The anti-lock brake controller is also known as the CAB (Controller Anti-lock
Brake).[7][8]
A typical ABS is composed of a central electronic control
unit (ECU), four wheel speed sensors — one for each wheel — and two or more
hydraulic valves within the brake hydraulics. The ECU constantly monitors the
rotational speed of each wheel, and when it detects a wheel rotating significantly
slower than the others — a condition indicative of impending wheel lock
— it actuates the valves to reduce hydraulic pressure to the brake at the affected
wheel, thus reducing the braking force on that wheel. The wheel then turns
faster; when the ECU detects it is turning significantly faster than the others,
brake hydraulic pressure to the wheel is increased so the braking force is reapplied
and the wheel slows. This process is repeated continuously, and can be
detected by the driver via brake pedal pulsation. A typical anti-lock system can
apply and release braking pressure up to 20 times a second.[citation needed]
The
ECU is programmed to disregard differences in wheel rotative speed
below a critical threshold, because when the car is turning, the two wheels
towards the center of the curve turn slower than the outer two. For this same reason,
a differential is used in virtually all roadgoing vehicles.
If
a fault develops in any part of the ABS, a warning light will usually be illuminated
on the vehicle instrument panel, and the ABS will be disabled until the
fault is rectified.
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