Anti-lock braking system
From Freepedia
An anti-lock braking system (commonly known as ABS, from the German name "Antiblockiersystem" given to it by its inventors at 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.
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History
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.
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)).
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) it moves the valves to decrease the pressure on the braking circuit, effectively reducing the braking force on that wheel.
The reason that ABS is popular is that in a typical emergency stop, most of the grip of the tire is used up in providing the braking force, leaving none to control the direction of motion of the car. If the driver turns the steering wheel, no grip is left to push the car sideways, so the car carries on in a straight line.
If ABS is fitted the brake pressure in the relevant wheels is reduced slightly, allowing some effective steering to take place.
There is also a secondary benefit. Inexperienced and tired drivers tend to panic in an emergency stop, and find it difficult to modulate the brake pedal pressure to provide maximum braking. Typically they lock the wheels up, which not only removes any steering, but actually INCREASES the braking distance (typically by 40%). ABS prevents this by making sure that all the wheels keep going round at some speed near the optimum for braking on typical roads (there are exceptions discussed later).
When activated, ABS causes the brake pedal to pulse noticeably. As most drivers rarely or never brake hard enough to cause brake lockup, and a significant number rarely bother to read the car's manual, this may not be discovered until an emergency (with the exception of drivers in cold climates who encounter icy conditions frequently). When drivers do encounter an emergency that causes them to brake hard and thus encounter this pulsing for the first time, many are believed to reduce pedal pressure and thus lengthen braking distances. Some manufacturers have therefore implemented brake assist systems that determine the driver is attempting an emergency stop and maintain or increase braking force in this situation.
ABS can improve safety and control for drivers in on-road situations if they know not to release the brakes when they feel the pulsing of ABS, but it does not, as some believe, reduce stopping distances on dry roads. It can also lead to subtle changes in driver behaviour (see risk compensation), and two experiments comparing accident rates of taxi drivers with and without ABS in Munich and Copenhagen found no effective difference: the drivers had consumed all the benefit as a performance gain. It has also been observed that many drivers in emergency situations do not take the evasive action which ABS should allow them to take. This may be due to the tendency to focus on the threat, which naturally leads the driver to keep moving towards that threat. Best practice would be to focus on the escape route.
Overall ABS is considered a valuable safety tool, but it is probably safest to view it as a way of maintaining control while braking rather than as a way of reducing stopping distances, even though this is a principal design effect in conditions of poor traction.
Effectiveness
On high-traction surfaces such as bitumen, whether wet or dry, most ABS-equipped cars are able to attain braking distances better (i.e. shorter) than those that would be easily possible without the benefit of ABS. An alert skilled driver without ABS should be able, through the use of techniques like cadence 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 Finnish car magazine, Tekniikan Maailma, tested a VW Golf V fitted with non-studded Continental ContiVikingContact 3 tires.
| locked wheels | ABS | |
|---|---|---|
| dry pavement | 45m | 32m |
| snow | 53m | 64m |
| ice | 255m | 404m |
Note, however, that this somewhat simplistic test compares ABS with locked wheels. As noted above, maximum braking effect is achieved with the wheels on the limit of friction, whereas ABS works by releasing the brakes as the wheels break traction, so a skilled driver should be able to exceed the braking performance of an ABS system. Few drivers, however, have the skill and practice necessary to do this correctly or instinctively, and a common response to an emergency is to initially under-brake initially then over-brake, a situation in which ABS (and brake assist) will work well.
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 Continental Teves terms Electronic Stability Control or what Bosch terms the "Electronic Stability Program" (ESP).
