146 Mobile 2003 Tractors Bosch Rexroth AG

Application Center Tractors

Head: Uwe Maier

Bosch Rexroth AG
Mobile Hydraulics
Robert-Bosch-Str. 2
71701 Schwieberdingen
Telephone: +49 (0) 711 811 - 344 67
Facsimile: +49 (0) 711 811 - 17 98
PC-Facsimile: +49 (0) 711 811 - 266 926
E-Mail: uwe.maier2@boschrexroth.de
Bosch Rexroth AG Tractors Mobile 2003 147

State of the Art and Trends of Working Hydraulics

in the Tractor System

1.Working platform
Tractor
As well as being machines that pull,
tractors also act as a „mobile power
source and control center“ for trailed
or semi-mounted equipment. This se-
cond function is implemented partly
by the PTO and partly by the working
hydraulics.
The working hydraulics of modern
tractors incorporate constant or varia-
ble displacement pumps to supply
pressure and valves to control the
hitch and any coupled implements.
At the start point of introduction of
the hydraulics into tractors were the
hydro-mechanical regulators and ma- Figure 1: Hydraulic components in the tractor
nually operated controls for the non-
electrical/non-electronic work func-
tions. Electrohydraulic control valves for the future. The drive function
and electronic/hydraulic control sys- components will be described in the
tems are now widely used in modern other technical articles for Mobile
tractors. And specific sensors and 2003.
electronic control units have become
well established throughout the trac-
tor performance range due to the in-
creasing automation of work func- 2. Main components of
tions. This change also applies to the the hydraulic system
components associated with the drive The mechanization of agriculture
function, such as steering, brake and started in the early years of the 20th
transmission. Figure 2: Tractors before the introduction of
century. One typical milestone was working hydraulics
Bosch Rexroth offers a complete range the development of the mechanical
of hydraulic and electronic compon- hitch by Harry Ferguson in the 1920s. the mechanical lift for mounted im-
ents for tractor hydraulics (Fig. 1). In 1955, 105,000 tractors were built in plements due to the increasing size of
This article contains a brief historical West Germany, although even in the tractors and their implements (Fig. 3).
review, covers the current status of early 1950s, tractors still had no hy- This ground-breaking innovation,
working hydraulics, including the draulics (Fig. 2). It was at this time with its increased functionality and
steering, and suggests possible trends that the hydraulic power lift was in- flexibility, started the trend towards
troduced. It was designed to replace the greater use of hydraulics.

Authors:
Gerhard Keuper BRM-MC/ETS
Uwe Maier BRM-MC/VTS2
148 Mobile 2003 Tractors Bosch Rexroth AG

One feature of all external gear pumps of oil and the pressure to be better ad-
Manual lever Return linkage Lifting arms
is the pulsation of the pumped medi- apted to the needs of the system.
um, which can amount to 14% with If we compare an axial piston displa-
12-tooth gears (Fig. 5). In 1998, cement pump from 1958 with one of
Bosch Rexroth launched the „Silence today’s variable displacement pumps,
pump“ in which the flank clearance of we will see that enormous progress
the gears is practically zero. This redu- has been made, particularly with re-
Hydraulic pump
Hydraulic Directional Power lift ces pulsation of the pumped medium spect to the reduced power to weight
cylinder 2 control block Adjustable stops for
valve by up to 75% compared with the stan- ratio combined with an increase in
limiting the lifting and
lowering movement dard solution. The volumetric ef- the permitted operating pressure
ficiency is also slightly higher. The ad- (Fig. 6).
Figure 3: Hydraulic power lift (around 1952)
vantages of this quiet and low-pulsati- Used in conjunction with hydraulic
on gear pump are being used increa- load or LS signals that are forwarded
2.1 History and current situation singly to supply pressure to the tractor via the valves, the variable displace-
for pumps steering, in order to reduce vibrations ment pump acts as a feedback control
within the system. pump, i.e. it constantly adjusts its dis-
2.1.1 Gear pumps placement to take account of current
A hydraulic pump was needed in or- 2.1.2 Variable displacement demand. Today, variable displacement
der to supply the power lift. Bosch de- pumps pumps are very common, particularly
veloped such a pump for up to 120 As the hydraulic power increased, new in medium and heavy-duty tractors,
bar pressure, and the first Bosch gear ways were needed to supply the pres- since the energy balance is better than
pump based on the plate design with- sure oil. The variable displacement that of constant displacement pumps.
out gap compensation (Fig. 4) was pump was further developed during
produced from 1953 onwards. the 1950s, which allowed the volume
The next step was a gear pump of
bushing type with gap compensation, Standard pump Silence pump
to make it much more efficient. This Gear with clearence Gear without clearence
has been in serial production since
1972, and is still produced today,
either as a solo or multiple pump. It is
also used as a motor. Hydraulic line Line of action
of action of front flank

