Anti-lock Braking System (ABS)
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. It is an
automated system that uses the principles of threshold braking and
cadence braking.
How ABS Works
ABS
includes a central electronic control unit (ECU), four wheel speed
sensors, and at least two hydraulic valves within the brake hydraulics.
The ECU constantly monitors the rotational speed of each wheel with the
help of wheel speed sensor; if ABS system detects that one wheel is
slowing down more rapidly than the others, a condition indicative of
impending wheel lock, it automatically reduces the brake pressure on
this wheel by opening release valve in hydraulic cylinder , 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
16 times per second.
ABS Braking Components
There are three main components of ABS:
1. Speed Sensors
2. Hydraulic Modulator
3. Controller
Speed sensors:
The
speed sensors, which are located at each wheel, or in some cases in the
differential, provide this information of wheel speed.
Hydraulic Modulator:
Hydraulic modulator has two main component :
Valves:
There is a valve in the brake line of each brake controlled by the ABS. On some systems, the valve has three positions:
In position one, the valve is open; pressure from the master cylinder is passed right through to the brake.
In
position two, the valve blocks the line, isolating that brake from the
master cylinder. This prevents the pressure from rising further should
the driver push the brake pedal harder.
In position three, the valve releases some of the pressure from the brake.
Pump:
When the ABS system operates the brake lines lose pressure. The pump re-pressurizes the system.
Controller:
The
controller is an ECU (Electronic control unit) type unit in the car
which receives information from each individual wheel speed sensor, in
turn if a wheel loses traction the signal is sent to the controller, the
controller will then limit the brake force (EBD) and activate the ABS
modulator which actuates the braking valves on and off.
