21-09-2013, 04:27 PM
Anti-Lock Braking System
Braking System.doc (Size: 157.5 KB / Downloads: 88)
AIM: - To design & develop Anti-Lock Braking System.
ABSTRACT
The project here is all about Anti-Lock Braking System.
An anti-lock braking system, or ABS is a safety 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, steel bridges, anything other than dry tarmac) and the creation of a "false sense of security" among drivers who do not understand the operation, and limitations of ABS.
How Antilock Brake Systems Work
Since most cars on the road today have some form of Antilock Brakes (ABS) I think we should take a look at how they work and clear up some mis-information about them.
As always, what I describe here is how most systems work in general. Since different manufactures have their own versions of ABS their values, specifications and part names will differ. If you are having a problem with the ABS on your vehicle you should always refer to the specific service and repair manuals for your vehicle.
Essential to know:
- The four-wheel ABS system can help to slightly reduce the braking distance in some situations. However, under certain conditions (e.g. on loose snow or gravel), the braking distance may be longer. The main purpose of ABS is to help the driver maintain steering control during hard braking.
- Some trucks may have the two-wheel ABS system that controls only rear wheels. The rear wheel ABS system does not help to maintain steering control during braking. To gain full advantage of the ABS system you need to know what type of ABS you have in your vehicle and operate it correctly. Check your owner's manual for details.
- Good tires play an important role; for example, if the car has bald or wide sport tires the ABS can hardly prevent it from skidding on the snow.
How the actual ABS system works
Major components of the typical ABS system include four speed sensors (one at each wheel), an electronic control unit (ABS computer) and a hydraulic control unit (see the picture). The ABS computer constantly monitors the signal from each wheel speed sensor. When it senses that any of the wheels are approaching lock up during braking, the ABS computer sends the signal to the hydraulic control unit, which modulates the braking pressure for a corresponding wheel(s) preventing it from locking up.
When the ABS is activated, it feels like rapid brake pedal pulsation with a buzzing noise, which is normal.
ABS system problems
If the ABS light comes on while driving, there is a problem with your ABS system and you need to visit your dealer as soon as possible.
ELECTRONIC BRAKE CONTROL MODULE:
The EBCM mounted on a bracket next to the master cylinder, contains a microprocessor and software for system operation.
ANTI-LOCK PRESSURE VALVE:
The Anti-Lock Pressure Valve (APV) is mounted to the combination valve under the master cylinder, has an isolation valve to maintain or increase hydraulic pressure and a dump valve to reduce hydraulic pressure.
VEHICLE SPEED SENSOR:
The Vehicle Speed Sensor (VSS) located on the left rear of the transmission on two-wheel drive trucks and on the transfer case of four-wheel drive vehicles, produces an AC voltage signal that varies in frequency according to the output shaft speed. On some vehicles the VSS is located in the rear differential.
BASE BRAKING MODE:
During normal braking, the EBCM receives a signal from the stop lamp switch and begins to monitor the vehicle speed line. The isolation valve is open and the dump valve is seated. This allows fluid under pressure to pass through the APV and travel to the rear brake channel. The reset switch does not move because hydraulic pressure is equal on both sides.
PRESSURE DECREASE:
During pressure decrease the EBCM keeps the isolation solenoid energized and energizes the dump solenoid. The dump valve moves off its seat and fluid under pressure moves into the accumulator. This action reduces rear pipe pressure preventing rear lock-up. The reset switch grounds to tell the EBCM that pressure decrease has taken place.
PRESSURE INCREASE:
During pressure increase the EBCM de-energizes the dump and isolation solenoids. The dump valve reseats and holds the stored fluid in the accumulator. The isolation valve 9pens and allows the fluid from the master cylinder to flow past it and increase pressure to the rear brakes. The reset switch moves back to its original position by spring force. This action signals the EBCM that pressure decrease has ended and driver applied pressure resumes.
SYSTEM SELF-TEST:
When the ignition switch is turned "ON," the EBCM performs a system self-test. It checks its internal and external circuit and performs a function test by cycling the isolation and dump valves. The EBCM then begins its normal operation if no malfunctions are detected.
Brake pedal pulsation and occasional rear tire "chirping" are normal during RWAL operation. The road surface and severity of the braking maneuver determine how much these will occur. Since these systems only control the rear wheels, it is still possible to lock the front wheels during certain severe braking conditions.