Steering Torque Measurement

The driver's steering torque is measured by a steering torque sensor integrated in the servo unit. The function is based on the magnetoresistive principle, these sensors are currently used in BMWs include wheel speed sensors (DSC III MK60) and Valvetronic position sensors (E65/N62 engine). The magnetoresistive 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 magnetoresistive 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 driver's 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.