Suc-
Pres- Pres- tion
sure sure side
side Suc-
tion side
side

Hydraulic
Line of action of
meshing
rear flank
Front flank delivery

Pulsation
Rear flank delivery

Figure 4: Plate pump (1953) Figure 5: External gear pump (standard vs. silence)
Bosch Rexroth AG Tractors Mobile 2003 149

2.2 History and current situation

of hitch control valves and
systems
A key function in any tractor is the
hitch control. Bosch Rexroth was and
still is the market leader as a supplier
of the required hitch control valves.
Serial production of the mechano-
Typ 716BV-1000 A10VSO28DFR hydraulically regulated hitch control
Year of manufacture 1958 ... 1970 1982 ... today
valve commenced as early as the start
Displacement Vg 28 cm3 28 cm3
of the 1960s (Fig. 8). This valve was
perm. operating pressure pb at Vgmax 150 bar 350 bar
already used to implement combina-
max. Flow Qmax 84 L/min 84 L/min
tion closed-loop control systems
max. Power
Pmax 19 kW 44 kW (draft and position). It was mounted
at Qmax and po
Weight m 26 kg 15 kg directly on the power lift block.
Power/weight ratio m/P 1,36 kg/kW 0,34 kg/kW

Figure 6: Progress in the development of variable displacement pumps

Type A10VNO pumps are used incre- Air filter with Lever for closed-loop
oil dipstick control unit
asingly in modern tractors. These are Hydraulic Lowering
Encoder axle filter choke
variable displacement pumps with Encoder
counterbored piston housing that of- rocker arm

fer an even better power to weight ra-

tio at pressures up to 210 bar. To Lever for
additional
achieve the necessary speeds, implement
Enco- Addi- Cover
A10VNO units are normally supplied der
Preselec- tional
Lifting tor lever imple-
via an upstream gear pump or a Gero- Encoder
shaft ment
spring
tor pump. One variant of this series is Closed-loop-
the A10CNO variable displacement Control unit
Lowering Lowering
pump. It was developed especially for choke valve
Check valve
tractors with a common oil supply for Eccentric
adjusting
Actua-
both working hydraulics and trans- pin
ting
mission. The variable displacement shaft
Shut-off
spool valve Wedge
and the feed pump section, plus pres-
Pilot spool
sure relief valves, filter connector, fil- valve Driver
ter bypass, and filter differential pres-
sure switches were integrated in a Pressure relief valve
compact, space-saving unit (Fig. 7).
This allows the optimum solution to Figure 7: A10CNO regulating pump for Figure 8: Power lift block with Bosch closed-
be created, even where the installation tractor LS systems loop control unit
space is limited.
150 Mobile 2003 Tractors Bosch Rexroth AG

