ZF Servolectric - Bosch Automotive Steering

ZF Servolectric®
Electric Power Steering System for Passenger Cars
and Light Commercial Vehicles
Electric Power Steering System
ZF Servolectric®
An electrifying
power solution
Top ride comfort and unsurpassed
precision – easy installation and
minimum power consumption: ZF
Lenksysteme GmbH has brought a
new quality to steering technology for passenger cars and light
commercial vehicles.
Its name: ZF Servolectric.
Contents
ZF Servolectric is superior to conventional hydraulic power steering in many respects. It works by
using an electronically controlled
electric motor which replaces the
conventional hydraulic system.
Control and steering assistance
are powered by a single, uncomplicated source: electric current.
The vehicle’s ride characteristics are
enhanced considerably because the
electrical control perfectly responds
to vehicle dynamics and handling.
At the same time, vehicle manufacturers can increase their productivity thanks to easy system integration, fast installation and optimum
adaptation to the vehicle.
Exciting benefits:
more useful functions, less
power consumption
Page
ZF Servolectric /Servo Unit on the Steering Column ....... 4
®
ZF Servolectric®/Servo Unit on a second Pinion ................ 6
ZF Servolectric®/Paraxial Servo Unit................................... 8
The System ........................................................................10
The Power Pack ................................................................12
The Torque Sensor ............................................................14
Unique matching Options ................................................15
Comprehensive Expertise for EPS Development ............16
Depending on vehicle type, ZF
Servolectric consumes over 90 %
less power than hydraulic steering
solutions. This means in particular
that, for the NEDC driving cycle,
the fuel consumption is reduced
by 0.4 l/100 km, and city driving
up to 0.8 l/100 km fuel savings can
be realized. The reason: ZF Servolectric only requires power when
the vehicle is actually being steered
(in contrast to conventional power
steering, permanent oil pressure is
not required). The servo unit aside,
there are no major components:
steering valve, steering pump, oil
reservoir and high-pressure hoses
are all dispensed with. This saves
weight and facilitates installation
in the vehicle.
Production Locations ........................................................17
Servolectric® is a registered trademark of
ZF Lenksysteme GmbH
2
Worth knowing
Standard values of steering rack force and mechanical
performance for all vehicle classes
Subcompact cars
Compact cars
Mid-size cars
Upper class Off-road vehicles
Luxury class
Upper mid-size class
Performance (W)
1000
Easier installation –
no maintenance
360
°/s
800
EPSapa
600
Insert, fasten, connect up the
electronics. It really is that easy: ZF
Servolectric is supplied as a complete, fully functional system
module. Dispensing with oil and
hydraulics makes it absolutely
maintenance free. A diagnostic
system enables checking of the
functions. And it can be integrated in electronic stability systems
(e.g. ABS, ESP etc.) without any
problems.
Enhanced steering comfort,
extreme precision
ZF Servolectric operates completely independently of the vehicle’s
engine. This means the power
EPSdp
400
eed at
Steering sp
EPSc
200
3
4
5
6
7
assistance can be easily adapted
to suit any vehicle type. And the
electronic control unit registers a
wide range of parameters such as
road speed, steering angle, steering torque or steering speed, for
example. Meanwhile, a programmable damping characteristic
guarantees superb absorption of
road bumps. Moreover, a steer-
Summary of advantages for the customer
The right solution for every vehicle
Safety
•
•
•
•
Stabilization functions
Lane departure warning
Obstacle avoidance assistance
…
Comfort
•
•
•
•
Straight line running correction
Parking assistant
Lane keeping assistant
…
Steering
•
•
•
•
Steering feel
Steering performance
Noise performance
…
Emissions
• CO2 saving:
10 g/km*)
20 g/km**)
Consumption
• Fuel saving:
0.4 l/100 km*)
0.8 l/100 km**)
*) New European Driving Cycle (NEDC) with 2-liter spark ignition engine
**) only city traffic
8
9
10
11
12
100°/s
standstill
13
14
15
16
Steering rack force (kN)
ing angle sensor, which can
optionally be integrated, underlines the high value added by this
steering technology.
