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Anti-lock Braking System(ABS)
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A. ABS ELECTRONIC CONTROL UNIT /
HYDRAULIC CONTROL UNIT (ECU / HCU) :
This is an Electro-hydraulic device for reducing,
holding and restoring pressure to one or more
wheel brake, independent of the brake pedal effort
applied by the driver.
It consists of:
i.
An Electronic Control Unit (ECU) and
ii.
A Hydraulic Control Unit (HCU).
i.
ECU :
It is a solid state electronic device containing
computer functions, sensor signal processing
circuits, output device drives for the various ABS
valves and components and Failure Detection
Logic.
The ECU is integrated in a single package with
the Hydraulic Control Unit (HCU). It has 2 Micro
controllers, which utilize the same program for
independent data processing and monitoring
functions. One of the Micro controllers has an
EEPROM to retain memory data in case battery
supply is interrupted. The EEPROM is a non-
volatile data storage device employed to store
error codes, which can be read out for diagnosis.
When an error is encountered ABS switches itself
off and triggers the ABS warning lamp. An
independent module controls the solenoid valve
relay and return pump motor.
ABS ECU senses the rotational speed of the
wheels and calculates the vehicle speed based on
the signal received from the wheel speed sensor.
During braking, deceleration will vary on pedal
pressure, the vehicle speed during braking, and
the road surface conditions. For example,
deceleration rate will be much higher on dry
asphalt compared to wet or icy surface.
ECU judges the slip condition between the wheel
and road surface by monitoring the change in
wheel’s rotational speed during braking. The ECU
controls the ABS actuator to deliver the optimum
hydraulic pressure to the brake cylinder to
precisely control the speed of the wheels,
maintaining the maximum brake force.