An anti-lock brake system (ABS) wheel speed sensor measures road-wheel speed and direction. This information is used for anti-lock brake (ABS), traction control (TCS) or vehicle stability (ASP) system ECUs to prevent the vehicle’s wheels from locking or spinning. This increases the driver’s control of the vehicle.
How it works:
There are two basic types of wheel speed sensor, passive and active;
1. Passive sensors;
These are similar in operation to Inductive cam/crank sensors; they consist of a soft iron, permanent magnetic pin around which is wound a fine copper wire coil. The unit fits in close proximity to a rotating trigger wheel. The trigger wheel is mounted onto a rotating assembly, such as the disc, drum or hub. When the trigger-wheel rotates past the pick-up assembly the alternating teeth generate a small inductive voltage in the copper windings. This voltage signal is used by the ECU as wheel speed information. Passive sensors generally only have two wires, one signal and one earth/shield.
2. Active sensors;
The operation of the active sensor can be likened to the Hall type sensor found in distributors etc. The pick-up assembly has an inbuilt amplifier and thus relies on a supply voltage which is normally 5v but it can be 12v.
The rotating element consists of a multi-pole (north, south, north, south …) magnetic ring, which can be located onto a rotating assembly as with the passive sensor. There is an increasing trend to incorporate it into the wheel bearing seal and to use magnetic powder instead of fixed magnets.
The rotating, alternating, magnetic poles generate a magnetic flux within the sensor element which then amplifies and regulates the signal for the ECU to use as wheel speed information. The output of an active sensor is digital with a square wave signal, it is capable of sending wheel speed information down to 0mph, whereas the passive sensor’s accuracy is usually dubious below, 25mph. Active sensors generally have three wires; one power supply, one signal and one earth/shield.
