This article has been accepted for publication in a future issue of this journal, but has not been

fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TTE.2020.3004694, IEEE
Transactions on Transportation Electrification
IEEE TRANSACTIONS ON TRANSPORTATION ELECTRIFICATION 1

Recent Developments in the Vehicle Steer-by-Wire

System
Seyed Abolfazl Mortazavizadeh, Ahmad Ghaderi, Mohamad Ebrahimi, and Masood Hajian
Department of Electrical and Computer Engineering, Isfahan University of Technology (IUT), Isfahan, Iran.
Corresponding Author: Mohamad Ebrahimi, Department of Electrical and Computer Engineering, Isfahan
University of Technology, Isfahan, 84156-83111, Iran. Email: mebrahim@iut.ac.ir

Abstract—The next generation of steering system in auto Tf c Free control torque

manufacturing industry would be steer-by-wire (SBW) in Tm2 RW motor load torque
which mechanical connection between vehicle steering wheel tp , tm Pneumatic and mechanical trail
and tires is removed. Numerous advantages are envisaged
for this modification but there are still some under-research Ts Measured torque on steering hand-wheel
technical and safety-related challenges to be addressed. This V Vehicle velocity
paper presents a thorough review on the history of vehicle Vabc1 , Vabc2 Three-phase stator voltages of HW and RW mo-
steering systems first. The structure and components of tors
the cutting edge technology SBW system are subsequently
described with the advantages and disadvantages discussed
in detail. The main challenges of this system including the I. I NTRODUCTION
required motor-drives equipped with advanced controllers
and reliability concerns are studied and proposed solutions Vehicle technology has rapidly evolved and its efficiency,
presented in literate are reviewed. Finally, a projection of safety and performance have been highly improved over the
the future of steering systems considering driverless vehicles time. Steering system transferring the drivers’ steering com-
with integrated steering and traction systems is briefly presented. mands to tries has experienced several evolution levels and has
a trend from a pure mechanical toward a pure electrical system
Index Terms—Steer by wire, Vehicle steering system, Vehicle [1]. The mechanical parts are being substituted by electrical
control system, Review.
sensors and actuators with the aim of improved performance
[2].
Vehicle steering system has an evolution history started
N OMENCLATURE from pure mechanical generation upgraded with, hydraulic
αF , αR Front and rear tires sideslip angles power-assisted system (HPS), electro-hydraulic power-assisted
β vehicle body sideslip angle steering (EHPS), electric power-assisted steering (EPS), and
θ1∗ , θ2∗ HW and RW motors’ reference angles moves toward SBW system [3].
θm1 , θm2 HW and RW motors’ actual angle Traditional steering systems consist of the main components
θroad Road angle of steering wheel, column, gear, rack and pinion. The system
θs Steering HW angle must transfer driver’s commands to vehicle tires and enforce
θV eh Vehicle moving direction respect to road angle them to follow the desired path determined by the driver.
a, b Distances from vehicle centre of gravity to front and Primary steering systems known as mechanical systems did
rear tires not support any kind of power steering option. All required
Cα,F , Cα,R Front and rear tires cornering coefficients effort had to be provided by driver’s hands.
Fc Torque due to friction coulomb force The invention of HPS system is recognized as a great
Gf eel Feeling gain improvement introduced in 1951 by Chrysler Imperial [4].
i2 RW motor stator current It needs less driver’s effort and achieves faster response
iabc1 , iabc2 Three-phase stator currents of HW and RW motors supported with more stability and safety compared to its
Iz Vehicle moment of inertia predecessor. The hydraulic oil helps in less vibration appeared
Jm2 RW moment of inertia on steering hand-wheel (HW).
k Ratio between HW and RW motors’ torque HPS was subsequently upgraded to EHPS system mainly
ka Scaling factor because of significant costs required for its design, assem-
Kt RW motor torque constant bling and maintenance [5]. EHPS employs an electrical pump
m Vehicle mass responsible for circulating hydraulic oil instead of a pulley
r Yaw rate constantly connected to the combustion crank resulting in less
T1∗ , T2∗ HW and RW motor reference torques fuel consumption [6] . Contrary to HPS, it can also work with
Tassist Assistance torque off engine. The fuel consumption of a commercial vehicle at
Ta Self-aligning torque long haul driving can be reduced by 1% if its HPS is replaced

