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What is Anti-Lock Braking System (ABS)? How does it works?

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Introduction

In the realm of automotive safety, few innovations have had as profound an impact as the Anti-Lock Braking System (ABS). Designed to prevent wheel lock-up during braking, ABS has become a standard feature in modern vehicles, revolutionizing the way we approach braking and significantly enhancing road safety. This blog takes a closer look at the evolution,mechanics, and benefits of ABS.
Illustration showing an "Anti-Lock Braking System" (ABS) at work. A yellow car on a road avoids skidding at 90 km/h. Inset of a foot on a brake pedal.

It is a safety system in automobiles. It prevents the wheels from locking while braking. The purpose of this is to allow the driver to maintain steering control under heavy braking and, in some situations, to shorten braking distances (by allowing the driver to hit the brake fully without skidding or loss of control).

The Birth of ABS

Before the advent of ABS, traditional braking systems faced a critical limitation – the tendency for wheels to lock up during hard braking.Locked wheels result in loss of steering control, leading to skidding and potential accidents. Recognizing this issue, engineers began developing a solution to maintain steering control under heavy braking conditions,ultimately giving rise to the Anti-Lock Braking System.

How Do Wheels Lock?

During braking, wheels lock if the brake force applied is more than the friction between the road and tyre. This often happens in a panic braking situation, especially on a slippery road. When the front wheels lock, the vehicle slides in direction of motion. When the rear wheels locks, the vehicle swings around. It is impossible to steer around an obstacle with wheels locked. Locked wheels can thus result in accident. Skidding also reduce tyre life.

What Does ABS Do?

  • The system detects when a wheel is about to lock and briefly releases brake pressure. Once traction returns, braking resumes. A toothed pole wheel on the hub rotates near a magnetic sensor, generating pulses that reflect wheel speed. These are sent to the ECU for real-time processing.
  • When a wheel starts to lock, the ECU signals the modulator valve solenoid to release brake pressure. Once traction is regained, the signal stops and pressure is reapplied. In its 'hold' state, the valve maintains constant pressure to optimize braking. This cycle—typically 5 to 6 times per second—continues until the vehicle stops. ABS ensures stability even if the brake pedal remains fully pressed, reducing the risk of skidding or accidents.
Diagram of an anti-lock braking system over a blue background, with labeled components like master cylinder and valves. Below is an orange car.

Components:

The anti-lock braking system consists of following components.

1.Wheel Speed Sensor

The wheel speed sensor consists of a permanent magnet and coil assembly.It generates electrical pulses when the pole wheel rotates. The rate at which the pulses are generated is a measure of wheel speed. The voltage induced increases with the speed of rotation of the wheel and reduces with increasing gap between the pole wheel and the sensor.

2.Pole Wheel

A pole wheel is a toothed ferrous component, usually with teeth on its face. If parallel sensor placement isn’t possible, teeth may be on the outer edge. Standard 9.00-20 or 10.00-20 tires use 100-tooth wheels, while smaller tires (under 9 inches) use 80-tooth wheels.

3.Sensor Extension Cable

The sensor extension cable is a two core cable which connects the wheel speed sensor to the Electronic Control Unit. The inner core sheathing is of EPDM rubber and the outer sheathing is polyurethane which provide abrasion resistance to the cable. The cable has a module plug with two pins is connected to the control assembly. The cable has two cores-brown and black in colour.

4.Electronic Control Unit

The ECU is the core of the ABS system. It receives input from wheel speed sensors and calculates wheel speed, deceleration, and acceleration. If a wheel begins to lock, the ECU activates the corresponding modulator valve to prevent it. The ECU is typically mounted in the driver’s cabin.

How ABS Works

1. Wheel Speed Sensors:

At the core of ABS are wheel speed sensors, typically located at each wheel or sometimes within the differential. These sensors continuously monitor the speed of each wheel.

2. Electronic Control Unit (ECU):

Wheel speed sensors send data to the Electronic Control Unit (ECU), the brain of the ABS. The ECU analyzes this in real time to detect any wheel about to lock.

3. Modulator Valves:

To prevent wheel lock-up, ABS utilizes modulator valves.These valves regulate brake fluid pressure to the brake calipers or wheel cylinders. If the system detects an imminent lock-up, it momentarily reduces brake pressure, allowing the wheel to continue rotating.

