Anti-lock Braking Systems: Concept, Principles, and Components

Anti-Lock Braking Systems (AB) are designed to maintain driver control and stability of the car during emergency braking. Locked wheels will slow a car down but will not provide steering ability. ABS allows maximum braking to be applied while retaining the ability to 'steer out of trouble' The theory behind anti-lock brakes is simple. A skidding wheel (where the tire contact patch is sliding relative to the road) has less traction than a non-skidding wheel. By keeping the wheels from skidding while you slow down, anti-lock brakes benefit you in two ways: You'll stop faster, and you'll be able to steer while you stop. An ABS system monitors four wheel speed sensors to evaluate wheel slippage. Slip can be determined by calculating the ratio of wheel speed to vehicle speed, which is continuously calculated from the four individual wheel speeds. During a braking event, the function of the control system is to maintain maximum possible wheel grip on the road - without the wheel locking - by adjusting the hydraulic fluid pressure to each brake by way of electronically controlled solenoid valves.

Auto producers worldwide are competing with each other to imagine more dependable contraptions thereby coming nearer to the fantasy of the 'Propelled wellbeing vehicle' or 'Extreme security vehicle', on which innovative work has been continuing for as far back as a few years. The greater part of the more up to date vehicle models offers ABS as either standard or discretionary gear.Wheel lockup amid braking causes slipping which thusly cause lost footing and vehicle control. This diminishes the controlling capacity to alter course. So the auto slides crazy. In any case, the street wheel that is as yet pivoting can be directed. That is the thing that ABS is about. With such a framework, the driver can brake hard, make the sly move and still be responsible for the vehicle in any street condition at any speed and under any heap. ABS does not diminish halting separation, but rather repays the changing footing or tire stacking by averting wheel lockup. Amid freeze braking when the wheels are going to lock up, sensors sense that the wheel has recently started turning slower than others on the vehicle. So they quickly diminish braking power on the influenced wheel. This anticipates sliding of the wheels on the asphalt. At the point when the wheel resumes moving, full braking power is again connected. ABS rehashes the procedure until there is never again any requirement for balanced braking. ABS acts speedier than any driver could, pumping the brakes a few times each second. Contingent upon the kind of framework, ABS changes the braking power at each wheel or set of wheels, though a driver's foot on the brake pedal works every one of the brakes without a moment's delay in typical braking.

The theory behind anti-lock brakes is simple. A skidding wheel (where the tire contact patch is sliding relative to the road) has less traction than a nonskidding wheel. If the vehicle has been stuck on ice and if the wheels are spinning then the vehicle have no traction. This is because the contact patch is sliding relative to the ice. By keeping the wheels from skidding while you slow down, anti-lock brakes benefit you in two ways: You'll stop faster, and you'll be able to steer while you stop. Good drivers have always pumped the brake pedal during panic stops to avoid wheel lock up and the loss of steering control. ABS simply get the pumping job done much faster and in a much precise manner than the fastest human foot.

The brakes of the vehicle not furnished with ABS will very quickly bolt the wheels when the driver all of a sudden applies the brake. For this situation, the vehicle slides instead of moves to a stop. The slipping and absence of control were caused by the locking of wheels. The discharge and reapply of the brake pedal will stay away from the locking of the wheels which thus evade the slipping. This is precisely what an electronically monitored slowing mechanism does.

When the brake pedal is pumped or pulsed the pressure is quickly applied and released at the wheels. This is called pressure modulation. Pressure modulation works to prevent the wheel locking. ABS can modulate the pressure to the brake as often as 15 times per seconds. By modulating the pressure to the brakes the friction between the tires and the road is maintained and the vehicle is able to come to the controllable stop. Steering is another important consideration. As long as a tire doesn’t slip it goes only in the direction in which it is turned. But once it is skid it has little or no directional stability. The Maneuverability of the vehicle is reduced if the front wheels are locked and the stability of the vehicle is reduced if the rear wheels are locked. ABS precisely controls the slip rate of the wheels to ensure maximum grip force from the tire and it thereby ensures maneuverability and stability of the vehicle.

Many different ABS are found on today’s vehicles. These designs are varied by their basic layout, operation, and components. The ABS components can be divided into two categories.

1. Hydraulic components
2. Electrical/electronic components

Besides these normal and conventional brake parts are part of the overall brake system.

