One of the most readily accepted applications of electronics in automobiles has been the antilock braking system (ABS). ABS is a safety-related feature that assists the driver in deceleration of the vehicle in poor or marginal braking conditions (e.g., wet or icy roads). In such conditions, panic braking by the driver (in non-ABS-equipped cars) results in reduced braking effectiveness and, typically, loss of directional control due to the tendency of the wheels to lock.
In ABS-equipped cars, the wheel is prevented from locking by a mechanism that automatically regulates braking force to an optimum for any given low-friction condition. The physical configuration for an ABS is shown in above Figure. In addition to the normal brake components, including brake pedal, master cylinder, vacuum boost, wheel cylinders, calipers/disks, and brake lines, this system has a set of angular speed sensors at each wheel, an electronic control module, and a hydraulic brake pressure modulator (regulator).
In order to understand the antilock braking system ABS operation, it is first necessary to understand the physical mechanism of wheel lock and vehicle skid that can occur during braking. below Figure illustrates the forces applied to the wheel by the road during braking.
The car is traveling at a speed U and the wheels are rotating at an angular speed w where
and where RPM is the wheel revolutions per minute. When the wheel is rolling (no applied brakes),
where R is the tire radius. When the brake pedal is depressed, the calipers are forced by hydraulic pressure against the disk. This force acts as a torque Tb in opposition to the wheel rotation. The actual force that decelerates the car is shown as Fb . The lateral force that maintains directional control of the car is shown as FL .
The wheel angular speed begins to decrease, causing a difference between the vehicle speed U and the tire speed over the road (i.e., wR). In effect, the tire slips relative to the road surface. The amount of slip (S) determines the braking force and lateral force.