10-07-2012, 11:08 AM
ANTI-LOCK BRAKE SYSTEM
[attachment=27318]
ABS Concept
The theory behind ABS is simple.
A skidding wheel has less traction than a non-skidding wheel.
Traction: It is the ability of the rear wheel to transmit the tractive effort without slipping.
Tractive Effort: A force provided by power unit & is mainly required to over come the tractive resistance.
By keeping the wheels from skidding while you slow down, anti-lock brakes benefits in two ways
Able to stop faster
Able to steer while stopping
ELECTRONIC control unit
Electronic control unit
Receives and interprets voltage pulses generated by the sensor pickup
It uses this information to determine:
Impending wheel lock up
When/how to activate the ABS modulator valve
ECU uses voltage pulses from each wheel speed sensor to determine wheel speed changes
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.
The fluid pressure that is generated by the pump is stored in the "accumulator.“
Wheel Speed Sensors
These are located at each wheel ,or in some cases in the differential.
These are used to know when a wheel is about to lock up
the wheel speed sensors has two main components:
the exciter
the pick up
1. Exciter:
It is a ring with notched teeth
Commonly the ring has 100 teeths
2. Pick up:
Commonly called “sensor”
It consists wire magnet assembly which emits pulses as exciter teeth passes through it
The strength of these pulses decreases with slight increase in gap between pick up and exciter
Working of sensor
Ac voltage is created when teeth moves through the magnetic field surrounding the sensor
Sensor is a switch with voltage supplied to it
A DC square wave is created as switch is turned on and off
Frequency increases with wheel speed
Modulator valves
An electro-pneumatic control valve that contains the solenoids
used to precisely modulate brake air pressure during an ABS event
ECU commands the actions of modulator valves
Allows free flow of air when no command is given
Modulator valve can do 2 functions:
Change the air pressure from brake chamber
Hold the existing pressure
Types of Anti-lock Brakes
According to the number of channels and the number of speed sensors:
Four-channel, four-sensor ABS:
There is a speed sensor on all four wheels and a separate valve for all four wheels.
The controller monitors each wheel individually to make sure it is achieving maximum braking force.
This is best scheme.
Three -channel, three -sensor ABS:
It has a speed sensor and the valve for each of the front wheels
One valve and one sensor for both rear wheels
The speed sensor for the rear wheels is located in the rear axle
This scheme commonly found on pick up trucks with four-wheel ABS
One -channel, one -sensor ABS:
It has one valve, which controls both rear wheels, and one speed sensor, located in the rear axle
This system is commonly found on pick up trucks with rear wheel ABS
In this system it is also possible that one of the rear wheels will lock, reducing brake effectiveness
ABS at Work
There are many different variations and control algorithms for ABS system.
The controller monitors the speed sensors at all times.
Its look for deceleration in the wheel.
Then it increases the pressure until it sees the deceleration.
This gives the system maximum breaking power.
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.
Disadvantages of ABS
It is very costly
Maintenance cost of a car equipped with ABS is more
The anti-lock brakes are more sensitive on the damper condition
[attachment=27318]
ABS Concept
The theory behind ABS is simple.
A skidding wheel has less traction than a non-skidding wheel.
Traction: It is the ability of the rear wheel to transmit the tractive effort without slipping.
Tractive Effort: A force provided by power unit & is mainly required to over come the tractive resistance.
By keeping the wheels from skidding while you slow down, anti-lock brakes benefits in two ways
Able to stop faster
Able to steer while stopping
ELECTRONIC control unit
Electronic control unit
Receives and interprets voltage pulses generated by the sensor pickup
It uses this information to determine:
Impending wheel lock up
When/how to activate the ABS modulator valve
ECU uses voltage pulses from each wheel speed sensor to determine wheel speed changes
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.
The fluid pressure that is generated by the pump is stored in the "accumulator.“
Wheel Speed Sensors
These are located at each wheel ,or in some cases in the differential.
These are used to know when a wheel is about to lock up
the wheel speed sensors has two main components:
the exciter
the pick up
1. Exciter:
It is a ring with notched teeth
Commonly the ring has 100 teeths
2. Pick up:
Commonly called “sensor”
It consists wire magnet assembly which emits pulses as exciter teeth passes through it
The strength of these pulses decreases with slight increase in gap between pick up and exciter
Working of sensor
Ac voltage is created when teeth moves through the magnetic field surrounding the sensor
Sensor is a switch with voltage supplied to it
A DC square wave is created as switch is turned on and off
Frequency increases with wheel speed
Modulator valves
An electro-pneumatic control valve that contains the solenoids
used to precisely modulate brake air pressure during an ABS event
ECU commands the actions of modulator valves
Allows free flow of air when no command is given
Modulator valve can do 2 functions:
Change the air pressure from brake chamber
Hold the existing pressure
Types of Anti-lock Brakes
According to the number of channels and the number of speed sensors:
Four-channel, four-sensor ABS:
There is a speed sensor on all four wheels and a separate valve for all four wheels.
The controller monitors each wheel individually to make sure it is achieving maximum braking force.
This is best scheme.
Three -channel, three -sensor ABS:
It has a speed sensor and the valve for each of the front wheels
One valve and one sensor for both rear wheels
The speed sensor for the rear wheels is located in the rear axle
This scheme commonly found on pick up trucks with four-wheel ABS
One -channel, one -sensor ABS:
It has one valve, which controls both rear wheels, and one speed sensor, located in the rear axle
This system is commonly found on pick up trucks with rear wheel ABS
In this system it is also possible that one of the rear wheels will lock, reducing brake effectiveness
ABS at Work
There are many different variations and control algorithms for ABS system.
The controller monitors the speed sensors at all times.
Its look for deceleration in the wheel.
Then it increases the pressure until it sees the deceleration.
This gives the system maximum breaking power.
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.
Disadvantages of ABS
It is very costly
Maintenance cost of a car equipped with ABS is more
The anti-lock brakes are more sensitive on the damper condition