Today, they have been largely superse- Draft

force
Position 1950s
ded by the technically more flexible
electrohydraulic hitch control systems
(EHR), which perform more func-
tions, and are standard equipment on
medium-sized and large tractors. Far-
mers now have access to many new
1990s
functions, such as ride control, slip ST OP

control, etc.
Rexroth supplies the components for
the entire system, such as hitch con-
trol valve, control unit, control panel
and sensors. (Fig. 9).
Since the EHR was introduced, the
hitch control valves have been conti-
nuously developed or replaced by new
technologies (Fig. 10). Important
Figure 9: Replacement of the hydromechanical control power lift by the EHR
steps have been the provision of an
EHR23 valve slice that can be integra-
ted in the directional control valve
block (Qnom up to 100 l/min) for me-
dium-sized to large tractors, and the
introduction of an EHR5 flange valve
(Qnom up to 60 l/min) for small trac-
tors. The most recent step has been to
introduce the on-board electronics
(OBE) in both versions, creating a
„mechatronic unit“ - the brain and
heart of the electrohydraulic hitch
control.
EHR 4 - OC
EHR 23- OC EHR 23- LS EHR 5- OC EHR 5- LS EHR 5- OBE EHR 23- ZM EHR 23- OBE
EHR 4 - LS

1984 1992 1995 1996 2000 2001 2002 2002

Figure 10: Development history of the EHR valves

Bosch Rexroth AG Tractors Mobile 2003 151

The EHR5-OBE and EHR23LS-OBE

hitch control valves with integrated
electronics (Fig. 11) are designed for EHR 23 ZM EHR 23 OBE
robust and harsh agricultural usage,
and offer all the well-established opti-
ons of an electronic hitch control for
the medium and heavy-duty power
classes:
- Position control
- Draft control
- Mixed control
Slice-type LS valve On Board Electronics
- Soil pressure control
pmax = 250 bar (3600 psi) CAN interface
- Clearance control Qmax (Lift) < 100 l/min (26 GPM) Flash memory
- Slip control Secondary pressure limiting valve Self-diagnostics
- Ride control
(transport mode) Figure 11-1: EHR23-OBE
- External control (attachments)
EHR 5 EHR 5-OC EHR 5 OBE

EHR 5-LS

Flange-type OC or LS valve On Board Electronics

pmax = 250 bar (3600 psi) CAN interface
Qmax (Lift) < 60 l/min (16 GPM) Flash memory
Self-diagnostics
Figure 11-2: EHR5-OBE
152 Mobile 2003 Tractors Bosch Rexroth AG

And with the latest, powerful hard- CAN Operating electronic/panel

ware and software, they also offer lea-
ding edge technology with a wealth of ST OP
EHR5 or
benefits (Fig. 12): Display Buttons Joystick Panel EHR23 Valve
- The control center of the hitch con- with OBE
trol system as a balanced, (On Board Electronics)

tested unit Rear button raise/lower

- Little wiring and assembly Angle or position sensor

work required
Draft sensor left/right
- Integral flash memory for
External sensor
software updates
- CAN communication for Pressure senor

operator input and diagnostics Radar sensor

- Temperature monitoring of the
electronics Figure 12: Sensor and operator interfaces on the EHR5 and EHR23 OBE units
The consistent use of electronics ex-
pertise from the Bosch automotive di-
visions creates the basis for their relia- 2.3.1 Mechanically actuated direc-
bility and long service life, and now tional control valves
allows powerful electronics to be inte- More demanding requirements, such
grated in Rexroth products. These are as
very important criteria for tractors - excellent sealing in the neutral posi-
and other agricultural machinery. tion,
- combined with limitation of the se-
2.3 History and current situation condary pressure due to thermal ex-
of directional control valves pansion
The introduction of hydraulic hitches
- kick-out and latching
meant that hydraulics could now be
- load-neutral function control,
used both for the attached imple- Figure 13: SRZ directional control valve
ments and their control system. The block in tractors from the early - load sensing technologies, both with
1960s constant displacement pump pres-
first SRZ directional control valves
were used back in 1953. The main fea- sure supply (OC-LS) and with
Since then, the hydraulic equipment
ture of the slice-based directional variable displacement pumps
for tractors and the increasing use of
control valves from the SRZ series was (CC-LS),
electrohydraulically actuated imple-
that they could be coupled directly to ments have led to a high level of auto- have led to the development of the la-
the hitch control valves (Fig. 13). Even mation. test series - the SB23 LS.
back in those days, they created an ex- This has a larger set of valve modules
tremely compact structure for the with functional blocks optimized for
entire valve system, which made them use in tractors. By constantly increa-
particularly attractive to the market. sing the range of functions, we have
been able to meet the needs of the
market (Fig. 14).
Bosch Rexroth AG Tractors Mobile 2003 153

