1. History
1929 Anti-lock braking systems were first developed for aircraft, by Gabriel Voisin,
1950 Dunlop's Maxaret introduced a system and still in use on some aircraft models‡
1960 A fully mechanical system used in the Ferguson P99 racing car, the Jensen FF and the Ford Zodiac , butsaw no further use; the system proved expensive and, in automobile use, somewhat unreliable

2. ANTI-LOCK BRAKES
An anti-lock braking system (ABS, from German: Antiblockiersystem) is a safety system that allows the wheels on a motor vehicle to continue interacting tractively with the road surface as directed by driver steering inputs while braking, preventing the wheels from locking up (that is, ceasing rotation) and therefore avoiding skidding.
An ABS generally offers improved vehicle control and decreases stopping distances on dry and slippery surfaces for many drivers; however, on loose surfaces like gravel or snow-covered pavement, an ABS can significantly increase braking distance, although still improving vehicle control.

3. Brake types
‡Anti-lock braking systems use different schemes depending on the type of brakes in use. They can be differentiated by the number of channels: that is, how many valves that are individually controlled—and the number of speed sensors.
Four-channel, four-sensor ABS
This is the best scheme. There is a speed sensor on all four wheels and a separate valve for all four wheels. With this setup, the controller monitors each wheel individually to make sure it is achieving maximum braking force.
Three-channel, three-sensor ABS
This scheme, commonly found on pickup trucks with four-wheel ABS, has a speed sensor and a valve for each of the front wheels, with one valve and one sensor for both rear wheels. The speed sensor for the rear wheels is located in the rear axle. This sys­tem provides individual control of the front wheels, so they can both achieve maximum braking force.
One-channel, one-sensor ABS
This system is commonly found on pickup trucks with rear-wheel ABS. It has one valve, which controls both rear wheels, and one speed sensor, located in the rear axle. This system operates the same as the rear end of a three-channel system.

4. FEATURES
The ABS electrically controls the brake fluid pressure to each wheel to prevent the wheel from locking during braking on slippery road surfaces, thereby enabling the driver to maintain the directional control.
If the ABS becomes inoperative, a fail-safe system is activated to ensure same level of braking performance as with a conventional brake system. In that case, the warning light comes on to indicate that the ABS is malfunctioning.
The ABS is a 4-sensor, 4-channel system. The front wheel system is an independent control design while the rear wheel system is a select-low control design.
Vehicles with Sports ABS have an additional function which independently controls the rear wheels according to the value sent from the lateral G sensor. (STi models)

5. (1) ABS control module and hydraulic control unit (ABSCM & H/U)
(6) Transmission control module (AT models only) (11) Stop light switch
(2) Two-way connector (7) Tone wheels (12) Master cylinder
(3) Diagnosis connector (8) ABS wheel speed sensor (13) Brake & EBD warning light
(4) ABS warning light (9) Wheel cylinder (14) Lateral G sensor (STi)
(5) Data link connector (for SUBARU select monitor)

6. COMPONENTS Master Cylinder Assembly Pressure Valve Speed
Sensor
Dump/Vent Valve
Anti Lock
Anti-Lock
Brake Module
12 V

7. Major components of the abs
Brake control module,
Solenoid valve assembly,
Speed sensor's
Wiring, and the amber ABS brake warning light.
Wheel Speed Sensors
Abs Control Module(ECU)
Hydraulic Modulator
Pump Motor & Accumulator

8. PRINCIPLE OF ABS CONTROL
When the brake pedal is depressed during driving, the wheel speed decreases and the vehicle speed does as well. The decrease in the vehicle speed, however, is not always proportional to the decrease in the wheel speed. The non-correspondence between the wheel speed and vehicle speed is called “slip” and the magnitude of the slip is expressed by the “slip ratio” which is defined as follows: Slip ratio = (Vehicle speed – Wheel speed)/Vehicle speed × 100%
When the slip ratio is 0%, the vehicle speed corresponds exactly to the wheel speed. When it is 100%, the wheels are completely locking (rotating at a zero speed) while the vehicle is moving.

9. (A) Slip ratio (1) Icy road
(B) Coefficient of friction between tire and road surface (2) Asphalt-paved road
(3) Control range by ABS

10. FUNCTIONS OF SENSORS AND ACTUATORS
NAME
FUNCTION
ABS wheel speed sensors
These sensors detect the wheel speed in terms of a change in the density of the magnetic flux passing through them and convert it into an electrical sig- nal. The electrical signal is sent to the ABSCM.
Tone wheels
They give a change in the magnetic flux density by the teeth around them- selves to let the ABS wheel speed sensors generate electrical signals.
G sensor
It detects a change in acceleration in the longitudinal direction of the vehicle and outputs it to the ABSCM as a voltage signal.
Lateral G sensor (STi)
Converts the variation in lateral G on the vehicle into a change in capacity of the sensor condenser, and outputs a varying voltage to the ABS ECU.
Stop light signal
It provides information on whether the brake pedal is depressed or not to the
ABSCM. The ABSCM uses it to determine ABS operation.
ABS warning light
It alerts the driver to an ABS fault. When the diagnosis connector and diag- nosis terminal are connected, the light flashes to indicate a diagnostic trouble code stored in the ABSCM.
Automatic transmission control module
It provides gear controls (fixing the speed at 3rd or changing power transmis- sion to front and rear wheels) in response to control signals from the ABSCM.
Brake warning light
It alerts the driver to an EBD fault. This warning light is also used for parking brake warning and brake fluid level warning.

11. Disadvantages OF ABS
1. ABS will not stop you as fast on very loose surfaces like gravel or snow. On such surfaces with ABS on it will not allow a wedge of snow/gravel to build up in front of the tires that would nornally help you stop faster.
2. All ABS systems have a speed below which they have to stop working. It might be 3 or 4 MPH. This is because the basic function of ABS is to keep the wheels turning and if the wheels keep turning the car will never stop. It also has to do with the pulse rate from the wheels speed sensors. In either case once the car get to this speed the ABS turns off and the driver must adapt what he is doing. A classic case wouldbe trying to stop on an icy road, once below that lower speed limit you could slide quite far!

12. PERCENTAGE OF USSAGE VS YEAR