The effect of air gap, wheel speed and drive angle on the anti-lock braking system efficiency

Anti-lock Braking Systems (ABS) is a safe system and equipped in almost modern cars. In ABS, wheel speed sensors (WSS) are important components, they provide essential wheel speed information not only for anti-lock brake systems, but also for traction control and stability control systems. The wheel speed sensors on most vehicles are magnetic and generate an alternating current (AC) signal that increases in frequency and amplitude with wheel speed. When the teeth on the tone ring rotate past the sensor tip, it changes the magnetic field and induces a current in the sensor windings. The result is a classic sine wave current pattern that changes with wheel speed. The strength of the signal can be affected by not only wheel speed but also the air gap between the sensor and tone ring, resistance inside the sensor, resistance in the external wiring and connector. A precise gap is necessary to produce a strong, reliable signal. But if the gap is too close, metal-to-metal contact between the sensor and tone ring would damage both. Conversely, an air gap that´s too wide may produce a weak or erratic signal or no signal at all. The paper presents the change of alternating current signal when changing sensor air gap, wheel speed and drive angle. By using PULSE LabShop and SignalAnalyser software, sensor KMI15, we observe the waveform on the scope. A scope will show the voltage and frequency of the signal, as well as any missing or distorted humps in the sine wave pattern. From this, we can determine the ABS brake efficiency.