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with an EHPS [7]. Regardless of all the benefits observed Steering Hand-Wheel
for EHPS, it still suffers from some critical disadvantages
mostly related to its limited design flexibility and requirement
of hydraulic oil.
The next commercial steering system introduced in 1996
was EPS using an electrical motor with compact structure and Hand-Wheel
power-on-demand performance [8]. The EPS system resolved Sensors
all the environmental and acoustic issues reported for EHPS
due to the use of hydraulic oil [9]. Various locations have been Hand-Wheel
considered for this electric motor installation. It can be placed Motor
on the steering column which is the oldest arrangement, at the
steering rack, right at the steering pinion with slightly higher
steering power, at a second pinion supplying higher power
compared with the other arrangements, or at an axis parallel
with rack [5], [10]. Road-Wheel
DC motors were extensively used in EPS systems due to Sensors
their control simplicity. However, more power demand needed Road-Wheel
for larger vehicles, rapid developments in power electron- Motor
ics technology, introduction of advanced control techniques Fig. 1: The SBW system structure.
of field-oriented control and direct torque control of AC
machines, as well as DC motors higher capital expenditure
(CAPEX) and operating expense (OPEX) resulted in more Q50 and Q60 produced by Nissan are examples of this system
trends toward permanent magnet synchronous motor (PMSM) implementation so far [17].
drive application [5]. This paper attempts to provide a detailed review on recent
Eventually, EPS became the best choice for passenger cars research and development in SBW system. Its outline is
due to benefits of lower weight and size, less complexity, as follows: In next section, the physics of SBW system is
minimum maintenance cost, and also more possible steering firstly discussed. The main components, pros and cons as
functions. Although, the EPS could improve fuel efficiency well as challenges observed so far are subsequently described.
up to 5% by power-on-demand function, and also passenger The solutions to address SBW challenges are discussed in
comfort using special functions such as lane keeping assistance section IV. The methods for reliability improvement as well as
system, smart parking assistance system and vehicle stability various control methods suitable for SBW implementation are
management, but still suffered from some design restrictions presented in detail in section V. The advantages, disadvantages
due to the use of steering column [11]. and possibility of application of each control method are
Fault-tolerant EPS system is being widely studied through discussed. Finally, a prediction of the future of vehicle steering
industrial researches and patents. The utilization of multi- system including application of wireless systems and steering
winding electric motors, fault-tolerant control methods, and system integrated with traction system is presented.
virtual sensor techniques relying on advanced estimation meth-
ods are presented by Ghaderi for EPS system [12]–[15].
II. SBW SYSTEM OVERALL STRUCTURE
Such improvements are extended to develop SBW as the next
generation of steering system. By-wire steering technologies started with fly-by-wire sys-
The SBW is a steering system with no steering column that tem used in Concord civilian aircraft in 1970s [18] where,
benefits from an electrical connection between steering wheel mechanical link was replaced with by-wire electrical connec-
and vehicle wheels. With no direct mechanical link between tion. Subsequently similar systems using this concept such
the steering wheel and the tires, most road bumps are not as brake-by-wire, throttle-by-wire, drive-by-wire, shift-by-wire
directly transmitted to the steering wheel resulting in more (also known as gear-by-wire) and steer-by-wire have been in-
driver comfort. Two individual control loops are required, troduced for automobile application [19]–[22]. In this section,
one for the steering wheel and another for the road wheel the overall structure of SBW system with all its subsystems
motor drives. The road wheels motor drive is in charge for is described.
appropriate transmission of driver’s commands to the tires Given that there is no mechanical link between steering HW
whereas the steering wheel motor drive should provide a and vehicle wheels, the system should achieve an amplified
reflection of the road surface for the driver [16]. transmission of the driver’s commands to vehicle wheels using
The mechanical link omission has major advantages of less an alternative solution and ensuring comfortable vehicle driv-
weight and space as well as more flexibility in design but there ing. Additionally, a haptic mechanism should be implemented
are still challenges against widespread use of such mechanism in the system to provide the driver with a realistic steering
including complexity of required control and less overall feeling.
reliability due to extensive application of sensors. Nevertheless The SBW comprises of two main subsystems called as
several concept designs as well as few number of commercial HW and road-wheel (RW) modules, as shown in Fig. 1. The
vehicles equipped with this system have emerged so far. Infiniti main duty of steering HW module is to receive the driver