Accumulator: An accumulator is used to store hydraulic fluid to maintain high pressure in the brake system and provide the residual pressure for power-assisted braking. Normally the accumulator is charged with nitrogen gas and is an integral part of the modulator unit.

Antilock hydraulic control valve assembly: This assembly controls the release and application of the brake system pressure to the wheel brake assemblies. It may be of an integral type and nonintegral type. In integral type, the unit is combined with the power boost and master cylinder unit into one assembly. The nonintegral type is mounted externally from the master cylinder /power booster unit and is located between the master cylinder and wheel brake assembly. Both types generally contain solenoid valve that controls the releasing, holding and applying of brake system pressure.

Booster pump: The booster pump is an assembly of an electric motor and pump. The booster pump is used to provide pressurized hydraulic fluid ABS. The pump's motor is controlled by the systems control unit.

Booster/Master cylinder assembly: It is referred to as the hydraulic unit, contains the valves and pistons needed to modulate hydraulic pressure in the wheel circuit during the ABS operations.

Fluid accumulator: Different than a pressure accumulator, fluid accumulator temporarily store brake fluid, that is removed from the wheel brake unit during ABS cycle. This fluid is then used by the pump to build pressure for the brake hydraulic system.

Hydraulic control unit: This assembly contains solenoid valve, fluid accumulator, pump and electric motor. The unit may have one pump and one motor or it has one motor and two pumps.

Main Valve: This is a two position valve and is also controlled by the ABS control module and is open only in the ABS model. When open pressurized brake fluid from the booster circuit is directed into the master circuit to prevent excessive pedal travel.

Modulator unit: The modulator unit controls the flow of pressurized brake fluid to the individual wheel circuits. Normally the modulator is made up of the solenoid that open and close valves, several valves that control the flow of fluid to wheel brake units and electrical relays that activate or deactivate the solenoids through the commands of the control module. This unit may also be called the hydraulic actuator, hydraulic power unit or the electro-hydraulic control valve.

Solenoid valves: The solenoid valves are located in the modulator unit and are electrically operated by signals from the control module. The control module switches the solenoids on or off to increase, decrease, or maintain the hydraulic pressure to the individual wheel units.

Wheel circuit valves: Two solenoid valves are used to control each circuit or channel. One controls the inlet valve of the circuit, the controls the outlet valve .the position is determined by the control module. Outlet valves are normally closed and inlet valves are normally open. Valves are activated when the abs control module switches 12 volts to the circuit solenoids. During normal driving, the circuits are not activated.

ABS control module: This small computer is normally mounted inside the trunk on the wheel housing, mounted to the master cylinder or is part of the hydraulic control unit. It monitors system operation and controls antilock function when needed. The module relies on input from the wheel speed sensors and feedback from the hydraulic unit to determine if the abs are operating correctly and to determine when the anti-lock mode is required.

Brake pedal sensor: The antilock brake pedal sensor switch is normally closed. When the brake pedal exceeds the antilock brake pedal sensor switch setting during an antilock stop, the antilock brake control module senses that the antilock brake pedal sensor switch is open and grounds the pump motor relay coil. This energizes the relay and turns the pump motor on. When the pump motor is running, the hydraulic reservoir is filled with high-pressure brake fluid and the brake pedal will be pushed up until the antilock brake pedal sensor switch closes. when the antilock brake pedal sensor switch closes, the pump motor is turned off and the brake pedal will drop some with each abs control cycle until the antilock brake pedal sensor switch opens and the pump motor is turned on again .this minimizes pedal feedback during abs cycling.

Pressure differential switch: It is located in the modulator unit. This switch sends a signal to the control module whenever there is an undesirable difference in the hydraulic pressures within the brake system.

Relays: Relays are electromagnetic devices used to control a high current circuit with a low current switching circuit. In abs, relays are used to switch motors and solenoids. A low current signal from the control module energizes the relays that complete the electrical circuit for the motor or solenoid.

It can be located on an axle shaft, differential gear or the hub of a wheel. This ring is used in conjunction with the wheel speed sensor. The ring has a number of teeth around its circumference. As the ring rotates and each tooth passes by the wheel speed sensor, an ac voltage signal is generated between the sensor and tooth.

It is mounted near the different toothed ring. As the rings teeth rotate past the sensor an ac voltage is generated. as the teeth move away from the sensor, the signal is broken until the next tooth comes close to the sensor .the end result is a pulsing signal that is sent to the control module. The control module translates the signal into wheel speed. The sensor is normally a small coil of wire with a permanent magnet in its center.

