An
anti-lock braking system, or
ABS is a safety system which prevents the wheels on a motorvehicle from locking up (or ceasing to rotate) while braking.
A rotating road wheel allows the driver to maintain steering control under heavy braking by preventing a skid and allowing the wheel to continue interacting tractively with the road surface as directed by driver steering inputs. ABS offers improved vehicle control and decreases stopping distances on dry and especially slippery surfaces. However, on loose surfaces like gravel and snow-on-pavement, it can slightly increase braking distance while still improving vehicle control.
[1]
Since initial widespread use in production cars, anti-lock braking systems have evolved considerably. Recent versions not only prevent wheel lock under braking, but also electronically control the front-to-rear brake bias. This function, depending on its specific capabilities and implementation, is known as electronic brakeforce distribution (EBD), traction control system, emergency brake assist, or electronic stability control.
Chrysler, together with the
Bendix Corporation, introduced a true computerized three-channel, four sensor all-wheel antilock brake system called "Sure Brake" on the 1971
Imperial.
[5] It was available for several years thereafter, functioned as intended, and proved reliable.
General Motors introduced the "Trackmaster" rear-wheel (only) ABS as an option on their
Rear-wheel drive Cadillac models in 1971.
[6][7]
In 1975, Robert Bosch took over a European company called Teldix (
contraction of
Telefunken and Bendix) and all patents registered by this joint-venture and used this acquisition to build the base of the ABS system introduced on the market some years later. The
German firms
Bosch and
Daimler-Benz had been co-developing anti-lock braking technology since the early 1970s, and introduced the first completely electronic 4-wheel multi-channel ABS system in
trucks and the
Mercedes-Benz S-Class in 1978.
[citation needed]
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The anti-lock brake controller is also known as the CAB (Controller Anti-lock Brake).
[9][10]
A typical ABS is composed of a central
electronic control unit (ECU), four
wheel speed sensors — one for each wheel — and two or more hydraulic valves within the brake
hydraulics.
The ECU constantly monitors the
rotational speed of each wheel, and when it detects a wheel rotating significantly slower than the others — a condition indicative of impending wheel lock — it actuates the valves to reduce hydraulic pressure to the brake at the affected wheel, thus reducing the braking force on that wheel. The wheel then turns faster; when the ECU detects it is turning significantly faster than the others, brake hydraulic pressure to the wheel is increased so the braking force is reapplied and the wheel slows.
This process is repeated continuously, and can be detected by the driver via brake pedal pulsation. A typical anti-lock system can apply and release braking pressure up to 20 times a second.
[citation needed]
The ECU is
programmed to disregard differences in wheel rotative speed below a critical threshold, because when the car is turning, the two wheels towards the center of the curve turn slower than the outer two. For this same reason, a
differential is used in virtually all roadgoing vehicles.
If a fault develops in any part of the ABS, a
warning light will usually be illuminated on the
vehicle instrument panel, and the ABS will be disabled until the fault is rectified.
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