A four wheeled WCM vehicle can be equipped with either two or four in-wheel motors.
Depending on which setup is chosen, various properties concerning handling, comfort and
active safety can be achieved. The most versatile behavior may be accomplished by the
adaption of four individually controlled tractive wheels, although such a setup also involves
an increased structural complexity, particularly regarding the control system. Each motor

8
is controlled individually, but since the specic torque applied to each tractive wheel
aects the motion of the vehicle, they have to be regulated as one system. Each wheel
need to deliver the amount of torque momentarily optimal for the specic vehicle corner
in relation to the other forces acting on the vehicle. The optimal torque distribution
is continuously calculated by one or several control units based upon driver input and
information from position sensors, velocity sensors, acceleration sensors, force and torque
sensors and pressure sensors. This setup supports ecient traction control systems and
therefore oers major improvements in terms of handling as well as safety, compared to
conventional vehicles, see section 3.1.

Electrical machines suitable for in-wheel operation Regarding in-wheel motor

applications, certain fulllments associated to geometry and torque output are required
by the electrical machines. Since the general idea of in-wheel motor drives includes that
the rim shall be capable to hold the major part of the machine body inside the boundaries
of its volume, certain geometrical properties need to be met. The diameter of the machine
may preferably be large in comparison to the depth, and the overall size and weight shall
be kept down, in order for the motor to t inside the rim and keep the unsprung mass
to a minimum. For direct drive to be feasible, the motor shall also be able to produce a
continuous high torque at low angular velocity.

In addition to these in-wheel motor specic aspects, the following demands presented by
Zeraoulia et al. [17], are general for electrical machines intended for vehicular tractive
applications:

1. High instant power and power density,

2. high torque at low speed for starting and climbing, high power at high speed for
cruising,

3. wide speed range, including constant-torque and constant-power regions,

4. fast torque response,

5. high eciency over the wide speed and torque ranges,

6. high eciency for regenerative braking,

7. high reliability and robustness for various vehicle operating conditions,

8. reasonable costs.

Permanent magnet (PM) brushless motors, also called synchronous motors, are partic-
ularly suitable for in-wheel motor direct drive applications. Characteristic for such a
machine is that the rotor is equipped with permanent magnets instead of windings, see
gure 5. There are various types of PM brushless motors. Generally, they are classied
according to the mounting position of the permanent magnets, surface-mounted or buried.
The surface-mounted type contains the magnets on the surface of the rotor, whilst the
buried magnet motor keeps the magnets embedded in the rotor core, see gure 4. The
former requires less magnet material compared to the latter, given an equal size of the

9
nder,
der, Tobias
TobiasSchneider,
Schneider,Markus
MarkusKlohr
Klohr
ment
ent of
ofElectrical
ElectricalEnergy
EnergyConversion
Conversion
mstadt
mstadtUniversity
Universityof ofTechnology
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machines. It also benets from a cheap and simple construction. However, the buried
Darmstadt,
Darmstadt, Germany
Germany
magnet type may achieve higher air-gap ux density, which in turn admits higher torque
abinder@ew.tu-darmstadt.de
binder@ew.tu-darmstadt.de
per rotor volume. Additionally, the risk of demagnetization is smaller. Buried magnet
PM motors represent the more rugged solution [17, 18].

hnical
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Figure 4: Magnet mounting positions [19].
nd
d discusses
discusses IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY, VOL. 55, NO. 6, NOVEMBER 2006

ed
d machines
machines machines
machines and and switched
switched reluctance
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PM brushless motors have a limited eld weakening capability, owing to the presence of
he
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becoming
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etetmachines, excitation,
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However,
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s Motor (PM Brushless Motor)

Fig.
Fig. 22 shows
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Figure 5: In-wheel PM brushless motor [17].
are also called a permanent
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10
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Rahman et. al investigates the axial ux permanent magnet (AFPM) motor, which is
a particular PM brushless motor, as part of a direct drive in-wheel setup. The axial
ux permanent magnet motor have favorable characteristics for in-wheel motor drive
applications in terms of eciency and specic torque. The torque density can be improved
by as much as two times compared to an induction motor, which is another motor type
utilized in vehicular propulsion systems. The highest amount of torque output is achieved
by designing the machine with a large diameter and high number of poles. Thus, the
geometry of AFPM machines makes them suitable for in-wheel placement [12].

2.2.2 Brakes
In conventional vehicles, friction brakes such as hydraulic, pneumatic or non-hydraulic
mechanical disc or drum brakes are the primarily utilized solutions for means of decelera-
tion. In terms of WCMs, the conventional drums and discs are still being used, although
in a dierent manner. Signicant amounts of energy are dissipated in form of heat when
conventional friction brakes are applied. Regenerative braking through in-wheel motors
makes it possible to re-use parts of this energy. The in-wheel motors and friction brakes
can be applied separately or together depending on required brake force and momentary
vehicular velocity. During deceleration from low velocity the friction brakes are primarily
used, since the in-wheel motors require a higher angular velocity to be able to supply
enough brake torque. During light deceleration from high velocity the in-wheel motors
are preferably used exclusively. When excessive brake force is required, friction brakes
and regenerative brakes are applied together.

Regenerative braking through in-wheel motors The primary function of in-wheel

motors is to supply propulsive torque to the driven wheels of a vehicle. However, they
also have the ability to work in a reversed manner, thereby decelerating the vehicle. The
in-wheel motors then work as generators, transforming the kinetic energy bound in the
vehicle movement into electricity. This electricity is either directly redistributed to other
components, or supplied to a battery pack or power electronics comprising capacitors. A
compact design and capability of fast charge and discharge makes the latter a favorable
solution for this matter. By temporarily storing the energy, it can be reused at a later
point of time, thereby reducing the overall energy consumption. A regenerative brake
setup such as the one described, when combined with conventional disc or drum brakes,
also prots in form of less brake pad/shoe wear. This advantageous side eect helps keep
friction brake maintenance and environmental impact to a minimum.

The dead time between driver input and initiation of vehicular deceleration is shortened
by use of regenerative brakes compared to conventional hydraulic brakes [20]. Hence,
active safety solutions can be improved, see subsection 3.1.3.

Brake-By-Wire There are two types of brake-by-wire solutions suitable for operation
in WCM vehicles, the electro-hydraulic brake (EHB) and the electro-mechanical brake
(EMB). Both types are friction brakes.

11
EHB is based on a conventional hydraulic brake system, implemented with BBW control.
It is realized by combining a hydraulic circuit with an electrical circuit. The latter com-
prises a control unit, wiring and sensors, in accordance with gure 6, as well as electrically
controlled valves [21]. The hydraulic circuit comprises a hydraulic pump connected to a
brake caliper, of which the pump is merely controlled by means of by-wire technology.
Alternatively it may, in excess of the system previously described, contain a complete con-
ventional brake system representing a direct hydraulic connection between brake pedal
and brake caliper, for means of backup. In case of electrical circuit malfunction, the elec-
tricity is cut, thereby opening a valve engaging the conventional hydraulic system [22].
This fault-tolerant function makes it suitable for use during the transition from conven-
tional brake systems to BBW, as the safety of a conventional brake system remains. Since
2001, Mercedes Benz AG implemented an EHB system into some of their passenger car
models [23], called the Sensotronic Brake System. It shall however be mentioned that a
problem concerning the system resulted in a recall of more than 680 000 vehicles in 2004
[24]. This was a signicant set-back for the customers' condence in both Mercedes and
BBW systems in general.

Figure 6: Block diagram of a basic electro-hydraulic brake system.

EMB is a pure BBW solution including an operational device coupled with a control
unit in connection with an electro-mechanical brake actuator [21], see gure 7. Through
the operational device, the driver communicates the vehicular deceleration rate that is
desired, after which the control unit calculates the appropriate brake actuator force to be
applied to each wheel. EMB involves faster response compared to EHB, owing to its fast
motor dynamics [25].

A WCM can contain either one of these two described brake setups, however EMB might
be the technology primarily used as the technology evolves, owing to easier adaption to
the vehicle structure and the absence of uids.

12
 Figure 7: Block diagram of a basic electro-mechanical brake system.

Electro-mechanical disc brakes As previously stated, WCMs usually comprise re-

generative brakes in combination with friction brakes. The purpose of the latter is merely
to assist the in-wheel motors decelerating function when required, such as during heavy
or low velocity braking. It is quite possible to utilize EHBs for this application. How-
ever, since EHBs involve hydraulic liquids, they require more maintenance and possibly
hydraulic connections between wheel module and vehicle body. EMB setups are more
easily implemented into WCM structures, owing to its pure electro-mechanical function.
It eliminates the need for hydraulic oil, thereby simplifying maintenance as well as the
connection structure of the WCM. Owing to its electro-mechanical function, the EMB can
easily be integrated into the structure of advanced active safety systems, see subsection
3.1.3. Although a hydraulic brake can be electrically controlled, as with EHB technology,
the faster processing of a pure electro-mechanical brake makes it more suitable for the
application.

There are however a couple of problems surrounding the electro-mechanical brakes. Dur-
ing heavy braking the brake pad needs to be applied towards the disc with high pressure.
To nd a linear actuator which can deliver sucient force, yet meet the demands of low
cost, compact design and low weight can be dicult. One solution is to use a rotary-to-
linear converter such as e.g. a ball screw device, in connection with a rotary actuator.
That way a low torque can be converted into a high force. However, such devices require
frequent maintenance, thereby lowering the overall reliability of the system [8].

The VDO Electronic Wedge Brake, thoroughly presented in subsection 4.3, utilizes an-
other solution which enables use of a simple linear actuator. The latter applies a force
onto the side of a wedge formed element mounted in between the actuator and brakepad,
see gure 22. The geometrical relations in between the length and width of the wedge
element induce an amplifying eect, thereby increasing the low force created by the linear
actuator into an adequate force applied onto the brakepad.

Another issue concerning electro-mechanical brake systems is the fact that most of them
require a 42 V electrical system to function properly [5]. Such a demand might inhibit
implementation into serial-produced vehicles, since 12 V is the standard voltage utilized
in conventional vehicles. Siemens VDO's Electronic Wedge Brake is however claimed to
function properly in connection to 12 V systems [26].

13
2.3 Vertical motion

A WCM may contain either a passive, a semi-active or an active suspension system or any
feasible combination. The main objective of this section is to discuss the active suspension
system, which oers several advantageous properties compared to passive and semi-active
systems. Referring to the WCM concepts presented in section 4, active suspension solu-
tions are widely used in terms of WCM technology.

An automotive suspension system have several tasks to fulll. It shall...

ˆ ...support the static load of the vehicle with passengers,

ˆ ...reduce the vehicle body vertical acceleration, in order to improve passenger com-
fort,

ˆ ...minimize the dynamic tire load during cornering and braking, in order to improve
handling,

ˆ ...minimize roll and pitch motions during maneuvering [27].

To fully understand the function of an active suspension system, one should possess knowl-
edge about the conventional passive equivalent. Hence, below follows a basic presentation
of the latter.

2.3.1 Passive suspension

The vertical motion of a vehicle is, in terms of conventional vehicles, passively controlled
by means of a suspension system, generally comprising springs such as leaf springs, coil
springs or air springs and dampers, usually containing hydraulic oil. Additionally, roll
motions can be inhibited by means of a torsion bar. The suspension links the wheels to
the vehicle body and allows relative movement.

The springs support the load of the vehicle body at all times, and absorb the dynamic
energy related to acceleration, cornering and braking. Hydraulic dampers dissipate the
kinetic energy absorbed by the springs into heat, through application of a velocity depen-
dent force, counteracting the induced spring deformation. This way, excessive suspension
movement and oscillation is inhibited.

In accordance with the principle function described, the main objectives when designing a
suspension system is to isolate the vehicle body from road disturbances while maintaining
continuous contact between road and tire. These properties, if successfully implemented,
admit optimized ride comfort and handling performance. However, when utilizing a pas-
sive suspension system, the result is always a compromise between these two conicting
characteristics. A softer suspension setup generally involves a more comfortable ride but
degraded road holding properties, while a stier setup generally involves better road hold-
ing properties but degraded ride comfort. Since the setup is invariable, it is a result of
expected operating conditions according to the developer.

