ZF Servotwin - Bosch Automotive Steering

ZF Servotwin®
The Electro-hydraulic Steering System for Commercial Vehicles
ZF Servotwin®
The future begins today.
ZF Lenksysteme is the worldwide
technology leader in the field of steering systems for commercial vehicles.
The responsibility to respond to the
complex challenges of the present
with solutions arises from this. Thus,
ZF Lenksysteme makes a big contribution to the commercial vehicle industry. The company from Baden-Württemberg uses fixed values, such as
safety, environmental protection and
comfort, as a guideline. The commercial vehicle industry faces extensive
changes.
Above all, the introduction of a
power steering that can be influenced electronically must be mentioned here. With the ZF Servotwin,
ZF Lenksysteme presents the first
electro-hydraulic steering system
worldwide for heavy commercial
vehicles. The Servotwin represents
the innovative combination of a
recirculating ball power steering
gear ZF Servocom® with an electronic drive and control unit. The system
provides speed-sensitive steering
assistance and active return to the
Content
Introduction............................................................................ 2
Mechanical design of the system............................................. 3
Hydraulic operation of the system........................................... 4
Electronical operation of the system........................................ 5
straight ahead position. These functions are already proven in passenger
cars and are now available for the first
time in the commercial vehicle sector
too. Also, the electronics fully replace
the – so far hydraulic – second steering circuit for commercial vehicles
with two steered axles. In addition to
the mentioned functions, the integration of driver assistance systems is
planned for the second generation
of the Servotwin. Functions known
from the passenger car, ranging from
lane keeping function and traffic jam
guidance to partly automated driving,
are feasible as a result of the networking of electronic components with
the steering system. Economy and
ecology are further key subjects of
the 21st century.
In 2014 the exhaust emission standard EURO VI became effective in the
EU for commercial vehicles. Important sales markets in Asia and North
America have introduced their own
exhaust emission standards. This
requires equally efficient solutions
from manufacturers and suppliers.
ZF Lenksysteme presents the newly
developed Servotwin.
System advantages..................................................................6
ZF-Servotwin® is a registered trademark of
ZF Lenksysteme GmbH
2
ZF Servotwin® The future begins today.
Mechanical design of the ZF Servotwin®
The Servotwin is basically made
up of two components: the triedand- tested recirculating ball power
steering gear Servocom and an electric motor with control unit and worm
gear. Via the gear, an additional
steering torque can be applied, in
parallel with the steering wheel
torque, by means of an electric motor.
The additional steering torque acts
on the input shaft of the Servocom.
Thus, the manual effort needed for
steering can be changed according
to the driving situation. Nevertheless,
the mechanical connection between
the steering wheel and the steering
linkage is maintained. If the electronic
system fails, hydraulic steering effort
assistance is always retained.
4
1
3
2
6
5
1 Housing
2 Piston
3 Worm
4 Ball chain
5 Sector shaft
6 Worm gear
7
8
7 Electric motor
8 Electronic control unit
Mechanical design of the ZF Servotwin®
3
Hydraulic operation of the system
8
7
6
4
1
5
3
9
2
9
ZL
10
ZR
1 Piston
11
8 Oil tank
2 Worm
9 Worm gear
3 Ball chain
10 Electric motor
4 Torsion bar
11 Electronic control unit (ECU)
5 Valve slide
ZL Power cylinder, left-hand
6 Flow limiting valve
ZR Power cylinder right-hand
7 Steering pump
4
Hydraulic operation of the system
The main steering assistance is
provided by a recirculating ball
power steering gear. The Servocom
is designed on the principle of a
classic hydraulic steering. An oil
circuit and a power cylinder provide the steering assistance for the
steering wheel rotation by
the driver. The Servocom
from ZF Lenksysteme GmbH
is a well-proven system of
which millions have been
made, and it has been a
benchmark in the commercial vehicle industry for many
years. But modern transport
makes ever-increasing demands
on the commercial vehicles. Only
an innovative steering system
with electronic interface can meet
those demands. Where the integral
hydraulic steering gear reaches its
limits, the intelligent Servotwin brings
to bear its strengths.
Electronical operation of the system
A rotary movement generated at
the steering wheel and the applied
steering torque are detected by
modern turning angle and torque
sensors and are transmitted to the
integrated electronic control unit
(ECU) of the Servotwin. Taking into
account further vehicle parameters,
e.g. speed and load condition, the
ECU determines in split milliseconds
the steering assistance required and
controls the electric motor. Adapted
to the particular driving situation, the
electric motor supports the steering
movement by adding or subtracting a
steering torque, thus optimizing the
steering operation electronically.