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commands and to convert the demanded angle or torque Q60 produced by Nissan motor. It is worthwhile mentioning
to an electronic signal using appropriate sensors located on that it keeps mechanical link between steering wheel and
steering HW motor shaft. The electronic signal is transmitted pinion using a clutch for customer’s assurance. It provides a
via connection wires to RW electronic control unit (ECU) in mechanical backup for SBW system and ensures mechanical
order to determine relevant command for the vehicle wheels connection existence between HW and RW in the cost of
motor-drive. The RW ECU also receives other information using a clutch resulting in higher weight and cost [10], [17],
from speed sensors, accelerometers, angle sensors and yaw [39]. Other kinds of mechanical backup for electrical actuators
rate sensors to determine corresponding commands to be sent are also discussed in [39]–[42]. The dual-motor driving SBW
to HW actuator [2], [23], [24]. system proposed in [42] benefits from three electrical motors
The electric motor in RW module receives commands in which two motors are dedicated to RW module located
from RW ECU and attempts to change the road-wheel angle at harsh position. These two motors are placed on pinion and
accordingly. This motor typically has an output power of 150 rack to provide higher torque and to improve the SBW system
to 1000 W [5], and same motor types used in EPS system reliability. Dual motor driving SBW system which benefites
could be applied [5]. In order to notify driver about vehicle from two RW motors provides higher torque and improves the
wheel conditions and forces applied on tires in various road system reliability. The most challenging issue in this structure
situations, sensors should be used in wheels and a feedback is how to synchronize the steering angle that is addressed in
system from the RW module to the HW module is to be [42] by using sliding mode control (SMC) and disturbance
established. These sensors are connected to the RW motor and observer under master-slave control.
the outputs are transferred to HW ECU to determine reference The complexity of reliable control system implementation
signals of HW motor drive. and the need for a haptic mechanism from tires to steering
Note that data communication delays between HW and RW hand-wheel should be mentioned as the main disadvantages.
sub modules highly impact the reliability of SBW steering sys- The latter is mainly due to the lack of mechanical connection
tem. If total delay exceeds the maximum affordable response between HW and vehicle tries which should be compensated.
time, reliability of the system will be compromised, and the car Also there are critical debates on the whole system safety,
security equipped with SBW is not guaranteed anymore [25]. stability and reliability to be further investigated [43]. Indeed
Digital communication architecture of vehicles is traditionally the application of control configured vehicle (CCV) principle
based on controller area network (CAN) bus operation which mainly employed in aircraft industry sector, improves the lay-
is an event-trigged technology [26]. However, the more recent out flexibility and the overall performance in SBW equipped
counterpart time-trigged technologies such as time-trigged vehicles [44].
CAN (TTCAN), Byteflight and Flexray are proposed for SBW
system implementation due to their higher precision and less
IV. SBW M AIN C HALLENGES
time delay [27]–[29]. An integrated approach for the design
of secure automotive cyber-physical system required for SBW Blind spots of SBW systems are recognized as the com-
implementation is also presented in [30]. plexity of system control and the overall system reliability.
According to DIN EN 50129, reliability of one unit is defined
III. SBW P ROS AND C ONS as the capability to accomplish a task under specific conditions
for a specified time [45]. Risk analysis accomplished by ISO
The elimination of mechanical connection proposed in SBW 26262 proposed an automotive safety integrity level (ASIL) of
system has several potential advantages listed below [18], D as the highest level for vehicle steering system [46]. ASIL
[31]–[38]: considers severity, possibility of exposure to different failures,
• Passive and active safety improvement and controllability of possible failures. The standard specifies
• Modular structure achievement and easier assembling different scales starting from ASIL-A to ASIL-D in which
• Less modifications for left- or right-hand driven vehicles components with ASIL-D level have the most restrictions on
• Availability of more steering functions such as au- acceptable failure rate [47]. In this way, achieving this level of
tonomous driving, variable steering feeling, dynamic sta- reliability requires either analytical or mechanical redundancy.
bilization The desired reliability may be achieved using mechani-
• Weight, space and cost reduction cal back-up similar to what experienced in Nissan motor
• Simpler axle geometry achievement commercial vehicles, Infiniti Q50 and Q60 [17]. However
• Improved handling capability mechanical back-up will increase the system cost, weight,
• Better design flexibility volume, and does not provide enough freedom in car design
• More convenient driving because of variable ratio steering either. Additionally, mechanical back-up relies on human in
(also known as active steering) the loop, which makes it unsuitable for autonomous vehicles
The SBW technique enables application of advanced sys- [37].
tems such as corner modules and four-wheel steering. It is According to ASIL-D standard, all components of steer-
considered in numerous concept deigns such as Fine-X by ing system including sensors, communication links, power
Toyota, F2000 Imagination by Daimler-Chrysler, Hy-wire by electronic converters, actuators and micro-controllers must
General Motors and Infiniti QX Inspiration [4], [17], [18]. A be fault-tolerant with regard to at least one failure [48].
limited version of SBW is implemented in Infiniti Q50 and Duplication is a recognized practice to develop fault-tolerance