Passive suspension systems are low cost and easy to manufacture [28]. Additionally, they
combine fairly good suspension performance with reliability and ease of implementation.

14
2.3.2 Semi-active suspension
A semi-active suspension system is basically a passive suspension system, with the ability
to vary the damping properties. The damping characteristics can be adjusted through
application of a low-power signal. The two most common technologies involve either a
solenoid-valve or an electro-magnetic eld, for the purpose of damper adaption. Both
solutions involve regulation of the hydraulic oil ow between the extension chamber and
compression chamber of the damper, see gure 8.

The solenoid-valve solution, in its simplest form, is based upon opening and closure of
an additional valve in the piston. The oil transition in between the two chambers during
damper action thereby increases or decreases respectively.

The electro-magnetic solution instead varies the viscosity of the specic magnetorheologi-
cal (MR) uid utilized as hydraulic medium, by activating an electro-magnetic eld. The
MR-uid contains iron particles, which during inuence of a magnetic eld form bres,
which in turn increase the viscosity of the uid. By varying the viscosity, the damper
coecient is altered.

Owing to the adaptable characteristics, semi-active suspension extends the range of opera-
tion compared to passive suspension [28]. Considering an MR-damper, the dynamic range
(the ratio between the peak damping force and the damping force without magnetic in-
uence) is often in the range of 5-10. This means that the damper can be widely adjusted
according to prevailing road conditions and desired comfort and handling properties.

(a) (b)

Extension Extension
Chamber Chamber
Piston Rod Gas Chamber
Piston Valve Inner Tube
Compression Outer Tube
Chamber
Gas Chamber Foot Valve

Fig.Figure
1-4 (a)8: Mono-tube conventional damper [28].
Mono-tube and (b) twin-tube dampers.

Mono-tube dampers are simpler in terms of manufacturing, lighter due to the fewer parts, requiring
higher gas pressure, and are more susceptible to damage of the cylinder compared to their twin-tube
counterparts. In contrast, twin-tube dampers can operate with lower gas pressure, but they are more
complex, and have issues with dissipating the generated heat.

On the other hand, variable dampers vary the damping rate by varying the size of the valve opening
by means of a servo-valve, shim-valving, piezoelectric actuators, solenoid-valve, or using MR-fluid
in which the viscosity of the oil is varied, instead of the size of the valve opening. The following
section describes the solenoid-valve
15
and MR dampers.

1.3.2 Semi-active Dampers

Semi-active dampers use an actively powered valve or other technologies that allow the damping
2.3.3 Active suspension
The principal function of an active suspension system is similar to the passive and semi-
active systems previously described. The main objectives are still optimal ride comfort and
handling performance. However, in contrary to passive systems, the vertical motion can
be actively controlled, actuated by means of one or several suspension actuators coupled
to a power source. The actuators are connected to suspension linkages, admitting active
control of the relative movement between vehicle body and wheels. Often, the suspension
actuators are coupled with passive components such as dampers and springs. These can
be mounted in series with, or parallel to, the electrical actuators. Depending on this setup,
the system attains quite dierent properties. This is further discussed in the following
sections.

An active suspension system can store and dissipate energy in the same manner as a
passive system, but also adds the ability to introduce energy to the system when necessary
[29]. Hence, the relative vertical movement between wheel and vehicle body can be more
freely assigned, compared to passive systems. To exemplify the opportunities enabled by
utilization of active suspension systems, consider the following. A vehicle is in motion
along a road. A camera mounted in front of the vehicle detects upcoming disturbances in
the road prole, and transmits this information to the suspension control unit. Based on
the road prole information, the control unit continuously adjusts the vertical distance
between the wheels and the vehicle body. The wheels follow the road prole, while the
body is kept unaected from the road irregularities. It shall be mentioned that this is an
idealized example. In reality there are boundary conditions set by the nite suspension
travel and invariant points in the suspension transfer function [27].

The active suspension components are dependent on input from a proper management
system. A closed loop control system ensures the calculation of a correct reference actuator
force. It involves actuators, power electronics converter, mechanical components and
instrumentation feedback [30]. For each instance, the suspension properties are adjusted
according to prevailing operating conditions. Hence, improved ride comfort and handling
performance is attainable.

Active suspension systems may overcome the need for setup compromises related to pas-
sive suspension systems. However, they generally involve drawbacks including high energy
consumption, high weight, high cost and non fail-safe operation [28]. Thus, commercial
vehicle implementation has so far been limited.

Active suspension systems are generally separated into two main types, high bandwidth
systems and low bandwidth systems. These two types may in turn be categorized accord-
ing to the actuator technology used. This section treats two kinds of suspension actuators,
hydraulic and electro-magnetic.

16
High bandwidth systems In terms of high bandwidth active suspension systems, the
actuator is generally mounted in between the sprung mass and the unsprung mass, often in
parallel with a spring, see gure 9. It aims to counteract both high disturbance frequencies,
with a maximum amplitude around the frequency 12 Hz, and low disturbance frequencies,
with a maximum amplitude around 3-4 Hz. Since the suspension can be actively controlled
over a broad bandwidth, it admits very good road holding and comfort properties. A
drawback with this particular type of suspension is that it generally consumes a signicant
amount of power. It also requires the use of broad bandwidth actuators, which are
generally more expensive than narrow operating actuators. Vehicle height regulation is
only possible by increasing the applied actuator force [27, 29, 31].

Figure 9: Principal setup for a high/low bandwidth active suspension system [27].

Low bandwidth systems In terms of low bandwidth active suspension systems, the
actuator is generally, in common with high bandwidth systems, mounted in between the
sprung mass and the unsprung mass. However, it is mounted in series with a passive part
comprising a spring and damper device, see gure 9. The active part of the suspension
counteracts disturbance frequencies in the lower range, while the passive part handles the
higher frequencies. This construction admits easier vehicle height regulation and lower
energy consumption compared to high bandwidth systems [27, 29, 31].

Hydraulic active suspension A hydraulic active suspension comprises hydraulic ac-

tuators for means of suspension. A hydraulic pump supplies the actuators with sucient
pressure. According to gure 10, the pump may be driven by an ICE if implemented in
a conventional vehicle. In terms of WCM vehicles, the pump is electrically driven, pow-
ered by batteries or any other feasible power source. Regulation of the actuator pressure

17
 IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY, VOL. 55, NO. 1, JANUARY 2006

agnets Linear Actuators Applicability

omobile Active Suspensions
is taken care of by utilizing a low-power electromagnetic actuator which connects to a
hydraulic valve. A control system including sensors and instrumentation together with a

EEE, Jorge Esteves, Senior Member,

power electronics IEEE,
converter Gil D.
completes Marques,
the Member,
construction [30]. IEEE,

andHydraulic
Fernando Pinasuspension
active da Silva have drawbacks in form of a complex structure due to a high
number of mechanical devices, and the hydraulic oil requirement.

in automobile suspension
ystems. However, current
sive and complex. Develop-
permanent magnet materi-
s analysis of the possibility
ators in order to improve
ion systems without exces-
n this paper, the layouts of
uspensions are compared.
ed, and some experimental
uspension could become a

omobile suspension, linear

ON

r actuators in automobile Fig. 1. Block

Figure 10: diagram of a hydraulic
Hydraulic single-wheel active
active suspension blocksuspension.
diagram [30].

oposed by other authors.

d unconstrained integra-
re very important Electro-magnetic
factors activethree
Currently, suspension
types of vehicle suspensions
A hydraulic damperare used: pas-kinetic energy
converts
into heat, whichsive,insemi-active and active.
turn dissipates without All systems
being implemented
utilized. Segal andinXio-Peo
auto- [32], shows
ctromagnetic actuators in
that roughly mobiles
200 W of today
power areisbased on hydraulic
dissipated in termsorofpneumatic
an ordinaryoperation.
sedan passenger car
However,
travelling a poor road atit 13.4
is found
m/s. that thesea regenerative
Hence, solutions do suspension
not satisfactorily
structure might be
agnets in electromagnetic
desirable. solve the vehicle oscillation problem, or they are very expensive
Tall in 1961 and Lyman
Electromagnetic
andsuspension
increase theadmits
vehicle’s energy consumption [14] and [18].
such a regenerative function by converting the kinetic
hors have proposed other
Significant improvement of suspension performance is
energy involved in the vehicle body vertical movement into electrical energy, which in
automobile suspensions
turn
achieved by active systems. However, these are only used in
However, the use ofcan be stored for latter use [28]. This function somewhat improves the high energy
rota-
a small number of automobile models because they are expen-
consumption issue that is generally associated with active suspension systems. Owing
convert the rotational into
sive and function
to the fully electrical complex.of this suspension type, it simplies implementation with
orce value.
WCM modules. In Fig. 1, a block diagram of a single-wheel hydraulic auto-
ny kind of gearbox. The
mobile active suspension is presented. The vehicle engine drives
sing linear actuators
Figurewere
11 shows a block diagram of a basic electromagnetic active suspension structure.
a hydraulic pump to power the suspension based on hydraulic
As previously stated, this construction requires less mechanical parts compared to the
actuators that create oscillation-damping forces between the ve-
ages of electrical suspen-equivalent. Hydraulic components such as pump and valves are needless. How-
hydraulic
hicle body and the wheel assembly. A hydraulic valve is driven
actuators haveever,also the
beenelectromagnetic actuator as well as the power electronics shown in gure 11 must
by a low-power electromagnetic actuator in order to handle
obviously be larger than the ones shown in gure 10. The electromagnetic actuator may
be systems
the actuator forces. The control system, together with a power
of either rotary or linear type. Rotary actuators may require some kind of gearbox to
hicle suspension
translate
electronics converter, drives the low-power electromagnetic
oad irregularities in orderthe rotary movement into linear and amplify the applied force, hence increasing
the structuralactuator.
complexity [30].
nd to produce continuous
In the last ten years, developments in power electronics,
icle handling quality [20].
permanent magnet materials, and microelectronic systems have
brought significant improvements 18
in the electrical drive domain.
vised June 28, 2004. This work Increases in dynamic and steady-state performances, reductions
I-14270/1998, named “Electro- in volume and weight, unconstrained integration with electronic
RAXIS Program and Portuguese control systems, reliability, and cost reduction are important
by Dr. M. Abul Masrur.
factors that justify the generalized use of electrical drives.
 MARTINS et al.: PERMANENT-MAGNETS LINEAR ACTUATORS APPLICABILITY IN AUTOMOBILE ACTIVE SUSPEN

Fig. 3. Model of a hydraulic

Fig. 2. Block diagram of an electromagnetic single-wheel automobile active

Figure 11: Electromagnetic active suspension block diagram [30].
suspension.

