ABS brakes

The objective of the ABS Braking systems is to stop an automobile in less distance than would be required by conventional—human operated—brakes no matter how slippery the driving surface.  Because it is impossible to measure the slipperiness in real time from the vehicle, the controller infers how slippery the surface is by sensing whether the wheels are rotating.  If the controller detects that a wheel has stopped turning, it reduces the braking force until the wheel begins to rotate again.  But the surface can change so the controller continually, according to a sophisticated algorithm, adjusts the braking force in order to guarantee that it always achieves its objective of stopping sooner than a purely human-operated braking system.  A key component of ABS is an electronically controllable hydraulic cylinder—the actuator of the brakes.

The ABS Braking system exemplifies the basic components of a control system, sensors to measure the outcome of control, actuators to effect the control, and a control method/scheme/algorithm to make the system work well.