Physics of Anti-Lock Braking System (ABS) and it's working

An Anti-Lock Braking system or Anti-Skid Braking system (ABS) is an automobile safety system

Symbol of ABS

 that prevents the wheels of motor vehicle from locking up, by maintaining a tractive contact with the road surface, thus avoiding uncontrolled skidding of vehicle.



Driving on expressways gives us endure thrilling experience. It gives us the opportunity to explore the vehicle's full potential. Each vehicle cross the speed of 100 Km/hr.

But the situation becomes very tensed as well as risky during monsoons or when you have to suddenly apply the brakes of the vehicle. Without an Anti-Lock brake system, the wheels of your vehicle stop spinning and it will begin to skid. You'll completely loose control over your vehicle and the results could be deadly.

Need of ABS:

ABS offers improved vehicle control and decreases stopping distances on dry and slippery surfaces; however, on loose gravel or snow covered surfaces, ABS can significantly increase braking distance although still improving vehicle steering control. The Anti-Lock braking systems have improved considerably. Recent versions not only prevent wheel lock under braking, but electronically control the front-to-rear brake bias. This function, depending on its specific capabilities and implementation, is known as Electronic Brakeforce Distribution (EBD), or traction control system.

The ABS is a four-wheel system that prevents wheel lock-up 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 an 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.
Apply steady and constant pressure-do not take your foot off. ABS modifies the brake fluid pressure, independent of the amount of the pressure being applied on the brakes, to bring the speed of the wheel back to the minimum slip level that is mandatory for optimal braking performance.               

 


    ABS has four major components:

1.   Speed Sensor                                                                                                                                       This sensor monitors the speed of each wheel and determines the necessary acceleration and deceleration of the wheels. It consists of an exciter (a ring with V-shaped teeth) and a wire coil/magnet assembly which generates the pulses of electricity as the teeth of exciter pass in front of it.   
   
                                                                                                                                                            
2.   Valves                                                                                                                                                    The valves regulate the air pressure to the brakes during the ABS action. There is a valve in the break line of each brake that is controlled by the ABS.                                                                     
   
   
   
    
   
   
                                                              
3.   Electronic Control Unit (ECU)                                                                                                       The ECU is an electronic control unit that receives, amplifies and filters the sensor signals for calculating the wheel rotational speed and acceleration. The ECU receives a signal from the sensors in the circuit and controls the brake pressure, according to the data analyzed by the unit.     
   
                                                                                                                                                
4.   Hydraulic Control Unit                                                                                                                        The Hydraulic Control Unit receives signals from the ECU to apply or release the brakes under the anti-lock conditions. The Hydraulic Control Unit controls the brakes by increasing the hydraulic pressure or bypassing the pedal force to reduce the braking power.                                                                                                                                         

    

Vehicles with ABS are equipped with a pedal-actuated, dual-brake system. 

Anti-Lock Brake systems (ABS) operate as follows:-

1)  When the brakes are applied, fluid is forced from the brake master cylinder outlet ports to the HCU inlet ports. This pressure is transmitted through four normally open solenoid valves contained inside the HCU, then through the outlet ports of HCU to each wheel.

2)  The primary (rear) circuit of the brake master cylinder feeds the front brakes.

3)  The secondary (front) circuit of the brake master cylinder feeds the rear brakes.

4)  If the Anti-Lock brake control module senses a wheel is about to lock, based on anti-lock brake sensor data, it closes the normally open solenoid valve for that circuit. This prevents any more fluid from entering that circuit.

5)  The anti-lock brake control module then looks at the anti-lock brake sensor signal from the affected wheel again.

6)  If that wheel is still decelerating, it opens the solenoid valve for that circuit.

7)  Once the affected wheel comes back up to speed, the anti-lock brake control module returns the solenoid valves to their normal condition allowing fluid flow to the affected brake.

8)  The anti-lock brake control module monitors the electromechanical components of the system.

9)  Malfunctioning of the anti-lock brake system will cause the anti-lock brake module to shutoff or inhibit the system. However normal power assisted braking as it is. 

10) Loss of hydraulic fluid in the break master cylinder will disable the anti-lock system. The 4-wheel anti-lock brake system is self monitoring.When the ignition switch is turned to ON position, the anti-lock brake module will perform a preliminary self-check on the anti-lock electrical system indicated by a three-second illumination by the yellow ABS indicator.

11) During vehicle operation, including normal and anti-lock braking, the anti-lock brake control module monitors all the electrical anti-lock functions.

12) When the vehicle is driven , as soon as the speed reaches 20 km/hr(12 mph), the anti-lock brake control module turns on the pump motor for approximately half to one second. At this time, a mechanical noise may be heard. This is normal function of the self-check by the anti-lock brake control module.

13) When the speed is below 20 km/hr(12 mph), the ABS turns off.

14) Malfunctions of anti-lock brake system and traction control system if occured, will cause the ABS warning indicator to be illuminated.