Subject: Automobile Engineering

Topic: Brakes, Steering & Front Axles

Difficulty: High

ABS stands for Anti-lock Brake system. ABS not only cause the vehicle to stop without deviating from its straight line path, this but also provide directional stability since there is skidding of wheels. ABS only operates under heavy braking or on slippery surfaces. When the front wheels of the vehicle are locked it movability is reduced, whereas in case of rear wheel locking, the vehicle stability is reduced. ABS calculate the required slip rate of the wheels accurately based on the vehicle speed & the speed of wheels & then controls the brake fluid pressure to achieve the target slip rate.

$Slip=1-\frac{w_w}{w_v}\hspace{0.2 cm}$ & $ \hspace{0.2 cm}w_v=\frac{V_V}{R_T} $

where :
$w_v,w_w$=angular velocity of vehicle & wheel.
$v_v$=linear velocity of vehicle
$R_T$=radius of tyre/wheel

Although ABS prevents complete locking of wheels, it allows some wheel slip in order to attain the best possible braking.

In an emergency stop with ABS, one should apply the pedal firmly & hold it while the ABS does all the work. There would be putstations of the pedal which may be quite violent but this is normal & the driver must not let off the pedal due to this.

A modern ABS consists of an electronic control unit (ECU) one sensor on each wheel, an electrically driven hydraulic pump & a pressure accumulator. Accumulator is used to store hydraulic fluid to maintain high pressure in the braking system & to provide residual pressure for power-assisted braking. Generally, the accumulator is charged with nitrogen gas. ECU monitors & controls the anti lock function when required. Its function is based on inputs from the wheel speed sensors & feedbacks from the hydraulic unit to determine whether the ABS is operating precisely & also to decide when the antilock operation is required.