As speed increases, the steering feel
becomes stiffer and thus more direct
by the action of the electric motor.
Different load conditions of the commercial vehicle can be compensated
by the steering system, which means
that the driver always gets optimum
steering feel.
Another new product feature is the
electrical steering lock stop which
is to protect the steering system
against overload at maximum wheel
lock. From a programmable steering
wheel position, the electric motor
reduces steering assistance or applies, if
required, a countertorque so that the
mechanical steering limiter cannot be
damaged.
Electronic control unit
Electronical operation of the system
5
Advantages of the ZF Servotwin®
Generation 1
Technical background
Energy saving
Reduction of fuel consumption in comparison to a conventional dual-circuit system:
0.6 l/100km, depending on vehicle application.
Active return to straight ahead
After cornering, the wheels are returned to straight ahead by the electric motor.
Safety
Should the hydraulic supply fail, steering assistance will, for safety reasons, be provided by the electric motor. The design of the Servotwin is that of a dual-circuit steering
system with separate energy supply.
Cross wind compensation
In the event of cross wind the Servotwin keeps the vehicle reliably on track. The
necessary steering correction is recorded by the ECU of the steering system and initiated
via the electric motor.
Generation 2*
Technical background
Speed-dependent steering
effort
The Servotwin permits precise, fast steering activities at high speed and at the same
time ease of handling of the commercial vehicle at low speed.
Lane keeping function
If the lane is left unintentionally, the vehicle is steered back into the lane by a corrective
steering operation and/or the driver is warned by means of a signal.
Traffic jam guidance
The system assists the driver with lateral guidance operations. The driver can intuitively
override the system at any time and retains full control.
Partly automated driving
The system carries out lateral guidance operations in special situations and is permanently monitored by the driver in the process, for example automatic coupling and
automatic reversing.
* permits assistance functions, implementation by the commercial vehicle manufacturer
6
Technical data
8298
Front axle load [kg]
5.500 – 8.500
Torque hydraulic [Nm]
6.500 – 7.600
Efficiency hydraulic
0,9
Torque electric [Nm]
125
Max. pressure [bar]
170
Rec. oil flow [dm3/min]
Max. 25
Weight dry [kg]
53
Advantages of the ZF Servotwin®
The many advantages impressively
demonstrate the conceptual superiority of the Servotwin. Thus, this innovative steering system meets the
ever-increasing demands of a modern
society on mobility and transport. The
Servotwin enables networking of the
steering system with the vehicle electronics and thus the implementation
of assistance systems. This increases
safety and comfort of the commercial
vehicle considerably.
One trend is clearly noticed at this
early stage: the importance of
driver assistance functions
on commercial vehicles
will continuously
increase.
For the Servotwin, the focus is initially
on model size 8298. This is designed
for heavy construction site vehicles,
commercial vehicles and buses in intercity traffic. Here, the emphasis is
on the parameters torque and front
axle load. In the future, it is imaginable to also equip commercial vehicle
classes of lower performance requirements with a Servotwin. While the
first generation of the Servotwin is
exclusively designed for trucks, the
second generation is additionally to
be used in buses. The Servotwin can
be adapted for both modern chassis
with independent wheel suspension
and rigid-axle systems.
Installation situation of the ZF Servotwin®
7
ZFLS 8298 C-U 06/14e Printed on paper bleached without use of chlorine
ZF Lenksysteme GmbH:
the systems partner
ZF Lenksysteme GmbH is one of the
largest independent manufacturers of power steering systems for
passenger cars and commercial vehicles. Renowned automotive manufacturers from all over the world
value us as a creative and efficient
systems partner for the development of new and innovative solutions.
ZF Lenksysteme GmbH
Richard-Bullinger-Straße 77
73527 Schwäbisch Gmünd
Germany
Phone: +49 (0)71 71 31- 0
Telefax: +49 (0)71 71 31- 32 22
www.zf-lenksysteme.com
As a joint venture of Robert Bosch
GmbH and ZF Friedrichshafen AG,
ZF Lenksysteme GmbH offers its
customers a unique source of expertise when it comes to integrating a wide range of top technologies in modules, system modules or
entire chassis systems.
The benefits for the manufacturer
are clear to see: even shorter development times and optimized
production processes – with quality
standards which just get better and
better.
Excitement of steering
A Joint Venture of
Robert Bosch GmbH and
ZF Friedrichshafen AG