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{{refimprove|date=December 2010}}

An '''anti-lock braking system''' ('''ABS''', from German: '''''A'''nti'''b'''lockier'''s'''ystem'') is a safety system that allows the [[wheel]]s on a [[motor vehicle]] to continue interacting [[traction (engineering)|tractively]] with the road surface as directed by driver steering inputs while [[brake|braking]], preventing the wheels from locking up (that is, ceasing rotation) and therefore avoiding [[Skid mark|skidding]].

An ABS generally offers improved vehicle control and decreases stopping distances on dry and slippery surfaces for many drivers; however, on loose surfaces like gravel or snow-covered pavement, an ABS can significantly increase braking distance, although still improving vehicle control.<ref name="monash.edu.au">{{cite web|url=http://www.monash.edu.au/muarc/reports/Other/RACV%20ABS%20braking%20system%20effectiveness.pdf |title=Effectiveness of ABS and Vehicle Stability Control Systems |format=PDF |date=April 2004 | publisher = Royal Automobile Club of Victoria |accessdate=2010-12-07|archiveurl=http://web.archive.org/web/20080824020216/http://www.monash.edu.au/muarc/reports/Other/RACV%20ABS%20braking%20system%20effectiveness.pdf|archivedate=2008-08-24}}</ref>

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]] (ESC).

== History ==
=== Early systems ===

The ABS was first developed for [[aircraft]] use in 1929 by the [[France|French]] [[automobile]] and [[aircraft]] pioneer, [[Gabriel Voisin]], as threshold braking on airplanes is nearly impossible.   These systems use a [[flywheel]] and valve attached to a hydraulic line that feeds the brake cylinders. The flywheel is attached to a drum that runs at the same speed as the wheel. In normal braking, the drum and flywheel should spin at the same speed. However, if a wheel were to slow down, then the drum would do the same, leaving the flywheel spinning at a faster rate. This causes the valve to open, allowing a small amount of brake fluid to bypass the master cylinder into a local reservoir, lowering the pressure on the cylinder and releasing the brakes. The use of the drum and flywheel meant the valve only opened when the wheel was turning. In testing, a 30% improvement in braking performance was noted, because the pilots immediately applied full brakes instead of slowly increasing pressure in order to find the skid point. An additional benefit was the elimination of burned or burst tires.<ref>{{cite web |url=http://www.flightglobal.com/pdfarchive/view/1953/1953%20-%201433.html |title=Non-Skid Braking |publisher=FLIGHT International |date=30 October 1953 |pages=587–588}}</ref>

In  1958, a [[Royal Enfield Super Meteor]] [[motorcycle]] was used by the [[Road Research Laboratory]] to test the Maxaret anti-lock brake.<ref name="Reynolds">{{cite book|last=Reynolds|first=Jim|title=Best of British Bikes|publisher=Patrick Stephens Ltd|year=1990|isbn=1-85260-033-0}}</ref> The experiments demonstrated that anti-lock brakes can be of great value to motorcycles, for which skidding is involved in a high proportion of accidents. Stopping distances were reduced in most of the tests compared with locked wheel braking, particularly on slippery surfaces, in which the improvement could be as much as 30 percent. Enfield's technical director at the time, Tony Wilson-Jones, saw little future in the system, however, and it was not put into production by the company.<ref name="Reynolds"/>

A fully mechanical system 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 unreliable in automobile use.

