CHAPTER 01Introduction
An anti-lock braking system (ABS) is a safety system on motor
vehicles which prevents the wheels from locking while braking.A
rotating road wheel allows the driver to maintain steering control
under heavy braking, by preventing a locked wheel or skid, and
allowing the wheel to continue to forward roll and create lateral
control, as directed by driver steering inputs. Disadvantages of
the system include increased braking distances under some limited
circumstances (snow, gravel, "soft" surfaces), and the creation of
a "false sense of security" among drivers who do not understand the
operation, and limitations of ABS.
CHAPTER 02History
Anti-lock braking systems were first developed for aircraft in
1929, by the French automobile and aircraft pioneer, Gabriel
Voisin, as threshold braking an airplane is nearly impossible.
An early system was Dunlop's Maxaret system, introduced in the
1950s and still in use on some aircraft models.
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, in automobile use, somewhat
unreliable.
However, a limited form of anti-lock braking, utilizing a valve
which could adjust front to rear brake force distribution when a
wheel locked, was fitted to the 1964 Austin 1800.
Chrysler, together with the Bendix Corporation, introduced a
crude, limited production ABS system on the 1971 Imperial. Called
"Sure Brake", it was available for several years, and had a
satisfactory performance and reliability record.
Ford also introduced anti lock brakes on the Lincoln Continental
Mark III and the Ford LTD station wagon, called "Sure Trak" in
1975.
The German firms Bosch and Mercedes-Benz had been co-developing
anti-lock braking technology since the 1930s, and introduced the
first completely electronic 4-wheel multi-channel ABS system in
trucks and the Mercedes-Benz S-Class in 1978.
ABS Systems based on this more modern Mercedes design were later
introduced on other cars and motorcycles. General Motors introduced
the "Trackmaster" ABS on their Cadillac models in 1971 as an option
that was operational on the rear wheels for RWD models.
In 1988 BMW became the world's first motorcycle manufacturer to
introduce an electronic/hydraulic ABS system, this on their BMW
K100.
In 1992 Honda launched its first ABS system, this on the ST1100
Pan European.
In 1997 Suzuki launched its GSF1200SA (Bandit) with ABS
Today ABS has become a standard equipment even for small
cars
CHAPTER 03Theory
When the car brakes (normally), the momentum of the car must be
reduced, so a backwards force needs to be transmitted to the car.
This is achieved by the wheels exerting a forward force on the
street which lies below the threshold of maximum static sliding
friction. The wheels keep sticking to the road because of this
friction.
If the driver brakes very hard (or accelerates extremely) it can
occur that the maximum static friction is surpassed and the wheels
lose their grip and begin sliding (or spinning). In this case the
dynamic sliding friction (which is less than the maximum static
friction) takes over
The amount of traction which can be obtained for an auto tire is
determined by the coefficient of static friction between the tire
and the road. If the wheel is locked and sliding, the force of
friction is determined by the coefficient of kinetic friction and
is usually significantly less. A tire that is just on the verge of
slipping (10 to 20% slippage) produces more friction with respect
to the road than one which is locked and skidding (100% slippage).
Once traction is lost, friction is reduced, the tire skids and the
vehicle takes longer to stop. So locked wheels are less effective
in stopping on a road
But in gravel, sand and deep snow, locked wheels dig in and stop
the vehicle more quickly. A locked tire allows a small wedge of
snow to build up ahead of it which allows it to stop in a somewhat
shorter distance than a rolling tire. That is why some vehicles
have an on/off switch for deactivating the antilock system when
driving on snow.
So, antilock brakes do not necessarily reduce the stopping
distance, and in fact may actually increase stopping slightly on
dry pavement. But on wet or slick pavement, antilock brakes may
reduce the stopping distance up to 25% or more, which could be the
difference between a safe stop and an accidentBut what ABS provides
is Directional stability which is very crucial.Directional
stability also depends on traction. As long as a tire does not
slip, it will roll only in the direction it turns. But once it
skids, it has about as much directional stability as a hockey puck
on ice because, regardless of the angle of the front wheels, the
vehicle continues to skid in whatever direction its momentum sends
it until either the driver releases the brakes. By minimizing the
loss of traction, antilock braking helps maintain directional
stability and steering control CHAPTER 04Basic Operation
The Anti-lock Braking System is designed to maintain vehicle
control, directional stability and optimum deceleration under
severe braking conditions on most road surfaces.
