An anti-lock braking system (ABS) is a safety anti-skid braking system used on aircraft and on land vehicles, such as cars, motorcycles, trucks and buses. ABS operates by preventing the wheels from locking up during braking, thereby maintaining tractive contact with the road surface.
ABS is an automated system that uses the principles of threshold braking and cadence braking which were once practised by skilful drivers with earlier non-ABS braking systems. ABS operates at a very much faster rate and more effectively than most drivers could manage. Although ABS generally offers improved vehicle control and decreases stopping distances on dry and slippery surfaces, on loose gravel or snow-covered surfaces, ABS may significantly increase braking distance, while still improving steering control. Since ABS was introduced in production vehicles, such systems have become increasingly sophisticated and effective. Modern versions may only prevent wheel lock under braking, but may also alter the front-to-rear brake bias. This latter function, depending on its specific capabilities and implementation, is known variously as electronic brakeforce distribution, traction control system, emergency brake assist, or electronic stability control (ESC).
The concept for ABS predates the modern systems that were introduced in the 1950s. In 1908, for example, J.E. Francis introduced his 'Slip Prevention Regulator for Rail Vehicles'.
In 1920 the French automobile and aircraft pioneer Gabriel Voisin experimented with systems that modulated the hydraulic braking pressure on his aircraft brakes to reduce the risk of tire slippage. These systems used 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, when a wheel slows 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.
The first patented system was created by German engineer Karl Wessel in 1928. Wessel, however, never developed a working product and neither did Robert Bosch who produced a similar patent eight years later.
By the early 1950s, the Dunlop Maxaret anti-skid system was in widespread aviation use in the UK, with aircraft such as the Avro Vulcan and Handley Page Victor, Vickers Viscount, Vickers Valiant, English Electric Lightning, de Havilland Comet 2c, de Havilland Sea Vixen, and later aircraft, such as the Vickers VC10, Hawker Siddeley Trident, Hawker Siddeley 125, Hawker Siddeley HS 748 and derived British Aerospace ATP, and BAC One-Eleven being fitted with Maxaret as standard. Maxaret, while reducing braking distances by up to 30% in icy or wet conditions, also increased tire life, and had the additional advantage of allowing take-offs and landings in conditions that would preclude flying at all in non-Maxaret equipped aircraft.
In 1958, a Royal Enfield Super Meteor motorcycle was used by the Road Research Laboratory to test the Maxaret anti-lock brake. 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.
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.
The first fully electronic anti lock system was developed in the late 1960s for the Concorde aircraft.
Chrysler, together with the Bendix Corporation, introduced a computerized, three-channel, four-sensor all-wheel ABS called "Sure Brake" for its 1971 Imperial. It was available for several years thereafter, functioned as intended, and proved reliable. In 1970, Ford added an antilock braking system called "Sure-track" to the rear wheels of Lincoln Continentals as an option; it became standard in 1971. In 1971, General Motors introduced the "Trackmaster" rear-wheel only ABS as an option on their rear-wheel drive Cadillac models and the Oldsmobile Toronado. In the same year, Nissan offered an EAL (Electro Anti-lock System) developed by Japanese company Denso as an option on the Nissan President, which became Japan's first electronic ABS.
1971: Electronically controlled anti-skid brakes on Toyota Crown In 1972, four wheel drive Triumph 2500 Estates were fitted with Mullard electronic systems as standard. Such cars were very rare however and very few survive today.
1976: WABCO began the development of anti-locking braking system on commercial vehicles to prevent locking on slippery roads, followed in 1986 by the electronic braking system (EBS) for heavy duty vehicles.
1982: Honda introduced electronically controlled multi-channel ALB (Anti Locking Brakes) as an option for the second generation of Prelude, launched worldwide in 1982. Additional info: The general agent for Honda in Norway required all Preludes for the Norwegian market to have the ALB-system as a standard feature, making Honda Prelude to be the first car delivered in Europe with ABS as a standard feature. The Norwegian general agent also included sun roof and other options to be standard equipment in Norway, adding more luxury to the Honda brand. However, the Norwegian tax system made the well-equipped car very expensive, and the sales suffered from high cost. From 1984 the ALB-system, as well as the other optional features from Honda, was no longer a standard feature in Norway.
