HEXA

Anti-lock Braking System(ABS)

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

Master cylinder

-Wheel cylinder

OUTLET

OFF

Wheel cylinder

-reservoir