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property using replication, redundancy or diversity [49], [50]. applied for SBW system control system implementation as are
Replication stands for providing multiple same units operating briefly discussed [80], [81]. A brief explanation is presented
in parallel while redundancy stands for providing multiple here for each control method.
similar units coming into operation following a severe fault
event leading to a unit failure. And diversity means application A. Model-based control technique
of multiple dissimilar units with similar working specification
Model-based approach is recognized as a torque sensorless
in parallel operation [26], [49]. In this way, fault detection and
technique relying on dynamic mathematical models of vehicle,
isolation mechanism is also necessary to determine whether or
steering system and human interactions with the steering
not one unit is faulty. It can be achieved either by signal-based
system [31], [56], [82]–[86]. Typically a 4th order model is
or model-based approaches [51].
used in the literature which is known as bicycle model [23],
Redundancy will decrease failure rate in the system, but im-
[87]–[89]. The observer outputs can be either used in control
poses further cost and increases the system weight and volume.
system architecture or as an analytical redundancy for actual
Another solution for increasing reliability without increasing
sensors [75], [90]. Nonlinear seven and ten degrees of freedom
system weight and cost, relies on analytical redundancy. It is
models for vehicle in literature are also reported [72], [91]. The
based on using dynamic mathematical model of the system,
studied system in [91] is equipped with a multi-sensor network
estimation techniques, or application of state observers [48],
algorithm in order to obtain the optimal state estimation of
[52], [53].
running vehicle. Also particle swarm optimization (PSO) is
Zheng in [54] used dual motor and micro-controller archi-
used for SBW control system parameter optimization [92].
tecture to achieve fault tolerance property against single point
The following second-order equation is used to approximate
failure and enable system self-reconfiguration. Using triple
the steering system [54]:
modular redundancy with voting system is also utilized for
the SBW micro-controllers in [55]. Anwar suggests analytical  
redundancy for predictive fault tolerant control of a SBW ¨ + bθm2
Jm2 θm2 ˙ + Fc sgn θm2
˙ + ka Ta = Tm2 (1)
system in [56], [57].
Based on the 4th order vehicle model, an observer is Note that that all the variables have been defined in the
designed to estimate RW angle based on motor currents in nomenclature for brevity. The self-aligning torque,Ta , would
[56]. The observed value of RW angle is then used as the an- be calculated from pneumatic and mechanical trail, tp and tm ,
alytical sensor reading replacing one of the redundant sensors. along with front-tire lateral force and sideslip angle.
Application of adaptive observer and model-based approach is Assuming small sideslip angles (less than 4 deg), front and
suggested to obtain fault diagnosis and reconfiguration strategy rear- tire sideslip angles, αF and αR , can be approximated
in [52]. It ensures real-time fault detection without the proba- using (2) and (3) to have a linear vehicle model [56].
bility of false and missed alarms because of noise issues even
with model uncertainties [58]. Using Takagi-Sugeno fuzzy a.r
αF = β + − θm2 (2)
H∞ controller instead of traditionally PID controller, SMC, V
imperialist competitive algorithm (ICA), and radial basis func-
b.r
tion neural network (RBFNN) algorithm are other solutions to αR = β − (3)
obtain more effective and robust system against uncertainty V
and noise issues [59]–[72]. Adaptive parameter estimation The overall vehicle and steering system state space model
addressing system nonlinearity and parameter variation is also would be as follows [57]:
suggested in [73], [74].
Application of robust observers, has been also studied in ẋ = Ax + Bi2 + EFc (4)
[75] to achieve fault tolerance property against power switches
open-circuit failure as well as current sensors fault in a PMSM  
β
drive [75]. Ito and Hayakawa [76] proposed a fault-tolerant  r 
control strategy using separate controllers targeting sensor y = Cx; x = 
 θm2  ;

