Electromechanical Power Steering
Electromechanical Power Steering
Electromechanical Power Steering (EPS)
The electromechanical power steering (EPS) differs from conventional hydraulic steering in that it has steering servo. The EPS supports the driver using an electrical servomotor instead of a hydraulic electric motor. The servomotor is only active when the vehicle is being steered. This means that the servomotor consumes no power during straight-ahead driving.
Electromechanical power steering offers the following advantages:
- lower steering forces on parking
- integrated, driving-speed-dependent steering servo (Servotronic)
- lower bumpiness when cornering as well as lower steering wheel vibrations
- active steering wheel return
- saves up to 0.3 l of fuel per 100 km, thus reducing CO2 emissions
- no hydraulic fluid necessary
The electromechanical power steering is available in the following equipment specifications:
1. electromechanical power steering (EPS): voltage supply with 12 V (as before)
2. electromechanical power steering (EPS) with Integral Active Steering (AL) and a defined combination of engine and transmission: Voltage supply with 12 V via jump start terminal point in engine compartment
3. Electromechanical power steering (EPS) with Integral Active Steering (AL) and a defined combination of engine and transmission (weight on front axle): Voltage supply with 24 V via auxiliary battery, cut-off relay and charging controller for auxiliary battery with DC/DC converter
Brief component description
The following components are described for the electromechanical power steering:
EPS unit
The EPS unit consists of the following components:
- EPS control unit
- Servomotor with integrated rotor position sensor
The EPS control unit is part of the electromechanical power steering. The EPS control unit is connected to the vehicle electrical system with 2 plug connections. The steering-torque sensor is connected via another plug connection to the EPS control unit. A number of characteristic curves for power assistance, active steering wheel return as well as damping characteristics are stored in the EPS control unit. The values calculated from the input variables added to each characteristic curve result in the necessary steering servo.
Depending on equipment specification, the EPS unit may also be supplied via terminal 30. The power distribution box in the junction box supplies the EPS control unit with terminal 15N.
Servomotor with rotor position sensor
The servomotor is a non-collector, synchronous direct current motor (permanent magnet). The servomotor drives the reduction gear. This transfers the power output of the servomotor to the rack.
There are 2 rotor position sensors (redundancy) on the printed circuit board of the control unit. Both sensors use the principle of the Hall effect (Hall sensor with magnet wheel). The magnet wheel is attached to the motor shaft. Rotor position sensor 1 determines the position of the servomotor. The sensor delivers a sine-wave signal and a cosine-wave signal. The signals are used to determine the rotor position of the servomotor. Rotor position sensor 2 is used for monitoring (plausibility check). Both sensors are supplied with voltage by the EPS control unit.
Reduction gearset
The reduction gear transfers the power output of the servomotor to the rack. The rotational ratio between servomotor and steering wheel is approx. 18.37:1 (13.37 with Integral Active Steering).
The reduction gear consists of a belt drive and a ball screw. The ball screw has a pitch of 9 mm per revolution (10 mm with Integral Active Steering). The servomotor drives the toothed belt. The belt drive has a reduction ratio of 2.85:1 (2.49:1 with Integral Active Steering). The toothed belt drives the ball screw. The ball screw has an internal ball return (5 rotations). The ball screw is noise-optimised.
Steering-torque sensor
The steering torque sensor digitally senses the steering torque applied by the driver. The working area is approx. 3 steering wheel turns from limit position to limit position.
The steering torque causes the torsion bar to twist. The torsion bar then transfers the steering torque to the pinion. The function of the steering-torque sensor is based on the magneto-resistive principle. This utilizes the effect that arises in response to changes in the magnetic field and changes in resistance. Different voltage signals are generated by these magneto-resistive elements and these are then forwarded to the EPS control unit. The EPS control unit uses this information to calculate the assistance torque required.
Jump start terminal point: EPS 12 V
The jump start terminal point is located to the right side of the engine compartment beside the spring strut dome. When EPS is used in conjunction with Integral Active Steering, the EPS unit is supplied with power via the jump start terminal point. This creates a relationship between engine and transmission.
The voltage supply via the jump start terminal point is assured by means of a safety fuse. This safety fuse can be checked relatively easily, i.e. it is accessible.
Capacitor box: EPS 12 V
The capacitor box is installed in the right-hand wheel arch. When EPS is used in conjunction with Integral Active Steering, the EPS unit is supplied with power via the capacitor box.
This creates a relationship between engine and transmission.
The voltage supply via the capacitor box is protected by another fuse.
The ignition capacitor damps the ripple in the vehicle voltage. This ripple is generated by the alternator (3-phase).
The EPS control unit uses a dedicated line to diagnose the ripple in the vehicle voltage.
System network: EPS 12 V
System network: EPS 24 V
The following other control units communicate with the electromechanical power steering:
ICM: Integrated chassis management
The DSC control unit supplies the driving speed via the wheel speed sensors.
The ICM control unit handles central signal processing for driving speed. This involves the relevant control unit being connected by FlexRay to the EPS.
SZL: Steering column switch cluster
The steering angle sensor is integrated in the steering-column switch cluster (SZL). The steering angle sensor delivers the steering angle as a signal. This signal is transmitted to the ICM control unit. The ICM control unit calculates the rack angle from this. On vehicles with Integral Active Steering, the actuating angle for active steering is included in the same calculation. The steering column switch cluster is connected via the FlexRay unit.
DME and DDE: Digital Motor Electronics or Digital Diesel Electronics
The engine control system delivers the signal "Engine running" on the FlexRay. For certain operating conditions (e.g. engine start), the engine speed is required as a signal.
