Wheel Speed Sensor: Description and Operation

GF42.45-P-5134SX Wheel rpm sensor, component description

Component Identification:

Location
The left and right front rpm sensor are located in the steering knuckle of the front axle.

The left and right rear rpm sensor are located in the wheel carrier of the rear axle.

Task
The rpm sensors supply the current wheel speeds as a square wave signal to the ESP control unit (N47-5). The ESP control unit evaluates these signals with regard to direction of rotation, wheel speed and fault detection.





The display shows the current curve through the active rpm sensor during an acceleration phase.

Function
A change in internal resistance inside the rpm sensor is caused by the strength and direction of an external magnetic field. Via an electronic analysis system accommodated in the speed sensor (supplied with voltage from the ESP control unit) a square wave signal is produced, the frequency of which depends on the rpm, however the amplitude is constant. Due to a different signal pattern when the wheel is moving to the left or right the direction of travel is detected. This signal is read in by the ESP control unit and transmitted on the chassis CAN.

The external magnetic field is produced by a multipole ring. This consists of an elastomer plastic with metal particles embedded in it. Via a special magnetization process these have been aligned so that various magnets arise, the polarity of which changes from North to South at the periphery.

Advantages compared with a passive (inductive) rpm sensor:
^ Sensing of low rpm possible (makes possible recording of speed from 0 km/h)
^ Improvement in signal quality (digital signal).
^ Signal largely insensitive to vibrations and temperature differences
^ Low weight and compact construction