2.4 LateralInmotion
Fig. 2, a block diagram of an automobile suspension us-
ing an electromagnetic linear actuator is shown. An electrical
The control ofgenerator
a vehicle's lateralamotion
feeding batteryisnow
a safety-critical
replaces the function. Thus,
complex and ex-it is of utmost
importance that the steering system maintains a proper function during
pensive hydraulic power supply. The hydraulic valve and actu- all possible op-
erating conditions. A conventional
ator have been removed rack andthe
from pinion steering
system. system is regarded as safe,
An electromagnetic
owing to its rigid mechanical
actuator driven bystructure.
the controlHowever, in terms
system through of WCM
a power vehicles, by-wire
electron-
based electrical steering systems are generally considered, since the utilization of such
ics converter is the main component of the wheel suspension
adapts well to the modular design. It allows the mechanical steering connections, gen-
system.
erally found in between the front wheels and the passenger cabin, to be removed. Fig. 4.With
Model of an electrom
The actuator and power electronics must be larger in this case.
an electrical steering system comes the possibility of improved handling performance and
However, the system is simpler since it has fewer devices and
active safety, see subsection 3.1, since the driver input can be optimized by utilization of
mechanical parts. Because it has no hydraulic devices, this is an 2) Electromagnetic Su
computational resources.
oil-free system. ator replaces the dampe
This paper shows that it is now possible to build an elec- with the spring an oil-fre
2.4.1 Steer-by-wire
tromagnetic actuator producing the required forces and with of an electromagnetic su
suitable power and dimensions for this application. The friction force of a
Steer-by-wire is a lateral motion XBW system, controlled by the driver in co-operation glected. So, the dynamic
with an electronic control unit, multiple sensors and one or several actuators. Regardingbecome
II. ACTUATOR REQUIREMENTS
conventional vehicles, when changing direction, the driver by own means compensates for
A. Active Suspension Models
varying road conditions and wheel slip by correcting the steering wheel angle. In terms of
ms z̈s = −ks (
SBW the procedure is quite dierent. The driver communicates an input signal through
1) Hydraulic Suspension: Fig. 3 presents a model for ve- mu z̈u = ks (zs
operation of the steering device, indicating which direction of motion is desired. The
hicles with independent suspensions, where ms represents a
and the electromagnetic
control unit gets information from multiple sensors regarding prevailing road conditions
quarter of the “sprung” mass of a vehicle, mu the “unsprung”
and vehicle position. It then compensates for these factors and sets the optimal dynamics
wheelas the hydrauli
mass of one wheel with the suspension and brake equipment,
angles, suspension setup and throttle/brake torque, required for the vehicle to move in
ks the spring stiffness, kt the tire stiffness, and bs the damper
the direction indicated by the driver. This process is very rapid and repeats continuously.
FA = −
coefficient. The variable
As described, the key dierence regarding
F represents the friction force [21].
f the maneuvering of a conventional vehicle com-
3) Control System: T
The dynamic behavior of a single-wheel suspension system
culation of an actuator
may be expressed by the following differential equations:
with expression (3) in th
19
ms z̈s = −ks (zs − zu ) − bs (żs − żu ) − Ff + FA (1) with (6) in the case of a
The actuator, power ele
mu z̈u = ks (zs − zu ) + bs (żs − żu ) − kt (zu − zr )+Ff − FA . ponents, and instrument
(2) closed-loop automatic co
pared to a steer-by-wire equivalent, is that the driver communicates the desired directional
change of the vehicle, rather than the steering angle of the wheels.

SBW systems can comprise individually steerable or pairwise steerable wheels, the former
being the technology primarily used in terms of WCM vehicles.

2.4.2 Steering device

Conventional vehicles generally have a mechanical rack and pinion setup connecting
the steering device with the front wheels. In terms of SBW, mechanical connections are
excluded, thereby allowing alternative variants of feasible SBW steering devices.

The conventional steering wheel is a good solution for controlling the direction of a WCM
vehicle, since drivers are already familiar with its function and operational feeling.

An alternative solution is to implement a joystick, see gure 12. Such a device can control
not only the direction of the vehicle, but also propulsion and brake forces. Hence, the
conventional foot pedals controlling acceleration and brakes can be excluded, allowing
the lower area of the cabin to be more freely designed. The absence of foot pedals also
improves safety, since there are less parts that may protrude the cabin during a collision.

Figure 12: Joystick steering device implemented in Mercedes-Benz SCL600 concept car
[33].

Despite which type of steering device is utilized, lateral displacement is enabled, owing
to the absence of mechanical connections between the device and the wheels. Thereby,
the steering wheel or joystick can be easily adapted to either right or left side steering.
Concerning conventional vehicles, left side steering vehicles are manufactured for right-
hand trac markets, and right side steering vehicles are manufactured for left-hand trac
markets. Such a dierentiation is not required when utilizing side-to-side adjustable SBW
steering devices. Another alternative to enable this feature is to have a joystick mounted

20
between the front seats, see gure 12, allowing access from both the left and the right
seat.

A haptic feedback device, as described in subsection 2.1, may be required in order for the
driver to be able to feel the road. Such a device functions as a feedback tool for the
driver, hence allowing a safer and more enjoyable ride.

Haas and Kunze [34] performed a comparison on the eect on driving performance during
operation of a joystick compared to a conventional steering wheel and foot pedals. Eight
male civilian volunteers with normal visual acuity drove a simulated military tank along
a simulated road with a speed of either 15 mph or 45 mph. The task was to keep the
center of the vehicle on the road while maintaining speed. The test was rst conducted
operating a conventional steering wheel in combination with brake and accelerator foot
pedals. Thereafter, the same test was conducted using a joystick as steering device. The
results indicated that a steering wheel did not provide any signicant advantages over a
joystick when it came to driving performance. During the lower test speed of 15 mph there
was no signicant dierence regarding the mean driving velocity. The higher test speed of
45 mph resulted in a small dierence between the two dierent steering setups, however
it was small enough to be discarded as of no practical signicance. Additionally, there
was no dierence in the driver's ability to keep the vehicle centered on the road among
the two controller types. The report further implies that a joystick steering device has
potential as an alternative control technology because it provides more control through
curves, possibly because it admits greater use of the hand, wrist and ngers to improve
the degree of driver control, compared to a conventional control setup.

2.5 Power source

Operation of a WCM vehicle is based upon electrical power. This power can be supplied
from several types of on-board power sources such as an ICE equipped with a generator,
hydrogen fuel cells or a pack of chemical batteries. In this section, focus is aimed at the
latter alternative, chemical batteries, since it represents a frequently utilized power source
in terms of electrical vehicles.

2.5.1 Chemical batteries

Batteries store energy electrochemically. They are built up by one or several low-voltage
battery cells, connected in parallel and/or in series depending on desired properties. The
battery can be charged by applying a voltage over the two terminals, after which an
internal chemical process takes place [35].

There is currently a major demand for high capacity batteries. Even so, the battery
technology evolves in a slow manner. The main reason for this is the lack of suitable
electrode materials and electrolytes. Research in the area is very time consuming [36].

One major benet when considering a battery package as primary vehicular power source
is its capability to adapt to various designs. Battery packages generally comprise a number
of cells, which can be placed in dierent locations of the vehicle, according to desired
weight distribution and design related boundary conditions. One particular alternative to

21
consider is the skateboard-design, see gure 13. By placing the batteries inside the oor
of the cabin, the chassis adapts a basic design similar to a skateboard, at with one wheel
mounted to each corner. Such a layout enables the cabin, including impact deformation
zones, to be more freely assigned, while at the same time lowering the center of gravity.

Figure 13: Skateboard-design chassis, GM AUTOnomy concept vehicle [37].

Battery types Many types of batteries exist, such as lead-acid (Pb-acid), nickel-
cadmium (Ni-Cd), nickel-metal hydride (Ni-MH) and lithium-ion (Li-ion). As the names
imply, they contain dierent chemical reactants, giving each battery type unique proper-
ties. There are several factors to consider in terms of the capability of a certain battery
type. Energy density, energy capability, round trip eciency, cycling capability, life span
and initial cost are all important [35].

−1
Ni-MH batteries, with a specic energy of 60 Wh kg , represent an interesting alternative
for vehicle traction purposes. These batteries can be specically designed for high power
outputs, allow fast recharging and oer a high number of charge cycles. Ni-MH batteries
can be found in several current hybrid-electrical vehicles, such as e.g. Toyota Prius [38, 35].

Energy-dense Li-ion batteries, such as being used in e.g. laptops, are currently being
implemented into electrical vehicles. As an example, the battery package installed in the
Tesla Roadster, a fully electrical vehicle currently on the market, comprises 6800 Li-ion
−1
cells with a total package mass of 450 kg. With a specic energy of about 180 Wh kg
and an eciency of around 90%, Li-ion batteries have a very high capacity compared to
other battery types. Although currently associated with a high manufacturing cost, it is
one of the most promising candidates for on-board power supply [38, 39].

However, there are some issues concerning the Li-ion technology. The cathode generally
contains cobalt, which can only be obtained from natural resources. It makes up 20
parts per million of the earth's crust. With the large amount of Li-ion batteries currently
being produced, this cathode material might not be sustainable. In addition, if all 800
million cars of the world would be replaced by electrical vehicles or plug-in hybrid vehicles
powered by 15 kWh Li-ion battery packages, 30% of all known lithium reserves would be
used. If such a large share of the lithium reserves is to be utilized, price increases are likely

22
to appear, since the extraction process is likely to become increasingly expensive as the
reserves decrease. However, recycling could ease these problem. Both Toyota and Tesla
Motors are currently developing recycling processes which are intended to enable reuse
of recycled metals such as cobalt and lithium in new batteries [40, 41]. Until now the
recycled lithium have been used solely for steel production, since the quality and purity
of the metal have been fairly low [40]. Owing to more precise extraction- and assorting-
processes, the lithium can now be reused for manufacturing new batteries [40]. According
to Armand and Tarascon [36] there are also possibilities to extract lithium from sea water
in practically unlimited quantities.

Another factor to consider in terms of Li-ion is related to safety. The presence of both a
combustible material and an oxidizing agent involves risks of re or explosion. This safety
risk can be minimized with improved monitoring and management [36].

Table 2 lists basic properties for the battery types discussed above.

Table 2: Battery properties [38].

Type Specific energy Specific power Number of cycles Efficiency

(Wh∙kg-1) (W∙kg-1) (%)
Pb-acid 30-40 80-300 500 82.5
Ni-Cd 50-60 200-500 1350 72.5
Ni-MH 60-70 200-1500 1350 70.0
Li-ion 125-180 80-2000 1000 90.0

−1
The specic energy of petrol, 10-12 kWh kg , is vastly more than any current battery
technology can oer. Therefore, regardless of the type, battery packages involve large
amounts of additional weight and volume compared to ICE based power systems. This
is valid even though electrical motors operate with a much higher eciency than ICEs.
For a fully electrical battery-powered vehicle to attain a range of operation comparable
with a conventional ICE vehicle, without adding a large amount of weight, a remarkable
progression in high-capacity battery technology is required. In addition, the battery
life-time needs to be extended in order to avoid high costs related to battery change.
These issues have to be resolved in order for chemical batteries to become a sustainable
competitive alternative as a primary vehicular power source.

23
3 The eects of wheel corner module technology
There are major construction dierences between WCM vehicles and conventional vehi-
cles. This results in quite dierent properties regarding vehicle handling, comfort and
safety as well as environmental and economical impact. The following sections discuss
these areas, in order to enlighten the possibilities and drawbacks involved with WCM
technology.

3.1 Vehicular aspects

The following sections describe the characteristics and possible issues related to handling,
comfort and safety of a typical WCM vehicle. The impact of an increased unsprung mass
aects all of these areas. It is therefore discussed separately in section 3.1.4.

3.1.1 Handling
Handling is dened as a vehicle's response to driver input [42]. Good handling performance
generally implies that the vehicle responds accurately to driver input. It shall behave in a
safe and controlled manner, even during aggressive driving. The sports car is a typically
well handling vehicle.

There are several factors to take into account when estimating the handling performance
of a vehicle. The driver's subjective experience of how well the vehicle reacts according
to operational inputs can be dicult to measure and may dier between drivers. As of
today, there is no standard of how to objectively measure handling performance.

P.E Uys et al. [43] investigate dierent methods generally utilized in terms of handling
performance estimation, in order to nd the most suitable objective measure. Investigated
methods include among others dynamic wheel load as a measure of driving safety, pitch
motion and roll angle as measures of steering stability and RMS tire contact force as
an indication of wheel hop and road holding capability. Lateral acceleration is generally
considered as a good measure of handling performance.

One of the key features of WCM technology is the opportunity to control all four wheels in-
dividually, enabled by the absence of mechanical connections between the wheels. Propul-
sion, braking, steering and suspension properties can all be set individually. This ability
can drastically enhance the handling capability, since it involves additional degrees of
freedom. Owing to the XBW structure, the handling properties can be set according to
driver characteristics and road conditions.

Individually controlled corner modules make it possible for the driver to change the han-
dling characteristics of the vehicle between understeering, neutralsteering and oversteer-
ing. Regardless of the necessity of such a function, it highlights the versatility enabled by
utilization of WCM technology.

Since all wheels have the ability to maintain individual steering angles, several opportu-
nities are enabled. The turning radius can be signicantly decreased [44], and steering
characteristics can easily be adjusted according to the driver's desire. Improved handling

24
is enabled since the steering properties, such as steering ratio and force feedback, may
automatically be set according to current driver characteristics and road conditions.

