Anti-Lock braking causes slipping which thusly cause lost footing

 

Anti-Lock 
Braking Systems (AB)  are designed
to maintain driver control and stability of the car during emergency braking.
Locked wheels will slow a car down but will not provide steering ability. ABS
allows maximum braking to be applied while retaining the ability to ‘steer out
of trouble’ The theory behind anti-lock brakes is simple. A skidding wheel
(where the tire contact patch is sliding relative to the road) has less
traction than a non-skidding wheel. By keeping the wheels from skidding while
you slow down, anti-lock brakes benefit you in two ways: You’ll stop faster,
and you’ll be able to steer while you stop. An ABS system monitors four wheel
speed sensors to evaluate wheel slippage. Slip can be determined by calculating
the ratio of wheel speed to vehicle speed, which is continuously calculated
from the four individual wheel speeds. During a braking event, the function of
the control system is to maintain maximum possible wheel grip on the road –
without the wheel locking – by adjusting the hydraulic fluid pressure to each
brake by way of electronically controlled solenoid valves.

 

                                                   

 

 

1.Introduction

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Auto producers
worldwide are competing with each other to imagine more dependable contraptions
there by coming nearer to the fantasy of the ‘Propelled wellbeing vehicle’ or
‘Extreme security vehicle’, on which innovative work has been continuing for as
far back as a few year. The greater part of the more up to date vehicle models
offer ABS as either standard or discretionary gear .Wheel lockup amid braking
causes slipping which thusly cause lost footing and vehicle control. This
diminishes the controlling capacity to alter course. So the auto slides crazy.
In any case, the street wheel that is as yet pivoting can be directed. That is
the thing that ABS is about. With such a framework, the driver can brake hard,
make the sly move and still be responsible for the vehicle in any street
condition at any speed and under any heap. ABS does not diminish halting
separation, but rather repays the changing footing or tire stacking by averting
wheel lockup. Amid freeze braking when the wheels are going to lockup, sensors
sense that the wheel has recently started turning slower than others on the
vehicle. So they quickly diminish braking power on the influenced wheel. This
anticipates sliding of the wheels on the asphalt. At the point when the wheel
resumes moving, full braking power is again connected. ABS rehashes the
procedure until there is never again any requirement for balanced braking. ABS
acts speedier than any driver could, pumping the brakes a few times each
second. Contingent upon the kind of framework, ABS changes the braking power at
each wheel or set of wheels, though a driver’s foot on the brake pedal works
every one of the brakes without a moment’s delay in typical braking.

                                              

2.CONCEPT OF
ABS

 

The theory behind anti-lock brakes
is simple. A skidding wheel (where the tire contact patch is sliding relative
to the road) has less traction than a nonskidding wheel. If the vehicle have
been stuck on ice and if the wheels are spinning then the vehicle have no
traction. This is because the contact patch is sliding relative to the ice. By
keeping the wheels from skidding while you slow down, anti-lock brakes benefit
you in two ways: You’ll stop faster, and you’ll be able to steer while you
stop. Good drivers have always pumped the brake pedal during panic stops to
avoid wheel lock up and the loss of steering control. ABS simply gets the
pumping job done much faster and in much precise manner than the fastest human
foot.

 

Fig2.1

 

 

 

 

 

 

 

 

 

3.PRINCIPLES
OF ABS

 

                                  Fig3.1

 

 The brakes of vehicle not furnished with ABS
will very quickly bolt the wheels, when the driver all of a sudden applies the
brake. For this situation the vehicle slides instead of moves to a stop. The
slipping and absence of control was caused by the locking of wheels. The
discharge and reapply of the brake pedal will stay away from the locking of the
wheels which thus evade the slipping. This is precisely what an electronically
monitored slowing mechanism does.

 3.1Pressure
modulation

When the brake pedal is
pumped or pulsed the pressure is quickly applied and released at the wheels.
This is called pressure modulation. Pressure modulation works to prevent the
wheel locking. ABS can modulate the pressure to the brake as often as 15 times
per seconds. By modulating the pressure to the brakes the friction between the
tires and the road is maintained and the vehicle is able to come to the
controllable stop.

Steering is another
important consideration. As long as a tire doesn’t slip it goes only in the
direction in which it is turned. But once it is skid it has little or no
directional stability. The Maneuverability of the vehicle is reduced if the
front wheels are locked and the stability of the vehicle is reduced if the rear
wheels are locked. ABS precisely controls the slip rate of the wheels to ensure
maximum grip force from the tyre and it there by ensures maneuverability and
stability of the vehicle.

 

   
4.ABS COMPONENTS

 Many different ABS are found on today’s
vehicles. These designs are varied by their basic layout,    operation and components. The ABS
components can be divided into two categories.

           1.Hydraulic components

         
2. Electrical/electronic components

           
Besides these normal and conventional brake parts are part of the
overall brake system.

  

4.1Hydraulic
components 

4.1.1Accumulator

· An accumulator
is used to store hydraulic fluid to maintain high pressure in the brake system
and provide the residual pressure for power assisted braking. Normally the
accumulator is charged with nitrogen gas and is an integral part of the
modulator unit. 