To be able to do this, even as the com-

Switchable latch plexity of the work increases, it is im-
portant to constantly develop and im-
prove the ergonomics of the driver’s
cab. This relates both to the design
and arrangement of the controls and
displays and to the decoupling of cli-
matic stresses and noise that can re-
duce output. At least partial automati-
Slice-type LS valve on is advantageous where complex
Pmax = 250 bar (3600 psi) workflows are involved. In this way,
Qnom = 80 l/min 21 (GPM) the complexity can still be clearly un-
Qmax = 100 l/min (26 GPM) derstood by the driver, and is thus a
2/3-way pressure compensator (option) requirement for optimum use of the
Variability of the flow rate implement. Possible examples are lin-
king the drive control and working
Mechanical and hydraulic actuation
hydraulic functions, and the automa-
Mechanically actuated check valves
tion of work sequences (e.g. automa-
Switchable latch
ted turning, raising and lowering the
plough at the field end).
Figure 14: Manually actuated SB23LS directional control valve Two developments were the key to ex-
tensive automation:
This module provides a more cost-ef- 2.3.2 Electrohydraulically - The provision of electrohydraulically
fective, functionally reliable and fu- actuated directional control val- controlled directional control valves
ture-proof solution for the working ves SB 23-EHS and
hydraulics. The slice-based basic valve Modern tractors have to operate eco- - The CAN bus
can be fitted with the usual variety of nomically, and the emphasis is placed (Controller Area Network).
control functions. Optional additional on optimizing the energy and power
functions, such as check valves, shock required to utilize the installed machi-
valves and pressure compensators en- ne power and working implements.
sure minimal leakage, ability to with- Nominal flow rate: Qnom 80 l/min at ∆p 7 bar
stand pressure peaks and load pressu- Qmax 100 l/min at ∆p 11 bar
re-independent oil flow. The module Operating pressure: pmax 250 bar
is supplemented with a manual actua- Supply
voltage: 12, 24 V
tor using lever and latch, plus a kick-
Max. power
out function. In the future, it will also consumption: 11 W
be possible to switch between five dif- Response time: < 70 ms from 0 to Qnom
ferent combinations of latch positions Hysteresis: Cannot be measured
and switching functions. Control pressure: 21 ... 24 bar
Max. control oil
flow rate: 2.2 l/min
Setpoint signals: SPA / PWM / CAN

Figure 15: SB23LS-EHS with electrohydraulic actuation

154 Mobile 2003 Tractors Bosch Rexroth AG

The CAN bus has gained ground due,

Shape of the programmable characteristic
in particular, to its successful use in
– Linear (x) to progressive (x3)
automobiles. It is widely available, so
– Different for both directions
it can also be used in tractors and
– For adapting fine control behavior
other agricultural machinery as the
platform for controlling functions. It
Gradient of the programmable characteristic
has been available for several years in – 0 ... 100%
the SB23 family of valves used in con- – Different for both directions
junction with the EHS electrohydrau- – For adapting to the specific hydraulic motor or cylinder
lic actuator (Fig. 15).
The task of the EHS is to convert an Programmable dynamic behavior
input signal into a position-controlled – Ramps 0.07 ... 4 s
displacement of the directional con- – Different for both directions
trol valve spool. This input signal can – For adapting acceleration and deceleration to different load
be transferred as a CAN message or, if conditions
the bus is not available, ratiometrical- Figure 16: SB23LS-EHS with programmable valve characteristics and dynamic behavior
ly or by pulse-width modulation
There are predifined characteristic
PWM. The valve converts the input
curves available for selecting the valve
signal into an appropriate load-inde-
characteristics, ranging from linear to
pendent flow to the consuming actua-
progressive behavior individually for
tor.
both actuating directions (Fig. 16).
The necessary functional elements are
For example, the desired fine mete-
combined to form a compact unit in
ring behavior can be set while, at the
the EHS module. The actuating device
same time, guaranteeing full modula-
is an electrohydraulic actuator consi-
tion in the large signal.
sting of 4/3 proportional pilot valve
The characteristic gradient can be sel-
and control piston. An inductive,
ected as required, again separately for
non-contact position sensor detects
both actuating directions. This means
the spool position. It is thus possible
that if a differential cylinder extends
both to control the position of the
at different speed in both directions,
spool and to reliably detect various er-
for example, this can be compensated
rors by differentiated self-diagnosis.
without changing the input. And set-
The sensor’s amplification circuit is
ting ramp times allows the rise times
housed, together with the control cir-
to be adapted to different load condi-
cuit and the bus coupler, on a com-
tions for the drive in use, allowing im-
mon electronic circuit board.
plements to be started up gently, for
example.
Bosch Rexroth AG Tractors Mobile 2003 155

