abs

Introduction The basic design of a braking system has been around and in use in other applications for many years. The brakes in a car use the simple principle of hydraulics. This principle reduces the amount of work required by the user. Figure 1 illustrates the basic design of a modern braking system.

ABS The problem with the traditional braking system is that the force exerted by the brakes on the wheel cannot exceed the force of friction between the wheel and the road. If the braking force exceeds the force of friction from the road the vehicle will begin to slide. This problem brought about the invention of the anti-locking breaking system (ABS). The ABS detects drastic changes in the speed of the wheels. When a sharp deceleration is detected the ABS will reduce the hydraulic pressure supplied to the braking system until the wheel begins to accelerate again. When the acceleration is detected the pressure is again increased until an unusual amount of deceleration is detected. The process is repeated until the user removes their foot from the brake pedal or the vehicle comes to a complete stop.

The ABS is a four-wheel system that prevents wheel lock-up by automatically modulating the brake pressure during an emergency stop. By preventing the wheels from locking, it enables the driver to maintain steering control and to stop in the shortest possible distance under most conditions. During normal braking, the ABS and non-ABS brake pedal feel will be the same. During ABS operation, a pulsation can be felt in the brake pedal, accompanied by a fall and then rise in brake pedal height and a clicking sound.

Background The ABS consists of speed sensors, valves, a pump, and a controller. The location of these devices within a vehicle

Speed Sensor Speed Sensor in an ABS The speed sensor is used to determine the acceleration or deceleration of the wheel. A picture of this sensor is shown in Figure Speed Sensor in an ABS

These sensors use a magnet and a coil of wire to generate a signal These sensors use a magnet and a coil of wire to generate a signal. The rotation of the wheel or differential induces a magnetic field around the sensor. The fluctuations of this magnetic field generate a voltage into the sensor. A schematic of this system is shown in Figure The ABS controller interprets this signal Since the voltage inducted on the sensor is a result of the rotating wheel, this sensor can become inaccurate at slow speeds. The slower rotation of the wheel can cause inaccurate fluctuations in the magnetic field and thus cause inaccurate readings to the controller.

Valves The valves within an ABS serve three distinct functions. The first function of the valves is to open and allow the hydraulic fluid from the brake pedal or the pump to reach the braking system. The second function of the valves is to maintain the current pressure provided to the braking system. This is accomplished by closing the valve to resist further pressure from the brake pedal. The third function of these valves is to reduce the amount of hydraulic pressure at the braking system. This is accomplished by opening the valves to allow the hydraulic fluid to be released from the braking system. A picture of a standard ABS valve and pumping system is show in Figure The majority of problems with the valve system occur due to clogged valves. When a valve is clogged it is unable to open, close, or change position. An inoperable valve will prevent the system from modulating the valves and controlling pressure supplied to the brakes.

Pump The pump in the ABS is used to restore the pressure to the hydraulic brakes after the valves have released it. A signal from the controller will release the valve at the detection of wheel slip. After a valve release the pressure supplied from the user, the pump is used to restore a desired amount of pressure to the braking system. The controller will modulate the pumps status in order to provide the desire amount of pressure and reduce slipping.

A picture of the pumping system is shown in Figure

Controller The entire system is observed and manipulated by the ABS controller. A detailed control system used in ABS is shown in Figure

Vehicles with ABS are equipped with a pedal-actuated, dual-brake system. The basic hydraulic braking system consists of the following: ABS hydraulic control valves and electronic control unit Brake master cylinder Necessary brake tubes and hoses The anti-lock brake system consists of the following components:  Hydraulic Control Unit (HCU). Anti-lock brake control module. Front anti-lock brake sensors / rear anti-lock brake sensors

Anti-lock Brake Systems (ABS) operate as follows: When the brakes are applied, fluid is forced from the brake master cylinder outlet ports to the HCU inlet ports. This pressure is transmitted through four normally open solenoid valves contained inside the HCU, then through the outlet ports of the HCU to each wheel. The primary (rear) circuit of the brake master cylinder feeds the front brakes. The secondary (front) circuit of the brake master cylinder feeds the rear brakes. If the anti-lock brake control module senses a wheel is about to lock, based on anti-lock brake sensor data, it closes the normally open solenoid valve for that circuit. This prevents any more fluid from entering that circuit. The anti-lock brake control module then looks at the anti-lock brake sensor signal from the affected wheel again. If that wheel is still decelerating, it opens the solenoid valve for that circuit. Once the affected wheel comes back up to speed, the anti-lock brake control module returns the solenoid valves to their normal condition allowing fluid flow to the affected brake. The anti-lock brake control module monitors the electromechanical components of the system. Malfunction of the anti-lock brake system will cause the anti-lock brake control module to shut off or inhibit the system. However, normal power-assisted braking remains. Loss of hydraulic fluid in the brake master cylinder will disable the anti-lock system. [li[The 4-wheel anti-lock brake system is self-monitoring. When the ignition switch is turned to the RUN position, the anti-lock brake control module will perform a preliminary self-check on the anti-lock electrical system indicated by a three second illumination of the yellow ABS wanting indicator. During vehicle operation, including normal and anti-lock braking, the anti-lock brake control module monitors all electrical anti-lock functions and some hydraulic operations. Each time the vehicle is driven, as soon as vehicle speed reaches approximately 20 km/h (12 mph), the anti-lock brake control module turns on the pump motor for approximately one-half second. At this time, a mechanical noise may be heard. This is a normal function of the self-check by the anti-lock brake control module. When the vehicle speed goes below 20 km/h (12 mph), the ABS turns off. Most malfunctions of the anti-lock brake system and traction control system, if equipped, will cause the yellow ABS warning indicator to be illuminated.