1. INTRODUCTION
Anti-lock
Braking System (ABS) is one of the earliest commercially available
crash-avoidance systems and is considered among the most meaningful vehicle
safety advances in the history of the automobile. The system’s value is in its
ability to prevent vehicle from skidding when braking. The control strategies
designed for ABS, use wheel angular velocity and angular acceleration signals
to detect wheel lock up. As long as the brakes are applied, if skidding is
detected the system automatically releases and reapplies the brakes many times
per second if necessary, to provide as much braking force as the friction
between the road and the tyre will allow.
The
system helps in better steering control and lessens the chances of sideways
skidding of vehicles in the critical situations.
1.1 NEED OF
ANTI-LOCK BRAKING SYSTEM
If
standard brakes are applied too hard, the wheels "lock" or skid,
which prevents them from giving directional control. If directional control
(steering) is lost, the vehicle skids in a straight line wherever it is going.
The ABS will not allow the tire to stop rotating; you can brake and steer at
the same time. The braking and steering ability of the vehicle is limited by
the amount of traction the tire can generate. ABS thus proves advantageous
under heavy braking and on slippery tracks for better control.
2.
TYPES
OF ANTI-LOCK BRAKES:
Anti-lock
braking systems use different schemes depending on the type of brakes in use.
We will refer to them by the number of channels that is, how many valves that
are individually controlled and the number of speed sensors.
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, 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.
3.
COMPONENTS OF ABS:
There
are four main components to an ABS system:
·
Speed sensors
·
Hydraulic modulator
·
Pump
·
Control Module
3.1
Speed Sensors:
The
wheel speed sensors (WSS) consist of a magnetic pickup and a toothed sensor
ring. The sensor(s) may be mounted in the steering knuckles, wheel hubs, brake
backing plates, transmission tailshaft 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
tailshaft or inside the differential on the pinion shaft.
The
sensor pickup has a magnetic core surrounded by coil windings. As the wheel
turns, teeth on the sensor ring move through the pickup’s magnetic field. This
reverses the polarity of the magnetic field and induces an alternating current
(AC) voltage in the pickup’s 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 ABS 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.
3.2
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.
3.3
Pump Motor & Accumulator:
A 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’s switched on and
off by the ABS control module. The fluid pressure that’s 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.
3.4
Control Module:
The 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.
4.
WORKING OF ABS:
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 kph) 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 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 can be explained by graph, under marginal condition of
braking, the brake pressure is applied increasing the slip. When peak value of
slip is attained, wheel torque reaches a maximum value. After the peak wheel
torque is sensed electronically, the electronic system commands that brake
pressure be reduced (via. Brake pressure modulator). As the brake pressure is
reduced, slip is reduced and the wheel torque again passes through maximum. The
wheel torque reaches a value below the peak on the low slip side and at this
point brake pressure is again increased as is seen in graph.
The system will continue to cycle, maintaining slip near the
optimal value as long as the brakes are applied and the braking conditions are
poor.
The driver can notice a
mechanical sound and feel some pulsation or increased resistance in the brake pedal;
this comes from the rapid opening and closing of the valves. Some ABS systems
can cycle up to 15 times per second.
To increase safety, most modern car brake systems are
broken into two circuits, with two wheels on each circuit. If a fluid leak
occurs in one circuit, only two of the wheels will lose their brakes and your
car will still be able to stop when you press the brake pedal.
The master cylinder
supplies pressure to both circuits of the car. It is a remarkable device that
uses two pistons in the same cylinder in a way that makes the cylinder
relatively failsafe. The combination valve warns the driver if there is a
problem with the brake system, and also does a few more things to make your car
safer to drive.
5.
PERFORMANCE OF ABS:
1.
This first series of graphs depicts locked conventional brakes vs. ABS brakes at maximum braking pressure on loose gravel.
· The
graph on the left shows that the vehicle with standard brakes had an initial
velocity of 12.4 m/s and demonstrated a negative acceleration with locked
wheels on loose gravel of -3.1 m/s/s.
·
The
graph on the right shows that the vehicle with anti-lock brakes had an initial
velocity of 11.1 m/s and demonstrated a negative acceleration of -2.3 m/s/s.
In
this situation locked standard brakes reduced stopping distance, or created a
more negative acceleration, than did the anti-lock braking system. This occurs because of the “damming” effect
created by the piling of loose gravel and dirt in front of the locked
wheels. The ABS wheels, which
essentially never stop rolling, remain on top of the loose gravel layer and do
not create a “damming” effect. The
benefit of ABS brakes in loose gravel or unpacked snow pertains to the driver’s
ability to maintain control of the automobile in emergency braking situations.
2.
