Anti-lock Braking System (ABS)

The anti-lock braking system is a system on motor vehicles which prevents the wheels from locking while braking. This system, more commonly known as ABS from the German name “Antiblockiersystem,” which was given to it by its inventors at Bosch, serves two purposes. First, it allows the driver to maintain steering control under heavy braking and secondly, in most situations, to shorten braking distances by allowing the driver to hit the brake fully without the fear of skidding or loss of control. Disadvantages of the system include increased braking distances under certain conditions and the creation of a “false sense of security” among drivers who don’t understand the operation and limitations of ABS.

Since its beginnings in production cars in 1978 (ABS version 2), it has made considerable progress. The latest version (8.2) not only handles the ABS function itself (i.e. preventing wheel locking) but also traction control, BAS (Braking Assist System), anti-understeer ESC (Electronic Stability Control), CBC (Cornering Brake Control), and others. The entire anti-lock braking system now weighs less than 2 kilograms, compared to the 6.3 kgs ABS version 2.

Anti-lock Braking System 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. The first car (worldwide) to have ABS fitted as standard (across the entire range) was the Ford Granada Mk 3 of 1985.

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

Anti-lock Braking System 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 any number of wheels are rotating considerably slower than the others (a condition that will bring it to lock) it moves the valves to decrease the pressure on the braking circuit, effectively reducing the braking force on that wheel. Wheel(s) then turn faster and when they turn too fast, the force is reapplied. This process is repeated continuously, and this causes the characteristic pulsing feel through the brake pedal.

The sensors can become contaminated with metallic dust and fail to detect wheel slip; this is not always picked up by the internal ABS controller diagnostics.

In modern Anti-lock Braking Systems, two more sensors are added to help ESC work – these are a wheel angle sensor, and a gyroscopic sensor. The theory of operation is simple. When the gyroscopic sensor detects that the direction taken by the car doesn’t agree with what the wheel sensor says, the ABS software will brake the necessary wheel(s) (up to three with ABS 8.1 or greater) so that the car goes the way the driver intends. The wheel sensor also helps in the operation of CBC, since this will tell the ABS that wheels on the outside of the curve should brake more than wheels on the inside, and by how much.

The electronic unit needs to determine when some of the wheels turn considerably slower than any of the others because when the car is turning the two wheels towards the center of the curve inherently move slightly slower than the other two, which is the reason why a differential is used in virtually all commercial cars.

Anti-lock Braking System Effectiveness

On high-traction surfaces such as asphalt, whether wet or dry, most ABS-equipped cars are able to attain braking distances better (i.e. shorter) than those without the benefit of ABS. An alert skilled driver without ABS should be able, through the use of techniques like cadence braking or threshold braking, to match or improve on the performance of a typical driver with an ABS-equipped vehicle. However, for a majority of drivers, in most conditions, in typical states of alertness, ABS will reduce their chances of crashing, and/or the severity of impact. The recommended technique for non-expert drivers in an ABS-equipped car, in a typical full-braking emergency, is to press the brake pedal as firmly as possible and, where appropriate, to steer around obstructions. In such situations, ABS will significantly reduce the chances of a skid and subsequent loss of control.

In gravel and snow, ABS tends to increase braking distances. On these surfaces, locked wheels dig in and stop the vehicle more quickly. ABS prevents this from occurring. Some ABS calibrations reduce this problem by slowing the cycling time, thus letting the wheels repeatedly briefly lock and unlock. The primary benefit of ABS on such surfaces is to increase the ability of the driver to maintain control of the car rather than go into a skid – though loss of control remains more likely on soft surfaces like gravel or slippery surfaces like snow or ice. On a very slippery surface such as sheet ice or gravel it is possible to lock multiple wheels at once, and this can defeat ABS (which relies on detecting individual wheels skidding). Availability of ABS should not deter drivers from learning to master cadence braking.

A June 1999 National Highway Traffic Safety Administration study found that ABS increased stopping distances on loose gravel by an average of 22%.

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 Control” (ESC).

Risk Compensation

ABS brakes are the subject of some widely-cited experiments in support of risk compensation theory, which support the view that drivers adapt to the safety benefit of ABS by driving more aggressively.

The two major examples are from Munich and Oslo. In both cases taxi drivers in mixed fleets were found to exhibit greater risk-taking when driving cars equipped with ABS, with the result that collision rates between ABS and non ABS cars were not significantly different.