2.4 Customer-specific control

block configurations
Modern tractors have extremely li-
mited space for installing valve blocks.
Cost-intensive hydraulic line connec-
tions must therefore be minimized.
The state of the art for directional
control valves as used in the working
hydraulics is the control block confi-
guration (Fig. 17).
We call a control block with EHR con-
trol valve, additional directional con-
trol valves, priority valves (if required)
and customer-specific connectors a
central modular control block („ZMS
block“). Such a valve block, which has Figure 17: Central modular control block (ZMS); configured from directional control and
very few external lines, is fully tested EHR valves; subplate with priority function
by Rexroth at the factory and is supp-
lied ready for installation. Today, such achieving this objective is to optimize which is mounted directly on the trac-
blocks are ideally mounted on the the installation space required by tor transmission. The filler neck and
tractor tail, over the rear axle, occasio- complex valve slices. Combined with oil return line from the valves to the
nally with a further block for customer-specific subplates and plug- oil tank in the drive are integrated in
functions at the front of the tractor. It in coupling housings, they then offer the subplate. The plug-in couplings in
is also a good idea to incorporate optimal conditions in terms of func- the customer-specific housing point
plug-in couplings for the attachments. tion, space required and cost-efficien- towards the tail of the tractor.
One important objective for produ- cy. If space is limited between the lifting
cing control units for tractor working Typical arrangements of such valve arms, the installation problem can be
hydraulics is to ensure that the design blocks are one-sided linking of the solved by fitting the valve slices hori-
of the individual modules (and of the valve slices with the input section at zontally (Fig. 20).
valve control blocks that can be confi- the side (Fig. 18), and two-sided lin-
gured from them) is as compact as king to a central subplate (Fig. 19),
possible, but still flexible. The key to

Figure 18: Directional control valve block Figure 19: Directional control valve block Figure 20: Directional control valve block
interlinked on one side (in this case with with central subplate, interlinked on both interlinked vertically (in this case with prio-
7xSB23LS-EHS, 2xEHR23, 1xpriority valve) sides (in this case with 4xSB23LS, EHR23) rity valve, EHR23-ZM, 4xSB23LS, trailer
brake valve)
156 Mobile 2003 Tractors Bosch Rexroth AG

