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Anti-lock Braking System(ABS)
7
ii.
BRAKING WITH EBD INTO OPERATION :
In case of a braking where the ABS ECU detects
a slip difference between the front and rear
wheels, EBD comes into operation. If the
difference in wheel slip between the front and rear
wheels exceeds a predetermined threshold the
ABS activates the pressure inlet valves for the
rear wheels, closing the valves and preventing
further pressure build up at that rear wheels.
If during the braking event, this slip ratio between
the front and rear axle reduces below the
threshold level the inlet valves will be de-activated
and switch back to their normally open state
allowing further pressure increase at the rear axle.
In this way the rear axle can follow more closely to
the Ideal braking force distribution curve for the
rear axle (See Figure a).
If the driver reduces the brake pedal effort during
EBD control, brake fluid can return to the TMC
through the one way valve located within the
modulator in parallel to the Inlet valve.
If the driver releases the brake, the inlet valves
will be deactivated and fluid can return to the TMC
via the inlet valve and the one way valve.
EBD control only acts upon the rear axle and can
only limit pressure being applied to a wheel, it
cannot perform pressure reductions.
Figure -: Pressure increase on rear axle
following ideal brake force distribution curve.
iii.
BRAKING WITH ABS IN OPERATION :
In case of braking where the ABS ECU detects a
significant difference in the wheel speed
information from an individual wheel. The
electronic control unit instructs the hydraulic
modulator to vary the brake pressure being
applied at the affected wheel(s). There are three
stages to this control.
1. Pressure Maintaining (Hold) Phase :
When the braking forces being applied are higher
than the forces that can be transmitted to the road
surface for a specific wheel, the ECU will detect
that the wheel is tending towards lock (
Slip
relative to other wheels and a calculated vehicle
reference speed)
. It will then activates the relevant
inlet valve within the hydraulic modulator,
switching it from its normally open position to
closed, hence preventing further pressure being
transmitted from the TMC to the affected wheel
brake. As the outlet valve is closed in this phase
the pressure at the wheel is maintained.
If the driver reduces the brake pedal effort during
ABS control, brake fluid can return to the TMC
through the one way valve located within the
modulator in parallel to the Inlet valve.
Solenoid
valve
Electricity
status
Valve
open-close
Open-close
channel
INLET
ON
CLOSE
Master cylinder
-Wheel cylinder
OUTLET
OFF
CLOSE
Wheel cylinder
-reservoir