One of the classifications of abs is the integral and nonintegral type. The integral type they combine the master cylinder, hydraulic booster, and as hydraulic circuit into the single hydraulic assembly. In nonintegral type they use a conventional vacuum-assist booster and master cylinder. In addition, they can be classified according to the control they provide.

This is the best scheme. There is a speed sensor on all four wheels and a separate valve for all the four wheels. With this set up the controller monitors each wheel individually to make sure it is achieving maximum braking force.

This scheme is commonly found on pick up trucks with four wheels 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 wheel is located in the rear axle. This system provides individual control of the wheels, so they can both achieve maximum braking force. The rear wheels, however, are monitored together, they both have to start to lock up before the abs will activate on the rear. With this system, it is possible that one of the rear wheels will lock during a stop, reducing brake effectiveness.

This scheme is commonly found on pick up 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 the rear channel system. The rear wheels are monitored together and both have to start to lock up before the abs kick in. in this system is also possible that one of the rear wheels will lock reducing brake effectiveness.

Advancements in ABS Some systems, which work with the ABS, are Automatic traction control and Automatic stability control, which are discussed below.

Automatic traction control (ATC) Automatic traction control systems apply the brakes when a drive wheel attempts to spin and lose traction. The system works best when one drive wheel is working on a good traction surface and the other is not. The system also works well when the vehicle is accelerating on slippery road surfaces, especially when climbing hills. ATC is most helpful on four wheel or all wheel drive vehicles in which loss of traction at one wheel could hamper driver control. During road operation, the ATC system uses an electronic control module to monitor the wheel speed sensors. If a wheel enters a loss of traction situation, the module applies braking force to the wheel in trouble. Loss of traction is identified by comparing the vehicle speed to the speed of the wheel. If there is a loss of traction the speed of the wheel will be greater than expected for the particular vehicle speed. ABS and ATC systems can be integral and use the common valves. These systems are designed to reduce wheel slip and maintain traction at the drive wheels when the road is wet or snow covered. The control module monitors wheel speed. If during acceleration the module detects drive wheel slip and if brakes are not applied, the control module enters into the traction control mode. The inlet and outlet solenoid valves are pulsed and allow the brake to be quickly applied and released.

In some systems when a loss of traction is sensed, it not only cycles the brakes but signals the engine control module to retard ignition timing and partially close the throttle as well, which in turn reduces engine output. Many systems are equipped with a dash-mounted warning light to alert the driver that the system is operating. There will also be a manual cut off switch so that the driver can turn off ATC operation.

Like ATC, the stability control systems are linked with the ABS. it can also be called the Electronic Stability Programme (ESP). Stability control systems momentarily apply the brakes at any one wheel to correct oversteer or understeer. The control unit receives signals from the typical sensors plus a yaw, lateral acceleration (G-force) and a steering angle sensor. The system uses the angle of the steering wheel and the speed of the four wheels to calculate the path chosen by the driver. It then looks at lateral G-forces and vehicle yaw to measure where the vehicle is going. (Yaw is defined as the natural tendency for a vehicle to rotate on its vertical center axis). So it is also called Yaw control.

Understeer is the condition in which the vehicle is slow to respond to steering changes. Oversteer occurs when the rear wheels try to swing around causing the car to spin. When the system senses understeer in a turn the brake at the inside rear wheel is applied. During oversteer, the outside front brake is applied. Relaying on the input from the sensors and computer programming the system calculates if the vehicle is going exactly in the same direction in which it is being steered. In case of any difference between what the driver is asking and what the vehicle is doing, the system corrects the situation by applying one of the right or left brakes.

• It allows the driver to maintain directional stability and control over steering during braking
• Safe and effective
• Automatically changes the brake fluid pressure at each wheel to maintain optimum brake performance.
• ABS absorbs the unwanted turbulence shock waves and modulates the pulses thus permitting the wheel to continue turning under maximum braking pressure.
• It is very costly
• Maintenance cost of a car equipped with ABS is more.

ABS has been so far developed to a system, which provides rapid, automatic braking in response to signs of incipient wheel locking by alternately increasing and decreasing hydraulic pressure in the brake line Statistics show that approximately 40 % of automobile accidents are due to skidding. These problems commonly occur on a vehicle with conventional brake system which can be avoided by adding devices called ABS If there is an ABS failure, the system will revert to normal brake operation. Normally the ABS warning light will turn on and let the driver know there is a fault.