Antilock Braking System Dr. Khisbullah Hudha Chapter 3 Antilock Braking System Dr. Khisbullah Hudha

WHEN THE WHEEL IS SAID TO BE LOCKED WHILE BRAKING? Wheel locking is occurred when the wheel has no rotational speed, but still displace either longitudinally or laterally.

WHY DOES THE BRAKE LOCK WHILE BRAKING?

Ideal Brake Torque Ideal brake torque should be able to maintain s=0.2 during braking (deceleration) Ideal brake torque should be able to maintain Vw as close as possible to Vv

PROBLEMS WITH WHEEL-LOCK IN BRAKING Purely Skidding Occurred in a condition where both brake torques and road roughness are distributed uniformly in all four wheels.   Combination of Skidding and unwanted yaw motion Occurred in a condition where either brake torques or road roughness are not distributed uniformly in all four wheels. Skidding and unwanted yaw may cause the driver lose control of the vehicle and may lead to a road accident.

ANTILOCK BRAKING SYSTEM The ABS is a four-wheel system that prevents wheel lock-up by automatically modulating the brake pressure during an emergency stop. By preventing the wheels from locking, it enables the driver to maintain steering control and to stop in the shortest possible distance under most conditions. During normal braking, the ABS and non-ABS brake pedal feel will be the same. During ABS operation, a pulsation can be felt in the brake pedal, accompanied by a fall and then rise in brake pedal height and a clicking sound.

STRUCTURE OF ABS

MAIN COMPONENTS OF ABS There are four main components to an ABS system: Speed Sensors The anti-lock braking system needs some way of knowing when a wheel is about to lock up. The speed sensors, which are located at each wheel, or in some cases in the differential, provide this information.   Valves There is a valve in the brake line of each brake controlled by the ABS. On some systems, the valve has three positions: - In position one, the valve is open; pressure from the master cylinder is passed right through to the brake. - In position two, the valve blocks the line, isolating that brake from the master cylinder. This prevents the pressure from rising further should the driver push the brake pedal harder. - In position three, the valve releases some of the pressure from the brake. Pump Since the valve is able to release pressure from the brakes, there has to be some way to put that pressure back. That is what the pump does; when a valve reduces the pressure in a line, the pump is there to get the pressure back up. Controller The controller is a computer in the car. It watches the speed sensors and controls the valves.

ABS Hardware Vehicle speed sensor Pump & valve modulator Wheel speed sensor DAQ & Controller

MODELING, SIMULATION AND CONTROL OF ANTILOCK BRAKING SYSTEM

Equations of Motion Longitudinal slip Tt = Tire torque; Tb = brake torque; Iw = wheel inertia & αw = wheel angular acceleration Ft = Long force acting on tire μ = coef. of friction Coef. Of friction as the function of long slip av= vehicle long speed

Hydraulic Brake Modeling

Hydraulic Brake Model

Control Structure of ABS