HEXA

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