Installation variants
for all classes
With just 3 basic types, ZF Servolectric covers all vehicle classes,
from subcompact to mid-size and
upper-class cars and vans. The
servo unit is mounted on the
steering column (EPSc), on a second pinion (EPSdp) or in parallel
to the steering rack (EPSapa),
depending on the installation
space available, the vehicle
power supply and the steering
rack force required.
Worth knowing
3
ZF Servolectric®
Servo Unit on the Steering Column
7 6
1
and its electronic control unit are
integrated in the steering column.
3
5
2
8
Ideal for vehicle manufacturers:
The steering column and servo
unit are delivered to the production line ready-to-install. They
are connected to the mechanical
rack and pinion steering gear via
the intermediate shaft with universal joints. The sensors and torsion bar are located next to the
worm gear. The torque produced
by the electric motor is converted,
via a worm gear, into an assistance torque and transmitted to
the intermediate shaft (see also
description on page 7).
4
Combined with a mechanical rack
and pinion steering gear, this
“wheel-to-wheel” steering system impressively demonstrates
the thorough system competence
of ZF Lenksysteme GmbH.
Even subcompact and compact car
drivers have now come to expect
supreme steering comfort – when
maneuvering through city traffic
or when parking, for example.
This is why ZF Lenksysteme GmbH
developed a Servolectric variant
that is specially tailored for vehicles with lower steering effort.
The basic principle: the servo unit
Configuration of ZF Servolectric
with servo unit on the steering
column (- - - non-visible part or
internal part)
1
2
3
4
5
6
7
8
4
Servo Unit on the Steering Column
Upper steering column
Steering intermediate shaft
Serrations for steering wheel
Torque sensor
Sensor cable
Electronic control unit
Electric motor
Worm gear
The result:
Drivers have all the benefits of ZF
Servolectric with an extremely
lightweight design that requires
very little space.
Picture on the right side of page 5:
“Wheel-to-wheel” steering system
with ZF Servolectric and mechanical
rack and pinion steering
An ideal combination
To achieve appropriate adaptation
of the electric power steering system to the vehicle, modular servo
units are available for different performance requirements. Another
way is to separate the electronic
control unit from the servo unit and
position it in a different place (see
below).
little space are the features of this
well-proven component of which
millions have been made. The ratio
of the rack and pinion steering can
either be constant or variable.
The ideal base for these steering
variants is the mechanical rack and
pinion steering from ZF Lenksysteme. High stiffness, good efficiency
and lightweight design requiring
7
6
2
1
3
5
4
Configuration of the mechanical
rack and pinion steering (- - - nonvisible part or internal part)
1
2
3
4
Housing
Steering rack
Input shaft connection
Drive pinion
5 Yoke
6 Bellows
7 Tie rod
Servo Unit on the Steering Column
5
ZF Servolectric®
Servo Unit on a second Pinion
7
13 2 1
5
7
3
4
The servo unit on a second pinion
is a further superb option, designed for mid-size or upper midsize cars. An option that allows
the ZF Servolectric to shine and to
show just how versatile it really is.
The installation of the servo unit
on the second pinion enables the
physical separation of the sensor
and the drive unit.
12
11
10
9
8
Configuration of ZF Servolectric
with servo unit on a second
pinion (- - - non-visible part or
internal part)
6
Servo Unit on a second Pinion
The fact that the drive pinion
ratio is independent of the steering ratio creates the opportunity
for a performance-optimized configuration. The increase in system
performance is in the order of
10 - 15 %. And superb crash safety
is guaranteed thanks to optimum
use of the available installation
space. By allowing a rotation of
the servo unit of 360° about the
axes of the rack and the drive
pinion, the position of the servo
unit can be defined to fit the particular application.
6
1
2
3
4
5
6
7
Housing
Steering rack
Input shaft connection
Torque sensor
Sensor cable
Steering pinion
Yoke
8
9
10
11
12
13
Electronic control unit
Electric motor
Worm gear
Bellows
Tie rod
Drive pinion (second pinion)
Excellent power transmission
The function of the sturdy worm
gear is to convert the torque provided by the electric motor into
assist torque and to transmit it to
the steering rack. The demands
made in the process on performance and comfort are very high.
