Anti-lock Braking system What is ABS and how does it work?

An anti-lock braking system (ABS) is a safety anti-skid braking system used on aircraft and on land vehicles, such as cars, motorcycles , trucks  and Buses .ABS operates by preventing the wheels from locking up during braking , thereby maintaining tractive  contact with the road surface Anti-lock braking systems are now present in all modern cars sold across world .it prevents the wheels from locking up and helps them maintain grip with the road below. ABS was first introduced as an anti-skid system for aircraft use in the 1950s. And in the 1970s, Ford and Chrysler proved that it can also be used in cars. Since in ABS installed vehicle the chance of skidding is very less and hence it provides a better steering control during braking. Without ABS system, even a professional driver can fail to prevent the skidding of the vehicle on dry and slippery surfaces during sudden braking. But with ABS system, a normal person can easily prevent the skidding of the vehicle and get better steering control during braking.

ABS Working principle

The basic theory behind anti-lock brakes is simple. It prevents the wheels from locking up, thus avoiding uncontrolled skidding. ABS generally offers improved vehicle control and decreases stopping distances on dry and slippery surfaces. 

Fig: Figure shows vehicle apllying brakes with or without ABS   

A skidding wheel (where the tire contact patch is sliding relative to the road) has less traction (grip of the tire on the road) than a non-skidding wheel. For example, if your car drives over a road covered in ice, it is unable to move forward and the wheels will keep spinning, since no traction is present. This is because the contact point of the wheel is sliding relative to the ice.

ABS modifies the brake fluid pressure, independent of the amount of 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.                                                                                                                                          

Components of ABS :

           

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 the exciter pass in front of it.

2) 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.

The majority of problems with the valve system occur due to clogged valves. When a valve is clogged it is unable to open, close, or change position. An inoperable valve will prevent the system from modulating the valves and controlling pressure supplied to the brakes.

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 that is 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

USES of Anti-Lock Braking system

There are many different variations and control algorithms for use in ABS. One of the simpler systems works as follows :

1) The controller monitors the speed sensors at all times. It is looking for decelerations in the wheel that are out of the ordinary. Right before a wheel locks up, it will experience a rapid deceleration. If left unchecked, the wheel would stop much more quickly than any car could. It might take a car five seconds to stop from 60 mph (96.6 km/h) under ideal conditions, but a wheel that locks up could stop spinning in less than a second.

2) The ABS controller knows that such a rapid deceleration of the car is impossible (and in actuality the rapid deceleration means the wheel is about to slip), so it reduces the pressure to that brake until it sees an acceleration, then it increases the pressure until it sees the deceleration again. It can do this very quickly, before the wheel can actually significantly change speed. The result is that the wheel slows down at the same rate as the car, with the brakes keeping the wheels very near the point at which they will start to lock up. This gives the system maximum braking power.

3) This replaces the need to manually pump the brakes while driving on a slippery or a low traction surface, allowing to steer even in most emergency braking conditions.

4) Then the ABS is in operation the driver will feel a pulsing in the brake pedal; this comes from the rapid opening and closing of the valves. This pulsing also tells the driver that the ABS has been triggered

Types of ABS Systems

Most people are familiar with the term “anti-lock brakes” or ABS. However, a much smaller percentage is aware that there are three types of ABS available.

1) Four Channel, Four Sensor

The four-channel, four-sensor ABS system allocates a different valve for each wheel and also a separate speed sensor for each of the four wheels. This is considered to be the best system because it monitors each individual wheel to ensure that the greatest braking force is achieved.

2) Three Channel, Three Sensor

The three-channel, three-sensor ABS provides both of the front wheels with an individual valve and speed sensor. However, the rear wheels must share one valve and one speed sensor, which is located in the rear axle. Pickup trucks featuring four-wheel ABS will typically have the three-channel, three-sensor ABS system.

The drawback to this particular ABS system is that because the rear wheels share one valve and speed sensor, they are monitored together. Therefore, both wheels must become locked in order to trigger ABS. If only one rear wheel locks, the brake’s effectiveness will be reduced.

3) One Channel, One Sensor

This ABS system has one valve and one speed sensor installed to monitor all four wheels and is located in the rear axle. Pickup trucks that have rear-wheel ABS typically feature a one-channel, one-sensor ABS system.

Like three-channel, three-sensor ABS, this system also carries the risk of one rear wheel locking, and thus lowering the effectiveness of the brakes. It is fairly easy to spot this system on a vehicle because there will only be one brake line that runs to the rear wheels through a T-fitting.

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