It does so by monitoring the rotational speed of each wheel and
controlling the brake line pressure to each wheel during braking.
This prevents the wheels from locking up. The ABS system has
following main components
Wheel Speed Sensors Abs Control Module Hydraulic Modulator Pump
Motor & Accumulator.
The sensors - one at each wheel since1985, but at both front
wheels and one at the rear differential on earlier models send a
variable voltage signal to the control unit, which monitors these
signals, compares them to its program information, and determines
whether a wheel is about to lock up. When a wheel is about to lock
up, the control unit signals the hydraulic unit to reduce hydraulic
pressure (or not increase it further) at that wheels brake caliper.
Pressure modulation is handled by electrically-operated solenoid
valves.
Regardless of manufacturer or the type of vehicle, all antilock
brake systems operate in a similar manner. Wheel speed sensors are
placed on each wheel that is to be controlled. Each speed sensor
usually has a toothed wheel, like a gear, that rotates at the same
speed as the vehicle wheel or axle. Mounted close to, but not
touching this toothed wheel, is a permanent magnet wrapped with a
coil of wire, called the pick-up coil (see illustration). As each
tooth rotates past the permanent magnet, it causes the magnetic
field to concentrate and increase slightly. This, in turn, induces
a small pulse of current in the coil of wire. The number of pulses
per second is directly proportional to the speed of the wheel. The
faster the wheel turns, the faster the teeth pass the magnet and
the higher the pulse rate. The pulsed output from the wheel speed
sensors goes to an electronic controller, which monitors each
wheel's speed relative to the speed of the other wheels. As long as
the brakes are not being applied and all of the monitored wheels
are rotating at roughly the same speed, the system takes no action.
If, however, the brakes are being applied and one or more of the
monitored wheels suddenly begins to reduce speed at a higher rate
than the othersindicating a loss of traction with the road and an
imminent wheel lockup and skidthe controller then activates the
antilock system.
The antilock brake system on any vehicle is simply an additional
monitoring and controlling function superimposed on the existing
vehicle brake system. ABS is not a second brake system, nor does it
replace the vehicle brake system. When all four wheels on an
automobile are monitored and controlled, the system is called a
four-channel ABS. If the front two wheels plus the rear axle (but
not each rear wheel individually) are to be controlled, the system
is called a three-channel ABS. On heavy trucks with two rear drive
axles, the ABS is commonly a four-channel system which controls the
front wheels and two of the four rear wheels. Trailers pulled by
heavy truck tractors may also have their own separate ABS which
must interconnect with the ABS on the tractor.
In an automobile, the brakes are actuated by hydraulic pressure.
The ABS controller operates solenoid valves built into the high
pressure side of the master brake cylinder. These valves are
normally open and do not interfere with braking. When the
controller senses that a wheel is locking up while braking, it
first activates a solenoid to close a valve in the affected wheel's
brake line which prevents the pressure from increasing any further.
If the locked wheel continues to lose speed, the controller
activates a second solenoid which bleeds pressure off the affected
brake line, in effect releasing the brake for that wheel regardless
of whether the driver is still pushing on the brake pedal. As soon
as the wheel regains traction and its speed increases, the
solenoids are de-activated and normal braking resumes. Of course,
if the conditions are such that the wheel starts to skid again, the
brake will promptly begin to lock up and the ABS will take over.
This cycle is repeated 12 to 15 times per second until either the
road condition changes or the driver releases the brakes. The
driver will be able to detect this rapid cycling as a vibration
felt through the brake pedal, but will not have to take any action.
The ABS will minimize the skid and will allow the driver to
maintain directional control of the vehicle.The brakes on a heavy
truck are actuated by air pressure, rather than hydraulic pressure.
The antilock brake system on a truck works in a manner similar to
the ABS on an automobile, except the antilock air pressure control
valves are located on the vehicle frame rail, near each wheel.
