Steering: Description and Operation

Electrical Power Steering (EPS)

The Electrical Power Steering (EPS) system is an electromechanical power-assisted steering system with speed-dependent steering power assistance. The servo unit of the system transfers the necessary momentum of the steering power assistance directly to the steering column. This momentum is transferred to the steering column by and electric motor and gearing integrated in the servo unit. The control module integrated in the servo unit calculates the necessary momentum for the steering power assistance from the steering momentum request of the driver and other input variables. The electric motor is activated with this data.

Brief description of components

EPS servo unit
The following components are integrated in the EPS servo unit:

Electric motor (asynchronous motor) with gearing for transmission of the momentum of the steering power assistance to the steering column

Sensors for recording the speed of the electric motor and the steering momentum

Control module for control of the steering power assistance from vehicle speed, steering momentum, steering-angle speed, steering angle and system temperature. The characteristic curves for the speed-dependent steering power assistance are also stored in the control module

The operating voltage range of the EPS system lies between 9 V and 16 V.

Control modules involved
The EPS control module receives the following signals across the CAN bus.

From the Dynamic Stability Control (DSC): - vehicle speed

From the Digital Engine Electronics (DME): - engine speed and status of terminal 15 (one message) - CAN status - status of Sport switch

From the steering-angle sensor (LWS): - steering-wheel angle - prefix of steering-wheel angle - status of steering-angle sensor (all signals in one message)

From the instrument cluster: - kilometer reading - switch day/night lighting (0=off, 1=on)

The instrument cluster receives the following signal from the EPS control module across the CAN bus. - activation EPS indicator lamp in the instrument cluster (0=off, 1=on)

FDC-/SMG button only in the case of option Dynamism Control (FDC)
The button is read in by the DME control module. The button lamp is activated by the EPS control module. The button enables the driver, among other things, to switch the EPS system to a sporting characteristic curve for steering power assistance (higher steering momentum).

SMG button without option Dynamism Control (FDC)
The button is read in by the DME control module. The button lamp is activated by the EPS control module.

Function
The EPS system creates and controls the servo assistance for the steering and its active return. Integrated in the system is Servotronic, the electronic control of the speed-dependent servo assistance. The EPS control module calculates the required servo assistance from the following input variables: - vehicle speed - driver steering momentum - steering angle - steering-angle speed - Sport switch (option FDC) - system temperature of EPS

The characteristic curves (Standard and Sport) for the assistance and damping characteristics are stored in the EPS control module. The values calculated from the input variables, together with the relevant characteristic curve, result in the necessary assistance momentum. The electric motor transfers the assistance momentum across gearing directly to the steering column. The EPS system is activated at signal engine speed > 300 rpm and terminal 15 = on. Terminal 15 = on means that terminal 15 local = on and DME signal status terminal 15 via CAN bus = on. The instrument cluster contains an indicator lamp for the EPS system. This warns the driver about significantly reduced steering force reduction or a complete cutoff of the assistance. The indicator lamp is activated by the EPS control module via the CAN bus.

Features of the EPS system

Protective functions"
In the case of overvoltage (> 17 V), the servo assistance is switched off immediately to protect the final stages. The indicator lamp in the instrument cluster is activated and an entry is made in the fault memory. If the voltage again falls below 16 V, the servo assistance returns to the currently requested value within 2 seconds. The indicator lamp goes out.

In the case of undervoltage (< 9 V), the servo assistance is switched off immediately. The indicator lamp in the instrument cluster is activated as soon as the assistance is completely taken away. An entry is made in the fault memory. If the voltage again rises above 10 V, the servo assistance returns to the currently requested value within 2 seconds. The indicator lamp goes out.

In the case of system excess temperature, the servo assistance is reduced to cool down the EPS system. If the reduction has no effect, the servo assistance is taken away completely and the indicator lamp in the instrument cluster is activated. An entry is made in the fault memory. When the system temperature again reaches the normal range, the servo assistance returns to the currently requested value within 2 seconds. The indicator lamp goes out.

Characteristics in the case of defective or missing CAN bus signals
Engine-speed signal from DME control module:
- Entry in the fault memory. Servo assistance is retained. In the case of a new start with the same condition, the servo assistance remains off and the indicator lamp is activated.

Vehicle speed from DSC control module:
- Entry in the fault memory. The EPS system provides substitute values for control of the servo assistance.

Message from steering-angle sensor:
- Entry in the fault memory. No active steering return.

Failure of all CAN bus signals while the vehicle is being driven (general bus fault for the CAN bus):
- Entry in the fault memory. Substitute value for speed. No active steering return. In the case of a new start with the same condition, the servo assistance remains off and the indicator lamp is activated.