Nov 5, 2014

Wheel Speed Measurement in Antilock Braking System (ABS)

The role of ABS is to ensure the steer-ability of the vehicle in challenging braking conditions where there is not adequate friction between the wheels and the road. ABS also minimizes the stopping distance by maximizing the braking force. ABS for each wheel of a vehicle mainly consists of an ABS sensor, an actuator, and a controller. The role of the ABS sensor is to provide the ABS controller with a good estimate of the wheel speed. The controller generates an appropriate control signal based on the speed. This signal is fed back to the actuator, which adjusts the braking force accordingly. The common technology for measuring wheel speed in ABSs is to use a magnetic WSS which is often referred to as the ABS sensor. This sensor usually consists of a rotating toothed ring (which is called the ABS ring), a permanent magnet, and a winding (which is called the pickup) that produces a voltage signal that contains the wheel speed information. The ABS ring (also called tone, trigger, or gear wheel) is an essential component of the WSS and generates the speed signal by modulating the magnetic field between the magnet and winding. The modulation frequency is directly related to the speed of the ABS ring, which is rotating with the vehicle wheel.

The importance of accurate wheel speed measurement has motivated brake manufacturers to produce accurate WSS using precision technologies. Several attempts have also been made to improve the accuracy of existing WSSs using signal-processing methods. For instance, a method was proposed in  to compensate for the mechanical inaccuracy of conventional ABS sensors. Their analysis showed that the irregularities of the tooth and gap width have the largest influence on the speed signal quality of conventional sensors. In addition to mechanical inaccuracies, the accuracy of wheel speed measurement is affected by noise. A frequency-domain least-mean-square adaptive filter was used in  to improve the wheel speed measurement. To further mitigate the effect of noise, the same author also proposed using an adaptive line enhancer algorithm to predict the response of a WSS buried in a broad-band noise . The experimental results of the aforementioned two works showed that it is necessary to further process the output of common WSSs to achieve required accuracy in practice where noise is present.

The ABS sensor has a simple and rugged design. However, the commonly used technology has several disadvantages as follows:
  •     The ABS sensor is not accurate at low speeds (lower than 5 km/h) due to very low signal-to-noise ratio .
  •     The ABS sensor, due to the size of the ABS ring, is relatively bulky compared to most sensors. Typical ABS rings have a diameter range of 44–115 mm and a width range of 6–30 mm.
  •     The ABS ring is vulnerable both to environmental factors such as rust and dirt and to operational factors such as wheel hub inspection or repair procedures.
  •     The size of the air gap between the ABS ring and the magnetic field impacts the accuracy of the sensor. This is difficult to adjust, and in some designs, the whole wheel hub has to be replaced when the air gap is unbalanced.
  •     ABS speed measurements are susceptible to mechanical faults in the wheel bearing assembly.
  •     ABS speed measurements are also susceptible to wiring faults (e.g., an open/short circuit or changes in wire resistance).
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