4. Pulsating Braking:

ABS operates by pulsating the brakes rapidly—applying and releasing brake pressure multiple times per second. This pulsating action prevents the wheels from completely locking up and maintains traction between the tires and the road.

Advantages

  1. It prevents the locking of the wheel and thus eliminates the chance of skidding.
  2. Skidding is eliminated, ensuring excellent braking control.
  3. ABS enhances steering control during braking.
  4. It reduces the chance of collision by 30 %.

Disadvantages

  • A vehicle equipped with ABS (Anti-lock Braking System) is costlier as compared with a vehicle without ABS.

Future Developments

ABS keeps changing as automotive technology advances. Future versions of ABS are expected to offer even more comprehensive control and safety under a variety of driving conditions when integrated with other safety features like traction control systems and Electronic Stability Control (ESC).

The ECU consists of 7 major circuits

  • Input circuit
  • Master circuit
  • Slave circuit
  • Driver circuit
  • Feedback circuit
  • Power supply circuit
  • Fail safe circuit

The functions of ECU,

  1. It receives wheel speed signal from the sensor. The wheel speed signals are processed and appropriate output signals are sent to the modular valves in the event of a wheel lock.
  2. It continuously monitors the status and operation of ABS components and wiring.
  3. It alerts the driver in the event of occurrence of any electrical fault in the ABS system by actuating a warning lamp.
  4. It disconnects the exhaust brakes during ABS operations.
  5. It enables the service technician to read the faults in the system either through a diagnostic controller or a blink code lamp.

Modulator Valve Cable

The Modulator valve cable has thee cores. There are two solenoid interface lines and a common ground line. The inner core sheathing is of EPDM type and the outer sheathing is polyurethane which provide abrasion resistance to the cable. The cable has a three pin moulded socket is connected to the modulator valve solenoid at one and an interlock connector with locking feature at the other end. The cores are brown, blue and green.

Modulator Valve

ABS Modulator valve regulate the air pressure to the brake chamber during ABS action. During normal braking it allows air to flow directly from inlet to delivery. Modulator valve cannot automatically apply the brakes, or increase the brake application pressure above the level applied by the driver through the dual brake valve.

There is an inlet port, Delivery port and Exhaust passage.

  1. The inlet port is connected to the delivery of quick release valve or relay valve.
  2. The delivery port is connected to the brake chamber.
  3. The exhaust passage vents air from the brake chambers.

The modulator valve has two solenoids. By energizing the solenoids, the modular valve can be switched to any of the following modes.

  1. Pressure
  2. Pressure hold
  3. Pressure release

Quick Release Valve

Quick release valve are fitted in air braking system to release the air from the brake chamber quickly after release of brake pedal. This prevents delay in brake release due to long piping runs or multiples of brake chamber being exhausted through the brake valve.

Relay Valve

Relay valve provides a means of admitting and releasing air to and from brake chamber quickly, in accordance with the signal pressure from the delivery of the dual brake valve. Air from the reservoir passes through the valve into the brake chamber. The pressure applied to the brake is equal to the signal pressure from the dual brake valve. When the brake pedal is released the signal pressure is released. The pressure in the brake chamber is released directly through the exhaust port of the relay valve.

Warning Lamp

Vehicle are fitted with an ABS warning lamp. It is a LED indicator lamp amber in colour and lights up when the system has detected any electrical fault. ABS warning lamp is located on the instrument panel in form of a driver.

Blink Code Lamp

This lamp is green in colour and is used to indicate the stored faults in the system to the service technician on operating a blink code switch. The nature of fault in the system can be diagnosed by the number of flashes.

Off Highway Switch

This optional switch, located in front of the driver, can be activated during off-highway operation. In this mode, ABS allows greater wheel slip for shorter stopping distances compared to standard ABS control.

Blink Code Switch

A momentary switch on the dash panel grounds the ABS Indicator Lamp output to activate the ECU's diagnostic blink code mode.

Conclusion

The Anti-Lock Braking System stands as a testament to the commitment of the automotive industry to enhance safety on the road. By preventing wheel lock-up and maintaining steering control during braking, ABS has become a fundamental safety feature in vehicles worldwide. As we move toward a future of smarter and more advanced transportation, ABS remains a critical component in the ongoing pursuit of safer and more efficient driving experiences.
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