To achieve this, it must be
ensured that the teeth of the
worm (5) and helical gear (1)
remain in mesh without backlash
during the entire service life of
the steering gear. For this purpose, a specially developed
spring damper element (2) is
used which provides optimum
spring load to the worm in any
driving situation.
safety device (3) is provided
between the helical gear and
drive pinion (6, universal shaft in
steering column version). This
provides an defined limit to the
torque that can be transmitted.
In unusual driving situations and
in case of abuse, the worm gear
is subjected to extreme loads. To
prevent damage to and any
blockage of the worm gear, in
those rare cases, an overload
4
6
3
To make sure that the worm can
move as necessary, a novel fixed
bearing (4) is fitted. Thanks to its
convex outer race, which is
accommodated in a concave steel
ring, the ball bearing can absorb
high axial and radial loads and
can nevertheless move easily.
7
5
2
1
Configuration of the worm gear
1
2
3
4
5
6
7
Helical gear
Spring damper element
Overload safety device
Fixed bearing
Worm
Drive pinion
Housing
Servo Unit on a second Pinion
7
ZF Servolectric®
Paraxial Servo Unit
2
1
5
3
4
ZF Servolectric with paraxial drive
is characterized by low system
friction and high efficiency. The
possible customer applications
range from dynamic sports cars
and upper mid-size cars to highload vehicles such as off-road
vehicles and vans.
Due to the combination of recirculating ball gear and toothedbelt drive, ZF Servolectric with
paraxial drive is ideally suited for
the customer’s differing performance requirements.
12
11
10
9
8
7
6
The wide range of positioning
possibilities of the servo unit
allows optimum use of the installation space on the vehicle. Thus,
this steering system helps to meet
the high crash requirements of
the automotive industry.
As with all products of ZF Lenksysteme GmbH, the focus for ZF
Servolectric with paraxial drive is
on full compliance with the customer requirements with respect
to function and cost. This goal is
achieved by a modular concept,
which minimizes the amount of
development and application
work.
Configuration of ZF Servolectric with
paraxial servo unit (- - - non-visible
part or internal part)
1
2
3
4
8
Housing
Steering rack
Input shaft connection
Torque sensor
Paraxial Servo Unit
5
6
7
8
9
10
11
12
Sensor cable
Steering pinion
Yoke
Electronic control unit
Electric motor
Servo gear system
Bellows
Tie rod
Minimum sliding forces with
low noise performance
To transform the rotational
movement of the electric motor
into a linear movement of the
steering rack (1), ZF Servolectric
with paraxial drive uses a drive
concept consisting of toothedbelt drive (2, 3, 4) and recirculating ball gear (5, 6, 7). Both drive
stages work with a very high efficiency.
Despite its ability to transmit
high torques safely, the slip-free
toothed belt (2) is characterized
by good noise performance.
6
The recirculating ball gear is a
system in which the ball chain (7)
is returned through a channel (6)
integrated in the ball recirculating nut (5).
The noise performance of the
ball thread during operation is
good. The low noise generation
of the recirculating ball gear
allows rigid connection of the
steering gear to the subframe of
the vehicle, which gives a very
direct steering feel.
7
5
4
1
2
3
Configuration of the servo gear
system
1
2
3
4
5
6
7
Steering rack
Toothed belt
Toothed disc, small
Toothed disc, big
Ball recirculation nut
Ball return channel
Balls/ball chain
Paraxial Servo Unit
9
The System
Electronic control unit
Perfect harmony
Two aspects are decisive in determining the effectiveness of ZF
Servolectric:
on the one hand, naturally, the
superior quality of the central
components; on the other hand,
perfect interaction between
these components and a whole
host of external signals. ZF Servolectric flexibly responds to any
technical environment and any
driving situation.
Electric motor
Diagnosis
10
System
Ignition
Engine speed
Intelligence: the key to easier
steering
Torque sensor
The registered data are used to
calculate the required steering
assistance and, based on the
calculated results, to control the
servomotor.
An ingenious system provides for
remarkably uncomplicated processes: ZF Servolectric operates
based on highly developed electronics and thus makes the
process of power steering fascinatingly easy – whichever installation variant you choose.