 

4.1.2Antilock
hydraulic control valve assembly

                                                                          

·
This assembly controls the release and
application of the brake system pressure to the wheel brake assemblies. It may
be of integral type and non integral type. In integral type the unit is
combined with the power boost and master cylinder unit into one assembly. The
non integral type is mounted externally from the master cylinder /power booster
unit and is located between the master cylinder and wheel brake assembly. Both
types generally contain solenoid valve that control the releasing, holding and
applying of brake system pressure.

 

 4.1.3 Booster pump

· The booster pump is an assembly of an electric motor
and pump. The booster pump is    used to
provide pressurized hydraulic fluid ABS. The pumps motor is controlled by
systems control unit.

 

4.1.4
Booster/Master cylinder assembly

· It is referred as the hydraulic unit, contains the
valves and pistons needed to modulate hydraulic pressure in the wheel circuit
during the ABS operations. 

 

 

4.1.5
Fluid accumulator

·.
Different than a pressure accumulator,
fluid accumulator temporarily store brake fluid, that is removed from the wheel
brake unit during ABS cycle. This fluid is then used by pump to build pressure
for the brake hydraulic system.

4.1.6 Hydraulic
control unit

·
This assembly contains solenoid valve,
fluid accumulator, pump and electric motor. The unit may have one pump and one
motor or it have one motor and two pumps. 

 

 

 

 

4.1.7
Main Valve

·
This is a two position valve and is also
controlled by ABS control module and is open only in the ABS mode. When open
pressurized brake fluid from the booster circuit is directed into the master
circuit to prevent excessive pedal travel.

 

4.1.8
Modulator unit

·
The modulator unit controls the flow of pressurized
brake fluid to the individual wheel circuits. Normally the modulator is made up
of solenoid that open and close valves, several valves that control flow of
fluid to wheel brake units and electrical relays that activate or deactivate
the solenoids through the commands of the control module. This unit may also be
called the hydraulic actuator, hydraulic power unit or the electro hydraulic
control valve.

 

4.1.9
Solenoid valves

·
The solenoid valves are located in the
modulator unit and are electrically operated by signals from the control
module. The control module switches the solenoids on or off to increase, decrease,
or maintain the hydraulic pressure to the individual wheel units.

    

 

4.1.10Wheel
circuit valves

· Two solenoid valves are used to control each circuit
or channel. One controls the inlet valve of the circuit, the controls the
outlet valve .the position is determined by the control module. Outlet valves
are normally closed and inlet valves are normally open. Valves are activated
when abs control module switches 12 volts to the circuit solenoids. During
normal driving the circuits are not activated.

 

 

5 Electrical
electronic components

 

 5.1 ABS control
module

·
This
small computer is normally mounted inside the trunk on the wheel housing,
mounted to the master cylinder or is part of the hydraulic control unit. It
monitors system operation and controls antilock function when needed. The
module relies on input from the wheel speed sensors and feedback from the
hydraulic unit to determine if the abs is operating correctly and to determine
when the anti lock mode is required.

 

5.2 Brake
pedal sensor

The antilock brake pedal
sensor switch is normally closed. When the brake pedal exceeds the antilock
brake pedal sensor switch setting during an antilock stop, the antilock brake
control module senses that the antilock brake pedal sensor switch is open and
grounds the pump motor relay coil.This energizes the relay and turns the pump
motor on. When the pump motor is running, the hydraulic reservoir is filled
with high pressure brake fluid and the brake pedal will be pushed up until
antilock brake pedal sensor switch closes. when the antilock brake pedal sensor
switch closes , the pump motor is turned off and the brake pedal will drop some
with each abs control cycle until the antilock brake pedal sensor switch opens
and the pump motor is turned on again .this minimizes pedal feedback during abs
cycling .

 

5.3 Pressure
differential switch

 It is located in the
modulator unit. This switch sends a signal to the control module whenever there
is an undesirable difference in the hydraulic pressures with in the brake
system.

 

 

      

5.4 Relays

 
Relays are
electromagnetic devices used to control a high current circuit with a low
current    switching circuit. In abs
relays are used to switch motors and solenoids. A low current signal from the
control module energizes the relays that complete the electrical circuit for
the motor or solenoid. 

 

5.5
Toothed ring

 It
can be located on an axle shaft, differential gear or a wheels hub. This ring
is used with conjunction with the wheel speed sensor. The ring has a number of
teeth around its circumference. As the ring rotates and each tooth passes by
the wheel speed sensor, an ac voltage signal is generated between the sensor
and tooth.

5.6 Wheel speed
sensor

 It
is mounted near the different toothed ring. As the rings teeth rotate past the
sensor an ac voltage is generated. as the teeth move away from the sensor, the
signal is broken until the next tooth comes close to the sensor .the end result
is a pulsing signal that is sent to the control module. The control module
translates the signal in to wheel speed. The sensor is normally a small coil of
wire with a permanent magnet in its center.

