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The overall purpose of ABS is to aid in maintaining vehicle stability during a hard braking, or a panic braking event. ABS will allow the driver to essentially maintain control of the vehicle and prevent the loss of the driver’s ability to steer the vehicle, by maintaining traction. This is especially beneficial in slippery conditions caused by ice or rain. To sum it up, directional stability is accomplished by maintaining traction.
Before explaining exactly how anti lock systems function, it is important to cover the basics of how the cars braking system actually works. Under normal braking conditions when the driver presses the brake pedal it will actuate the master cylinder, which will in turn open a series of valves on the solenoid block. This allows hydraulic pressure to be applied to the brake callipers; the brake calliper then applies pressure to the brake pads which applies pressure to the rotor thus slowing the rotational speed of the wheel. Without ABS it is possible that enough hydraulic pressure can be applied to the rotor allowing the wheel to lock, causing the vehicle to slide.
At the heart of the ABS system is the ABS controller, this reads and processes signals from wheel speed sensors attached to each of the brake assemblies. Configurations can very from vehicle to vehicle but generally the wheel speed sensors read signals off of what is called a sensor ring. Other components of the ABS system include the previously mentioned solenoid block and valves. Also located on the solenoid block is a pump. The valves have three different positions. Under normal braking conditions, the valves are open allowing hydraulic pressure to the brake callipers. Position number two will close the valve blocking hydraulic pressure to the brake callipers, and valve position number three will vent the hydraulic pressure. The pumps role is to pressurize the lines back up when valve position number three is actuated. Once an increase in rotational speed is detected by a wheel sensor, it is fed back to the ABS control module. The ABS control unit will then react based on a number of factors such as overall vehicle speed, wheel speed increase, ect. The combination of the three mentioned valve positions, essentially modulate brake pressure preventing wheel lock. These modulations in brake pressure can occur multiple times per second.
ABS technologies have proved worthy in many cases, yet as manufacturers continue to make safety improvements, we can expect to see the emergence of new and more efficient ABS technologies.