ANTI LOCK BREAKING SYSTEM (ABS)
ANTI LOCK BREAKING SYSTEM (ABS)
It is a safety system in automobiles. It prevents the wheels
from locking while braking. The purpose of this is to allow the driver to
maintain steering control under heavy braking and, in some situations, to
shorten braking distances (by allowing the driver to hit the brake fully
without skidding or loss of control).
How Do Wheels Lock?
During braking, wheels lock if the brake force applied is more
than the friction between the road and tyre. This often happens in a panic
braking situation, especially on a slippery road. When the front wheels lock,
the vehicle slides in direction of motion. When the rear wheels locks, the
vehicle swings around. It is impossible to steer around an obstacle with wheels
locked. Locked wheels can thus result in accident. Skidding also reduce tyre
life.
What Does ABS Do?
The system detects when the wheel are about to lock and
momentarily release the pressure on locking wheel. The brakes are reapplied as
soon as the wheels haverecovered
.
A toothed wheel (pole wheel) is fitted to the rotating wheel
hub. A magnetic sensor mounted on each wheel in close in close proximity to the
teeth, generates electrical pulses when the pole wheel rotates. The rate at
which the pulses are generated (frequency) is a measure of wheel speed. This
signal is read by electronic control unit (ECU). When a wheel is lock, the ECU
sends an electrical signal to the modulator valve solenoid, which release
pressure from the brake chamber. When the wheel recovers sufficiently, the
brake pressure is reapplied again by the switch off signal to the modulator
valve.
The modulator valve has an addition ‘hold’ state which maintains
pressure. In break in the chamber, thus optimizing the braking process. The
cycling of modulator valve (5 to 6 times per second) is continued till the
vehicle comes to a controlled stop.With ABS, the vehicle remains completely stable even when the
driver continues to press the brake pedal during braking, thus avoiding
accidents.
ANTI LOCKING BREAK SYSTEM
Components:
Wheel Speed Sensor
The wheel speed sensor consists of a permanent magnet and coil
assembly. It generates electrical pulses when the pole wheel rotates. The rate
at which the pulses are generated is a measure of wheel speed. The voltage
induced increases with the speed of rotation of the wheel and reduces with
increasing gap between the pole wheel and the sensor.
Pole Wheel
Pole Wheel is a toothed wheel made of ferrous material. It
normally has teeth on the face. In some cases where it is not possible to
install the sensor parallel to the axle, the pole wheels are designed with
teeth on periphery. The pole wheel fitted on standard 9-20, 10-20 tires has
normally 100 evenly spaced teeth. 80 evenly spaced teeth pole wheels are used
for the vehicles having the tyre diameter less than 9mm.
Sensor Extension Cable
The sensor extension cable is a two core cable which connects
the wheel speed sensor to the Electronic Control Unit. The inner core sheathing
is of EPDM rubber and the outer sheathing is polyurethane which provide
abrasion resistance to the cable. The cable has a module plug with two pins is
connected to the control assembly. The cable has two cores-brown and black in
colour.
Electronic Control Unit
The ECU is the core component of the ABS system. Wheel speed
sensor signal are the input to the Electronic Control Unit. The ECU computers
wheel speeds, wheel deceleration and acceleration. If any wheel tends to lock,
the ECU actuates the corresponding Modulator valve to prevent wheel lock. The
ECU is normally mounted in driver's cabin.
Modulator Valve Cable
The Modulator valve cable has thee cores. There are two solenoid
interface lines and a common ground line. The inner core sheathing is of EPDM
type and the outer sheathing is polyurethane which provide abrasion resistance
to the cable.
The cable has a three pin moulded socket is connected to the
modulator valve solenoid at one and an interlock connector with locking feature
at the other end. The cores are brown, blue and green.
Modulator Valve
ABS Modulator valve regulate the air pressure to the brake chamber
during ABS action. During normal braking it allows air to flow directly from
inlet to delivery. Modulator valve cannot automatically apply the brakes, or
increase the brake application pressure above the level applied by the driver
through the dual brake valve
.
Quick Release Valve
Quick release valve are fitted in air braking system to release
the air from the brake chamber quickly after release of brake pedal. This
prevents delay in brake release due to long piping runs or multiples of brake
chamber being exhausted through the brake valve.
Relay Valve
Relay valve provides a means of admitting and releasing air to
and from brake chamber quickly, in accordance with the signal pressure from the
delivery of the dual brake valve. Air from the reservoir passes through the
valve into the brake chamber. The pressure applied to the brake is equal to the
signal pressure from the dual brake valve. When the brake pedal is released the
signal pressure is released. The pressure in the brake chamber is released directly
through the exhaust port of the relay valve.
Warning Lamp
Vehicle are fitted with an ABS warning lamp. It is a LED indicator lamp amber in colour and lights up when the system has detected any electrical fault. ABS warning lamp is located on the instrument panel in form of a driver.
Blink Code Lamp
This lamp is green in colour and is used to indicate the stored
faults in the system to the service technician on operating a blink code
switch. The nature of fault in the system can be diagnosed by the number of
flashes.
Off Highway Switch
This is an optional switch in front of the driver which can be
switched ON when the vehicle is operating off highway. In this mode, ABS
control will; allow higher wheel slip to achieve shorter stopping distance than
with normal ABS control.
Blink Code Switch
A momentary switch that grounds the ABS Indicator Lamp output is
used to place the ECU into the diagnostic blink code mode and is typically
located on the vehicle's dash panel.
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