Anti-lock Braking System
One of the most unnerving things that can happen in a vehicle while
braking is that you apply brakes and one or more of the wheels locks up. This
has two possible effects. It can make the car slew to one side or, if the car
happens to skid in a straight line, the steering becomes useless and you lose
all directional control. This causes longer stopping distance in some cases.
This problem can be solved with the use of Anti-Lock Braking System.
INTRODUCTION
An anti-lock braking system or anti-skid
braking system (ABS) is an automobile safety system that
allows the wheels on a motor vehicle to
maintain tractive contact with the road surface
according to driver inputs while braking, preventing the
wheels from locking up (ceasing rotation) and avoiding uncontrolled skidding.
ABS DESIGN
MAJOR
COMPONENTS
1. Speed Sensor: It monitors the speed of each
wheel and determines the necessary acceleration and deceleration of the
wheels. It consists of an exciter (a ring with V-shaped teeth) and a
wire coil/magnet assembly, which generates the pulses of electricity as the
teeth of the exciter pass in front of it.
2. Valves: The valves regulate the air pressure to the
brakes during the ABS action. There is a valve in the brake line of each brake
that is controlled by the ABS. In the first position, the brake valve is
open and it allows the pressure from the master cylinder to be transferred to
the brakes. In the second position, the brake valve remains closed and
pressure from the master cylinder to the brakes is constrained.
In the
third position, the valve releases some of the pressure on the brakes. The
third step is repeated until the car comes to a halt. The resistance that you
feel when braking suddenly at high speeds is actually the brake valves
controlling the pressure that is being transferred to the brakes from the
master cylinder.
3. Electronic Control Unit (ECU): The ECU is an electronic
control unit that receives, amplifies and filters the sensor signals for
calculating the wheel rotational speed and acceleration. The ECU receives
a signal from the sensors in the circuit and controls the brake pressure,
according to the data that is analyzed by the unit.
4. Hydraulic Control Unit: It receives signals from the ECU to apply or release the brakes
under the anti-lock conditions. The Hydraulic Control Unit controls the brakes
by increasing the hydraulic pressure or bypassing the pedal force to reduce the
braking power.
OPERATION
Conventional
The
ECU constantly monitors the rotational speed of each wheel; if
it detects a wheel rotating significantly slower than the others, a condition
indicative of impending wheel lock, it actuates the valves to reduce hydraulic
pressure to the brake at the affected wheel, thus reducing the braking force on
that wheel; the wheel then turns faster. Conversely, if the ECU detects a wheel
turning significantly faster than the others, brake hydraulic pressure to the
wheel is increased so the braking force is reapplied, slowing down the wheel.
This process is repeated continuously and can be detected by the driver via
brake pedal pulsation.
Some anti-lock systems can apply or release
braking pressure 15 times per second. Because of this, the wheels of cars
equipped with ABS are practically impossible to lock even during panic braking
in extreme conditions.
The ECU is programmed to
disregard differences in wheel rotative speed below a critical threshold,
because when the car is turning, the two wheels towards the centre of the curve
turn slower than the outer two. For this same reason, a differential is
used in virtually all road going vehicles.
If a fault develops in
any part of the ABS, a warning light will usually be illuminated on the vehicle
instrument panel, and the ABS will be disabled until the fault is rectified.
Modern
ABS applies individual
brake pressure to all four wheels through a control system of hub-mounted
sensors and a dedicated micro controller. ABS
is offered or comes standard on most road vehicles produced today and is the
foundation for electronic stability control systems, which are rapidly
increasing in popularity due to the vast reduction in price of vehicle electronics
over the years.
Electronic stability
control 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.
Advantages of
Anti-lock Braking System (ABS)
§ Anti-lock braking system (ABS) guarantees stable braking characteristics
on all road surfaces, hence avoids overturning of the vehicle.
§ ABS reduces friction on wheels and road, thus increases
the efficiency of tires (up to 30%).
§ The Vehicle with ABS can be stopped at a lesser distance than a non ABS
vehicle.
§ Steering control is effective, i.e., the vehicle can be steered smoothly
while braking. Thus minimizes the accidents.
§ A driver without experience can drive ABS vehicle effectively, then an
experienced driver on the non ABS vehicle.
Disadvantages of Anti-lock Braking System (ABS):
§ Initial cost for Anti-lock braking system (ABS) vehicle is high.
§ Maintenance issues arise as the whole braking system is controlled by
engine control unit.
§ On concrete roads, the ABS vehicle stopping distance might be
needed more.
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