Anti-lock Braking System

     An anti-lock braking system（ABS) is a safety system that allows the wheels on amotor vehicle to continue interacting tractively with the road surface as directed by driversteering inputs while braking, preventing the wheels from locking up（that is, ceasingrotation) and therefore avoiding skidding.

    Stopping a car in a hurry on a slippery road can be very challenging. Anti-lock brakingsystems (ABS) take a lot of the challenge out of this sometimes nerve-wracking event. Infact，on slippery surfaces，even professional drivers can't stop as quickly without ABS asan average driver can with ABS.

    The ABS System

    The theory behind anti-lock brakes is simple. A skidding wheel(where the tire contactpatch is sliding relative to the road) has less traction than a non-skidding wheel. If youhave been stuck on ice, you should know that if your wheels are spinning you have notraction. This is because the contact patch is sliding relative to the ice. By keeping thewheels 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.

    There are four main components to an ABS system:

    Speed Sensors

    The anti-lock braking system needs some way of knowing when a wheel is about tolock up. The speed sensors，which are located at each wheel，or in some cases in thedifferential，provide this information.

    Valves

    There is a valve in the brake line of each brake controlled by the ABS. On somesystems，the valve has three positions：

    In position one, the valve is open; pressure from the master cylinder is passed rightthrough to the brake.

    In position two，the valve blocks the line, isolating that brake from the mastercylinder. This prevents the pressure from rising further should the driver push the brakepedal harder.

    In position three, the valve releases some of the pressure from the brake.

    Pump

    Since the valve is able to release pressure from the brakes，there has to be some wayto put that pressure back. That is what the pump does；when a valve reduces the pressurein a line，the pump is there to get the pressure back up.

    Controller

    The controller is a computer in the car. It watches the speed sensors and controls thevalves.

    ABS at Work

    There are many different variations and control algorithms for ABS systems. We willdiscuss how one of the simpler systems works.

    The controller monitors the speed sensors at all times. It is looking for decelerationsin the wheel that are out of the ordinary. Right before a wheel locks up, it will experience。rapid deceleration. If left unchecked, the wheel would stop much more quickly than anycar could. It might take a car five seconds to stop from 60 mph（96. 6 kph) under idealconditions，but a wheel that locks up could stop spinning in less than a second.

    The ABS controller knows that such a rapid deceleration is impossible，so it reducesthe pressure to that brake until it sees an acceleration, and then it increases the pressureuntil it sees the deceleration again. It can do this very quickly, before the tire can actuallysignificantly change speed. The result is that the tire slows down at the same rate as thecar, with the brakes keeping the tires very near the point at which they will start to lockup. This gives the system maximum braking power.

    When the ABS system is in operation you will feel a pulsing in the brake pedal；thiscomes from the rapid opening and closing of the valves. Some ABS systems can cycle up to15 times per second.

    Anti-Lock Brake Types

    Anti-lock braking systems use different schemes depending on the type of brakes inuse. We will refer to them by the number of channels一that is, how many valves that areindividually controlled一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 separatevalve for all four wheels. With this setup，the controller monitors each wheel individuallyto 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 speedsensor and a valve for each of the front wheels, with one valve and one sensor for both rearwheels. 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 achievemaximum braking force. The rear wheels, however, are monitored together; they bothhave to start to lock up before the ABS will activate on the rear. With this system，it ispossible that one of the rear wheels will lock during a stop, reducing brake effectiveness.

    One-Channel，One-Sensor ABS

    This system is commonly found on pickup trucks with rear-wheel ABS. It has onevalve, which controls both rear wheels and one speed sensor, located in the rear axle.

    This system operates the same at the rear end of a three-channel system. The rearwheels are monitored together and they both have to start to lock up before the ABS kicksin. In this system, it is also possible that one of the rear wheels will lock，reducing brakeeffectiveness.

    This system is easy to identify. Usually there will be one brake line going through aT-fitting to both rear wheels. You can locate the speed sensor by looking for an electricalconnection near the differential on the rear-axle housing.