CHAPTER 05ComponentsWHEEL SPEED SENSORSThe wheel speed sensors
(WSS) consist of a magnetic pickup and a toothed sensor ring
(sometimes called a "tone" ring). The sensor(s) may be mounted in
the steering knuckles, wheel hubs, brake backing plates,
transmission tail shaft or differential housing. On some
applications, the sensor is an integral part of the wheel bearing
and hub assembly. The sensor ring(s) may be mounted on the axle hub
behind the brake rotor, on the brake rotor itself, inside the brake
drum, on the transmission tail shaft or inside the differential on
the pinion shaft.The wheel speed sensor pickup has a magnetic core
surrounded by coil windings. As the wheel turns, teeth on the
sensor ring move through the pickup magnetic field. This reverses
the polarity of the magnetic field and induces an alternating
current (AC) voltage in the pickup windings. The number of voltage
pulses per second that are induced in the pickup changes in direct
proportion to wheel speed. So as speed increases, the frequency and
amplitude of the wheel speed sensor goes up.The WSS signal is sent
to the antilock brake control module, where the AC signal is
converted into a digital signal and then processed. The control
module then counts pulses to monitor changes in wheel speed. On
applications where the wheel speed sensor is not part of the hub or
wheel bearing assembly, it can be replaced if defective. Sensor
problems can be caused by an accumulation of debris on the end
(they are magnetic), incorrect air gap or faults in the wiring or
connectors.ABS CONTROL MODULEThe ABS electronic control module
(which may be referred to as an EBCM "Electronic Brake Control
Module" or EBM "Electronic Brake Module") is a microprocessor that
functions like the engine control computer. It uses input from its
sensors to regulate hydraulic pressure during braking to prevent
wheel lockup. The ABS module may be located in the trunk, passenger
compartment or under the hood. It may be a separate module or
integrated with other electronics such as the body control or
suspension computer. On the newer ABS systems (Delphi DBC-7, Teves
Mark 20, etc.), it is mounted on the hydraulic modulator.The key
inputs for the ABS control module come from the wheel speed sensors
and a brake pedal switch. The switch signals the control module
when the brakes are being applied, which causes it to go from a
"standby" mode to an active mode. When ABS braking is needed, the
control module kicks into action and orders the hydraulic unit to
modulate brake pressure as needed. On systems that have a pump, it
also energizes the pump and relay.Like any other electronic control
module, the ABS module is vulnerable to damage caused by electrical
overloads, impacts and extreme temperatures. The module can usually
be replaced if defective, except on some of the newest systems
where the module is part of the hydraulic modulator
assembly.HYDRAULIC MODULATOR:The hydraulic modulator or actuator
unit contains the ABS solenoid valves for each brake circuit. The
exact number of valves per circuit depends on the ABS system and
application. Some have a pair of on-off solenoid valves for each
brake circuit while others use a single valve that can operate in
more than one position. On Delco VI ABS systems, small electric
motors are used in place of solenoids to drive pistons up and down
to modulate brake pressure. On some systems, the individual ABS
solenoids can be replaced if defective, but on most applications
the modulator is considered a sealed assembly and must be replaced
as a unit if defective.
Hydraulic modulator has a hydraulic modulator block including a
reservoir and a damper; and an electronic control block detachably
attached to the surface of the hydraulic modulator block. The
reservoir and the damper are overlapped with each other such that a
surface of the hydraulic modulator block becomes entirely
substantially flat. With this, the hydraulic modulator becomes
simple in construction. The hydraulic modulator has a solenoid
valve; an electronic control circuit board; and an electric wiring
pattern prepared by pressing a metal plate having a first major
surface formed with a first tin layer and a nickel layer and a
second major surface formed with a second tin layer, into a
three-dimensional shape such that the electric wiring pattern is
formed with (1) a first terminal having a surface that is formed
with the first or second tin layer electrically connected with the
solenoid valve, (2) a connector terminal having first and second
surfaces respectively formed with the first and second tin layers,
and (3) a second terminal having a surface that is formed with the
nickel layer electrically connected with the electronic control
circuit board. Thus, each terminal has a secure electrical
connection with another member
PUMP & ACCUMULATORA high pressure electric pump is used in
some ABS systems to generate power assist for normal braking as
well as the reapplication of brake pressure during ABS braking. In
some systems, it is used only for the reapplication of pressure
during ABS braking.The pump motor is energized via a relay that is
switched on and off by the ABS control module. The fluid pressure
that is generated by the pump is stored in the "accumulator." The
accumulator on ABS systems where the hydraulic modulator is part of
the master cylinder assembly consists of a pressure storage chamber
filled with nitrogen gas. Should the pump fail (a warning light
comes on when reserve pressure drops too low), there is usually
enough reserve pressure in the accumulator for 10 to 20
power-assisted stops. After that, there is no power assist. The
brakes still work, but with increased effort.On ABS systems that
have a conventional master cylinder and vacuum booster for power
assist, a small accumulator or pair of accumulators may be used as
temporary holding reservoirs for brake fluid during the
hold-release-reapply cycle. This type of accumulator typically uses
a spring loaded diaphragm rather than a nitrogen charged chamber to
store pressure.