In this example, the subplate incorpo- The CHP is flanged directly to the 2.5 History and current situation
rates options for adapting to different transmission, so no tank lines are re- of steering systems
pressure supplies with constant or va- quired. The length of line to the stee- When development of hydrostatic
riable displacement pumps. As an op- ring, rear or front hydraulics can be steering systems started in the early
tional extra, the trailer brake valve optimized, thus reducing the amount 1960s, the tractor was one of their
may also be mounted on the subplate. of piping on the machine. The volu- main fields of application. The me-
This will avoid having to provide a metric flow of the variable displace- chanical steering systems previously
suitable attachment flange at another ment pump is distributed by the shor- used that were based on bevel, rack or
point or external pipes. Another cu- test route. worm gearing were limited with re-
stomer-specific feature is the housing As increasing demands are placed on spect to the possible steering forces.
for the plug-in couplings. the main brake systems, some of the Although the hydro-mechanical stee-
If tractors have a lot of hydraulic brake systems currently in use have ring gear from the heavy goods vehic-
equipment, particularly if the hydrau- reached their limit of performance. le brought about a substantial impro-
lic drives are arranged at the front and The trend is moving towards remotely vement, the design freedom of the
the rear, it may be sensible to divide powered brake systems as used by tractor was still limited due to the me-
up the valve control blocks accordin- Rexroth in construction machinery chanical connecting and power trans-
gly. While the valve sections for the for some years now. These remotely mission elements required.
rear hitch and the additional func- powered brakes can also be supplied It was only with hydrostatic steering
tions are combined in the ZMS block, via the CHP. The LT59 section is that the desirable design options de-
a modular central hydraulic block designed as an accumulator charging manded by the market became possi-
(abbreviated to CHP = Central Hy- valve. ble, particularly in order to overcome
draulic Program) arranged centrally the high steering forces. This meant
in the middle of the tractor incorpo- that the steering system no longer li-
rates valve sections for controlling mited the size of a tractor.
priority functions (e.g. steering, trailer
brake, supplying the transmission
control (Fig. 21). This cascade arran-
gement ensures that the oil flow is dis-
tributed with the following priority:
1.Steering Trailer brake valve
Working hydraulics
2.Trailer brake valves External po-
3.Low-pressure supply for transmissi- wer brake

on or control oil for the directional

control valve
Steering
4. Working hydraulics and hitch control unit
If the steering system is constructed
independently, then the above order
of priority is not required. If the trai- Axial piston
pump Central Hydraulic
ler brake valve still has to be supplied Low-pressu-
CHP program re supply
as a matter of priority, then a pipe val-
ve (LT42), incorporating the priority
and trailer brake functions in a single
housing, is used. Figure 21: Ideal topological distribution of control blocks in the tractor central hydraulic
system
Bosch Rexroth AG Tractors Mobile 2003 157

The energy needed for steering is pro- Increased loads per axle and the kine- 3. Trends for the future
vided by the hydraulic pump. By tur- matic conditions lead to correspon-
The future of hydraulics lies in the
ning the steering wheel, the driver dingly high steering pressures. Becau-
ability to provide a compact, highly-
simply operates a „control unit“, also se of these, and due to the standards
integrated and energy-optimized con-
known as a „steering unit“. This com- and directives governing the steerabi-
cept of individual functions on the
bines directional control, dosing and lity of the machine, it may be necessa-
one hand, and in the linking of the
manual pump functions, and routes ry to use steering units with reduction
hydraulics with the electronics on the
hydraulic oil to a steering actuator ratios in limp-home mode (Fig. 22).
other hand. Open and closed-loop
(steering cylinder or motor). Rexroth has developed the LAGU
control elements and intelligent signal
steering unit with chamber cut-out
processing will increasingly help to
and the LAGZ steering unit with rotor
optimize the generation and control
set cut-out for these situations. In ser-
of hydraulic power in tractors and
LAGC vo mode, the LAGU and LAGZ units
other mobile working machines.
work no differently to other steering
The technical challenges for electro-
units. The transmission ratio from
hydraulic components are primarily
steering wheel angle to wheel turning
lying in the provision of cost-effective
angle is constant in each mode. In
and robust signal converters (actua-
limp-home mode, however, the dis-
tors such as pilot valves or solenoids
LAGU placement is automatically reduced.
and sensors). The challenges within
This in turn reduces the necessary
the system are associated with the net-
drive force = manual force on the
working of subfunctions and subsys-
steering wheel by 50% or more com-
tems (Fig. 23). These and future de-
pared with a traditional steering unit.
velopments will be driven by the re-
LAGZ sulting potential to increase efficiency
and avoid unnecessary redundancy.
The CAN bus developed by Bosch has
therefore become established as a
platform for mobile applications.