In 1985 the Ford Scorpio was introduced to European market with a Teves electronic system throughout the range as standard. For this the model was awarded the coveted European Car of the Year Award in 1986, with very favourable praise from motoring journalists. After this success Ford began research into Anti-Lock systems for the rest of their range, which encouraged other manufacturers to follow suit.
Since 1987 ABS has been standard equipment on all Mercedes Benz automobiles. 
In 1988, BMW introduced the first motorcycle with an electronic-hydraulic ABS: the BMW K100. Yamaha Introduced the FJ1200 model with optional ABS in 1991. Honda followed suit in 1992 with the launch of its first motorcycle ABS on the ST1100 Pan European. In 2007, Suzuki launched its GSF1200SA (Bandit) with an ABS. In 2005, Harley-Davidson began offering an ABS option on police bikes.
The anti-lock brake controller is also known as the CAB (Controller Anti-lock Brake).
Typically ABS includes a central electronic control unit (ECU), four wheel speed sensors, and at least two hydraulic valves within the brake hydraulics. The ECU constantly monitors the rotational speed of each wheel; if it detects the wheel rotating significantly slower than the speed of the vehicle, 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 systems can apply or release braking pressure 15 times per second. Because of this, the wheels of cars equipped with ABS are practically impossible to lock even during panic braking in extreme conditions.
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.
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 systems, which are rapidly increasing in popularity due to the vast reduction in price of vehicle electronics over the years.
Modern electronic stability control 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 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.
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.
The speed sensors of ABS are sometimes used in indirect tire pressure monitoring system (TPMS), which can detect under-inflation of tire(s) by difference in rotational speed of wheels.
The majority of problems with the valve system occur due to clogged valves. When a valve is clogged it is unable to open, close, or change position. An inoperable valve will prevent the system from modulating the valves and controlling pressure supplied to the brakes.
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.
A 2004 Australian study by Monash University Accident Research Centre found that ABS:
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 possible without the benefit of ABS. In real world conditions, even an alert and experienced driver without ABS would find it difficult 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 such as gravel or on slippery surfaces such as 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.
According to the NHTSA,
"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."
When activated, some earlier ABS systems caused the brake pedal to pulse noticeably. As most drivers rarely or do not brake hard enough to cause brake lock-up, and drivers typically do not read the vehicle's owners manual, this may not be noticeable until an emergency. 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, sometimes causing the system to enter its ice mode, where the system severely limits maximum available braking power. 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 Munich study, half a fleet of taxicabs 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.
On a motorcycle, an anti-lock brake system prevents the wheels of a powered two wheeler from locking during braking situations. Based on information from wheel speed sensors the ABS unit adjusts the pressure of the brake fluid in order to keep traction during deceleration to avoid accidents. Motorcycle ABS helps the rider to maintain stability during braking and to decrease the stopping distance. It provides traction even on low friction surfaces. While older ABS models are derived from cars, recent ABS are the result of research, oriented on the specifics of motorcycles in case of size, weight and functionality. National and international organizations evaluate Motorcycle ABS as an important factor to increase safety and reduce motorcycle accident numbers. The European Commission passed legislation in 2012 that made the fitment with ABS for all new motorcycles above 125 cm3 to be mandatory from 1 January 2016. Consumer Reports said in 2016 that "ABS is commonly offered on large, expensive models, but it has been spreading to several entry-level sport bikes and midsized bikes".
In 1988, BMW introduced an electronic/hydraulic ABS for motorcycles, ten years after Daimler Benz and Bosch released the first four-wheel vehicle ABS for series production. Motorcycles of BMW K100 series were optionally equipped with the ABS, which added 11 kg to the bike. It was developed together with FAG Kugelfischer and regulated the pressure in the braking circuits via a plunger piston. Japanese manufacturers followed with an ABS option by 1992 on the Honda ST1100 and the Yamaha FJ1200.