In the final step, the motor transmits the optimum servo torque –
via worm gear or recirculating
ball gear – to the steering column
(EPSc), second pinion (EPSdp) or
steering rack (EPSapa) in the
mechanical rack and pinion steering system.
The rack and pinion principle has
proved the ideal steering solution to date, and the ZF Servolectric provides a modern interpretation of this proven concept.
Vehicle speed
If the vehicle power supply fails,
the driver can continue to steer
the vehicle, due to the mechanical connection between the steering wheel and the steered
wheels.
What happens, step by step:
As soon as the driver turns the
steering wheel, sensors register
the corresponding steering torque
and steering speed with absolute
precision.
Functional block diagram of ZF Servolectric
Vehicle speed, power supply,
engine speed, ignition
Motor torque
Electronic
control unit
Electric motor
Power supply
Steering force
Gearbox
Mechanical
steering unit
Wheels,
vehicle
Torque sensor
Torque applied by driver
mechanical flow
Torque applied by driver
flow of current
System
11
The Power Pack
with respect to noise performance
and torque ripple for the requirements of an electric power steering system.
2
3
Thanks to their graded performance classes, the power pack
ranges of the modular design can
be adapted, as needed, to a wide
variety of customer requirements.
1
A high-precision converter
of electrical energy
The electronically commutated
brushless electric motor (1) converts the current defined by the
electronic control unit (2) into a
torque.
On the paraxial variant, the
motor torque is transmitted, via a
toothed disc (3) on the motor
shaft, to the servo gear system
(toothed-belt drive and recirculating ball gear). On the steering
column variant and the double
pinion variant, the torque is
transmitted to the worm gear by
means of a coupling.
The newly developed generation
of brushless motors provides the
assist calculated by the electronic
control unit with high precision
in any driving condition. The latest
materials enable high efficiency
with extremely small size. In
addition, the drive was optimized
Configuration of the power pack for
a paraxial drive
1 Brushless electric motor
2 Electronic control unit
3 Toothed disc, small
12
Power Pack
Example of a power pack for the
drive of a worm gear
Electronic control unit:
the heart of precision
The steering signal is taken by the
torque sensor and passed on to the
ECU.
Optimum assistance and motor control are calculated.
At the heart of ZF Servolectric is
the highly precise electronic control unit: an extremely high-performance microhybrid unit that
has a multi-layer ceramic base.
The ECU houses all the components – 32-bit microcontroller with
on-chip flash memory and RAM, as
well as the output stage for the
motor – in what is a fully integrated, hermetically sealed unit.
In order to ensure optimum
response, the electronic control
unit has all the relevant information permanently “in view”, including stored vehicle data as well as
current operating parameters
(such as vehicle speed, engine
status or steering speed).
And, as one would expect, ZF
Servolectric can be networked with
vehicle management systems via
the CAN bus or FlexRayTM without
any problems whatsoever: a new
dimension in intelligent driving!
1
2
Configuration of the electronic control unit
1 Control electronics with heat sink
and intermediate plate
2 Input filter unit
Power Pack
13
The torque sensor sits on the
steering pinion (5). A pole wheel
(9) is fitted on the input shaft (2)
4
9 3
2
5
7
8
6
6.0
12
5.0
8
4.0
4
3.0
0
2.0
-4
1.0
-8
0.0
-10
-8
-6
-4
which is connected to the steering pinion by means of the torsion bar (1). The measuring range
covered by the sensor is between
+/- 8 and +/- 10 Nm. If the steering
torques are higher, a mechanical
angle limiter prevents overload
of the torsion bar. When the
driver applies a torque on
the steering wheel, the
torsion bar is rotated
as is the magnet relative to the sensor.
1
The sensor consists of magnetoresistive elements
which change
their resistance
when the field
direction changes.
In the process,
the voltage follows a sine and
cosine curve when
the magnet is rotated. The direct rotation
angle of the torsion bar is
1 Torsion bar
2 Input shaft
3 Sensor module
Torque Sensor
0
2
4
6
8
Torsion bar angle [°]
Configuration of the torque sensor
on the steering pinion (- - - nonvisible part or internal part)
14
-2
then calculated by means of an
inverse tangent function.
