Theory of ABS

THEORY OF ANTI-LOCK BRAKING
The ability to slow or stop a vehicle depends upon the braking forces applied to the wheels and the frictional contact that exists between the tires and the road surface. Very low frictional forces exist when the tire is locked (or skidding). A locked tire also causes a loss or the lateral locating forces that effect directional control of the vehicle. The result of a locked tire (or tires) is the loss of steering control and stability. The major forces that affect how easy a tire will lock include:
^ The braking force applied from the vehicles braking system.
^ Environmental factors - rain - ice - snow - etc.
^ Type and condition of the road surface.
^ Condition of the tires (tread and design).

The anti-lock braking system is designed to allow the maximum amount of braking force to be applied to the wheels without allowing the wheels to lock or skid. The advantages that ABS provides includes:
^ Driving stability - by maintaining the lateral locating forces between the tires and the road surface
^ Steerability - allowing the driver to continue to steer the vehicle while stopping (even during panic stops) or accident avoidance maneuvers.
^ Provides optimum braking distances - the rolling wheels transfer higher frictional forces to slow the vehicle.





In order to prevent the wheels from locking during braking, yet provide the optimum braking force for maximum braking efficiency, the ABS braking system must:
^ Have the ability to monitor the wheel rotation rates
^ Be able to regulate the braking forces applied to the wheels.

The ABS system carries out these functions with an electronic control system. The components of the ABS system include:
^ The electronic control module
^ The wheel speed sensors
^ The brake hydraulic unit
^ The brake master cylinder

The four wheel speed sensors are used as inputs to the control module. The module uses these signals to determine wheel speed, wheel acceleration and deceleration. ABS controlled braking starts when the module detects that one or more wheels are about to lock.

Once activated, the ABS control module pulses the brakes on the affected wheel rapidly (215 times a second). This allows the vehicle to be slowed down while still maintaining steerability and directional stability.

The ABS pulses the brakes through solenoids mounted in the hydraulic unit. The solenoids regulate the pressure to the affected wheel through three phases of control:
^ Pressure Hold
^ Pressure Drop
^ Pressure Build