Anti Lock Braking System

Updated : 03-07-2017 Published : by :
Mechanical Engineering Seminars

Anti-Lock Braking Systems (ABS) are designed to maintain driver control and stability of the car during emergency braking. ABS allows maximum braking to be applied while retaining the ability to 'steer out of trouble'.

Anti-Lock Braking Systems (ABS) are designed to maintain driver control and stability of the car during emergency braking. Locked wheels will slow a car down but will not provide steering ability. ABS allows maximum braking to be applied while retaining the ability to 'steer out of trouble'

The theory behind anti-lock brakes is simple. A skidding wheel (where the tire contact patch is sliding relative to the road) has less traction than a non-skidding wheel. By keeping the wheels from skidding while you slow down, anti-lock brakes benefit you in two ways: You'll stop faster, and you'll be able to steer while you stop.

An ABS system monitors four wheel speed sensors to evaluate wheel slippage. Slip can be determined by calculating the ratio of wheel speed to vehicle speed, which is continuously calculated from the four individual wheel speeds. During a braking event, the function of the control system is to maintain maximum possible wheel grip on the road - without the wheel locking - by adjusting the hydraulic fluid pressure to each brake by way of electronically controlled solenoid valves.

Car manufacturers world wide are vying with each other to invent more reliable gadgets there by coming closer to the dream of the ‘Advanced safety vehicle’ or ‘Ultimate safety vehicle’, on which research and development has been going on for the past several year. Most of the newer vehicle models offer ABS as either standard or optional equipment .Wheel lockup during braking causes skidding which in turn cause a loss of traction and vehicle control. This reduces the steering ability to change direction. So the car slides out of control. But the road wheel that is still rotating can be steered. That is what ABS is all about. With such a system, the driver can brake hard, take the evasive action and still be in control of the vehicle in any road condition at any speed and under any load. ABS does not reduce stopping distance, but compensates the changing traction or tyre loading by preventing wheel lockup.

During panic braking when the wheels are about to lockup, sensors sense that the wheel has just begun turning slower than others on the vehicle. So they momentarily reduce braking force on the affected wheel. This prevents sliding of the wheels on the pavement. When the wheel resumes rolling, full braking force is again applied. ABS repeats the process until there is no longer any need for modulated braking. ABS acts faster than any driver could, pumping the brakes several times per second. Depending on the type of system, ABS adjusts the braking force at each wheel or set of wheels, whereas a driver’s foot on the brake pedal operates all the brakes at once in normal braking.

The brakes of vehicle not equipped with ABS will almost immediately lock the wheels, when the driver suddenly applies the brake. In this case the vehicle slides rather than rolls to a stop. The skidding and lack of control was caused by the locking of wheels. The release and reapply of the brake pedal will avoid the locking of the wheels which in turn avoid the skidding. This is exactly what an antilock braking system does.

When the brake pedal is pumped or pulsed the pressure is quickly applied and released at the wheels. This is called pressure modulation. Pressure modulation works to prevent the wheel locking. ABS can modulate the pressure to the brake as often as 15 times per seconds. By modulating the pressure to the brakes the friction between the tires and the road is maintained and the vehicle is able to come to the controllable stop.

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