Antilock brake controller

Bosch) is a system on motor vehicles which prevents the wheels from locking while braking. The purpose of this is twofold: to allow the driver to maintain steering control and to shorten braking distances.

It is important to note that there is some question as to whether ABS systems are able to achieve either of these goals in "real world" situations.

History

Anti-lock braking systems were first developed for aircraft. An early system was Dunlop's Maxaret system, introduced in the 1950s and still in use on some aircraft models. This was a fully mechanical system. It saw limited automobile use in the 1960s in the Ferguson P99 racing car, the Jensen FF and the experimental all wheel drive Ford Zodiac, but saw no further use; the system proved expensive and in automobile use somewhat unreliable. A purely mechanical system developed and sold by Lucas Girling was factory-fitted to the Ford Fiesta Mk III. It was called the Stop Control System.

The German firm of Bosch had been developing anti-lock braking technology since the 1930s, but the first production cars using Bosch's electronic system became available in 1978. They first appeared in trucks and German limousines from Mercedes-Benz. Systems were later introduced on motorcycles.

Operation

The anti-lock brake controller is also known as the CAB (controller anti-lock brake).

A typical ABS is composed of a central electronic unit, four speed sensors (one for each wheel), and two or more hydraulic valves on the brake circuit. The electronic unit constantly monitors the rotation speed of each wheel. When it senses that one or more wheel is rotating slower than the others (a condition that will bring it to lock), moves the valves to decrease the pressure on the braking circuit, effectively reducing the braking force on that wheel.

Effectiveness

It is generally claimed that fitting a car with ABS does not increase braking distance on high-traction surfaces such as bitumen (even when wet). One argument states that while it is true that its main function is to decrease braking force on selected wheels, it does so only to wheels that were locking, and so were not contributing effectively to the braking action. Bringing the wheel up to the real vehicle speed with the others will instead offer the optimum braking action.

How much such systems actually reduce braking distances on bitumen is a subject of debate and depends, in any case, on driver skill. A moderately skilled driver capable of cadence braking would get little benefit from an ABS system compared to a novice driver.

The recommended technique for non-expert drivers, in a typical full-braking emergency, in a straight line on a highway, is to press the brake as firmly as possible and to steer around the obstructions. However, in real-world emergencies of this sort, a novice driver will rarely (if ever) retain the presence of mind to do anything but stomp hard on the brake pedal and steer straight ahead.

Some automotive engineers argue that is a better practice to design a car so that it is more difficult to lock up the brakes in the first place. This is done by making the car lightweight and balancing the braking system so that the force applied by the driver during a "panic stop" would be insufficient to lock the wheels.

In gravel or snow, there is no question that ABS increases braking distances. On these surfaces, the action of the wheels locking causes them to dig into the material and create a buildup of ahead of the locked wheels which helps stop the vehicle more quickly. ABS prevents this from occurring. Some ABS controllers attempt to mitigate this problem by slowing the cycling time, thus letting the wheels repeatedly, briefly, to lock and then unlock again.

Another effect of ABS on slippery surfaces is that it helps the driver to maintain control of the car under braking rather than going into a skid. With 4-wheel ABS, the driver is able to brake and steer at the same time in order to avoid an obstacle, without having to worry about entering into a skid. The problem is that in certain situations, it is actually better for the car to go into a spin in order to reduce the stopping distance by coverting forward motion into rotating motion.

Another application issue with ABS is that, when activated, the system causes the brake pedal to pulse significantly. A significant number of drivers rarely or never brake hard enough to cause brake lockup, and rarely bother to read the car's manual. Those drivers may be unaware of this phenomena until they are required to perform a "panic stop". When drivers do encounter an emergency that causes them to brake hard and encounter this unexpected pulsing for the first time, many are believed to panic and reduce pedal pressure, thus lengthen braking distances and contributing to a higher level of accidents than the capabilities of ABS would otherwise promise. Some manufacturers have therefore implemented "brake assist" systems that determine the driver is attempting a crash stop and maintain braking force in this situation.

Traction control

The ABS equipment may also be used to implement traction control on acceleration of the vehicle. If when accelerating, the tire loses traction with the ground, the ABS controller can detect the situation and apply the brakes to reduce the acceleration so that traction is regained. Manufacturers often offer this as a separately priced option even though the infrastructure is largely shared with ABS. More sophisticated versions of this can also control throttle levels and brakes simultaneously, leading to what Bosch terms the "Electronic Stability Program" (ESP).

See also: car safety.