Wheel speed sensor
A wheel speed sensor (WSS) or vehicle speed sensor (VSS) is a type of tachometer. It is a sender device used for reading the speed of a vehicle's wheel rotation. It usually consists of a toothed ring and pickup. Automotive wheel speed sensor Purpose The wheel speed sensor was initially used to replace the mechanical linkage from the wheels to the speedometer, eliminating cable breakage and simplifying the gauge construction by eliminating moving parts. These sensors also produce data that allows automated driving aids like ABS to function. Construction The most common wheel speed sensor system consists of a ferromagnetic toothed reluctor ring (tone wheel) and a sensor (which can be passive or active). The tone wheel is typically made of steel and may be an open-air design, or sealed (as in the case of unitized bearing assemblies). The number of teeth is chosen as a trade-off between low-speed sensing/accuracy and high-speed sensing/cost. Greater numbers of teeth will require more machining operations and (in the case of passive sensors) produce a higher frequency output signal which may not be as easily interpreted at the receiving end, but give a better resolution and higher signal update rate. In more advanced systems, the teeth can be asymmetrically shaped to allow the sensor to distinguish between forward and reverse rotation of the wheel. A passive sensor typically consists of a ferromagnetic rod which is oriented to project radially from the tone wheel with a permanent magnet at the opposite end. The rod is wound with fine wire which experiences an induced alternating voltage as the tone wheel rotates, as the teeth interfere with the magnetic field. Passive sensors output a sinusoidal signal which grows in magnitude and frequency with wheel speed. A variation of the passive sensor does not have a magnet backing it, but rather a tone wheel which consists of alternating magnetic poles produce the alternating voltage. The output of this sensor tends to resemble a square wave, rather than a sinusoid, but still increases in magnitude as wheels speed increases. An active sensor is a passive sensor with signal conditioning circuitry built into the device. This signal conditioning may be amplifying the signal's magnitude; changing the signal's form to PWM, square wave, or others; or encoding the value into a communication protocol before transmission. Variations The vehicle speed sensor (VSS) may be, but is not always, a true wheel speed sensor. For example, in the Ford AOD transmission, the VSS is mounted to the tailshaft extension housing and is a self-contained tone ring and sensor. Though this does not give wheel speed (as each wheel in an axle with a differential is able to turn at differing speeds, and neither is solely dependent on the driveshaft for its final speed), under typical driving conditions this is close enough to provide the speedometer signal, and was used for the rear wheel ABS systems on 1987 and newer Ford F-Series, the first pickups with ABS. Special purpose speed sensors Road vehicles Wheel