An anti-lock braking system or ABS is a safety “electronic” controlled system on motor vehicles which prevents the wheels from locking while braking. A rotating road wheel allows the driver to maintain steering control under heavy braking, by preventing a locked wheel or skid, and allowing the wheel to continue to forward roll and create lateral control, as directed by driver steering inputs. Disadvantages of the system include increased braking distances under some limited circumstances (ice, snow, gravel, "soft" surfaces), and the creation of a "false sense of security" among drivers who do not understand the operation, and limitations of ABS.

The anti-lock brake controller is also known as the CAB (Controller Anti-lock Brake). A typical ABS is composed of a central electronic control unit (ECU), four wheel speed sensors (one for each wheel), and two or more hydraulic valves within the vehicle brake circuit. The ECU constantly monitors the rotational speed of each wheel. When it senses that any number of wheels are rotating considerably slower than the others (a condition that is likely to bring it to lock - see note below), it actuates the valves to decrease the pressure on the specific braking circuit for the individual wheel, effectively reducing the braking force on that wheel. This process is repeated continuously, and this causes the characteristic pulsing feel through the brake pedal. The sensors can become contaminated with metallic dust, or other contaminants, and fail to correctly detect wheel slip; this is not always picked up by the internal ABS controller diagnostic. In this occurrence, the ABS warning light will usually be illuminated on the instrument panel, and the ABS will be disabled until the fault is rectified.


Automatic, electronic regulation of brake pressure to prevent the wheels from locking up during braking. driver maintains the ability to steer vehicle.


The outputs from these ABS control unit are connected to various other circuits like speed sensors and Hydraulic Control Unit. Before going to know about the block diagram we have to know what is difference between the normal braking mode and ABS braking mode.


The hydraulic components of ABS of vehicle are shown in this figure. It consists of brake calipers, lines or hoses a master cylinder and hydraulic control unit. The hydraulic control is controlled by the ABS “electronic control unit”.


The speed sensors are of two types they are one is “Inductive Wheel Speed Sensor” and the other is “Active Wheel Speed sensor”.

When an inductive pickup is used: AC voltage is created when teeth move through the Magnetic field surrounding the

A newer version sensor: Magnetic field near sensor alternates N-S-N& Sensor is a switch with voltage supplied to it. A DC square wave is created as switch is turned on and off.


In normal braking mode equal pressures are applied on all the four wheels, no wheel speed difference. Pressure created by driver operation of the master cylinder. So all the four wheels experience the same pressure.
HL -Rear left; HR -Rear right; VL -Front
Left; VR -Front right


Vehicle Speed Signal indicates wheel is about to lock. Pressure still applied, but can’t crease because ABS control module has activated valve y8.


To reduce pressure at locking wheel (the wheel that has stopped spinning):ABS control module: "Activates return pump "Activates Valve y8 so no more pressure can be applied "Activates valve y9 so pressure at brake can be reduced.


The entire ABS system is considered to be a hard real-time system, while the sub-system that controls the self diagnosis is considered soft real-time. As stated above, the general working of the ABS system consists of an electronic unit, also known as ECU (electronic control unit), which collects data from the sensors and drives the hydraulic control unit (HCU), mainly consisting of the valves that regulate the braking pressure for the wheels.

A possible solution is the use of the CAN bus system, which has been, and is still in use in many ABS systems today. The communication with the valves of the HCU is usually not done this way. The ECU and the HCU are generally very close together. The valves, usually solenoid valves, are controlled directly by the ECU. To drive the valves based on signals from the ECU, some circuitry and amplifiers are needed (which would also have been the case if the CAN-bus was used).

The sensors measure the position of the tyres, and are generally placed on the wheel-axis. The sensor should be robust and maintenance free, not to endanger its proper working, for example an inductive sensor. These position measurements are then processed by the ECU to calculate the differential wheel rotation. The hydraulic control unit is generally integrated with the ECU (or the other way around), and consists of a number of valves that control the pressure in the braking circuits. All these valves are placed closely together, and packed in a solid aluminum alloy block. This makes for a very simple layout, and is thus very robust.

The central control unit generally consists of two microcontrollers, both active simultaneously, to add some redundancy to the system. These microcontrollers are chosen to be power-efficient, to avoid heating of the controller which would reduce durability. Most notably, the algorithms that drive the HCU as a function of the inputs, or control the brakes depending on the recorded wheel spin.

The requirement for the system to be hard real-time can therefore be reduced to stating that the software should be hard real-time. The required calculations to drive the HCU have to be done in time. Choosing a microcontroller that can operate fast enough is therefore the key, preferably with a large margin. The control system is thus comfortably fast enough, and is limited by the valves.


Modern Electronic Stability Control (ESC or ESP) systems are an evolution of the ABS concept. Here, a minimum of two additional sensors are added to help the system work: these are a steering wheele sensor, and a gyroscopic sensor. The theory of operation is simple: when the gyroscopic sensor detects that the direction taken by the car does not coincide with what the steering wheel sensor reports, the ESC software will brake the necessary individual wheel(s) (up to three with the most sophisticated systems), so that the vehicle goes the way the driver intends. The steering wheel sensor also helps in the operation of Cornering Brake Control (CBC), since this will tell the ABS that wheels on the inside of the curve should brake more than wheels on the outside, and by how much.

The ABS equipment may also be used to implement traction control system (TCS, ASR) on acceleration of the vehicle. If, when accelerating, the tire loses traction, the ABS controller can detect the situation and take suitable action so that traction is regained. Manufacturers often offer this as a separately priced option even though the infrastructure is largely shared with ABS. More sophisticated versions of this can also control throttle levels and brakes simultaneously.
Mercedes-Benz was the first to offer this electronic traction control system in 1985.
• Reduced the risk of multiple vehicle crashes by 18 percent
• Reduced the risk of run-off-road crashes by 35 percent.
• The recommended technique for non-expert drivers in an ABS-equipped car, in a typical full-braking emergency, is to press the brake pedal as firmly as possible and, where appropriate, to steer around obstructions. In such situations, ABS will significantly reduce the chances of a skid and subsequent loss of control.

The Anti-lock braking system is a microcontroller braking system. It is a high reliability and low power consumption. It allows the driver to maintain steering control under heavy braking, by preventing a locked wheel or skid, and allowing the wheel to continue to forward roll and create lateral control, as directed by driver steering inputs.

In vehicles not equipped with ABS, the driver has to manually pump the brakes to prevent wheel lockup. In vehicles not equipped with ABS, the driver has to manually pump the brakes to prevent wheel lockup. In vehicles equipped with ABS, your foot should remain firmly planted on the brake pedal, while ABS pumps the brakes for you so you can concentrate on steering to safety."

Some manufacturers have therefore implemented a brake assist system that determines that the driver is attempting a "panic stop" and the system automatically increases braking force where not enough pressure is applied. Nevertheless, ABS significantly improves safety and control for drivers in most on-road situations.