Individually controlled active suspension admits a higher level of comfort as well as im-
proved handling capability, through instant adaption to prevailing road conditions. With
individual rideheight adjustment comes the possibility of active roll- and pitch-angle man-
agement systems, as seen in e.g. Volvo Autonomous Corner Module (ACM). It also en-
ables the ground clearance of the vehicle to be altered according to driving style and road
properties. An aggressive driving style might benet from decreased ground clearance,
while driving on a rough bumpy road might benet from increased ground clearance [45].

In one of the WCM concepts further to be presented, Volvo's ACM, the camber angles
of all four wheels can be individually controlled. This feature improves the handling
versatility even more, by enabling increased lateral acceleration during cornering.

A WCM vehicle can be either two- or four-wheel driven. The latter alternative enables
more advanced handling properties, but also increases the complexity of the management
architecture. See subsection 2.2.1 for more information on this topic.

3.1.2 Comfort
Comfort is related to a person's subjective experience of well-being. When it comes to
automotive comfort the aecting factors are e.g. temperature, noise, body roll, pitch
and vibrations. A particular area in terms of automotive comfort is ride comfort. Ride
refers to the vehicle's vibrational response to road disturbances [42]. Hence, ride comfort
corresponds to a passenger's subjective experience of a vehicular journey in relation to the
induced environment in terms of mechanical vibration [42]. The root mean square (RMS)
of the vertical acceleration is generally used as an objective measure of ride comfort. Also
angular motions such as pitch and roll aect the experienced comfort.

As stated in subsection 2.3.3, active suspension systems may decrease the need for setup
compromises generally related to passive suspension systems. WCM vehicles equipped
with such systems are capable of combining optimized ride comfort with optimized han-
dling performance. Active suspension systems can instantly and continuously adapt to
the ground contact preferences for each wheel, resulting in an increased level of comfort.
Active roll and pitch management systems can eectively counteract angular motions,
which would otherwise deteriorate the level of comfort.

One shall notice, that implementation of WCM technology may have varying impact on
ride comfort. An increased unsprung mass, as discussed in subsection 3.1.4, can lead to
increased vertical accelerations.

Noise is dened as unwanted sound. There are several sources of noise in a conventional
vehicle. The internal combustion engine, aerodynamical vortices and tire-road friction all
create unwanted sounds. To a great extent, these sounds can be excluded from the cabin
by utilization of sound-isolating materials. However, sound isolation involves additional
weight and should only reduce the sound level to a certain level, since some sounds are
desirable from a safety aspect.

Consider a battery powered WCM vehicle equipped with in-wheel motors. Electrical
motors with associated components involve a signicantly lower noise level compared to an

25
ICE with corresponding performance. Hence, the noise level can be reduced, particularly
at low velocities. As the velocity increases, other sources of noise related to the tire-road
contact and aerodynamics have an increased eect. Since the body of a WCM vehicle
can be more freely designed compared to a conventional vehicle, see subsection 2.2.1,
aerodynamics can be improved and unwanted sound reduced. As for noise related to the
tire-road contact, active camber variation allows for the rolling resistance to be minimized
[45]. Decreased rolling resistance generally involves decreased noise generation.

3.1.3 Safety
Vehicular safety can be divided into two sections, passive safety and active safety. The
former includes safety equipment such as air-bags, deformation zones and seat belts. This
equipment aims to minimize injuries in event of an accident. Active safety on the other
hand, aims to prevent accidents from occurring in the rst place. Anti-lock braking sys-
tems (ABS), electronic stability programs and collision avoidance programs are examples
of active safety.

The utilization of XBW systems, such as the WCM, enable new safety solutions, passive
as well as active.

A major advantage concerning WCM vehicles compared to conventional vehicles, is the

absence of steering rack and pinion. It eliminates the risk of such components intruding
the driver compartment during a collision, with severe personal injury as a possible result
[46]. Furthermore, owing to the absence of a mechanical steering, additional space is left
for impact absorbing collapse zones, improving passive safety for the driver as well as
passengers [46]. Removal of the traditionally used ICE enhances passive safety in the
same manner.

Regarding active safety, the introduction of WCMs enables implementation of novel ad-
vanced collision avoidance programs. All maneuvering inputs of a WCM vehicle involve
electronic control units. In event of a hazardous situation, these control units can operate
without inuence of the driver, based on information from sensors and cameras. For ex-
ample, if an object is detected in the vehicular path, the control unit have the ability to
automatically, without inuence of the driver, brake the vehicle and if necessary steer to
avoid collision. Such a system has the benet of reacting much more agile than a human
driver. In addition to that, it has the ability to, with much more accuracy, calculate
the optimal path to avoid collision with both the obstacle in front, and any surrounding
objects. Obviously, such a collision avoidance system must involve a major fault tolerance
[46].

Regenerative brakes can improve active safety functions compared to conventional hy-
draulic disc brakes, due to faster excitation of brake torque. Sakai and Hori [20] in-
vestigates this possibility. In order to visualize the dierence in process time between
hydraulic brake systems and in-wheel motor regenerative brake systems, the conventional
ABS function is regarded. In terms of hydraulic ABS systems, a solenoid valve engages or
disengages the hydraulic pressure onto the brake caliper in order to keep the wheel move-
ment unblocked. According to the report, the hydraulic system involves a process time of
more than 60 ms, including solenoid dead time of at least 10 ms, and a response delay in

26
the hydraulic circuit (rst order delay) of at least 50 ms. This may be compared to the
stated regenerative brake process time of 1.1 ms, split into a dead time of 100 µs and rst
order delay of 1 ms. Through simulations, Sakai and Hori noted a 20% shorter braking
distance when combining a conventional hydraulic brake system with a regenerative brake
system, compared to the hydraulic brake system unaided.

The human factor is likely the primary cause of a majority of the trac accidents occurring
today. Table 3 highlights the usefulness of automated systems by listing some of the most
common driver errors, and which automated solutions that can prohibit these errors.
Not all of these errors are hazardous by themselves, however several such errors could
potentially attract the driver's attention from the road, resulting in an accident [47].

Table 3: Common driver errors with corresponding automated solutions [47].

Driver error Automated solution

Get into wrong lane Navigation system
Forget which gear Automatic gear shift
Only “half eye” on road Fully automated drive system
Distracted, need to brake hard Anti-lock braking system
Plan route badly Navigation system
Fail to recollect recent road Navigation system
Wrong exit from roundabout Navigation system
Intended lights, switched wipers Automated lighting
Forget light on main beam Automated lighting
Usual route taken by mistake Navigation system
Misjudge speed of oncoming vehicle Collision avoidance system
Queuing, nearly hit car in front Collision avoidance system
Miss motorway exit Navigation system
Manoeuvre without checking mirror Collision avoidance system
Fail to see pedestrian crossing Collision avoidance system
Brake too quickly Anti-lock braking system
Hit something when reversing Collision avoidance system
Misjudge gap in car park Collision avoidance system
Try to pass vehicle turning left Collision avoidance system
Attempt to drive off in third gear Automatic gear shift

With a complex technology like WCM being introduced, new safety issues arise. Proper
function of critical backup systems have to be regarded and guaranteed. Proving the
safety of a new technology obviously is very time consuming since no experience or docu-
mentation from prior work exists. Even though one of the particular advantages regarding
WCM technology is improved safety, incredible amounts of time is required for testing and
evaluation in order to treat possible problems related to the technology itself. Routines
used for testing conventional vehicles might not be well suited and developing new ones
take time. Fault tolerant behavior in terms of XBW systems is of utmost importance,
particularly concerning the steering and brake systems, see subsection 2.1.1.

27
3.1.4 Unsprung mass
The unsprung mass of a WCM vehicle is generally higher compared to an equivalently
powered conventional vehicle. A major part of this weight increase is caused by the
implementation of electrical in-wheel motors. Generally, the in-wheel motors are part
of the unsprung mass. Bridgestone's Dynamic-Damping In-Wheel Motor Drive System
however, separates the in-wheel motor from the unsprung mass by utilizing a separate
motor suspension, see subsection 4.2.

The main reason for in-wheel motors being part of the unsprung mass rather than the
sprung mass, is that such a construction is less complex. A permanent magnet electrical
motor with an output of 10-15 kW approximately weighs 10-15 kg, if a specic power
output of 1 kW/kg is assumed [48]. If such a motor is implemented into each wheel of a
vehicle, the unsprung mass of that vehicle increases with approximately 40-60 kg.

One of the major concerns regarding WCMs, is to what extent this increased unsprung
mass aects the handling and ride comfort of the vehicle. It is known that an increased
unsprung mass tends to increase the vertical accelerations in the vehicle body, which in
turn deteriorates comfort. In addition, the dynamic wheel load uctuations are likely to
increase which in turn involves handling performance degradation. However, novel active
suspension technology involves the ability to signicantly reduce such issues. If the result
is benecial compared to conventional vehicles may be highly dependent on the overall
module setup.

Vos et al. [48] investigated the eect of increased unsprung mass on ride comfort and
handling safety, by adding weights to the wheels of an ICE vehicle. According to this study,
the ride comfort decreased with 10-25% depending on road surface, when a 15 kg weight
was added to each of the front wheels. An equal experiment conducted with the same
weights moved to the rear wheels resulted in a 1-8% ride comfort deterioration. Based on
a validated computer model of the same ICE vehicle, they modied it to correspond to
a battery electrical vehicle. This was done by adding a total mass of 160 kg and shifting
the weight distribution. When equal tests were simulated with this latter model, the
ride comfort was equal to, or slightly better, than the original ICE vehicle without added
weights. However, the dynamic wheel load had increased with up to 40%, which would
likely aect the handling safety. This experimental data indicates that there is indeed a
connection between deteriorated comfort, handling and unsprung mass.

Anderson and Harty [49], investigated the eect of increased unsprung mass in a similar
manner to the above. In addition to the objective experiments, subjective measurements
were conducted by experts. Concluded in the report is that an increased unsprung mass
does indeed aect both ride comfort and road holding capability. However, according to
Anderson and Harty, the dierence is not greater than what could be overcome by the
application of normal engineering processes within a product development cycle. Further-
more, implementation of individual motor control oer such improved vehicle dynamics,
that the overall improvements regarding both comfort and handling could be substantial,
despite an increased unsprung mass.

Since the extent of this matter is unclear based on these two fairly contradicting reports,
simulations have been conducted in an attempt to get an improved view of the reality.
The results are presented and discussed in section 5.

28
3.2 Environmental aspects

WCM technology is based on electrical traction. The environmental advantages consider-

ing a WCM vehicle compared to a conventional or even hybrid vehicle can be substantial,
provided the energy comes from environmentally friendly electrical power production.
Consider the currently available fully electrical Tesla Roadster, a vehicle that utilizes a
Li-ion battery package as power source. It has a total drive-train eciency of 88% [50].
This number can be compared to the eciency of a conventional ICE vehicle, which is
generally around 35% at its best. In addition, WCM vehicles have an advantage compared
to conventional electrical vehicles, such as the Tesla Roadster. The in-wheel motors are
generally constructed for direct drive operation, hence it does not require an eciency
deteriorating gearbox.

Several environmentally hazardous liquids are related to motorized vehicles, such as oil,
petrol and battery acid. Owing to the introduction of novel electrical functions for means
of e.g. braking and steering, the need for systems containing hydraulic oil is eliminated.
Considering a battery-powered WCM vehicle, motoroil, petrol and diesel can also be
excluded. These factors improve the environmental impact of WCM vehicles. However,
one shall have in mind, that introducing additional battery power to a vehicle adds a
signicant amount of battery chemicals into the system. In case of battery leakage,
e.g. in connection to a vehicle collision, these chemicals are harmful to the environment.
Additionally, depending on the battery life time, it may need to be replaced at some point
during the vehicle life time. Since manufacturing a battery has a negative environmental
impact, this aspect has to be taken into account when considering how environment-
friendly a specic vehicle is.

By continuously optimizing the wheel angles to the demands, the rolling resistance can be
kept to a minimum [45]. Hence, tire and road wear can be reduced, resulting in decreased
air pollution and increased tire life time.