Continental presented its first Motorcycle Integral ABS (MIB) in 2006. It has been developed in cooperation with BMW and weighed 2.3 kg. While the first generation of motorcycle ABS weighed around 11 kg. The current generation (2011) presented by Bosch in 2009 weighs 0.7 kg (ABS base) and 1.6 kg (ABS enhanced) with integral braking.
Wheel speed sensors mounted on front and rear wheel constantly measure the rotational speed of each wheel and deliver this information to an Electronic Control Unit (ECU). The ECU detects on the one hand if the deceleration of one wheel exceeds a fixed threshold and on the other hand whether the brake slip, calculated based on information of both wheels, rises above a certain percentage and enters an unstable zone. These are indicators for a high possibility of a locking wheel. To countermeasure these irregularities the ECU signals the hydraulic unit to hold or to release pressure. After signals show the return to the stable zone, pressure is increased again. Past models used a piston for the control of the fluid pressure. Most recent models regulate the pressure by rapidly opening and closing solenoid valves. While the basic principle and architecture has been carried over from passenger car ABS, typical motorcycle characteristics have to be considered during the development and application processes. One characteristic is the change of the dynamic wheel load during braking. Compared to cars, the wheel load changes are more drastic, which can lead to a wheel lift up and a fall over. This can be intensified by a soft suspension. Some systems are equipped with a rear wheel lift off mitigation functionality. When the indicators of a possible rear lift off are detected, the system releases brake pressure on the front wheel to counter this behavior. Another difference is that in case of the motorcycle the front wheel is much more important for stability than the rear wheel. If the front wheel locks up between 0.2-0.7s, it loses gyrostatic forces and the motorcycle starts to oscillate because the increased influence of side forces operating on the wheel contact line. The motorcycle becomes unstable and falls.
Piston Systems The pressure release in this system is realized through movement of a spring-tensioned piston. When pressure should be released, a linear motor pulls back the plunger piston and opens up more space for the fluid. The system was used for example in the ABS I (1988) and ABS II (1993) of BMW. The ABS II differed in size and an electronically controlled friction clutch was mounted on the shaft instead of a plunger. Further displacement sensors record the travel distance of the piston to allow the control unit a more precise regulation. Honda also uses this system of pressure modulation for big sports and touring bikes. 
Valve and Pump Systems The main parts which are part of the pressure modulation system are solenoid inlet and outlet valves, a pump, motor and accumulators/reservoirs. The number of the valves differs from model to model due to additional functionalities and the number of brake channels. Based on the input of the ECU, coils operate the in- and outlet valves. During pressure release the brake fluid is stored in accumulators. In this open system approach the fluid is then brought back in the brake circuit via a pump operated by a motor which is felt through pulsation on the brake lever.
Different from cars, planes or trains, motorcycle rear and front wheels are controlled separately. If the rider only brakes with one wheel, this braked wheel tends to lock up faster than if both brakes had been applied. A Combined Braking System therefore distributes the brake force also to the non-braked wheel to lower the possibility of a lock up, increase deceleration and reduce suspension pitch.
With a single [rear] CBS the brake pressure applied on the rear brake (pedal) is simultaneously distributed to the front wheel. A delay valve cuts the hydraulic pressure to assure that only when strong braking is applied, pressure is also created at the front wheel. Honda's first street motorcycle with a combined braking system (then called Unified Braking) was the 1983 GL1100. This system was derived from a 1970s RCB1000 world endurance race bike.
Larger models with two front discs use a dual CBS System. The system was first installed by Moto Guzzi in 1975. Here, applied brake pressure at the front is also applied to the rear wheel and vice versa. If the front lever is applied, pressure is built up at 4 of the 6 pots in the 2 calipers at the front. A secondary master cylinder at the front wheel distributes remaining pressure to the rear wheel through a proportional control valve and acts on 2 of the 3 calipers. If strong brake force is applied at the rear wheel force is also distributed to 2 of the 6 pots of the front wheel. More modern dual CBS use front and rear calipers (and all pots) according to a preset load ratio of front to rear. The proportioning was originally controlled by complex all-hydraulic systems interlinking the front and rear, with a fixed delay or by sensing weight distribution changes. As early as 2001 an electrohydraulic system was introduced by BMW.