CHAPTER 06Anti-Lock Brake Types
There are several ABS systems used by the auto manufacturers,
every make and model chooses what system is best suitable for their
vehicle depending on size and weight, we will show you the most
common systems used today.
Open and closed systems:Open anti-lock system : Open anti-lock
system is one in which the brake fluid released from the brakes
during ABS stop is not returned to the brake during the ABS stop;
instead, the fluid is stored in an accumulator during the ABS stop,
then returned to the master cylinder reservoir afterwards.
A disadvantage of the open systems is that the brake pedal will
drop during a long ABS stop as fluid flows from the brake
lines.Some open systems have a pump that restores fluid to the
master cylinder to keep the pedal from sinking, but the pump is not
involved in the actual anti-lock function.
This type is used in simple-real wheel-only ABS designs.
Closed system: Closed system has some means, generally an
electrically powered pump, to restore hydraulic pressure that's
bled off during an ABS stop.
The pump supplies fluid to an accumulator, where it's stored
under pressure until is needed to increase brake line pressure. In
some cases, pump pressure is applied to the brakes during the ABS
stop, with the amount and timing of pressure application controlled
by a solenoid valve
By The Number Of Channels :
Anti-lock braking systems by the number of channels -- that is,
how many valves that are individually controlled -- and the number
of speed sensorsFour-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, 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.
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 easy to identify. Usually there will
be one brake line going through a T-fitting to both rear wheels.
You can locate the speed sensor by looking for an electrical
connection near the differential on the rear-axle housing
Integrated systems and Non-integrated systems
Integrated systems:
An integrated system gets its name from the fact that the major
hydraulic components like the brake booster and the hydraulic
modulator are integrated into a unit with the master cylinder.
Other components, such as the accumulator and hydraulic modulator,
may also be part of the assembly. Many of these systems have no
vacuum booster.In such systems, the ABS pump provides brake boost
as well as the pressure necessary for anti-lock brake operation.The
pump forces fluid into one or more accumulators, where is stored at
very high pressures, typically 2000 to 3000 psi until it is needed.
On systems without a vacuum booster, the booster is a valve,
controlled by the driver's foot on the brake pedal, which regulates
the amount of boost applied.
Non-integrated systems:
Non-integrated systems, also known as "add-on" ABS, are
installed in conventional brake systems between the master cylinder
and the wheel brakes. A vacuum booster is used.The master cylinder
is very much alike, or in some cases identical, to the master
cylinder used with non-anti-lock brakes.The hydraulic modulator is
installed near the master cylinder. The brake fluid lines from the
master cylinder connect to the hydraulic modulator. Brake lines run
from the hydraulic modulator to each of the wheel brakes.
During normal braking, it's as if the hydraulic modulator
weren't there, hydraulic pressure from the master cylinder flows
uninterrupted through the modulator to the brakes. During an ABS
stop, the hydraulic modulator rapidly changes the hydraulic
pressure at the wheel brakes, holding it steady, reducing it, or
letting it increase. Fluid pressure is reduced by allowing some of
the high pressure to return to its source. This low pressure fluid
in an ABS system is commonly referred as "decayed" fluid.
Hydraulic circuits:
Primary:The primary circuit is normally operated by the master
cylinder piston closest to the rear of the master cylinder, and
thus in direct contact with the booster pushrod.
Secondary:The secondary circuit is operated by the master
cylinder piston closest to the front of the master cylinder.
In the case of front-rear split circuits, the primary circuit
operates both front brakes and the secondary circuit operates both
rear brakes, in diagonally split circuits, the primary operates one
front brake and diagonally opposite rear, while the secondary
operates the remaining two wheels.
CHAPTER 07Modern developments
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 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
CHAPTER 08Disadvantages
Increased braking distances under some limited circumstances
(ice, snow, gravel, "soft" surfaces),
Creation of a "false sense of security" among drivers who do not
understand the operation, and limitations of ABS.
The anti-lock brakes are more sensitive on the damper condition.
the influence of the worn components on the performance of the
vehicle with anti-lock brakes is more significant than without
anti-lock brakes, the stopping distance with defective shocks is by
meters longer for the presented simulation scenario.
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