3.3 Economical aspects

Implementation of WCM technology may result in economical benets in favor of both

manufacturers and consumers. One major economical advantage concerns the develop-
ment and production stages. Many automotive manufacturers oer similar sized vehi-
cle models with quite diverse bodies and dynamic properties. In terms of conventional
vehicles, all models require their unique development processes, since there are major
dierences in terms of desired handling characteristics and construction. WCM tech-
nology enables diverse models to be constructed around a common platform, according
to skateboard design, see subsection 2.5. For example, a two-seated sports car and
a small four-seated city vehicle can be built on the same platform. By utilizing dif-
ferently specied WCMs and software the dynamic properties are individualized. The
sports car's WCMs can be equipped with more powerful in-wheel motors and wider tires,
to enable improved handling performance compared to the city vehicle. The bodywork
and interior is obviously also individualized. By standardizing the connection structure
between the WCM and the vehicle body, one type of WCM will t many dierent vehicle
models. To manufacture vehicles based on modularization can lower costs signicantly,

29
since it involves a decreased number of unique parts. In addition, improved opportunities
for individualization is enabled. The customers can, according to personal preference,
choose from a selection of bodywork, WCMs, and interiors, to create a unique vehicle at
a relatively low cost.

Conventional vehicles generally require workshop service every 15000-30000 km or once

per year. Since such workshop visits often involve high costs, consumer expenses can
here be signicantly reduced. A major reason for the frequent maintenance related to
conventional vehicles is replacement of motor oil and other uids like e.g. brake uid.
With WCM technology comes the ability of extended service intervals, since electrical
equipment require less maintenance compared to e.g. hydraulic systems and ICEs [8].

If WCM technology reaches serial production, novel production technology and manufac-
turing equipment will be required, involving vast investments. This is a major reason why
WCM implementation, in terms of serial produced vehicles, probably will not be possible
for a number of years. In order for the automotive industry to stay protable, the WCM
technology will have to be introduced step by step. That is also likely what is currently
taking place. Novel technical solutions such as hybrid vehicle technology, rear wheel steer-
ing and various XBW systems are each year being introduced from an increased number
of automotive manufacturers.

30
4 Concept analysis
Several dierent WCM concepts are currently being developed by a number of car man-
ufacturers, tire manufacturers and other research institutions. This section summarizes
a selection of these concepts by providing information gathered from various patents and
articles. Concerning most of the concepts, the major part of the available technical in-
formation is of general character rather than specic. Hence, the following descriptions
are typically general, with the exception of certain technical solutions which are described
more in detail.

4.1 Michelin Active Wheel

Michelin have been developing the Active Wheel, see gure 14, since 1996. In 2004 it
was rst showcased in the concept vehicle HY-Light, containing a hydrogen fuel cell as
primary power source. The Active Wheel was later implemented into Heuliez Will, a
concept vehicle based on Opel Agila, which was closer to production than the HY-Light.
In this form the vehicle contained two water-cooled in-wheel motors, used for front wheel
drive with a total rated power of 30 kW. Each wheel module weighed 43 kg [4, 51].

Figure 14: Michelin Active Wheel [4].

1
The in-wheel motor (97) contained in the Active Wheel, see gures 15 and 16, is unsprung
[1]. Generally, in-wheel motors are constructed for direct drive applications. However,
regarding the Active Wheel, torque is transferred from the traction motor to the hub (15)
via a pinion (95) and gearwheel (18) [52]. Since the torque is transferred to the wheel via a
xed gear, the motor can be constructed for maximum torque output at a higher angular
velocity compared to a direct drive motor. Thus, a more compact design is enabled.

For retardation means the in-wheel motors can be used, regenerating power back to the
batteries. When excess brake power is required, a brake caliper (90) coupled with a
conventional brake disc (17) is available. The former is stated to be of hydraulic or
electro-mechanical type [52].

1 Numbers within brackets mark component numbers contained in one of the gures 16 and 15, or
both.

31
For suspension purposes Michelin's WCM houses a coil spring (80) to hold the static
load of the car. In addition, a suspension actuator (87) is installed to actively handle
uctuations. The construction comprises a wheel-barrier connected bar (camber lever)
(4) which contains a rack (45), engaging a pinion (85) which is connected to an electro-
mechanical machine (87). Thus, the linear motion of the wheel is converted to a rotational
motion, which is then transferred onto the electro-mechanical machine. The latter is
composed by a rotor (86) and a stator (88), of which the stator is integral with the guide
member (6) [52].

Steering of the wheel is realized using a conventional steering lever (61) and rod (62), which
could be connected to either a conventional mechanical steering setup or an electrical
actuator attached to the chassis [53].

Figure 15: Side view of patent drawing covering the Active Wheel, Michelin [52].

32
Figure 16: Front view of patent drawing covering the Active Wheel, Michelin [52].

33
Michelin also has a patent covering active camber variation. Figure 17 shows a patent
drawing illustrating a solution for actively controlled camber angle variation, mounted to
a simplied wheel structure. In this construction an arm (70) is mounted to the chassis of
the vehicle (1). Via this arm the loads of the latter are transferred to the wheel integrated
suspension. The upper connection point (52) of the wheel support (5) is connected to a
camber lever through a parallelogram linkage. A jack (45) connected between the chassis
and camber lever controls the desired camber angle. This construction admits a maximum
◦ ◦
camber variation in the order of ±15 to 20 , which is far more than seen in conventional
vehicles [53].

Furthermore, the patent presents a solution as where the passenger compartment integral
with the camber lever, making it comprise a variable roll angle in relation to the chassis.
The passenger compartment is tilted synchronous with the camber angle. It is stated
in the patent that this function improves comfort as well as handling safety. The latter
is improved since tilting the passenger compartment in a bend, displaces the center of
gravity toward the inside of the bend, thus minimizing the load transfer to the wheels
located on the outside of the bend [53].

34
 (a) Perspective view

(b) Front view

Figure 17: Patent drawings covering a solution for active camber adjustment, Michelin
[53].

35
4.2 Bridgestone Dynamic-Damping In-Wheel Motor Drive Sys-

tem

Bridgestone have been developing their in-wheel motor systems since 2000 in cooperation
with Kayaba Industry Co. and Akebono Brake Industry Co.. The Bridgestone Dynamic-
Damping In-Wheel Motor Drive System, see gure 18, was rst announced in September
2003 [54].

Figure 18: Bridgestone Dynamic-Damping In-Wheel Motor Drive System [55].

As described in subsection 3.1.4, increased unsprung mass may deteriorate handling per-
formance and ride comfort. To address this problem, Bridgestone developed an alternative
suspension structure. In this structure, the traction motor is individually suspended in
order not to increase the unsprung mass. Figure 19 shows a patent drawing covering the
key components of the construction. The non-rotational part (3a) of the in-wheel motor
(3), is mounted to a knuckle (5) on the axle (6), via springs (22, 24) and dampers (23, 25).
This suspension setup separates the in-wheel motor from the unsprung mass. A exible
coupling (10), consisting of three hollow disc-like plates (11A,B,C) and guides (12A,B),
transfers the torque from the in-wheel motor to the wheel hub (4). This coupling admits
relative vertical movement of up to 50 mm between rotor and wheel [56, 57].

Regenerative function of the in-wheel motors handles deceleration. When additional brake
force is required, a disc-brake (8) is available. The brake-caliper type is not specied in
the patent [56]. The suspension system comprises a coil-spring and damper (7).

36
 Figure 19: Front view drawing covering patent US0247496A1, Bridgestone [56].

According to Bridgestone, the individually suspended design of their in-wheel motor en-
hances ride comfort as well as road-holding capability, by making the in-wheel motors
own vertical movements oset the vibrations from road and tires [57]. Nagaya et al. [58]
investigated this matter. Three test vehicles were driven on a road containing projections
with a height of 10 mm and width of 20 mm in a line with intervals of 5 m. The following
motor congurations were examined and compared:

1. A conventional electrical vehicle with an unsprung mass of 40 kg per wheel corner,

and the motor as part of the sprung vehicle body. Abbreviated Conv-EV in gure
20a,

2. an in-wheel motor electrical vehicle with an unsprung mass of 70 kg per wheel

corner, of which 30 kg was assigned to the in-wheel motor. Abbreviated IWD-EV
in gure 20a,

3. a dynamic-damper motor electrical vehicle with an unsprung mass of 40 kg and

an individually suspended 30 kg in-wheel motor per wheel corner. Abbreviated
ADM-EV in gure 20a.

The results, see gure 20a, indicate that the ground-contact load uctuation of the tire
is reduced in terms of the dynamic-damper setup, compared to conventional electrical

37
vehicles as well as other electrical vehicles equipped with unsprung in-wheel motors. This
implies improved handling performance. Improved ride comfort is also indicated, see gure
20b. According to this plot, the dynamic-damper solution involves decreased vertical
accelerations in the vehicle body. The accelerations are signicantly reduced around 10-
15 Hz. The resonance frequencies of the unsprung mass, the wheel hop frequencies, are
generally located in this frequency band. Since the individual motor suspension involves
reduced unsprung mass, thus reduced vertical acceleration of the unsprung mass, the
major improvements are found in this specic frequency band.

(a) Tire force load uctuations.

(b) Vertical acceleration of vehicle body.

Figure 20: Bridgestone test results for electrical vehicle equipped with single motor
mounted to vehicle body (Conv-EV), unsprung in-wheel motors (IWD-EV) and Bridge-
stone Dynamic Damping In-Wheel Motors (ADM-EV) [58].

38
4.3 Siemens VDO eCorner

The Siemens VDO eCorner represents an advanced WCM containing a direct drive in-
wheel motor, an active suspension system and an electro-mechanical disc brake.

The eCorner, see gure 21, is propelled by a direct drive in-wheel motor (2). From gure
21, it is visible that the rotor is integral with the rim.

This machine also has the capability to decelerate the vehicle with a regenerative eect
[13].

Figure 21: Siemens VDO eCorner [59].

When excessive brake power is required, a friction brake device called the Electronic
Wedge Brake (EWB), is available. Its function is based on a patent protected wedge
element design, intended to simplify the brake actuator structure compared to other
electro-mechanical brake solutions [60]. Figure 22a shows the principal function of the
EWB.

Electro-mechanical brake constructions generally utilize rotational actuators. A compli-

cating factor is that the rotational movement of the actuator needs to be transformed
into linear movement, prior to application onto the brake pads. This can be achieved by
utilization of e.g. a ball screw device, however such a solution increases the structural
complexity. From this point of view, the use of a linear actuator would be favorable.
However, during heavy braking, linear actuators generally lack the amount of power that
is required.

Siemens VDO claim to have solved this issue, by implementing a wedge formed element
(4) in connection to one of the brake pads (11), see gure 22a. A description of the

39
principal function follows. First, an actuator force F, see gure 22a, is applied onto the
side of the wedge element (4). The latter is pushed against another wedge formed element
(9), forcing the brake pad to move towards the brake disc (7). A friction force is generated
between the brake pad and the rotating disc. Owing to this force, the brake pad is pulled
further into the intermediate space between the wedge element (9) and the brake disc, as
well as towards the brake disc. This results in an additionally increased friction force. As
described, the function of the wedge brake is based on a self-amplifying process.

Owing to the self-amplifying function, the actuator requirements decrease. Less actuator
power is required to produce a certain brake pad force compared to a design where the
actuator moves perpendicular to the brake pad/disc. Hence, a linear actuator provides
sucient force even during heavy braking.

Figure 22b shows the actual design of the EWB. Here, several rolling bodies (21) make
contact between two elements (19, 20). Each of these elements contains a number of spaces
formed as v-shaped bearing halves, which each function according to the same principle
as the wedge elements previously described. Rolling bodies are placed in between these
spaces, in order to reduce friction between the wedge elements. This design makes the
brake force more evenly distributed over the brake pad contact surface, and admits equal
braking properties independent of brake disc rotational direction [60].

The linear actuators (16, 17) regulate the brake force by moving in a reciprocating manner.
Wedge brakes are generally associated with self-locking, as a result of an uncontrolled self-
amplifying eect. In order to avoid this problem, very precise control is required.

The EWB benets from a more compact and simple design, reduced weight and less
maintenance compared to a rotational brake actuator setup [60]. It is compatible with
a 12 V electrical system in contrary to most of the other electrical brake systems in
development [26]. It can also be used as an automated parking brake [61].