 

 

6.
TYPES OF ANTILOCK BRAKE SYSTEMS

One
of the classifications of abs is integral and non integral type. Integral type
they combine the master cylinder, hydraulic booster and abs hydraulic circuit
in to single hydraulic assembly. In non integral type they use a conventional
vacuum-assist booster and master cylinder. In addition they can be classified
according to the control they provide.

 

 

6.1
Four channel, four sensors ABS

This
is the best scheme. There is speed sensor on all four wheels and a separate
valve for all the four wheels. With this set up the controller monitors each
wheel individually to make sure it is achieving maximum braking force.

 

6.2Three
channel, three sensor ABS

This
scheme is commonly found on pick up trucks with four wheels 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 wheel is located in the
rear axle. This system provides individual control of the 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.

 

6.3   One
channel, one sensor abs

This
scheme is commonly found on pick up 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 the rear channel
system. The rear wheels are monitored together and both have to start to lock
up before the abs kicks in. in this system is also possible that one of the
rear wheels will lock reducing brake effectiveness.

 

7.
ADVANCEMENTS IN ABS

 
Some systems, which work with the
ABS, are Automatic traction control and Automatic stability control, which are
discussed below.

 

7.1AUTOMATIC
TRACTION CONTROL (ATC)

Programmed footing control frameworks(ATC) apply the
brakes when a drive wheel endeavors to turn and lose footing. The framework
works best when one drive wheel is taking a shot at a decent footing surface
and the other isn’t. The framework likewise functions admirably when the
vehicle is quickening on tricky street surfaces, particularly when climbing
slopes. ATC is most useful on four wheel or all wheel drive vehicles in which
loss of footing at one wheel could hamper driver control. Amid street operation
the ATC framework utilizes an electronic control module to screen the wheel
speed sensors. On the off chance that a wheel enters lost footing circumstance,
the module applies braking power to the wheel stuck in an unfortunate
situation. Loss of footing is distinguished by contrasting the vehicle speed
with the speed of the wheel. In the event that there is lost footing the speed
of the wheel will be more noteworthy than anticipated for the specific vehicle
speed. ABS and ATC frameworks can be necessary and utilizations the basic
valves. These frameworks are intended to diminish wheel slip and keep up
footing at the drive wheels when the street is wet or snow secured. The control
module screens wheel speed. On the off chance that amid increasing speed the
module distinguishes drive wheel slip and if brakes are not connected, the
control module goes into the footing control mode. The delta and outlet
solenoid valves are beat and enable the brake to be immediately connected and discharged

In
a few systems when lost footing is detected, it cycles the brakes as well as
signs the motor control module to impede start timing and somewhat shut the
throttle also, which thus lessens motor yield. Numerous frameworks are
furnished with a dash mounted cautioning light to alarm the driver that the
framework is working. There will likewise be a manual cut off switch so the
driver can kill ATC operation.

 

 

 

 

7.2 AUTOMATIC
STABILITY CONTROL

 

Like
ATC, the solidness control frameworks are connected with the ABS. it can
likewise be called Electronic Stability Program (ESP). Soundness control
frameworks quickly apply the brakes at any one wheel to remedy over cow or
under cow. The control unit gets signals from the run of the mill sensors in
addition to a yaw, horizontal speeding up (G-constrain) and a guiding edge
sensor. The framework utilizes the point of the guiding haggle speed of the
four wheels to compute the way picked by the driver. It at that point takes a
gander at sidelong G-powers and vehicle yaw to gauge where the vehicle is
going. (Yaw is characterized as the characteristic propensity for a vehicle to
turn on its vertical focus pivot). So it is likewise called Yaw control..

                                Fig7.1

                                            
fig7.2

 

 

Under
steer is the condition in which the vehicle is slow to respond to steering
changes. Over steer occurs when the rear wheels try to swing around causing the
car to spin. When the system senses under steer in a turn the brake at the
inside rear wheel is applied. During over steer the outside front brake is
applied. Relaying on the input from the sensors and computer programming the
system calculates if the vehicle is going exactly in the same direction in
which it is being steered. In case of any difference between what the driver is
asking and what the vehicle is doing, the system corrects the situation by
applying one of the right or left brakes.

 

 

 

 

8.ADVANTAGES
OF ABS

·        
It allows the
driver to maintain directional stability and control over steering during
braking 

·        
Safe and
effective

·        
Automatically
changes the brake fluid pressure at each wheel to maintain optimum brake
performance. 

·        
ABS absorbs the
unwanted turbulence shock waves and modulates the pulses thus permitting the
wheel to continue turning under maximum braking pressure.

 9.DISADVANTAGES

·        
It is very
costly 

·       
Maintenance cost
of a car equipped with ABS is more.

 

10.CONCLUSION

ABS
has been so far created to a framework, which gives fast, programmed braking in
light of indications of nascent wheel bolting by on the other hand expanding
and diminishing water powered weight in the brake line Statistics demonstrate
that around 40 % of car crashes are because of slipping. These issues usually
happen on vehicle with traditional stopping mechanism which can be evaded by
including gadgets called ABS If there is an ABS disappointment, the framework
will return to ordinary brake operation. Regularly the ABS cautioning light
will turn on and let the driver know there is a blame.