redundancy reduction with guaranteed system safety. Sensor- ˙ ;
θm2
less control techniques for motor-drives employed in SBW
technology are recognized as other solution for reliability  −Cα,f −Cα,r Cα,r b−Cα,f a Cα,f 
improvement against position and speed sensor faults [77]– mV −1 + mV 2 mV 0
 Cα,r b−Cα,f a −Cα,r b2 −Cα,f a2 Cα,f a
[79]. 0

A=

 Iz Iz V Iz 

 0 0 0 1 
(tp +tm )Cα,f a(tp +tm )Cα,f −(tp +tm )Cα,f b
V. T HE SBW C ONTROL T ECHNIQUES Jm2 Jm2 V Jm2 − Jm2
Due to the lack of mechanical connection in the SBW   
system, two important tasks of path tracking and realistic 0 0
 0   0 
haptic feedback should be achieved using a proper control
 
 0 ;C = 0 0 1 0 ;E =  0 ;
B=   
method. The following categories of model-based, torque-map, Kt 1
current measurement, and torque sensor-based methods can be Jm2
− Jm2

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Zhu proposed internal model controller (IMC) for SBW Nevertheless, it is the most accurate and reliable method
system control providing better performance compared with proposed so far [81], [83], [100].
traditional PID controllers [93]. Fast micro-controllers are The torque sensor based methods proposed for SBW sys-
needed to calculate force feedback and the predictions are used tem are classified to four individual techniques. Their block
to reduce SBW system input-output response time [57], [81]. diagrams, applicability, as well as pros and cons are discussed
here.
B. Torque map control method 1) Method I: Torque sensors for both HW and RW sub sys-
tems: This method employs two torque sensors for both HW
This method employs a map or look-up table for steering
and RW sub systems. The driver compares vehicle direction,
wheel torque determination using different signals such as
θV eh , with road path ahead, θroad , and applies a torque to the
vehicle velocity, yaw rate, etc [94], [95]. The technique relies
steering HW.
on physical properties of vehicle and dynamics of the steering
The steering command, Ts , is received by a torque sensor
system to create a realistic steering feel and cannot be modified
placed on steering hand-wheel. The RW motor-drive is sub-
easily [87]. It is a torque sensorless method and benefits from
sequently controlled using a closed-loop controller provided
no heavy calculations concerned with model based approach.
by the reference torque, T2∗ , which is calculated in the RW
The torque map control method developed by Se-Wook
ECU using measured Ts and is in charge for vehicle direction
Oh determines the steering wheel motor torque using vehicle
change. The load-torque includes the friction between tires
speed and steering wheel angle [94].
and the road surface, Tm2 , and is measured by another torque
Another torque map method is presented by Fankem in
sensor placed on RW motor shaft. The output of this sensor
which HW angular position and speed as well as vehicle speed
is used for establishing a haptic control mechanism enabling
are used in various driving tests to develop the required look
the driver with a real driving perception. In this way, the HW
up table [96]. The proposed control method is supported with
motor drive is also controlled using a closed-loop controller
extra parameters tuning enabling high degree of customization
with torque reference, T1∗ , calculated in HW ECU using Tm2
of the steering feel in a very intuitive approach. The correlation
signal transmitted to HW sub system. The block diagram of
between rack force and steering feel is used to design a torque
this SBW control method is shown in Fig 2.
map.
Given that the driver rotates steering wheel to the left or
The vehicle handling performance is improved in terms of
