Principle of Operation

Principle of Operation
EPS controls servo assistance for steering. In addition to the measuring the drivers steering torque, the EPS Control Module also monitors further inputs such as:
^ Vehicle speed
^ Dynamic Driving Control (Sport) button
^ Steering angle velocity
^ Internal system temperature
^ Steering angle
The EPS calculates an assistance set point. The electric motor is activated via the integrated power electronics and the torque is transmitted through the worm gear to the driven gear (attached to the steering column output shaft).
The Servotronic function (vehicle speed dependent steering assistance) is also integrated. The corresponding assistance and damping characteristics are stored in the EPS Control Module. The required assistance torque is gradually increased when the engine is started and reduced (with a delay) when the engine is switched off.

Steering Torque Measurement





The drivers steering torque is measured by a steering torque sensor integrated in the servo unit. The function is based on the magneto resistive principle, these sensors are currently used in BMWs include wheel speed sensors (DSC 111 MK60) and Valvetronic position sensors (E65 N62 engine). The magneto resistive elements resistance changes as a function of the magnetic field acting on them.
The input shaft of the upper steering spindle is connected by a torsion bar to the output shaft. A magnet wheel is mounted on the input shaft. The magneto resistive element is mounted on the output shaft.
The magnetic field lines are deflected by the magnet wheel as a result of the rotation of the input shaft with respect to the output shaft (slight twist due to the resistance from the tires on the road, steering gear, etc.). This deflection generates a resistance change (in one of the resistors) causing a voltage change in the evaluation electronics.
Two output signals (different voltage values) are generated which are constantly monitored for plausibility by the EPS Control Module. Based on this voltage change, the EPS calculates the extent of the drivers steering torque. The leads for signal transmission, power supply and ground run in a coil spring cassette mounted on the pinion shaft. The coil spring cassette is located in the worm gear housing.

Steering Angle Measurement





To be able to perform active steering wheel resetting (return to center), the EPS Control Module requires the following:
^ Steering wheel center position
^ Present steering wheel angle
The above information is input to the EPS Control | Module by the steering angle sensor (LWS), in addition to the DSC system requirement. The steering angle sensor is located on the lower steering column in the passenger compartment.

Note: The steering angle sensor must be calibrated (like the E46 DSC).

Dynamic Driving Control Function









When the Dynamic Driving Control function is selected with the Sport button, the engine management system directs the request signal via the PT CAN Bus to the EPS Control Module.
The EPS Control Module switches to sporty vehicle handling. This provides higher steering and holding forces.
1. MS45 ECM
2. Dynamic Driving Control Sport button
3. EPS servo unit and control module
4. Steering angle sensor (LOOS)
5. DSC control module

Indicator Light


The instrument cluster contains an indicator light for the EPS system. This light alerts the driver to significantly reduced steering effort assistance or to a complete shutdown of assistance. This may be caused by:
^ Fault in the EPS Control Module or an associated control module (DSC, ECM).
^ Undervoltage/overvoltage
^ Overloading of EPS