Anti-Lock Braking System

ABS or Anti-Lock braking system has become a common word these days. Most of the cars and almost all the premium segment motorcycles are offering ABS as an option. It is a common belief that Anti-lock brakes are more effective and safer as compared to the normal brakes. It is also said that ABS decreases the braking distance and provides handling stability during braking. Considering the effectiveness of ABS, the Government of India made it compulsory for all 125cc and above motorcycles to be equipped with Anti-Lock Braking System. In this post, we will look at the working of ABS and try to understand the basic phenomenon behind it.

Why Do We Need the Anti-Lock Braking System?

The very first question which may arise in your mind is "Why at all one need ABS ?"  To answer this question let's consider a moving car. When in motion, all the wheels of a car are in perfect rolling condition. Perfect rolling means, there is no slippage between the ground and the wheel. This will occur only when the point of contact of the wheel would remain stationary with respect to the ground. During perfect rolling, the linear and the rotational velocities of the point of contact completely cancel out each other, making the point stationary with respect to the ground. This rolling condition helps in the steering of the vehicle. During panic or severe braking, the wheels of the vehicle get locked i.e. completely stopped. This causes the wheels to slip on the ground instead of rolling. This slippage makes the, steering very difficult and the vehicle continues its path in spite of steering. This condition is very dangerous and many times lead to road accidents. Here come the roles of Anti-Lock Braking System. This system consistently monitors the wheel speed and prevent the wheel locking by reducing the braking force on the wheels.

Components Of Anti-Lock Braking System

The main components of the Anti-Lock Braking System are:

1. Speed Sensor

2. Hydraulic Unit

3. ABS Control Unit

Speed Sensor 

Speed Sensor used in ABS is a Hall effect based sensor mounted on the wheels. A toothed disc is also attached to the wheels just in the front of the sensor. The toothed disc rotates along with the wheel at an equal speed. The rotation of disc induces varying magnetic flux in the sensor. Each time a tooth crosses the sensor, the magnetic field inside it changes. These magnetic pulses are supplied to the ABS control unit for processing.

ABS Control Unit

ABS Control unit is a programmed electronic circuit, which acts as the brain of the Anti-Lock Braking System. It receives the signals from the Speed Sensor and determines the rotational speed of the wheel. Based on its calculations, it gives signals to the Hydraulic unit to control the braking force on a particular wheel.

Hydraulic Unit

The Hydraulic unit is a combination of electric pump and solenoid valves. It is connected in braking circuit between the master cylinder and the brakes. It helps in varying braking force on the wheels depending upon the signals given by the ABS Control Unit. 

How Does The Anti- Lock Braking System Works?

The ABS Control Unit consistently monitors the speed of the wheels through the Speed Sensors. Whenever it finds that a wheel's speed has become zero due to intense breaking, it gives signals to the Hydraulic Unit to reduce the amount of braking force on that wheel. The hydraulic unit than decreased the braking force on that wheel, preventing it from getting locked. The Hydraulic Unit repetitively applies and releases the braking force on the wheel by opening and closing the solenoid valves. This repetitive application and releasing of brakes occur at a rate of almost 15-20 times a minute. This prevents the wheel from getting completely locked and prevent it from slipping. Since the wheel continues its rotational motion the driver is able to steer the vehicle properly.

How Does Anti-Lock Braking System Improves Vehicle Stability?

On a track with varying traction, the brake application causes a different amount of frictional forces to act on different wheels. This may generate a force couple, causing the vehicle to Yaw. The Anti-Lock Braking System prevents this yawing by controlling the amount of braking force on each wheel. This makes the vehicle stable on wet tracks.

How Anti-Lock Braking System Reduces The Braking Distance?

It is evident from the previous paragraph that ABS prevents wheel locking by converting a continuous severe braking force in the form of pulsating force. But how does this reduce braking distance? While braking, sliding friction acts on the wheels. The sliding friction is proportional to the friction coefficient of the material. The coefficient of friction for a tyre is maximum around a slip ratio if 12%. The Anti-Lock Braking System is designed to keep the slip ratio around 12%. This causes a greater frictional force to act on the wheels, reducing the braking distance.

Types Of Anti- Lock Braking Systems

For cars three types of Anti-Lock Braking Systems are available. These systems are categorised depending upon the number of sensors and the valves present in the system

1. Four-Channel Anti- Lock Braking Systems

This Type Of Anti-Lock Braking Systems consists of 4-speed sensors. Each wheel has a separate speed sensor and a solenoid valve to vary the brake force. This system is complex but is very effective from a safety point of view.

2. Three Channel Anti- Lock Braking Systems

This Type Of Anti-Lock Braking Systems consists of 3-speed sensors. Each front wheel has a separate speed sensor and a solenoid valve. While both the rear wheels share a common speed sensor and a common solenoid valve. This speed sensor collects its data from the rear axle instead of the rear wheels.

3. Single Channel Anti- Lock Braking Systems

A Single channel  Anti- Lock Braking System
consists of only one speed sensor. The front wheels neither have speed sensor nor any solenoid valve. While the rear wheels share a common speed sensor and a common solenoid valve. This speed sensor collects its data from the rear axle instead of the rear wheels. This system is very simple in terms of design. However, it is not very effective from a safety point of view.

ABS Vs CBS

Both these terms seem to be similar but are quite different. ABS which means Anti-Lock Braking system is an intelligent braking system. It controls the brake force using intelligent sensors and a processing unit. It consistently monitors the wheel speed and distributes the brake force as per requirements. The CBS at other hands means Combi-Braking  System. It is completely a mechanical system and distributes the brake force using a mechanical force distribution unit. The ratio of force distribution between the front and the rear wheels remains fixed and is not affected by the real-time driving conditions. It is currently being used in lower-end motorcycles while ABS is generally used in high power motorcycles or Cars. The main function of ABS is to eliminate wheel locking. The CBS at other hands may reduce the chances of wheel locking, but can not eliminate it completely. It is operated only when the rear brakes are applied while the ABS may be functional with front and rear both the brakes. However, one major advantage of CBS over ABS is its simple design and lower cost. 

Problems In Anti- Lock Braking Systems

The Anti-lock Braking system has a large number of components and hence requires greater maintenance. The most common problem you may face with ABS is the malfunction of speed sensors. The speed sensors may get covered by dust or debris causing it to malfunction. This may generate false signals causing the brake to engage and disengage unnecessarily. In such case you must truly to clean the speed sensors and the area around them. The other major problem in Anti-Lock Braking System may be generated due to damaged wires. The wires connecting the speed sensor and the ABS control unit may get damaged, limiting the anti-lock capabilities of the brakes. The other major problems may occur due to the malfunction of hydraulic pump, corrosion of ABS module or leakage of brake fluids. All these problems may cause your brakes to underperform and glow the ABS lights on your dashboard. In any such instance, you must avoid panic braking and try to reach the nearest mechanic as soon as possible.