Figure 22: Overview of LAG steering systems Joystick Display

LAG hydrostatic steering systems are

currently used in various models:
- Open center - non-reaction
- Open center - reaction or
- Closed Center - non-reaction and
Sensors
- Closed Center - reaction.
Viewed as part of the overall system,
these components represent the best Control block with
technical solution to the „steering“ Variable displacement pump OBE
problem. Figure 23: CAN-based signal linking in the tractor
158 Mobile 2003 Tractors Bosch Rexroth AG

3.1 Pumps 3.1.2 Variable displacement

pumps
3.1.1 Gear pumps Electrical actuation doesn’t stop at the
The two-stage pump is a financially valves. In the future it will also make
attractive solution to the increased de- its mark on the pressure supply. The
mand to save energy. It improves the old LS systems may be replaced with
power balance compared with a solo electrohydraulic flow matching sy-
pump (Fig. 24). In periods during stems („EFM systems“).
which the working hydraulics are not With this system, the volumetric flows
in use, and only consumers such as required by the individual consumers
steering and brake valve, which re- are added and converted into a set-
quire a low volumetric flow, have to point value for the pumped flow in
be supplied continuously, only section the form of electrical signals. The
1 pumps. Pump section 2 of the two- pump is then electrically actuated
stage pump is only switched to additi- using this setpoint value. A suitable
onal supply by the LS signal when the valve switching concept allows the
demand increases. The two pump sec- flow to be distributed individually to
tions are connected via the valve sec- the consumers.
tion for the two-stage function. This Rexroth can supply electrically actua-
produces a compact unit that is ready ted pumps based on the A10VO or
for installation, without the need for A10VNO pump series for such future
external connecting lines. concepts (Fig. 25). For a desired flow 800
Control current [mA]

rate supplied by the pump a propor- 700

1200 600
tional solenoid is used to adjust the
1000 500
swivel angle of the pump’s swash pla- 800 400
te. The swivel angle has to be set as a 600 300
400 200
function of the detected speed. 200 100
Advantages of EP control: 0 0,2 0,4 0,6 0,8 1,0
EP 1 EP2
- High level of accuracy due to direct (12 V) (24 V) V gmin V gmax

and electro-proportional actuation

Figure 25: Variable displacement
with hydromechanical feedback
pump with EP control
- Compact design
- High system efficiency
- Can be combined with all common 3.2 Mobile valves
hydromechanical pressure and flow
SB23-EHS electrically actuated direc-
regulators.
tional control valves have become well
established in medium and heavy-
Working pressure: p 252 bar
duty tractors. A new generation of
Displacement: V 16+16 cm3/U
these valves - called „SB23LS - EHS2“
Max. speed: n 2800 U/min
for 2nd generation - will make even
Pressure output: with 2 or 1 output
better use of the benefits of electrohy-
as required
draulics, and will lead to even greater
integration (Fig. 26).
Figure 24: Two-stage pump
Bosch Rexroth AG Tractors Mobile 2003 159

One example of such an application is Another typical use of pressure sen-

the automatic turning of the raised sors is the weighing of loads on the
plow in the turn space (Fig. 27). The tractor hitch (Fig. 28). This uses the
pressure measurement allows the lifting cylinder pressure measure-
dead-center positions of a simple dif- ments. If the kinematic behavior is
ferential cylinder to be easily detected known, a special algorithm can be
from the rise in pressure, and then used to calculate the load to a high le-
Figure 26: 2nd generation SB23LS-EHS
used to electronically switch the valve vel of accuracy (<2% error) with the
The flow range is extended to Qnom = to be controlled. The conventional hy- center of gravity in any position, even
100 l/min, or Qmax=140 l/min, thus draulic switching to mimic a reversing when the tractor is moving across the
meeting the increased demand for gre- cylinder can thus be omitted, which field.
ater volumetric flows. Proven features, will simplify the implement drives ac-
such as position-controlled valve cordingly.
spools, diagnostic functions, CAN bus,
etc, will be retained in the new on-
board electronics. The availability of pz
cost-effective pressure sensors from the pH
automotive field will allow the range of SP
functions to be greatly extended, howe-
ver. Two optional sensors for measu- GL Measuring range (calibration measure) h
ring the pressure in the working con- h x SP
nections and an optional further pres-
sure sensor for measuring the pump, pH pH
LS or return pressure are new means of y SP
checking and regulating the pressure at
the connectors to the implement. This h
Measuring range (continuous measure)
will mean that many more work func-
tions can be open and closed-loop con- Figure 28: Use of pressure sensors for additional functions,
trolled from the tractor. Example: Weighing with the hitch