=== Modern systems ===

[[Chrysler]], together with the [[Bendix Corporation]], introduced a computerized, three-channel, four-sensor all-wheel ABS called "Sure Brake" for its 1971 [[Imperial (automobile)|Imperial]].<ref>{{cite web |url=http://imperialclub.org/~imperialclub/Yr/1973/Data/49.htm |title=Chrysler Imperial Sure Brake system description |publisher=ImperialClub.org}}</ref> It was available for several years thereafter, functioned as intended, and proved reliable. In 1971, [[General Motors]] introduced the "Trackmaster" rear-wheel only ABS as an option on their [[Rear-wheel drive]] [[Cadillac]] models.<ref>{{cite web |url=http://www.welovecadillacs.com/history.html |title= History |publisher=We Love Cadillacs }}</ref><ref>{{cite web |url=http://www.motorera.com/cadillac/cad1970/1972/CAD72.HTM |title=1972 Cadillac Fleetwood History |publisher=MotorEra.com}}</ref><ref>[http://history.gmheritagecenter.com/wiki/index.php/1972,_First_Automotive_Anti-lock_Brake_System_(ABS) 1972, First Automotive Anti-lock Brake System (ABS)]</ref> In the same year, [[Nissan]] offered an EAL (Electro Anti-lock System) as an option on the [[Nissan President]], which became [[Japan]]'s first electronic ABS.<ref>{{cite web |url=http://www.jsae.or.jp/autotech/data_e/5-4e.html |title=Electro antilock system (installed in Nissan President) |work=240 Landmarks of Japanese Automotive Technology |publisher=Society of Automotive Engineers in Japan, Inc.}}</ref>

[[Image:Absbrakes.jpg|thumb|ABS brakes on a BMW motorcycle]]

In 1988, [[BMW]] introduced the first [[motorcycle]] with an electronic-hydraulic ABS: the [[BMW K100]]. [[Honda]] followed suit in 1992 with the launch of its first motorcycle ABS on the [[ST1100]] Pan European. In 2007, [[Suzuki]] launched its [[Suzuki Bandit Series|GSF1200SA (Bandit)]] with an ABS. In 2005, Harley-Davidson began offering ABS as an option for police bikes.  In 2008, ABS became a factory-installed option on all Harley-Davidson Touring motorcycles and standard equipment on select models.{{Citation needed|date=October 2010}}. Recently(2011) TVS has launched the first Indian two wheeler with ABS in its much famous bike Apache RTR ABS(180 cc).

== Operation ==

The anti-lock brake controller is also known as the CAB (Controller Anti-lock Brake).<ref>{{cite web|author=KI4CY |url=http://dodgeram.org/tech/specs/glossary.htm |title=Ram Glossary of abbreviations and terms |publisher=Dodgeram.org |date=2003-02-13 |accessdate=2010-12-07}}</ref>

A typical ABS includes a central [[electronic control unit]] (ECU), four [[wheel speed sensor]]s, and at least two hydraulic valves within the brake [[Hydraulic brake|hydraulics]]. The ECU constantly monitors the [[rotational speed]] of each wheel; if 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. Conversely, if the ECU detects a wheel turning significantly faster than the others, brake hydraulic pressure to the wheel is increased so the braking force is reapplied, slowing down the wheel. This process is repeated continuously and can be detected by the driver via brake pedal pulsation. Some anti-lock system can apply or release braking pressure 16 times per second.<ref name="howstuffworks ABS 2010">Nice, Karim. [http://auto.howstuffworks.com/auto-parts/brakes/brake-types/anti-lock-brake1.htm How "Anti-Lock Brakes Work"]. [http://www.howstuffworks.com/about-hsw.htm howstuffworks]. Retrieved October 2, 2010.</ref>

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 (mechanics)|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.

The modern ABS applies individual brake pressure to all four wheels through a control system of hub-mounted sensors and a dedicated [[micro-controller]]. ABS is offered or comes standard on most road vehicles produced today and is the foundation for [[Electronic stability control|ESC systems]], which are rapidly increasing in popularity due to the vast reduction in price of vehicle electronics over the years.<ref name="absfaq">{{cite web|url=http://www.abs-education.org/faqs/faqindex.htm|title=ABS Frequently Asked Questions|publisher=ABS Education Alliance|date=2004-05-03|accessdate=2009-10-22}}</ref>

Modern [[electronic stability control]] (ESC or ESP) systems are an evolution of the ABS concept. Here, a minimum of two additional sensors are added to help the system work: these are a [[steering wheel]] angle sensor, and a [[Gyroscope|gyroscopic]] sensor. The theory of operation is simple: when the gyroscopic sensor detects that the direction taken by the car does not coincide with what the steering wheel sensor reports, the ESC software will brake the necessary individual wheel(s) (up to three with the most sophisticated systems), so that the vehicle goes the way the driver intends. The steering wheel sensor also helps in the operation of [[Cornering Brake Control]] (CBC), since this will tell the ABS that wheels on the inside of the curve should brake more than wheels on the outside, and by how much.