The eCorner suspension system comprises an active suspension system. A coil spring
holds the static load of the vehicle, while an electrical actuator handles load uctuations
[13]. The steering angle of each eCorner module can be individually set.

(a) Principal descriptive drawing. (b) Actual design.

Figure 22: Drawings covering patent US0230330A1, Electronic Wedge Brake by Siemens
VDO [60].

40
4.4 Volvo Autonomous Corner Module

The Autonomous Corner Module, see gure 23, is Volvo's WCM concept, invented 1998
[62]. It contains solutions for electro-mechanical propulsion and braking as well as active
steering, suspension and camber variation [63].

Figure 23: ACM front view [45].

Figure 24 is a descriptive patent drawing, showing the dierent components of the inven-
tion. Propulsion is handled by means of an in-wheel motor (9). According to Zetterström
[45], a power output of 10-15 kW per wheel, i.e. a total power output of 40-60 kW, is
sucient for giving a 1000 kg city-car a fair performance. An estimated weight for a
10 kW motor is about 10 kg, given aluminum or metal matrix lightweight materials are
assumed to be used for the rotor and stator [45].

Regenerative braking is supported by the in-wheel motor. When excessive brake force is
required, an integrated friction drum brake actuated by means of an electro-mechanical
rotational actuator (21), is available. It is operated by means of by-wire technology and
houses a recirculating ball screw actuator. During braking, the friction material coated
brake shoe (23) is pushed against the inside of the rotor (24), reducing its angular velocity,
see gure 24. A spring device locks the ball screw in any position when electrically
unpowered. During braking, an electromagnet unlocks the ball screw, enabling it to
operate. This locking mechanism can be used as a parking brake. Alternatively, the front
and rear wheels respectively can be set to maximum toe in or toe out position, disabling
movement of the car [45].

A planetary hub gear (7,12) is mounted between the in-wheel motor and the friction
brake device and corresponding wheel, thus dimensioned to transfer the loads not only

41
from the traction motor, but also from the drum friction brake. The gear ratio of about
2:1 amplies the traction and brake torque when distributed to the wheel [63, 45].

The suspension consists of a passive as well as an active part. The passive part comprises
springs that handle the static load. They are either conventional coil springs or composite
leaf springs running transversely between the left and right lower link arms. The active
part levels the body height and handles anti pitch and roll systems, by use of a combined
rotary damper and rubber torsion spring unit (27), with integrated pretensioning function.
Such a system can be utilized e.g. for stiening the suspension before entering a corner,
thereby obtaining better cornering characteristics [63, 45].

Two linear recirculating ball actuators (16, 16') are, via separate rods (17, 17'), connected
to attachment points (18, 18') in the armature (10). The structure enables these actuators
to adjust not only steering and toe angles individually, but also the camber angle. The
latter is set through parallel displacement of the two steering rods. Advanced algorithms
are used in order to maintain optimal steering, toe and camber angles adapted to driving
style, tire characteristics and prevailing road conditions. Steering angles of up to 22±◦

are admitted [63, 45].

42
 (a) Front view.

(b) Perspective view.

Figure 24: Drawings covering patent EP1144212B1, Volvo [63].

43
4.5 MIT Media Lab's Robot Wheel 5

Figure 25: MIT Media Lab's Robot Wheel 5 [44].

Students, researchers and professionals at MIT Media Lab have developed several early
stage WCM concepts, which are dierent iterations of an original model. They are all
called Robot Wheels. This section treats the Robot Wheel 5, see gure 25, which is the
fth iteration of the Robot Wheel original concept. The main purpose with this WCM is
to reduce the unsprung mass and structural complexity as much as possible, by keeping
the number of components to a minimum. As the name implies, the Robot Wheel 5 has
the potential of operating as an autonomous unit. The invention has been realized as a
1:2 scale concept model consisting of four corner modules attached to a simple structural
frame, see gure 26 [64]. The components described in this section are related to the
scaled version and are likely to be dierent in terms of a full sized model.

Figure 26: MIT Media Lab's WCM concept vehicle, scale 1:2 [44].

The Robot Wheel 5 comprises an in-wheel motor (145) for propulsion means, see gure
27. The scaled concept vehicle utilizes a permanent magnet, 3-phase, brushless outrunner
motor comprising 12 poles [64].

Regenerative in-wheel motor brake torque is applied for retardation means, and is claimed
to be sucient for all situations [65]. In dierence to other WCM concepts, no friction
brake is comprised in the design.

44
 (a) Exploded view.

(b) Perspective view.

Figure 27: Drawings covering patent US0116572A1, M.I.T Media Lab [65].

Similar to other WCM concepts, individual steering angles can be set by means of steering
actuators (130). The servo motor utilized in the 1:2 scaled model does however not provide
enough torque to steer the wheel when subjected to the weight of the prototype vehicle. In
the following iteration this motor is stated to be replaced by a more powerful alternative
[64, 66].

The suspension comprises a double wishbone setup, with exible connection arms (125),
fabricated out of carbon ber, reinforced with aluminum proles. The double wishbone
setup connects to the front axle (105), containing a battery package (110), via a connector.
The connector comprises a wheel side component (120) and a vehicle side component
(115), which establish a structural connection when linked to each other. In addition,
a power connection and a data connection is established. This suspension structure is
merely adequate for the downscaled model. A full-sized version is likely to comprise an
alternative suspension setup [65].

The most signicant benet of the invention is its steering capabilities. The Robot Wheel
◦
5 is able to work in a steering range of up to 150 , by the use of ve dierent drive modes,

45
see gure 28. It comprises a traditional ±30◦ front-wheel-steering mode (a), a four-wheel-
parallel-steering mode (b), a four-wheel-converse-steering mode (c), a spin-on-the-spot
◦
mode (d) and a 90 sideways-movement mode (e) [65, 66].

(a) Drive mode 1. (b) Drive mode 2. (c) Drive mode 3.

(d) Drive mode 4. (e) Drive mode 5.

Figure 28: MIT Media Lab's Robot Wheel steering modes [44].

4.6 General Motors Wheel Module

During 2002 General Motors (GM) presented two concept vehicles based on XBW tech-
nology, the AUTOnomy and the Hy-Wire. Both these vehicles were powered by means of
hydrogen fuel-cells.

The AUTOnomy concept vehicle is propelled by four in-wheel motors. It is built around
a hydrogen fuel-cell stack contained inside a skateboard platform, see subsection 2.5.
Acceleration, braking, steering and suspension are controlled by means of electrical signals.

GM Hy-Wire represents a drivable update of the AUTOnomy. It does not contain all
components related to propulsion, brakes and steering within each corner module, hence
it does not represent a pure WCM vehicle. However, owing to the skateboard-design and
extensive XBW technology utilized, it will be further discussed.

The Hy-Wire was the rst drivable vehicle to combine a hydrogen fuel-cell stack with
XBW technology. As previously stated, it is based on a skateboard design, and utilizes
XBW technology in terms of propulsion, braking and steering. Rather than in-wheel
motors, one single electrical motor located in the skateboard platform propels both front
wheels, leaving the rear wheels unpropelled. The skateboard platform, which also contains
the 94 kW fuel-cell stack, is 279 mm thick. This structure enables a unique interior design
of the vehicle, see gure 29. The Hy-Wire is controlled via an alternative steering device,
called the X-drive. To accelerate, either the right or the left handgrip is twisted. Brake
levers are located on the handgrips. The X-drive can be laterally adjusted between the
right and left side of the vehicle by means of a sliding device. The total weight of the Hy-

46
Wire is 1898 kg. With a total length of 5 m, the weight is in the same order of magnitude
as a corresponding conventional ICE vehicle [67].

Figure 29: GM Hy-Wire interior [67].

A patent led in 2002, covers a corner module developed by General Motors. It comprises
a brake system (9), a steering system (not visible in the patent drawing), a suspension
system (28) and an in-wheel traction motor (50), see gure 30. In terms of frame connec-
tions the wheel module houses a load-bearing mechanical coupling (32). A control signal
receiver, including a vehicle attachment interface, is integrated with the corner module.
Braking, steering, suspension and propulsion are controlled by means of a steering de-
vice connected to the control signal interface, i.e. the system is based purely on XBW
technology.

The in-wheel motor comprises a rotor (12) which is rigidly mounted to the hub, and a
stator (18) connected to the motor housing (26). The former contains two rotor discs (14,
16).

For means of retardation an electro-hydraulic friction brake system (9) is available, com-
prising a brake disc (8) and a caliper (6). A hydraulic line is operatively connected to a
brake actuator (36) at one end, and the brake caliper (34) at the other. The actuator is
operated according to input from a control signal receiver (38). An electrical signal trans-
mitted from the brake control device via the control signal receiver, causes the actuator
to apply a hydraulic force onto the caliper [68].

The suspension system contains an upper suspension arm (29) and a lower suspension
arm (31). The upper arm is connected to a suspension actuator (48), which operates the
suspension by application of a mechanical force, based on input from the control signal
receiver (38). The suspension is actively controlled according to prevailing conditions.

47
 Figure 30: Drawing covering patent US007597169B2, General Motors [68].

4.7 Summary of concepts

Throughout the concept analysis, specic technical solutions regarding wheel corner mod-
ules have been pointed out. This section summarizes the most advantageous solutions.

The Bridgestone Dynamic-Damping In-Wheel Motor Drive System utilizes an individual

motor suspension, which separates the motor from the unsprung mass. Comfort and
handling deterioration related to an uncreased unsprung mass are thus minimized.

In terms of the Siemens VDO eCorner, a highly-developed electro-mechanical brake called

the Electronic Wedge Brake have been discussed. It represents a cheap and energy-ecient
friction brake alternative that eliminates the requirement of hydraulic brake circuits.

48
Volvo's Autonomous Corner Module integrates active camber variation as part of the
concept. Owing to this function, handling performance can be improved in order to
enhance driving experience as well as safety.

The Wheel Robot 5 from MIT Media Lab is capable of working in a wide steering range,
thus improving the handling versatility. This versatility can be benecial during e.g.
urban driving, where the space available for vehicular operation is often very limited.

General Motors' Hy-Wire combines XBW technology with a hydrogen fuel-cell. The
vehicle is constructed around a skateboard-design, which allows the interior and bodywork
to be more freely designed, compared to conventional vehicle structures.

49
5 Modeling and simulation
A major concern regarding increased unsprung mass is deteriorated ride comfort and han-
dling performance. Referring to subsection 3.1.4, there are varying conclusions regarding
the extent of this matter. To clarify this uncertainty, simulations to assess the ride com-
fort and handling performance of a vehicle with dierent amounts of unsprung mass are
conducted. Two dierent models are assembled, one quarter car model (QCM) and one
full car model (FCM).

5.1 Quarter car model

Simulations are conducted based on a quarter car model, see gure 31. The model, as the
name implies, represents a quarter of a car. It involves parameters for the sprung mass
ms , unsprung mass mu , tire stiness kt , tire damper coecient ct , spring stiness ks and
damper coecient cs . The degrees of freedom, u, zu and zs , are all vertically directed.
Simulations based on a QCM represent a very simplied method to estimate the dynamics
of a vehicle.

Figure 31: QCM, block diagram.

Equations 1 and 2 describe the dynamics of the QCM.

ms z̈s + ċs żs + ks zs = cs żu + ks zu (1)

mu z̈u + (cs + ct ) żu + (ks + kt ) zu = cs żs + ks zs + ct u̇ + kt u (2)

A QCM is assembled in the dynamic modeling program Dymola, by utilization of basic

modelica blocks representing tire, unsprung mass, spring, damper and sprung mass. The

50
tire, the spring and the damper are modeled by use of linear spring and damper blocks.
The sprung and unsprung masses are modeled as punctual masses. The properties are set
through denition of each modelica block according to the vehicle parameters presented
in table 4. This data represents a mid-sized vehicle, with dimensions in the same order of
magnitude as e.g. a Volkswagen Golf Mk 6. Since the QCM represents one corner of the
vehicle, only one suspension setup is possible for each simulation. These simulations aim
to estimate the inuence of rear-wheel mounted motors, since in-wheel motor systems are
likely to rst be implemented onto the rear wheels once serial-production is possible [16].
Hence, the spring and damper constants are set according to the rear suspension of the
vehicle.