right by imposing a hand torque command, the HW electric
the change of velocity using a reactive torque map and variable
machine operates in regenerative braking mode by developing
gear ratio in two loops by Zhai [97].
an opposing torque proportional to the real load torque applied
on the RW machine. On the other hand, the RW electric
C. Direct current measurement control method machine is supposed to generate a constant multiple, k, of
Direct current measurement approach as the third SBW the torque command applied by the driver which is exerted on
control technique employs inexpensive current sensors and rack and pinion. Thus, the RW machine operates in motoring
calculates generated torque using motor phase currents [56], mode.
[98], [99]. The measured current signals are used to artificially Alternatively, if the driver decides to steer the vehicle
create haptic feedback mechanism and to provide a redundancy in straight ahead direction, the driver will not apply any
property and fault diagnostic data for SBW system [81]. This hand torque on the steering wheel. Consequently, the RW
technique offers simple algorithm using inexpensive sensors. motor drive operates in regenerative braking mode with zero
The SBW system developed by Nguyen and Ryu benefits reference torque in an effort to return back the wheels angle
from a direct current measurement control method and consists to zero. At the same time, the HW motor drive is to produce a
of two angular position sensors [81]. The first sensor measures positive electromagnetic torque applied on the steering hand-
steering HW angle which is used to establish a reference wheel in order to return the wheel back to zero angle. Hence,
angle for RW motor drive. The second sensor is employed the HW motor drive operates in motoring mode. Given that
for RW angle determination and a position tracking loop is bidirectional power flow is required for both HW and RW mo-
designed for RW motor drive control using a traditional PID tor drives, voltage source converters are used supporting fast
controller. In order to control HW motor drive, the actual load power reversal with fixed DC link voltage polarity provided
is estimated using current sensors employed in the RW motor by the battery.
drive system. The calculated torque is subsequently translated The HW and RW motor angles, θm1 and θm2 , are also
to HW motor drive reference torque using (5) [81]. needed for high performance advanced vector control imple-
mentation employed for AC motor drives. The motors stator
voltages, Vsabc1 and Vsabc2 , and currents isabc1 and isabc2 are
T1∗ = Gf eel (Kt .i2 − Tassist ) + Tf c (5)
measured using appropriate sensors.
Since RW motor is installed on the vehicle front axle near
D. Torque sensor based control approaches vehicle wheels, its torque sensor works in harsh conditions
Torque sensor based method needs costly torque sensors to and is exposed to severe noise, pollution, high temperature,
establish two closed-loop or open-loop controllers for both RW vibration and damage leading to reduced reliability. In this way
and HW sub systems. Note that torque sensors cost as well as utilization of two torque sensors seems a rational arrangement
their operational restrictions are the most challenging issues. but increases the SBW system total costs. Theretofore, meth-

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``

𝑉𝑠𝑎𝑏𝑐1 𝑖𝑠𝑎𝑏𝑐1 𝑉𝑠𝑎𝑏𝑐1 𝑖𝑠𝑎𝑏𝑐1

𝑇𝑚2 𝑇1∗ 𝜃𝑟𝑜𝑎𝑑 𝑇𝑚2 𝑇1∗ 𝜃𝑟𝑜𝑎𝑑
1/𝑘 1/𝑘
ECU 1
-+ ECU 1
-+
Gate Gate
Pulses Pulses

𝜃𝑉𝑒ℎ 𝜃𝑉𝑒ℎ
𝜃𝑚1 𝜃𝑚1

𝑉𝑠𝑎𝑏𝑐2 𝑖𝑠𝑎𝑏𝑐2 𝑉𝑠𝑎𝑏𝑐2 𝑖𝑠𝑎𝑏𝑐2

𝑇𝑠 𝑇2∗ 𝜃𝑠 𝜃2∗
𝑘 ECU 2 Gate
Pulses 1/𝑘 ECU 2 Gate
Pulses

𝜃𝑚2 𝜃𝑚2

Fig. 2: The block diagram of control method I. Fig. 4: The block diagram of control method III.

