Electromechanical Power Steering
Electromechanical Power Steering
With the E92, an electromechanical power steering (EPS: Electronic Power Steering) is introduced in the BMW 3 Series. The EPS will be introduced with diesel engine M57TU2 in 09/2006. Gradually, the EPS will be introduced as a CO2 reducing measure on all other models.
The electromechanical power steering (EPS) differs from conventional hydraulic steering in that it has steering power assistance. The EPS supports the driver with an electrical actuator motor instead of a hydraulic drive. The actuator motor is only active when the steering is used. 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 power steering assistance (Servotronic)
- lower bumpiness when cornering as well as lower steering wheel torsional vibrations
- active steering wheel return
- saves up to 0.2 l of fuel per 100 km, thus reducing CO2 emissions
- no hydraulic fluid necessary
On the E92, the drive for the EPS is arranged parallel to the axle. The arrangement parallel to the axle has advantages compared to arrangements on the pinion or steering column. The arrangement parallel to the axle has advantages with regard to the steering track rod forces that can be achieved, the vibrational properties as well as the acoustic pattern.
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 motor position sensors
The EPS control unit is part of the electromechanical power steering. The EPS control unit is connected with 2 plug connections to the vehicle power system. The steering momentum sensor is connected via another plug connection at the EPS control unit.
A number of characteristic curves for servo 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 power assistance.
The rear fuse carrier (luggage compartment) supplies the servo unit with terminal 30.
Servomotor with motor position sensors
The servomotor is a non-collector, synchronous DC motor (permanent magnet). The servomotor drives the reduction gear. This transfers the power output of the servomotor to the rack.
There are 2 motor position sensors (redundancy) on the board of the control unit. Both sensors use the principle of the Hall effect (Hall sensor with magnetic wheel). The magnetic wheel is attached to the motor shaft. The motor position sensor 1 determines the position of the actuator motor. The sensor delivers a sine-wave signal and a cosine-wave signal. The signals are used to determine the rotor position of the servomotor. The motor position sensor 2 is used for monitoring (plausibility check). Both sensors are supplied with voltage by the EPS control unit.
Reduction gear
The reduction gear transfers the power output of the servomotor to the rack. The revolution ratio between the servomotor and steering wheel is approx. 21:1.
The reduction gear consists of a belt drive and a ball screw. The ball screw has a rise of 7 mm per revolution. The actuator motor drives the toothed belt. The belt drive has a reduction of 2.85:1. 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 momentum sensor digitally senses the steering momentum applied by the driver. The operating range is 4 steering-wheel turns (corresponds to 1440°).
The steering momentum sensor contains 2 sensors (redundancy). Both sensors use the principle of the Hall effect (Hall sensor with magnetic wheel).
The steering momentum causes the torsion bar to twist. In doing so, the torsion bar transfers the steering momentum to the pinion. Both sensors deliver a sine-wave signal and cosine-wave signal. The steering momentum applied by the driver is determined from the signals. The two sensors work independently of one another. The 2nd sensor is present due to the higher availability of the system (redundancy). The sensors digitise the determined data. The signals are transferred across a redundant twin-wire connection. Both sensors are supplied with voltage by the EPS control unit.
The following other control units communicate with the electromechanical power steering:
DSC: Dynamic stability control
The driving stability control system delivers the driving speed as a bus signal. To this end, the corresponding control unit is connected via the PT-CAN (Powertrain CAN) with 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. The plausibility of the steering-angle-sensor signal is checked against the signal of the motor position sensor. If the deviation is too great, the steering power assistance is switched off with a fault code memory entry. The EPS calculates the steering angle and steering-angle speed from the signal of the motor position sensor. The signal of the steering-angle sensor is only required for initialisation and the plausibility check.
The steering-column switch cluster is connected via the PT-CAN.
DME and DDE: Digital Motor Electronics or Digital Diesel Electronics
The engine management system delivers the signal "Engine running" on the PT CAN. For certain operation conditions (e.g. engine start), the engine speed is required as a signal.
KOMBI: Instrument panel control unit
In the event of failure of the electromechanical power steering (EPS), a yellow Check Control symbol appears in the LC display. Simultaneously, the fixed-position indicator lamp lights up in the instrument cluster. The Check- Control symbol has the following meaning: "EPS without function"
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
- Over-temperature protection of the EPS
- Undervoltage or overvoltage
- Failure of external signals with influence on steering power 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 power assistance
- Active steering return
- Active road feedback
- Overvoltage detection and undervoltage detection
- Over-temperature protection
- Detection of overload
- End stop as software function
Steering power assistance
Integrated in the system is the Servotronic, the electronic control of the speed-dependent steering power assistance. The EPS control unit determines the required steering power 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 variables added to each characteristic curve result in the necessary steering power assistance. The servomotor and reduction gear provide the steering power assistance.
Important input variables for steering power assistance are:
- Driving speed
- Steering momentum on the driver side via steering momentum sensor
- Current onboard supply 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 setting as soon as the driver no longer applies steering momentum. 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 gyrostabiliser 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 momentum.
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 momentum for the active road feedback.
Overvoltage detection and undervoltage detection
In the case of overvoltage greater than 17 Volts, the steering power 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 Volts, the steering power assistance returns to the currently requested value. The Check Control symbol goes out when 100 % of the requested steering power assistance is reached.
In the case of undervoltage less than 10.5 Volts, the motor output is reduced (linear) to 9 Volts. Below 9 Volts, steering power assistance is no longer provided. The Check Control message is issued at 40 % of the steering power assistance (with fault code memory entry). When the voltage returns to above 10 Volts, the steering power assistance returns to the currently requested value. The Check Control symbol goes out when 100 % of the requested steering power assistance is reached.
Over-temperature protection
As over-temperature protection, the EPS reduces the specified 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
Detection of overload
The EPS detects overload when the steering presses against the kerb. The following signals are included in the detection:
- Steering angle
- Steering-angle speed
- Driving speed
- Steering momentum 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-angle speed
Notes for Service department
General Information
IMPORTANT: Learning the end stops.
After replacement of the steering, the end stops of the steering gear must be relearned. Incorrectly learned end stops can lead to sudden disappearance of steering power assistance in the end stop.
The following conditions 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 code memory entry in the dynamic stability control (DSC) and the steering-column switch cluster (SZL).
7. Indicator lamp and Check Control symbol light up in the instrument cluster.
8. Steering wheel in straight-ahead setting.
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 effort until the steering wheel no longer turns.
After approx. 5 seconds, the indicator lamp and Check Control symbol in the instrument cluster go out.
Diagnosis instructions
The following service function is available in the BMW 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.
Conditions required for switch-on and conditions required for switch-off
The steering power assistance is activated under the following conditions:
- Ignition on (terminal 15 wake-up)
- Engine running
When the engine is at a standstill and the driving speed is 0, the steering power assistance is disabled.
No liability can be accepted for printing or other errors. Subject to changes of a technical nature.