Table 4: Parameters for QCM model.

Vehicle parameter
Sprung mass (per corner) [kg] 325
Unsprung mass (per corner) [kg] 40
Spring constant [N/m] 56000
Damper constant [Ns/m] 1276
Tire spring constant [N/m] 200000
Tire damper constant [Ns/m] 100

5.1.1 Simulation
Simulations are performed, in order to estimate to what extent an increased unsprung mass
inuences ride comfort and handling performance of a vehicle. Hence, all simulations are
conducted for two setups:

1. The original setup containing 40 kg unsprung mass,

2. an alternative setup containing 60 kg unsprung mass.

Two dierent road models are utilized. One contains a at surface with a bump prole,
intended to resemble a speed bump. The other one contains a at surface with a ramp,
intended to resemble a curb. As visualized in gure 32, the bump has an amplitude of
0.05 m and a length of 0.10 m. The curb has an amplitude of 0.05 m and a rise angle of
◦
60 , i.e. a length of 0.02 m. For all simulations, the QCM hits the disturbance prole 5 s
after the simulation start time.

Figure 32: Simulated road disturbances.

51
In the QCM simulations, the road input signal is the road amplitude as a function of time.
The input signal for the bump prole consists of one half of a sine wave. By altering the
sine wave frequency, dierent vehicle velocities are resembled. These velocities, including
their disturbance frequencies, are listed in table 5. Regarding the curb prole simulations,
variation of the vehicle velocity is performed by variation of the ramp rise time. Simulated
runs are performed for each of the two road proles for a selection of velocities between
1 km/h and 90 km/h. The velocities are chosen to cover a wide spectrum, in order to
enable a complete analysis of the output data.

Table 5: Simulated vehicle velocity and corresponding bump prole disturbance frequency.

Velocity [km/h] 1 3 5 7 10 20 30 50 90
Disturbance frequency [Hz] 1.39 4.17 6.94 9.72 13.89 27.78 41.65 69.44 125

The eigenfrequencies of the system are calculated according to equations 3, 4 and 5. They
are relevant for a proper analysis of the simulation output data.

Sprung mass:

s s
ks 56000
ωs = = = 13.1 rad/s = 2.1 Hz, (3)
ms 325

which corresponds to a vehicle velocity of 1.5 km/h.

Unsprung mass 40 kg:

s s
kt + ks 200000 + 56000
ωu_l = = = 80.0 rad/s = 12.7 Hz, (4)
mu 40

which corresponds to a vehicle velocity of 9.1 km/h.

Unsprung mass 60 kg:

s s
kt + ks 200000 + 56000
ωu_h = = = 65.3 rad/s = 10.4 Hz, (5)
mu 60

which corresponds to a vehicle velocity of 7.5 km/h.

5.1.2 Results and analysis

The simulation output contains the vertical acceleration of the sprung mass and the tire
contact force, both as functions of time. These are plotted for each simulation setup. A
representative set of plots are shown and described in the following subsection.

RMS calculations provide a single value for a certain time frame. Thus, it is an appropriate
method to facilitate direct comparisons between dierent simulations. RMS values based
on the simulation output data are calculated for the vertical acceleration of the sprung

52
mass (vehicle body), as a measure of ride comfort. Increased vertical acceleration indicates
deteriorated comfort [48]. In order to estimate the handling performance, RMS values
for the tire contact force uctuations are calculated. Increased force uctuation indicates
deteriorated handling performance [48]. All the RMS calculations are based on a 3 s time
frame starting from the instance where the vehicle hits the road disturbance. This time
frame includes the major part of the vertical acceleration and tire force uctuations.

In terms of a FCM, a measure called the ride comfort index, can be calculated as the
RMS of the vertical, lateral and longitudinal acceleration of the sprung mass. The major
accelerations of the sprung mass, during longitudinal translation over a road bump or a
curb, are vertically directed. Hence, in terms of these simulations, the ride comfort index
and the RMS vertical acceleration are comparable. A reduction of the ride comfort index
of 5 - 10 % is generally regarded as a respectable ride comfort improvement [48].

Figure 33 shows the RMS of the vertical acceleration of the sprung mass a a function of
the vehicle velocity, during longitudinal translation over the bump prole. The vertical
acceleration of the sprung mass continuously decreases as the velocity increases.

Two lines connecting the RMS vertical acceleration values are implemented, see gure 33,
2
accentuating the dynamic characteristics of the system. An oset of 0.5 m/s is added, in
order to separate the line diagram from the bar diagram to increase visibility. In addition,
values for 1.5 km/h, 7.5 km/h and 9.1 km/h are added, marked with x in gure 33. These
velocities correspond to the eigenfrequencies of the sprung and unsprung mass, according
to equations 3, 4 and 5. When an object is subjected to a force acting with a frequency
that is close to its eigenfrequency, even a small excitation force can induce a signicant
oscillation amplitude. The damping factor is low in connection to the eigenfrequency, due
to a natural tendency to oscillate at this specic frequency. Due to this, the RMS vertical
acceleration around these certain velocities are amplied. At 1.5 km/h, the RMS vertical
acceleration is close to its peak value. Above 9.1 km/h, the RMS vertical acceleration
decreases signicantly.

53
For each simulated vehicle velocity, it can be seen that the vertical acceleration is depen-
dent upon the amount of unsprung mass. Figure 34 shows the percentage increase of the
RMS vertical acceleration caused by an increased unsprung mass. At velocities of 7 km/h
and below, an increased unsprung mass causes increased RMS vertical acceleration of the
sprung mass. At 10 km/h and higher, an increased unsprung mass results in decreased
RMS vertical acceleration, during transition of this certain road disturbance prole. Since
the bump prole inputs are based on sine waves with altered frequencies, these results
indicate that the extent to which the unsprung mass inuences ride comfort is dependent
upon the disturbance frequency.

Figure 33: RMS vertical acceleration of the sprung mass, bump prole, QCM.

Figure 34: Percentage increase of the RMS vertical acceleration, bump prole, QCM.

54
Regarding the curb prole, the RMS vertical acceleration, see gure 35, increases in
connection to increasing velocity from 1 km/h up to about 5 km/h, regardless of the
amount of unsprung mass. Above 5 km/h, the RMS vertical acceleration remains fairly
stable. It shall be noted, that the sprung mass will have a vertical displacement of 0.05
m, regardless of the velocity, since the amplitude of the disturbance prole remains at
0.05 m until the simulation end time.

According to gure 36, an increased unsprung mass is likely to cause deteriorated comfort
during longitudinal translation over a curb, since the vertical acceleration is higher for all
simulated velocities.

Figure 35: RMS vertical acceleration of the sprung mass, curb prole, QCM.

Figure 36: Percentage increase of the RMS vertical acceleration, curb prole, QCM.

55
The following characteristic description is valid for both the bump and the curb prole
simulations, and all simulated vehicle velocities.

In gure 37, the vertical acceleration of the sprung mass during longitudinal translation
over the bump prole at a velocity of 3 km/h can be seen. A low frequency uctuation of
around 2 Hz is visible, based on a period time of about 0.5 s (measured between second
and third peak at about 5.5 and 6.0 s). In addition, a higher frequency uctuation of
around 10 - 15 Hz is present, explicitly visible at about 5.2 s. These uctuations are
connected to the eigenfrequencies of the sprung (2.1 Hz) and unsprung mass (40 kg: 12.7
Hz, 60 kg: 10.4 Hz) respectively. The 60 kg curve involves a phase delay compared to the
40 kg setup.

Figure 37: Vertical acceleration of the sprung mass, bump prole, QCM.

The RMS value of the tire-ground contact force uctuation is a measure of the handling
performance of a vehicle. Figure 38 shows this variable as a function of the simulated
vehicle velocity, during longitudinal translation over the bump prole. The force uctua-
tion is higher in terms of the 60 kg setup compared to the 40 kg setup, for all simulated
velocities. This indicates deteriorated handling performance.

56
The blue and red lines in gure 38 correspond to the RMS force uctuation, added with
an oset of 200 N in order to separate the line diagram from the bar diagram. Referring
to the description of gure 33, amplied RMS force uctuations are visible around the
velocities corresponding to the eigenfrequencies of the sprung and unsprung mass.

The highest increase of the RMS tire force uctuation, is found at a simulated vehicle
velocity of 5 km/h, see gure 39. At this velocity, an increased unsprung mass has a
maximum negative impact on handling performance.

Figure 38: RMS tire force uctuation, bump prole, QCM.

Figure 39: Percentage increase of the RMS tire force uctuation, bump prole, QCM.

57
At 1 km/h, corresponding to a disturbance frequency of 1.39 Hz, the RMS values of both
the tire contact force uctuation, see gure 38, and the vertical acceleration of the sprung
mass, see gure 33, is signicantly high. This is likely an impact from the eigenfrequency
of the sprung mass, which is found at 2.1 Hz. It can be seen in gures 40 and 41, that the
vertical acceleration and force uctuation amplitude after the rst period is signicantly
higher at 1 km/h compared to 10 km/h, despite a lower tire force in the rst period. The
at spot that is visible at a time of 5.0 - 5.1 s in gure 41 is caused by the tire losing
contact with the ground.

Figure 40: Vertical acceleration of the sprung mass as a function of time, bump prole,
QCM.

Figure 41: Tire force uctuation as a function of time, bump prole, QCM..

58
According to gure 42, the RMS value of the tire force uctuation increases signicantly
as the velocity rises from 1 km/h up to about 7 km/h, for both setups. At velocities above
7 km/h, the RMS value is stabilized.

Figure 43 indicates that during longitudinal translation over a curb, the handling perfor-
mance is signicantly deteriorated for the 60 kg setup, compared to the 40 kg setup. This
is valid for all simulated vehicle velocities. A peak RMS tire force uctuation ratio value
can be noted at 3 km/h. Above 3 km/h, the handling deterioration due to an increased
unsprung mass decreases slightly in connection to increasing velocity.

Figure 42: RMS tire force uctuation, curb prole, QCM.

Figure 43: Percentage increase of the RMS tire force uctuation, curb prole, QCM.

59
5.1.3 Discussion
It is a common perception that an increased unsprung mass deteriorates the ride comfort
as well as the handling performance of a vehicle. The presented results indicate that the
impact of increased unsprung mass is highly dependent upon the disturbance frequency
of the road surface. When subjected to a disturbance frequency above the eigenfrequency
of the unsprung mass, the simulation output data shows a decreased RMS vertical accel-
eration, i.e. enhanced comfort, for a vehicle with an increased unsprung mass.

In section 3.1.4, a report by Vos et al. [48] is discussed. In that report, it is concluded
that an increased unsprung mass on the rear wheels deteriorates ride comfort with 1 -
8 %. Vos et. al performs simulations with a real vehicle on highway, cobblestones and
belgian blocks. The exact disturbance proles of these roads are not presented, hence a
direct comparison to the results attained in this thesis may be misleading. However, it
is noted that for velocities below 10 km/h, the ride comfort deterioration noted in the
performed simulations is in the same order of magnitude, 2 - 10 %.

The negative impact from an increased unsprung mass is generally higher in terms of han-
dling performance, compared to ride comfort. A deterioration of the former is indicated
for both road disturbance proles and all velocities.

It shall be noted that all the conducted simulations are based on simplied models. For
example, the inuence of the geometry of the wheel is not taken into account. This might
well aect the accuracy of the results.

A full car model is assembled in order to evaluate this matter to a further extent.

60
5.2 Full car model

A full car model is assembled in order to evaluate the results attained in subsection 5.1.
The model is based on a template of a compact car, available in the dynamic model-
ing program Dymola. The vehicle parameters are set according to table 6, which are
equivalent to the parameters utilized for the QCM in subsection 5.1.

Table 6: Vehicle parameters for the FCM.