1. Retracting the 2. TDC: Changeover 3. End stop as kick-

turning cylinder with directional out
control valve

Turning cylin-
der
pUmst pmax

Plow frame

Turning knuckle

Figure 27: Use of pressure sensors to control implements. Example: turning a plow
160 Mobile 2003 Tractors Bosch Rexroth AG

3.3 System functions

Tractor Attachment
Attachments that are supplied with oil
via additional valves inside the tractor Operator input Electronic
can be used to control the functions and display control
of their own drives or actuators by
sending electrical signals.
To do this, the directional control val- CAN
ves of the additional hydraulic system
are more or less „loaned“ by the trac- Electronic Directional control valves
with pressure sensors
tor to the attachment. All the necessa- control
ry communication takes place over
the LBS bus (CAN-based bus for trac- Drives,
tor-implement system data communi- Sensors
cation). In the future, the pressure
sensors integrated in the directional Figure 29: Control of additional functions from the implement
control valves will be used by the elec- („attachment controlling the tractor“)
tronic controls in the attachments to
3.4 Steering systems ving conditions. Rexroth is working
optimize their work functions wit-
As the complexity of work sequences on a modular system concept based
hout these attachments having to have
increases, the driver will need more on existing and proven function
sensors of their own (Fig. 29).
help with steering - or the function blocks that takes account of the cur-
The implements will be able to use
may even have to be automated. Elec- rent guidelines for steering systems.
this in conjunction with the powerful
trically controllable steering will be Figure 30 shows the concept of exter-
electro-proportionally actuated SB23-
needed for this. One additional bene- nal volumetric flow amplification for
EHS2 directional control valves to op-
fit here will be the possibility of am- the steering unit as an example of an
timize their functions, without increa-
plifying the steering (or rather the vo- alternative steering system.
sing the size of the hydraulic circuits
lumetric flow) in relation to the dri-
on those implements. In fact the hy-
draulics may even be simplified. In
this way, the implement manufactur-
ers will demonstrate their expertise
more in terms of the control devices
Control unit Sensor
on the implements, which will go a
long way towards making them stand
out from the competition. Sensor Valve LAG

Sensor

Figure 30: Electronic-hydraulic steering, Add-On solution

Bosch Rexroth AG Tractors Mobile 2003 161

The movement of the steering wheel 4. Summary

is detected, by the steering sensor on
Hydraulics have been used in tractors
the steering column or on the steering
for around 50 years. This has made a
unit, in the form of the angular
great contribution to the mechanizati-
velocity and direction. It is then ana-
on of agriculture, and has reduced or
lyzed and sent as a digital or analog
even done away with much of the
signal to an electronic control unit.
physically exhausting work. It would
Alternatively, the input signals may be
not have been possible to increase the
generated in a GPS track guidance sy-
efficiency of agriculture to such an ex-
stem. The electronic control unit con-
tent without hydraulics.
trols a proportional directional con-
In the meantime, electronics and bus
trol valve that supplies oil to the stee-
technology have gained a foothold as
ring cylinder as required. This system
other sources of innovation in the
concept aims to improve the ergono-
tractor. There are now some new and
mics and driver comfort, which will
technically and financially attractive
ultimately lead to greater work out-
solutions for the tractor functions,
put, will take the strain off the driver
which are very complex. They also
and result in further or improved ap-
meet today’s more stringent environ-
plications for the tractor.
mental requirements. The determi-
ning factor will be the ability of supp-
liers and tractor manufacturers to
combine the different technologies in
the best way.
In the past, the previously separate
companies Rexroth, Brueninghaus
Hydromatik and Bosch have led the
field in working with the customer to
ensure the success of hydraulic drive
technology in the tractor. Now that we
have merged to form Bosch Rexroth
AG, we aim to use our wide-ranging
experience of hydraulic power genera-
tion and control, electronics and
transmission technology to provide
our customers with even more com-
prehensive support in the form of a
broad range of tailored products.