Anti-lock braking system (ABS)
Anti-lock 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 a driver could manage.
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, ABS can significantly increase
braking distance, although still improving vehicle control.
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 brake force distribution(EBD), traction
control system, emergency brake assist, or electronic stability control(ESC).
anti-lock brake controller is also known as the CAB (Controller Anti-lock
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 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 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
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
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 of ABS
are four main components of ABS:
A speed sensor is used to determine the acceleration or
deceleration of the wheel. These sensors use a magnet and a coil of wire 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 controller.
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.
position three, the valve releases some of the pressure from the brake.
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
release 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 pumps status in order to provide the desired amount of pressure and reduce
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 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 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 steering even in the
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.