Instrument panel: Instrument panel
In the event of failure of the electromechanical power steering (EPS), a yellow Check Control symbol appears in the LC display. Simultaneously, the fixed indicator lamp lights up in the instrument cluster. The Check Control symbol has the following meaning: Steering behaviour. Drive with care.
Possible causes for the Check Control message lighting up can be:
- Fault in the EPS control unit, in an integrated sensor or in the servomotor
- EPS overheat protection
- Undervoltage or overvoltage
- Failure of external signals with influence on steering servo assistance
- Faulty or incomplete initialisation of the steering (e.g. learning end stops)
System functions
The following system functions are described for the electromechanical power steering (EPS):
- Steering servo
- Active steering return
- Active road feedback
- Overvoltage detection and undervoltage detection
- Overheat protection
- Identification of overload current
- End stop as software function
- Parking manoeuvring assistant
Steering servo
Integrated in the system is the Servotronic, the electronic control of the speed-dependent steering servo assistance. The EPS control unit determines the required steering servo assistance from various input variables.
Characteristic maps for the support and damping characteristics are stored in the control unit. The values calculated from the input parameters combine with the characteristic curve to produce the required level of steering servo assistance. The servomotor and reduction gear provide the steering servo assistance.
Important input variables for steering servo assistance are:
- Driving speed
- Steering torque on the driver side via steering-torque sensor
- Current vehicle voltage
- Operating temperature
The EPS adapts the deviation of the signals from the motor position sensor and steering angle sensor (e.g. curbed roadway with straight-ahead driving).
Active steering return
After cornering, the active steering return brings the steering back to the straight-ahead position as soon as the driver no longer applies steering torque. To achieve this, the EPS control unit requires the steering angle from the steering angle sensor. The active steering return ensures a harmonious return of the steering across the entire operating temperature. Active steering return from the end positions is particularly important at low temperatures. This increases operating convenience. A gyrostabilizer is integrated in the active steering return.
Active road feedback
Information on the road surface is e.g. changes in the friction coefficient or properties. The EPS provides this information by changing the steering torque.
The following variables are used to calculate the acceleration at the front axle:
- Driving speed
- Yaw rate
- Lateral acceleration
In turn, the EPS calculates a proportion of the steering torque for the active road feedback.
Overvoltage detection and undervoltage detection
EPS 12 V In the case of overvoltage greater than 17 V, the steering servo assistance switches off.
EPS 24 V In the case of overvoltage greater than 33 V, the steering servo assistance switches off.
The EPS control unit stores a fault. A Check Control symbol lights up in the instrument cluster. When the voltage returns to below 16 V (32 V), the steering servo assistance returns to the currently requested value. The Check Control symbol goes out when 100 % of the requested steering servo assistance is reached.
In the case of undervoltage less than 9.5 V, the motor output is reduced (linear) to 9 V. Below 9 Volts, steering servo assistance is no longer provided. The Check Control message is issued at 40 % of steering servo assistance (with fault entry). When the voltage returns to above 10 Volts, the steering servo assistance returns to the currently requested value. The Check Control symbol goes out when 100 % of the requested steering servo assistance is reached.
With the 12 V EPS with Integral Active Steering, engine performance is reduced 10% beforehand in a linear fashion.
Overheat protection
As overheat protection, the EPS reduces the nominal engine torque depending on the output stage temperature between 0 % and 100 %. Here, the EPS control unit carries out the reduction in the following steps:
- 100 % at 95 °C
- 60 % at 110 °C
- 0 % at 115 °C
Identification of overload current
The EPS detects overload current when the steering presses against the curb. The following signals are included in the detection:
- Steering angle
- Steering wheel angular velocity
- Driving speed
- Steering torque applied by driver
End stop as software function
The EPS has an end stop on the left and right as a software function. This protects the mechanical end stops (buffers) against wear at an early stage. The software function uses the following signals to calculate the end stops:
- Steering angle
- Steering wheel angular velocity
Parking manoeuvring assistant
The parking assist consists of the PMA control unit and 2 ultrasonic sensors which take charge of measuring out the parking space. The steering movements required for parking the car which are calculated by the control unit are then carried out by the electromechanical power steering system (EPS).
Notes for Service department
General notes
IMPORTANT: Learn end stops
After replacement of the steering, the end stops of the steering box have to be relearned. Incorrectly learned end stops can lead to sudden elimination of the steering servo in the end stop.
The following preconditions must be met on learning the end stops:
1. Vehicle parked on a level surface.
2. Front wheels free for steering wheel movements.
3. Engine running.
4. Vehicle stationary.
5. Brake pedal not pressed and parking brake not engaged.
6. No fault memory entry in the dynamic stability control (DSC) and the steering column switch cluster (SZL).
7. Indicator light and Check Control symbol light up in the instrument panel.
8. Steering wheel in straight-ahead position.
On learning the end stops, the steering wheel must be fully turned slowly once to the left and once to the right. The steering speed must be below 1 steering wheel turn per second. In the end stops, slowly increase the steering force until the steering wheel no longer turns.
Then return steering wheel to its centre position and let go of the steering wheel.
After approx. 5 seconds, the indicator light and Check Control symbol in the instrument panel go out.
Diagnosis instructions
The following service function is available in the diagnosis system:
- End stop
This service function is used to delete the values for the end stops (e.g. after incorrect learning). At the same time, the deviation to the steering angle is deleted.
Notes on encoding/programming
After replacement, the EPS control unit must be coded.
Switch-on and switch-off conditions
The steering servo assistance is activated under the following conditions:
- ignition ON (terminal 15N via FlexRay and voltage on the EPS control unit)
- Engine running or electrically operational (hybrid drive)
When the engine is at a standstill and the driving speed is 0, the steering servo assistance is disabled.
We can assume no liability for printing errors or inaccuracies in this document and reserve the right to introduce technical modifications at any time.