An anti-lock braking system or anti-skid braking system (ABS) is an
automobile safety system that allows the wheels on a motor vehicle to
maintain tractive contact with the road surface according to driver
inputs while braking, preventing the wheels from locking up (ceasing
rotation) and avoiding uncontrolled skidding. It is an automated
system that uses the principles of threshold braking and cadence
braking which were practiced by skillful drivers with previous
generation braking systems. It does this at a much faster rate and
with better control than many drivers could manage.
ABS generally offers improved vehicle control and decreases stopping
distances on dry and slippery surfaces; however, on loose gravel or
snow-covered surfaces, ABS can significantly increase braking
distance, although still improving vehicle steering control.
Since initial widespread use in production cars, anti-lock braking
systems have been improved 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).
1.1 Early systems
1.2 Modern systems
8 See also
10 External links
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 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, 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
The first patented system was created by German engineer Karl Wessel
in 1928. Wessel, however, never developed a working product and
Robert Bosch who produced a similar patent eight years
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
Hawker Siddeley HS 748 and derived
British Aerospace ATP, and
BAC One-Eleven being fitted with
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
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
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
'60s for the
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
Nissan offered an EAL (Electro Anti-lock System) developed by
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.
1971: First truck application: "Antislittamento" system developed by
Fiat Veicoli Industriali and installed on
Fiat truck model 691N1.
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
Mercedes-Benz W116 became the first production car to use an
electronic four-wheel multi-channel anti-lock braking system (ABS)
from Bosch as an option from 1978 on.
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
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.
BMW introduced the first motorcycle with an
electronic-hydraulic ABS: the
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
Suzuki launched its GSF1200SA (Bandit) with an ABS. In 2005,
Harley-Davidson began offering an ABS option on police bikes.
In 1993, Lincoln became one of the first automobile companies to
provide standard four-wheel anti-lock brakes on all of their
The anti-lock brake controller is also known as the CAB (Controller
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
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
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
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.
There are four main components of ABS: wheel speed sensors, valves, a
pump, and a controller.
ABS speed sensors
A speed sensor is used to determine the acceleration or deceleration
of the wheel. These sensors use a magnet and a Hall effect sensor, or
a toothed wheel and an electromagnetic coil to generate a signal. The
rotation of the wheel or differential induces a magnetic field around
the sensor. The fluctuations of this magnetic field generate a voltage
in the sensor. Since the voltage induced in the sensor is a result of
the rotating wheel, this sensor can become inaccurate at slow speeds.
The slower rotation of the wheel can cause inaccurate fluctuations in
the magnetic field and thus cause inaccurate readings to the
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
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.
The pump in the ABS is used to restore the pressure to the hydraulic
brakes after the valves have released it. A signal from the controller
will release the valve at the detection of wheel slip. After a valve
releases the pressure supplied from the user, the pump is used to
restore a desired amount of pressure to the braking system. The
controller will modulate the pump's status in order to provide the
desired amount of pressure and reduce slipping.
The controller is an ECU type unit in the car which receives
information from each individual wheel speed sensor. If a wheel loses
traction, the signal is sent to the controller. The controller will
then limit the brake force (EBD) and activate the ABS modulator which
actuates the braking valves on and off.
There are many different variations and control algorithms for use in
ABS. One of the simpler systems works as follows:
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 mph
(96.6 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.
This replaces the need to manually pump the brakes while driving on a
slippery or a low traction surface, allowing to steer even in most
emergency braking conditions.
When the ABS 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.
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.
1) Four-channel, four-sensor ABS
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.
2) 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. Older vehicles with four-wheel ABS usually use this type.
3) 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.
4) Two-channel, four sensor ABS
This system, commonly found on passenger cars from the late '80s
through the mid 1990s, uses a speed sensor at each wheel, with one
control valve each for the front and rear wheels as a pair. If the
speed sensor detects lock up at any individual wheel, the control
module pulses the valve for both wheels on that end of the car.
5) One-channel, one-sensor ABS
This system is commonly found on pickup trucks, SUVs, and vans 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
A 2004 Australian study by Monash University Accident Research Centre
found that ABS:
Reduced the risk of multiple vehicle crashes by 18 percent,
Increased 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 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
A June 1999
National Highway Traffic Safety Administration
National Highway Traffic Safety Administration (NHTSA)
study found that ABS increased stopping distances on loose gravel by
an average of 27.2 percent.
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
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
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
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
Insurance Institute for Highway Safety
Insurance Institute for Highway Safety released a study in 2010
that found motorcycles with ABS 37% less likely to be involved in a
fatal crash than models without ABS.
ABS are 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.
Anti-lock braking system
Anti-lock braking system for motorcycles
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Media related to Anti-lock braking systems at Wikim