The ABS equipment may also be used to implement a [[traction control system]](TCS) on acceleration of the vehicle. If, when accelerating, the tire loses traction, the ABS controller can detect the situation and take suitable action so that traction is regained. More sophisticated versions of this can also control throttle levels and brakes simultaneously.

== Components ==

There are four main components to an ABS: speed sensors, valves, a pump, and a controller.

; Speed sensors: The anti-lock braking system needs some way of knowing when a wheel is about to lock up. The speed sensors, which are located at each wheel, or in some cases in the differential, provide this information.
; Valves: There is a valve in the brake line of each brake controlled by the ABS. On some systems, the valve has three positions:
* In position one, the valve is open; pressure from the master cylinder is passed right through to the brake.
* In position two, the valve blocks the line, isolating that brake from the master cylinder. This prevents the pressure from rising further should the driver push the brake pedal 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 way to put that pressure back. That is what the pump does; when a valve reduces the pressure in a line, the pump is there to get the pressure back up.
; Controller: The controller is an ECU type unit in the car which receives information from each individual wheel speed sensor, in turn if a wheel loses traction the signal is sent to the controller, the controller will then limit the brakeforce (EBD) and activate the ABS modulator which actuates the braking valves on and off.

== Use ==

There are many different variations and control algorithms for use in an ABS. One of the simpler systems works as follows:<ref name="howstuffworks ABS 2010"/>

# The controller monitors the speed sensors at all times. It is looking for decelerations in the wheel that are out of the ordinary. Right before a wheel locks up, it will experience a rapid deceleration. If left unchecked, the wheel would stop much more quickly than any car could. It might take a car five seconds to stop from 60&nbsp;mph (96.6&nbsp;km/h) under ideal conditions, 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 reduces the pressure to that brake until it sees an acceleration, then it increases the pressure until it sees the deceleration again. It can do this very quickly, before the tire can actually significantly change speed. The result is that the tire slows down at the same rate as the car, with the brakes keeping the tires very near the point at which they will start to lock up. This gives the system maximum braking power.
# When the ABS system is in operation the driver will feel a pulsing in the brake pedal; this comes from the rapid opening and closing of the valves. This pulsing also tells the driver that the ABS has been triggered. Some ABS systems can cycle up to 16 times per second.

== Brake types ==

Anti-lock braking systems use different schemes depending on the type of brakes in use. They can be differentiated by the number of channels: that is, how many valves that are individually controlled—and the number of speed sensors.<ref name="howstuffworks ABS 2010"/>

; Four-channel, four-sensor ABS: This is the best scheme. There is a speed sensor on all four wheels and a separate valve for all four wheels. With this setup, the controller monitors each wheel individually to make sure it is achieving maximum braking force.
; Three-channel, four-sensor ABS: There is a speed sensor on all four wheels and a separate valve for each of the front wheels, but only one valve for both of the rear wheels.
; Three-channel, three-sensor ABS: This scheme, commonly found on pickup trucks with four-wheel ABS, has a speed sensor and a valve for each of the front wheels, with one valve and one sensor for both rear wheels. 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 achieve maximum braking force. The rear wheels, however, are monitored together; they both have to start to lock up before the ABS will activate on the rear. With this system, it is possible that one of the rear wheels will lock during a stop, reducing brake effectiveness. This system is easy to identify, as there are no individual speed sensors for the rear wheels.
; One-channel, one-sensor ABS: This system is commonly found on pickup trucks with rear-wheel ABS. It has one valve, which controls both rear wheels, and one speed sensor, located in the rear axle. This system operates the same as the rear end of a three-channel system. The rear wheels are monitored together and they both have to start to lock up before the ABS kicks in. In this system it is also possible that one of the rear wheels will lock, reducing brake effectiveness. This system is also easy to identify, as there are no individual speed sensors for any of the wheels.

== Effectiveness ==
A 2003 [[Australia]]n study by Monash University Accident Research Centre found that ABS:<ref name="monash.edu.au"/>

* Reduced the risk of multiple vehicle crashes by 18 percent,
* Reduced the risk of run-off-road crashes by 35 percent.