The high safety demands on electric steering systems require
detection of all faults occurring
on the sensor and creation of a
safe condition of the steering system. The sensor data are transmitted to the electronic control
unit via a very rugged digital
interface.
Optionally, the torque sensor can
also accommodate an index magnet (7) and an index sensor (8).
The index sensor delivers a signal
to the ECU for each full steering
wheel turn. In combination with
the data from the rotor position
sensor and the wheel speeds, the
electronic control unit is able to
calculate the steering angle with
a resolution < 0.05°.
4
5
6
7
8
9
Clock spring
Steering pinion
Plug
Index magnet (optional)
Index sensor (optional)
Pole wheel
-12
10
Steering torque [Nm]
The torque sensor measures the
torque applied by the driver at
the steering wheel. Based on this,
the control unit calculates the
steering assistance for the motor.
Signal 1 [V]
Signal 2 [V]
The Torque Sensor
Unique matching Options
The use of ZF Servolectric allows
one to adjust the steering characteristics (e.g. steering effort, selfcentering and damping) by programming the control unit, thus
adapting them perfectly to different vehicle philosophies. By
using identical hardware components, this electronic programming enables universal adaptation of the steering system to
very different types of vehicles
and applications (for example
sedan, SUV, sports coupé or van).
The result is a reduction in the
multitude of parts and part numbers.
A sporty steering system is characterized by best possible steering precision and handling char-
Assistance torque [Nm]
Examples of characteristics of steering system matching on vehicles
Matched for sporty driving
Matched for comfort
4
3
Vehicle speed
0 km/h
Vehicle speed
0 km/h
2
max
max
1
0
0
1
2
3
4
5
6
7
8
Steering torque [Nm]
acteristics. One factor for achieving this character is the ability to
reduce the steering assist; when
the driver must apply more steering effort, road feel and road
feedback become more direct,
which is desired. Conversely,
higher steering assistance is programmed when the vehicle is
designed for more comfortable
driving.
Matching Options
15
Comprehensive Expertise
for EPS Development
Our company is headquartered in
Schwäbisch Gmünd, Germany.
This is also where all development work on the Servoelectric
system is carried out. By concentrating our expertise in mechanical systems, software, electrics,
electronics and electric motors in
one location, we are able to
respond to our customers’ needs
more quickly.
Positioned as the technology and
innovation leader in the steering
market, ZF Lenksysteme serves as
a reliable and professional partner to vehicle manufacturers
around the globe. A rigorous
focus on satisfying the needs of
vehicle manufacturers and end
customers lays the foundation
for the company’s performance,
while ground-breaking techno-
logical solutions form the basis
for the market success of its
products. The Servolectric electromechanical steering system, of
which we have already produced
millions of units, is an impressive
example.
Comprehensive expertise for EPS development at one location
Benchmarking
Calculation,
simulation
Mechanical
design
Development
of control units
Analysis,
inspection
Motor
development
Vehicle
testing
Schwäbisch Gmünd
Sensor system
development
NVH
testing
Safety
System
networking
Software
development
Comprehensive Expertise
16
Production Locations
16 locations
9 countries
4 regions
Locations
Hydraulic
EPS
Pumps
Steering columns
steering gears
Cars
CVs
Cars
Cars
CVs
Cars
CVs
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•
•
•
•
•
•
•
•
•
•
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•
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Germany
France
Great Britain
•
U.S.A.
Brazil
•
•
•
China
Malaysia
Hungary
17
•
•*
*) only mechanical rack and pinion steering gears
Production Locations
•
)
•
India**)
• planned
•
•
•
**) minority holding
ZFLS 7802P-MBA 08/09e · 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.
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.
[Steering the right way]
A Joint Venture of
Robert Bosch GmbH and
ZF Friedrichshafen AG
Technical modifications reserved.
ZF Lenksysteme GmbH
Richard-Bullinger-Straße 77
D-73527 Schwäbisch Gmünd
Germany
Phone: +49 (0)71 71) 31 - 0
Telefax: +49 (0)71 71) 31 - 32 22
www.zf-lenksysteme.com