𝑉𝑠𝑎𝑏𝑐1 𝑖𝑠𝑎𝑏𝑐1
𝜃𝑚2 𝜃1∗
3) Method III: Angle and torque sensor for HW and RW
𝑘 𝜃𝑟𝑜𝑎𝑑
-+ sub systems: In this control system, an angle sensor measures
Gate
ECU 1 Pulses

𝜃𝑉𝑒ℎ
𝜃𝑚1 steering angle command applied by driver’s hands, as shown
in Fig. 4. The angle is used to obtain the position reference,
𝑉𝑠𝑎𝑏𝑐2 𝑖𝑠𝑎𝑏𝑐2
θ2∗ , required for RW motor drive vector control external loop.
𝜃𝑚2
𝜃𝑠 𝜃2∗
The load-torque acting on RW motor is also measured using
𝑘 ECU 2 Gate
Pulses
a torque sensor, Tm2 , and is used to calculate the torque
𝜃𝑚2 reference, T1∗ , required for HW motor drive vector control
torque regulation loop.
Fig. 3: The block diagram of control method II. Using a torque sensor in addition to voltage, current and
angular position sensors for RW motor drive is a disadvantage
of this control architecture indeed. In particular, the torque
ods to eliminate one of these sensors have been investigated sensor has to be installed in a harsh location exposed to
[101]. different kinds of failures near the RW motor as discussed.
A model reference control with described method is pre- A control method of this type is presented in [106].
sented and simulated for SBW system by Lorincz [45]. A This control technique architecture optimized with
hardware-in-loop (HIL) simulation of SBW equipped with the hysteresis-based steering feel is studied in [33], [107]. A
discussed control technique is presented in [102]. two-port network with the described control method is used
A SBW system using the studied control method improved in [54], [81] for SBW system implementation. Direct current
with torque sensorless HW motor drive is presented by Cheon measurement approach is used to obtain an estimation of HW
and Nam in which an estimation of the driver’s applied torque and RW motors generated torque using the discussed control
is obtained using a disturbance observer [103]. method in [101]. Wu employed this control method along
2) Method II: Angle sensors for both HW and RW sub with an active rack force compensator to improve tracking
systems: Method II employs two angle sensors instead of performance of the SBW system [108].
torque sensors used in method I. Since the angle sensors 4) Method IV: Torque and angle sensor for HW and RW
are already necessary for vector control of motor drives, the subsystems: The SBW control is implemented using torque
method benefits from sensors savings resulting in lower costs and angle sensors for HW and RW sub systems, respectively.
with improved reliability. Its block diagram is shown in Fig. In this way, the torque sensor installed close by the HW motor
3. shaft measures driver steering torque command. The measured
The driver rotates steering HW in order to correct the torque, Ts , is used for vector control of RW motor drive as
vehicle moving direction. The desired steering command, θs , is shown in Fig. 5. A torque regulation loop within the RW motor
received by a relevant angle sensor placed right on the steering drive field-oriented control (FOC) compares the reference
hand-wheel. It is used to obtain the reference signal, θ2∗ , of the torque, T2∗ , with the actual generated electromagnetic torque
position loop designed for the RW motor drive vector control generated by the RW motor. Note that no RW torque sensor is
block. The RW motor angle measured by its angle sensor, θm2 , used and the actual torque is to be calculated using machine
installed on RW motor shaft, is also used to create reference dynamic model.
position, for HW motor drive controller, θ1∗ . The RW motor angular position, θm2 , is measured using a
Active front steering for SBW via composite nonlinear position sensor. It is used to obtain the position reference, θ1∗ ,
feedback control with the described control method is studied for HW motor drive controller. The HW angle actual value
in [104]. is same as HW motor’s angle attached to the hand wheel via
Han and Baek studied side-slip angle and the yaw rate a gear box which is either estimated using machine dynamic
control in the SBW system using a receding horizon yaw model or obtained using an extra angle sensor for improved
moment control (RHYMC) technique with two control inputs, reliability.
the steering wheel angle and the external yaw moment [105]. Torque sensor installation for HW motor drive compared to

