Wheel Speed Sensor: Description and Operation

Wheel Speed Sensors
One Wheel Speed Sensor is located in each of the four wheel hubs. The sensors are of the magneto resistive type.

The active sensing of the Magneto resistive Sensor is particularly suitable for advanced stability control applications in which sensing at zero or near zero speed is required. A permanent magnet in the sensor produces a magnetic field with the magnetic field stream at a right angle to the sensing element. The sensor element is a ferromagnetic alloy that changes its resistance based on the influence of magnetic fields. As the high portion of the pulse wheel approaches the sensing element a deflection of the magnetic field stream is created. This creates a resistance change in the thin film ferromagnetic layer of the sensor element. The sensor element is affected by the direction of the magnetic field, not the field strength. The field strength is not important as long as it is above a certain level. This allows the sensor to tolerate variations in the field strength caused by age, temperature, or mechanical tolerances.

The resistance change in the sensor element affects the voltage that is supplied by the evaluation circuit. The small amount of voltage provided to the sensor element is monitored and the voltage changes (1 to 100 mv) are converted into current pulses by the evaluation module.

- Signal Low - 7 mA
- Signal High - 14 mA

The sensor is supplied 12V by the control unit. Output voltage from the sensor is approximately 10V. The control unit counts the high and low current pulses to determine the wheel speed.