Vehicle parameter
Total mass of vehicle [kg] 1460
Unsprung mass (per corner) [kg] 40
Moment of inertia around x-axis [kg∙m2] 542
Moment of inertia around y-axis [kg∙m2] 2480
Moment of inertia around z-axis [kg∙m2] 2656
Wheel base [m] 2.65
Track width front [m] 1.54
Track width rear [m] 1.53
Center of gravity (x,y,z from front center ground) [m] 1.08, 0, 0.54
Tire dimension 205/60-16
Tire spring constant [N/m] 200000
Tire damper constant [Ns/m] 100
Suspension type front McPherson
Suspension type rear Multilink
Spring constant front [N/m] 33000
Spring constant rear [N/m] 56000
Damper constant front [Ns/m] 1288
Damper constant rear [Ns/m] 1276
Stabilizer stiffness front [Nm/rad] 1897.8
Toe in front [rad] 0.0018

In terms of a QCM, the vertical movement of the road, the sprung and the unsprung
mass are the only degrees of freedom. The vehicle body is represented by a point mass
placed on top of the suspension. During simulation of the FCM, longitudinal and lateral
translation as well as pitch, roll and yaw motions are introduced as additional degrees
of freedom, see gure 44. The vehicle body is represented not only by a mass, but also
involves geometric boundaries. Owing to these additional variables, the FCM is likely to
better resemble a real vehicle, compared to the QCM. Hence, the inuence on ride comfort
and handling performance due to an increased unsprung mass can be further evaluated.

61
 Figure 44: FCM, degrees of freedom [69].

5.2.1 Simulation
The FCM is simulated in a test rig, according to gure 45. Each wheel is placed on top
of a cylinder, which can be vertically displaced. The input signals to each cylinder are
equivalent to the QCM simulations, see subsection 5.1.2.

Figure 45: Test rig for FCM simulations.

Consider a vehicle approaching a road disturbance. The front and rear axle reaches the
road disturbance at dierent moments. Hence, a pitch motion is initiated. The time
interval between the moments of impact depends on the velocity of the vehicle. To
resemble a vehicle in longitudinal motion over a disturbance prole, this time interval is

62
implemented into the simulation model. The two cylinders that support the front wheels
are displaced before the two cylinders that support the rear wheels, according to table 7.

Table 7: Time between the front and rear wheels reach the road disturbance.

Velocity [km/h] 1 3 5 7 10 20 30 50 90
Time interval [s] 9.54 3.18 1.91 1.36 0.95 0.48 0.32 0.19 0.11

Each simulation is conducted for two vehicle setups:

1. The original setup with an unsprung mass of 40 kg per vehicle corner,

2. an alternative setup with an unsprung mass of 40 kg per corner at the front and 60
kg per corner at the rear, intended to resemble a vehicle with rear mounted in-wheel
motors.

All other vehicle parameters are kept constant.

In subsection 5.1.2, the vertical eigenfrequency of the rear unsprung mass was calculated.
The vertical eigenfrequencies of the sprung mass and the front unsprung mass for the
FCM are calculated in equations 6 and 7 respectively. All eigenfrequencies of the system
are summarized in table 8.

s s
ks 2 · 56000 + 2 · 33000
ωs = = = 11.7 rad/s = 1.9 Hz, (6)
ms 1300
s s
kt + ks 200000 + 33000
ωu_f = = = 76.3 rad/s = 12.1 Hz, (7)
mu 40

Table 8: Eigenfrequencies of the system.

Mass Eigenfrequency [Hz]

Sprung mass 1.9
Unsprung mass front 40 kg/corner 12.1
Unsprung mass rear 40 kg/corner 12.7
Unsprung mass rear 60 kg/corner 10.4

5.2.2 Results and analysis

The main objective in terms of the output data analysis is the vertical acceleration of the
sprung mass, as a measure of ride comfort, and tire contact force uctuation, as a measure
of handling performance. The vertical acceleration of the sprung mass is measured at the
center of gravity.

63
The unsprung mass at the front corners remains unchanged throughout all simulations.
The increased unsprung mass at the rear corners mainly aects the contact forces on the
rear tires. Hence, in order to evaluate the impact on handling performance, the contact
force is measured at one of the rear wheels. The contact forces for the left and right rear
tire are equivalent.

RMS values are calculated and analyzed for the vertical acceleration of the sprung mass
and the rear tire contact force uctuation, based on a 3 s time frame. Consider the two
model setups in gure 46. When the front wheels reach the road disturbance, vertical
acceleration of the sprung mass is initiated. Until the rear wheels are displaced at 5.95 s,
the acceleration of the two setups is principally equivalent. Regarding the contact forces
of the rear tires, the same pattern is noted, see gure 47. In order to simplify a direct
comparison of the two dierent model setups, the RMS time frame is therefore initiated
when the rear wheels reach the road disturbance. The described characteristics are valid
for both disturbance proles and all simulated velocities.

Figure 46: Vertical acceleration of sprung mass, rear wheels hit the road disturbance at
5.95 s.

Figure 47: Rear tire contact force, rear wheels hit the road disturbance at 5.95 s.

64
Figure 48 shows the RMS vertical acceleration of the sprung mass as a function of vehicle
velocity, during longitudinal translation over a bump prole. For both setups the vertical
acceleration remains fairly constant between 1 km/h and 10 km/h. At velocities above 10
km/h, it decreases signicantly in connection to increasing vehicle velocity. This indicates
that the overall ride comfort deterioration caused by the road disturbance decreases as
the velocity increases.

The RMS vertical acceleration of the sprung mass depends on the amount of unsprung
mass at the rear wheels. Figure 49 shows the percentage increase of the RMS vertical
acceleration, caused by an increased unsprung mass. It can be seen that below 10 km/h
the RMS vertical acceleration increases about 2 - 5 %. This indicates deteriorated ride
comfort. At higher velocities the RMS vertical acceleration is reduced with up to 25 %,
indicating signicantly enhanced ride comfort.

Figure 48: RMS vertical acceleration of the sprung mass, bump prole, FCM.

Figure 49: Percentage increase of the RMS vertical acceleration, bump prole, FCM.

65
Figure 50 shows the RMS vertical acceleration of the sprung mass as a function of the
vehicle velocity, during longitudinal translation over a curb prole. The RMS vertical
2
acceleration varies between 0.58 and 1.13 m/s , depending on the vehicle velocity and the
amount of unsprung mass. The highest value is noted at 90 km/h. Hence, at this velocity
the ride comfort is likely to be signicantly deteriorated.

Figure 51 shows the percentage increase of RMS vertical acceleration caused by an in-
creased unsprung mass. Deteriorated comfort is indicated at all simulated vehicle veloc-
ities. It is noted that the impact of increasing the unsprung mass is signicantly high
at 1 km/h, compared to the other simulated velocities. At this specic velocity, the in-
creased unsprung mass involves a 4% increase regarding the RMS vertical acceleration of
the sprung mass. At other simulated velocities, the equivalent value is merely 0.1 % - 0.7
%.

Figure 50: RMS vertical acceleration of the sprung mass, curb prole, FCM.

Figure 51: Percentage increase of the RMS vertical acceleration, curb prole, FCM.

66
Figure 52 shows the RMS tire force uctuation at one of the rear wheels as a function of
the vehicle velocity, during longitudinal translation over a bump prole. It can be seen
that the RMS force uctuation increases with increasing velocity from 1 km/h to about
7 km/h, where it reaches a peak value. Above 10 km/h, it decreases with increasing
velocity, indicating less deterioration of handling performance when passing the bump at
velocities higher than 10 km/h.

According to gure 53, an increased unsprung mass deteriorates handling performance at

all simulated vehicle velocities. At 5 km/h the handling performance deterioration is at
its peak, with an RMS tire force uctuation increase of 18 %.

Figure 52: RMS tire force uctuation, bump prole, FCM.

Figure 53: Percentage increase of the RMS tire force uctuation, bump prole, FCM.

67
Figure 54 shows the RMS tire force uctuation at one of the rear wheels as a function
of the vehicle velocity, during longitudinal translation over a curb prole. The RMS
force uctuation increases with increasing velocity up to about 7 km/h. The diagram
indicates that the handling performance deterioration caused by the road disturbance is
fairly constant at velocities above 7 km/h.

An increased unsprung mass deteriorates handling performance at all simulated vehicle

velocities, according to gure 55. It has the most negative impact at a vehicle velocity
of 3 km/h, with an 18 % increase of the RMS tire force uctuation. The lowest value is
noted at 1 km/h, where the RMS tire force uctuation is increased by about 10 %.

Figure 54: RMS tire force uctuation, curb prole, FCM.

Figure 55: Percentage increase of the RMS tire force uctuation, curb prole, FCM.

68
5.2.3 Discussion
The overall characteristics of the FCM simulation output data are in concurrence with the
QCM simulations. The results from the QCM simulations indicate that the inuence on
ride comfort due to an increased unsprung mass, is dependent of the frequency of the road
disturbance. The FCM simulations conrm this indication. At longitudinal translation
over a bump prole, which is based on a sine wave input signal, ride comfort improvements
are indicated in connection to a disturbance frequency above the eigenfrequency of the
unsprung mass. All other simulations indicate deteriorated ride comfort in connection to
an increased unsprung mass.

Figure 39 and 43 show the percentage increase of the RMS tire force uctuation, attained
from the QCM simulations, whilst gure 53 and 55 contain equivalent data for the FCM
simulations. The characteristics of the QCM diagrams referred to, are very similar to
corresponding FCM diagrams. Signicant dierences are however noted when comparing
gure 36 with 51, which also contain results from the QCM and the FCM simulations
respectively. Both gures show the percentage increase of the RMS vertical accelera-
tion caused by an increased unsprung mass, regarding translation over the curb prole.
Whereas the QCM simulations contain fairly constant acceleration values, regardless of
vehicle velocity, the FCM values vary signicantly.

It is noted that the overall values of the RMS vertical acceleration of the sprung mass are
signicantly lower in terms of the FCM, compared to the QCM. This is likely due to a
dierent measurement point. The acceleration of the QCM is measured right on top of
the suspension. In terms of the FCM, the measurement point is placed in the center of
gravity, i.e. at a location between the front and rear axle. If the front wheels are xed and
the rear wheels are displaced 0.05 m vertically, the center of gravity is merely displaced
0.02 m. Hence, also the acceleration caused by this displacement is lower at the center of
gravity, compared to right above the rear axle.

69
6 Conclusions
WCM vehicles enable signicantly improved handling performance, safety and ride com-
fort compared to conventional vehicles, owing to the x-by-wire technology and an addi-
tional number of actuators related to motion control. Longitudinal, vertical and lateral
motions can be individually set for each comprised corner module.

Several WCM concepts are currently being developed by automotive manufacturers and
subcontractors. In order for serial production to be attainable, additional major invest-
ments in terms of research, testing and manufacturing equipment are required.

Implementation of in-wheel motors is likely to cause an increased unsprung mass in terms

of the WCM vehicle, compared to a conventional equivalent. The simulation results
are summarized in table 9. They indicate that an increased unsprung mass involves
deteriorated handling performance, regardless of the vehicle velocity or road disturbance
prole. In addition, a majority of the simulations indicates deteriorated ride comfort. At
disturbance frequencies above the eigenfrequency of the unsprung mass, a signicant ride
comfort improvement is however indicated.

Table 9: Percentage increase of RMS values, caused by an increased unsprung mass.

Simulation model Road disturbance RMS vertical acceleration RMS tire force fluctuation
min [%] max [%] min [%] max [%]
QCM Bump -13 +10 +0.1 +21
FCM Bump -25 +6 +0.3 +18
QCM Curb +2 +2.7 +5.8 +12
FCM Curb +0.15 +4 +10 +17.2

A vehicle is continuously exposed to dierent disturbance frequencies during operation.

Hence, the overall comfort experience is dependent not only of one single type of input, but
upon a wide variety of disturbances. Regarding a majority of the conducted simulations,
an increased unsprung mass causes deteriorated ride comfort. In addition, some of the
simulations where enhanced comfort is indicated, represents uncommon driving situations.
For example, speed bumps or curbs are rarely crossed at high velocities. Owing to these
circumstances, it is likely that the overall ride experience is deteriorated in connection
to an increased unsprung mass. Hence, the mass of in-wheel motors must be kept to a
minimum, or an individual motor suspension may be required.

70
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