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𝑉𝑠𝑎𝑏𝑐1 𝑖𝑠𝑎𝑏𝑐1 safety improvement [121]–[127]. These system benefit from

𝜃𝑚2 𝜃1∗ 𝜃𝑟𝑜𝑎𝑑 closer response to ideal steering characteristics, as well as
1/𝑘 Gate
ECU 1 Pulses
-+
𝜃𝑉𝑒ℎ improved overall stability [128]. An integrated steering and
𝜃𝑚1
traction technology is also expected by developing an in-wheel
steering and drive mechanism with a fault-tolerant property
𝑉𝑠𝑎𝑏𝑐2 𝑖𝑠𝑎𝑏𝑐2 [123], [129], [130]. Using 4-wheel drive technology also
𝑇𝑠 𝑇2∗
ECU 2 Gate
enables a complementary solution to hardware redundancy for
𝑘 Pulses

the SBW actuator faults by adapting a control based safety

𝜃𝑚2
strategy called differential drive assisted steering [131], [132].
Its working principle is based on torque vectoring technique
that independently modulates torques applied to the wheels of
Fig. 5: The block diagram of control method IV. vehicle.
Finally, it is not unexpected indeed to see future vehicles
with increased number of sensors in the steering systems to
previous method proves superior design since it would be a
reduce human factor impacts on vehicle control [2].
smaller sensor with safer installation location.
SBW implementation using this method is presented by
VII. C ONCLUSION
Scicluna using rotor flux oriented FOC controlled PMSM
motor drives [109]. Zheng propose this method for SBW The recent research outcomes presented in SBW system
optimized implementation after detailed analysis of various application to vehicle manufacturing industry is thoroughly
control techniques discussed [110]. reviewed. The SBW system is a key to the development of
the intelligent automobiles and autonomous vehicles. Modular
structure, weight and space reduction, more design flexibility
VI. F UTURE OF V EHICLE S TEERING S YSTEMS
and improved handling capability are achieved using SBW
The SBW system is recognized as the next-generation of system in the cost of complex control system and less reli-
steering systems in vehicle manufacturing industry. It is one ability to be further investigated. Considering serious efforts
of the core elements of autonomous driving technology indeed put into the subject from both academia and industry, it is
and can be used in traditional combustion, hybrid, or electric concluded that the prospect of the SBW system is quite
vehicles [111]. Since the reliability in steering system is promising.
quite crucial, work on this area will be continued to achieve The reliability in the SBW system is studied widely by
higher standards. Application of advanced fault estimators researchers and various techniques are suggested. Using me-
and fault-tolerant control methods such as model predictive chanical back-up, duplication, replication, diversity, hardware
control (MPC) are examples of possible solutions in this redundancy, software redundancy, and adaptive observers are
field [65], [112]–[114]. Given that SBW would be finally and presented in the literature.
widely accepted and developed, using other forms of steering Different control techniques for safe and reliable system
elements such as joy-sticks will be used instead of steering implementation are presented and classified in four groups of
hand-wheel leading to more simplified vehicle control [115]. model-based, torque-map, current measurement, and torque
Additionally, implementation of advanced driver assistance sensor-based methods. The sensor-based control methods as
system (ADAS) that has been already applied in accelerating the best choice is discussed in four groups considering dif-
and braking systems will find its application on SBW systems ferent controller input and output variables. The advantages
what will increase the area of using autonomous and autopilot and disadvantages of each control system architecture are
driving systems [109], [116], [117]. For example, Sun used presented and discussed.
iterative learning control in order to ease high-precision motion It is envisaged that sensor based control with HW and RW
tracking in SBW system [118]. motor drives respectively equipped with torque and position
In terms of communication platforms required, the con- sensors would be the optimized choice for SBW implementa-
troller area network with flexible data rate (CAN-FD) would tion considering both reliability and overall cost. The 4-wheel
be further developed and used instead of current platforms SBW system with integrated steering-traction system as well
because of higher data rates required for smart automotive as wireless steering system are also introduced as further steps
systems and better cyber security property [119]. forward in this field.
The 48 V DC-link voltage is rapidly becoming a new
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