13. a) Wheel Speed Sensor (WSS) Teeth on the sensor ring rotate past the magnetic sensor, causing a reversal of the magnetic field polarity, resulting in a signal with frequency related to the angular velocity of the axle.
14. Electronic Control Unit The signal from the WSS is proportional to angular velocity. By differentiating this signal, acceleration of each wheel is known. If a wheel is decelerating too quickly the brake pressure is modulated. A fifth input to the ECU is from a brake pedal switch. This signal can shift program execution from a standby to an active state
15.
16. Valves and Brakes The valves modulate the brake pressure up to 20 times per second, effectively realizing the ideal tire slip percentage. ABS ‘pumps’ the brakes much faster than any driver could.
17. 7. How does ABS work? Basically, there are sensors at each of the four wheels (or in the case of the less sophisticated three-channel system, one on each of the fronts and only one for the pair of rears). These sensors watch the rotation of the wheels. When any one of the wheels stops rotating due to too much brake application, the sensors tell the car's computer, which then releases some of the brake line pressure that you've applied - allowing the wheel to turn again. Then, just as fast as it released the pressure, the computer allows the pressure to be applied again - which stops the rotation of the wheel again. Then it releases it again. And so on. With most ABS, this releasing and re-application - or pulsing - of the brake pressure happens 20 or more times per second. Practically speaking, this keeps the wheel just at the limit - the threshold - before locking up and skidding. ABS prevents you from ever locking up the brakes and skidding - no matter how hard you apply the brakes. Obviously, this is going to mean much more steering control. With ABS, all you have to do in an emergency is quickly squeeze the brake pedal as hard as you can and hold it there. And when I say hard, I mean HARD . Let the system do the finesse work for you. This may not be as easy as it sounds. After years of being told (and practising) not to press too hard on the brake pedal, this may not feel very natural.
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