This second series of graphs depicts a driver pumping standard brakes to create maximum brake pressure and maintain steering control vs. ABS brakes with maximum brake pressure.
·
The
graph on the left shows that the vehicle with standard brakes had an initial
velocity of 9.7 m/s and while pumping the brakes in order to maintain control
of the car produced a negative acceleration of -1.9 m/s/s.
·
The
graph on the right shows that the vehicle with anti-lock brakes had an initial
velocity of 11.1 m/s and demonstrated a negative acceleration of -2.3 m/s/s.
From this data we see that the ABS system creates a more
negative acceleration than the standard brakes do while pumping the brakes in
order to maintain control on loose gravel.
Studies also show that pumping the brakes and attempting to steer
simultaneously requires tremendous coordination and the average driver is not
capable of doing both effectively. While
the average driver can probably pump her brakes to the point of locking and
back again about 2 times per second, modern ABS systems do the pumping
automatically at a rate of 18 times per second.
- This
third series of graphs depicts locked standard brakes vs. ABS brakes on
dry pavement.
Fig.
8 Locked standard brakes vs. ABS brakes
(
on dry pavement ).
·
The
vehicle with standard brakes had an initial velocity of 14.4 m/s and with
locked wheels on pavement produced a negative acceleration of –3.8
m/s/s.
·
The
vehicle with ABS brakes had an initial velocity of 16.0 m/s and with maximum
brake pressure produced a negative acceleration of –4.2 m/s/s.
On
dry pavement ABS brakes stop the vehicle faster and allow the driver to
maintain control of the automobile while stopping. ABS brakes create a greater
negative acceleration by taking advantage of the difference between the
coefficient of static friction and the coefficient of kinetic friction. Experimentally it is shown that the static
friction coefficient is larger than the kinetic friction coefficient. This means that when the wheels are skidding
there is less friction between the wheels and the pavement than when the brakes
are applied with as much pressure as possible without locking the wheels.
6. ADVANCEMENTS IN ABS:
6.1
FUZZY LOGIC:
Fuzzy logic, a more
generalized data set, allows for a class with continuous membership gradations.
This form of classification with degrees of membership offers much wider scope
of applicability, especially in control applications. Although fuzzy logic is
rigorously structured in mathematics, one advantage is the ability to describe
systems linguistically through rule statements. Such rules lend themselves to
development of an ABS braking system based on fuzzy logic.
ABS systems are nonlinear and dynamic in nature they are a
prime candidate for fuzzy logic control. Inputs to the Fuzzy ABS are derived
from wheel speed. Acceleration and slip for each wheel may be calculated by
combining the signals from each wheel. These signals are then processed in the
Fuzzy ABS system to achieve the desired control. The Intel Fuzzy ABS utilizes a
high performance, low cost,16-bit 8XC196Kx architecture to take advantage of
improved math execution timing.
Due to the nature of fuzzy logic, influential dynamic factors
are accounted for in a rule based description of ABS. This type of
"intelligent" control allows for faster development of system code.
The
Inputs to the Intel Fuzzy ABS are represented in the diagram above and consist
of:
1. The Brake: This block represents the
brake pedal deflection/assertion. This
information is acquired in a digital or analog format.
2. The 4 W.D: This indicates if the vehicle is in the
4-wheel-drive mode.
3. The Ignition: This input
registers if the ignition key is in place, and if the engine is running or not.
4. Feed-back: This block represents the set
of inputs concerning the state of the
ABS.
5. Wheel speed: In a typical application
this will represent a set of 4 input signals that convey the information
concerning the speed of each wheel. This information is used to derive all
necessary information for the control algorithm.
In the Intel Fuzzy ABS an embedded 87C196JT microcontroller is
used in conjunction with Inform Software Corporation fuzzyTECH(R) software.
Rules constitute the base of the algorithm and are evaluated in sequence, one
after the other. Upon completion of all rule processing the final system output
is calculated.
In
contrast, if a custom dedicated fuzzy parallel processor were to be used, rules
could be evaluated in parallel. The parallel processing method suggests a fast
processing cycle. However, in this case data acquisition and data output
continues using conventional peripherals. The time gained in parallel rule
processing can be lost in acquiring and manipulating data via external
peripherals.
The best solution continues to use a software fuzzy
algorithm on a microcontroller with fast internal peripherals. In this case,
sequential rule processing is transparent to the system and the process appears
to have been done in parallel. The MCS(R) 96 family of microcontrollers is
equipped with high performance internal peripherals that make data acquisition
and data conditioning of outputs fast and easy to handle. This and the wide
range of addressing modes, broad availability of interrupts and a powerful set
of instructions make Intel microcontrollers immanently suitable for fuzzy logic
applications.
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