When the front wheels of the vehicle are locked, it's maneuverability is reduced, whereas in rear wheel locking, vehicle stability is reduced. ABS calculates required slip rate accurately based on wheel speed and then controls fluid pressure to achieve target slip rate. Although ABS prevents complete locking of wheels, in practice it allows some wheel slip to attain best possible braking.
Modern anti lock braking system not only cause vehicle to stop without deviating from straight line path, it also provides directional stability as there is no wheel skidding.
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| Path of vehicle with and without A |
Skidding is avoided by releasing braking pressure just before wheels lock up and then reapplying it.
This releasing and reapplying the brakes in succession is what an anti lock braking system does. This process is called pressure modulation. These systems can modulate the pressure about 15 times per second.
How it works
A modern ABS consists of an electronic control unit (ECU), one sensor on each wheel, an electrically driven hydraulic pump and pressure accumulator.
Accumulator is used to store hydraulic fluid to maintain high pressure in the braking system and to provide residual pressure for power assisted braking. Generally accumulator is charged with nitrogen gas.
ECU monitors and controls the anti lock function when required. It's function is based on inputs from wheel speed sensors and feedback from the hydraulic Unit to determine whether ABS is operating precisely and also to decide when anti lock operation is required.
In some anti lock braking system, a lateral acceleration sensor is also provided to monitor the side (lateral) movement of the vehicle while taking a turn. This ensures proper braking during turns also.
Accumulator is used to store hydraulic fluid to maintain high pressure in the braking system and to provide residual pressure for power assisted braking. Generally accumulator is charged with nitrogen gas.
ECU monitors and controls the anti lock function when required. It's function is based on inputs from wheel speed sensors and feedback from the hydraulic Unit to determine whether ABS is operating precisely and also to decide when anti lock operation is required.
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| ECU |
In some anti lock braking system, a lateral acceleration sensor is also provided to monitor the side (lateral) movement of the vehicle while taking a turn. This ensures proper braking during turns also.
How it feels while driving
The feel of the brake pedal in case of a vehicle with ABS is different from that in case of a conventional braking system without ABS. The pedal feel in case of ABS equipped brakes is quite similar to that of conventional power brake system.
How to use
In an emergency stop with ABS, one should apply the brake pedal firmly and hold it while the ABS does all the work. There would be pulsations of pedal which may be quite violent, but this is normal and the driver must not let off the pedal due to this.
Types
Two types of anti lock braking system are available
1) two wheel system which controls rear wheel brakes only and are often used in light trucks
2) four wheel system
1) two wheel system which controls rear wheel brakes only and are often used in light trucks
2) four wheel system
Two wheel system can be subdivided into one and two channel types.
In one channel system, both rear wheels brakes are modulated simultaneously and have one common centrally located speed sensor for both wheels. In two channel system, two rear wheel brakes are controlled independently of each other.
The four channel system is, likewise, three and four channel types.
In three channel system, there are independent hydraulic circuits for each of the front wheels and a single circuit for rear wheels. The most effective four wheel system is however, the four channel system in which there are separate sensors and circuits for each wheel.
This means it can selectively do pressure modulation only to the wheels that need it. For instance, if three of the wheel tyres are gripping well, but front wheel is about to skid, the ABS can unlock the front left brake and pulse it to try to retain grip.
In one channel system, both rear wheels brakes are modulated simultaneously and have one common centrally located speed sensor for both wheels. In two channel system, two rear wheel brakes are controlled independently of each other.
The four channel system is, likewise, three and four channel types.
In three channel system, there are independent hydraulic circuits for each of the front wheels and a single circuit for rear wheels. The most effective four wheel system is however, the four channel system in which there are separate sensors and circuits for each wheel.
This means it can selectively do pressure modulation only to the wheels that need it. For instance, if three of the wheel tyres are gripping well, but front wheel is about to skid, the ABS can unlock the front left brake and pulse it to try to retain grip.
Main manufacturers
Main manufacturers of ABS are Bendix, Bosch, delco moraine, Kelsey-Hayes and Lucas Girling.
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