CBS helps to reduce the danger of wheel locks and fall downs but in certain situations it is possible that CBS causes a fall down. If brake pressure is distributed from the rear wheel to the front wheel and the friction of the surfaces changes suddenly (puddle, ice on the street) the front wheel might lock even if only the rear brake has been applied. This would lead to a loss of stability and a fall down. CBS is therefore combined with ABS to avoid this on a motorcycle. Different approaches are possible to realize this combination: Without active pressure Build up Single Version: A third additional channel links the rear wheel circuit through a delay valve to the front brake. Strong brake pressure at the rear wheel (or both wheels) pressurises both brake circuits however this pressure is adjusted according to wheel speed and brake slip.
The dual version combines Hondas Dual CBS with a secondary master cylinder and a proportional control valve [with Piston ABS] A modulator regulates the pressure for eachWith Active Pressure Build up In 2009, Honda introduced the electronic controlled combined ABS for its high performance sports bikes which utilizes brake by wire technology. The brake input of the rider is measured by pressure sensors and the information is provided to an ECU. Together with the information of the wheel speed sensors the ECU calculates the optimal distribution of pressure to prevent lockups and to provide best possible deceleration. Based on this output a motor for each wheel operates a pump which builds up and regulates the brake pressure on the wheel. This system offers a fast reaction time because of the brake by wire functionality.
The MIB (Motorcycle integral Braking system) from Continental Teves and the eCBS (electronic CBS) in the enhanced Motorcycle ABS from Bosch are results of another approach. These systems are based on the pump and valve approach. Through additional valves, stronger pumps and a more powerful motor the system can actively build up pressure. The input pressure of the rider is measured with pressure sensors at the lever and pedal. The pump then builds up additional pressure adjusted to riding conditions. A partial integral System is designed for working in one direction only: front->rear or rear->front. A fully integral system works in both directions.
Because these systems are electronically controlled and are able to build up pressure actively, they offer the opportunity to adjust the motorcycle braking behavior to the rider. CBS and ABS can be switched off by experienced riders and also different regulation modes with higher and lower thresholds can be chosen. Like the rain or slick mode in the BMW S1000RR.
A report of the European Transport Safety council showed that riding a motorcycle is 20 times more dangerous than driving a car the same distance. And the accidents situation in Germany from 1990 to 2011 showed that total traffic fatalities decreased drastically (11 000 to 4 009) but motorcycle fatalities remained constant.
The Insurance Institute for Highway Safety (IIHS) conducted a study on the effectiveness of ABS for motorcycles and came to the conclusion that motorcycles above 250 cm3 without ABS are 37 percent more likely to be involved in fatal crashes and a study of the Swedish Road Administration came to the conclusion that 48 percent of all severe and fatal motorcycle accidents above 125 cm3 could be avoided due to motorcycle ABS.
These studies caused the EU commission to initiate a legislative process in 2010 that was passed in 2012 and led to ABS for motorcycles above 125 cm3 becoming mandatory from 2016 onwards. Organizations like the Federation International de l'Automobile and the Institute of advanced Motorists (IAM) demanded the implementation of this legislation already for 2015. On the other hand, some motorcycle riders are protesting against a compulsory ABS for all bikes because they call for a possibility to switch the system off, for off road usage or for other reasons. In 2011 the United Nations (UN) started the Decade of Action for Road Safety. The main goal is to save 5 million lives until 2020 through global cooperation. One part of their global plan is to: Encourage universal deployment of crash avoidance technologies with proven effectiveness such as Electronic Stability Control and Anti-Lock Braking Systems in motorcycles.
ABS is required on all new passenger cars sold in the EU since 2004. In the United States, the NHTSA has mandated ABS in conjunction with Electronic Stability Control under the provisions of FMVSS 126 as of September 1, 2013.
Prius c is equipped with Toyota's Anti-skid Braking System (ABS).