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.
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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
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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
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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
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°/s
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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
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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
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Stabilization functions Lane departure warning Obstacle avoidance assistance …
Comfort
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Straight line running correction Parking assistant Lane keeping assistant …
Steering
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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
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100°/s standstill
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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
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ZF Servolectric® Servo Unit on the Steering Column
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1 and its electronic control unit are integrated in the steering column.
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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)
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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
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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
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ZF Servolectric® Servo Unit on a second Pinion
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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.
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Configuration of ZF Servolectric with servo unit on a second pinion (- - - non-visible part or internal part)
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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.
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Housing Steering rack Input shaft connection Torque sensor Sensor cable Steering pinion Yoke
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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
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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.
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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
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ZF Servolectric® Paraxial Servo Unit
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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.
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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
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Housing Steering rack Input shaft connection Torque sensor
Paraxial Servo Unit
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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.
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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.
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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
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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
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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
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The Power Pack
with respect to noise performance and torque ripple for the requirements of an electric power steering system.
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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.
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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
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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!
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Configuration of the electronic control unit 1 Control electronics with heat sink and intermediate plate 2 Input filter unit
Power Pack
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The torque sensor sits on the steering pinion (5). A pole wheel (9) is fitted on the input shaft (2)
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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.
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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
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Torsion bar angle [°]
Configuration of the torque sensor on the steering pinion (- - - nonvisible part or internal part)
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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°.
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Clock spring Steering pinion Plug Index magnet (optional) Index sensor (optional) Pole wheel
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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
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Vehicle speed 0 km/h
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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
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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
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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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Germany France Great Britain
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U.S.A. Brazil
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China Malaysia Hungary
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*) only mechanical rack and pinion steering gears
Production Locations
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India**)
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• **) 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