On high-traction surfaces such as [[bitumen]], or [[concrete]], many (though not all) 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. In real world conditions even an alert, skilled driver without ABS would find it difficult, even through the use of techniques like [[threshold braking]], to match or improve on the performance of a typical driver with a modern ABS-equipped vehicle.  ABS reduces 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, sand and deep 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. Some vehicle manufacturers provide an "off-road" button to turn ABS function off. 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 comparing all four wheels, and detecting individual wheels skidding).  Availability of ABS relieves most drivers from learning [[threshold braking]].

A June 1999 [[National Highway Traffic Safety Administration]] (NHTSA) study found that ABS increased stopping distances on loose gravel by an average of 22 percent.<ref>[http://web.archive.org/web/20080411112845/http://www.nhtsa.dot.gov/staticfiles/DOT/NHTSA/NRD/Multimedia/PDFs/VRTC/ca/capubs/NHTSAabsT4FinalRpt.pdf NHTSA Light Vehicle Antilock Brake System Research Program Task 4: A Test Track Study of Light Vehicle ABS Performance Over a Broad Range of Surfaces and Maneuvers, Jan 1999] PDF</ref>

According to the NHTSA,

<blockquote>
"ABS works with your regular braking system by automatically pumping them.  In vehicles not equipped with ABS, the driver has to manually pump the brakes to prevent wheel lockup.  In vehicles equipped with ABS, your foot should remain firmly planted on the brake pedal, while ABS pumps the brakes for you so you can concentrate on steering to safety."
</blockquote>

When activated, some earlier ABS systems caused the brake pedal to pulse noticeably.  As most drivers rarely or never brake hard enough to cause brake lock-up, and a significant number rarely bother to read the car's manual, this may not be discovered until an emergency.  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, contributing to a higher level of accidents than the superior emergency stopping capabilities of ABS would otherwise promise.  Some manufacturers have therefore implemented a [[brake assist]] system that determines that the driver is attempting a "panic stop" (by detecting that the brake pedal was depressed very fast, unlike a normal stop where the pedal pressure would usually be gradually increased, Some systems additionally monitor the rate at the accelerator was released) and the system automatically increases braking force where not enough pressure is applied.  Hard or panic braking on bumpy surfaces, because of the bumps causing the speed of the wheel(s) to become erratic may also trigger the ABS.  Nevertheless, ABS significantly improves safety and control for drivers in most on-road situations.

Anti-lock brakes are the subject of some experiments centred around [[risk compensation]] theory, which asserts that drivers adapt to the safety benefit of ABS by driving more aggressively. In a study in Munich, Germany, half a fleet of [[taxicab]]s was equipped with anti-lock brakes, while the other half had conventional brake systems. The crash rate was substantially the same for both types of cab, and Wilde concludes this was due to drivers of ABS-equipped cabs taking more risks, assuming that ABS would take care of them, while the non-ABS drivers drove more carefully since ABS would not be there to help in case of a dangerous situation.<ref>{{cite web|url=http://psyc.queensu.ca/target/chapter07.html |title=7. Remedy by engineering? |publisher=Psyc.queensu.ca |year=1994 | author = Gerald J. S. Wilde |accessdate=2010-12-07}}</ref> A similar study was carried out in Oslo, with similar results.{{Citation needed|date=November 2008}}

== Regulation ==
{{Expand section|date=April 2011}}
ABS systems are required on all new passenger cars sold in the EU since 2007.  In the USA, the NHTSA has repeatedly considered mandating anti-lock brakes on light vehicles, but has held off due to concerns over testing procedures and real-world crash data that failed to meet expectations.<ref>{{cite web|url=http://www.nhtsa.gov/Research/Light+Vehicle+Brake+Systems|work=DOT 809 747|publisher=NHTSA|accessdate=7/6/2011}}</ref>

== See also ==

* [[Electronic brakeforce distribution]]
* [[Brake Assist]]
* [[Electronic Stability Control]]
* [[Automobile safety]]
* [[Risk compensation]]

== References ==
{{Reflist|30em}}

== External links ==
{{wiktionary|ABS}}
{{Commons category}}


{{Components}}
{{Automotive-tech-stub}}
[[Category:Automotive safety technologies]]
[[Category:Vehicle braking technologies]]
[[Category:Mechanical power control]]