Anti-lock Braking System(ABS) 8 2. Pressure Reduction Phase: If the ECU detects that the % Slip relative to the other wheels and vehicle reference speed is still increasing during the pressure hold phase and the wheel is tending further towards lock, it will start the pressure reduction phase, as the existing brake pressure in the wheel brake is too high. It will activate the relevant outlet valve within the hydraulic modulator, switching it from its normally closed position to open. This will allow fluid to pass from the wheel brake into the accumulator chamber within the modulator. At the same time the ECU will activate the return pump integrated into the modulator to draw fluid from the accumulator and wheel brake and return it to the TMC. This fluid being returned to the TMC is what is felt as the pulsations at the pedal, and the amplitude of the pulsations is relative to the amount of fluid that needs to be reduced. Pressure reduction will continue until the controlled wheel speed returns to the vehicle reference speed. Solenoid valve Electricity status Valve openclose Open-close channel INLET ON CLOSE Master cylinder -Wheel cylinder OUTLET ON OPEN Wheel cylinder - reservoir 3. Pressure increase Phase: As the ECU detects that the speed of the controlled wheel has increased to the vehicle reference speed and hence the % slip reduced. It will sequentially deactivate the outlet and inlet valves switching them to their normal positions. With the outlet valve closed further pressure reduction is prevented and the inlet valve can be opened to allow brake fluid from the TMC to enter the wheel brake increasing the brake pressure once again. If the driver releases the brake at any time during ABS control the inlet valves will be deactivated and fluid can return to the TMC via the inlet valve and the one way valve. Solenoid valve Electricity status Valve openclose Open-close channel INLET OFF OPEN Master cylinder - Wheel cylinder OUTLET OFF CLOSE Wheel cylinder - reservoir iv. FAIL-SAFE FUNCTION : If the ABS system detects any fault that may adversely affect its performance it will switch to a fail safe shut down mode. When this happens the electricity supply to the solenoid valves is deactivated, stopping control signal output, this causes the valves to switch to their default state. (Open for inlet valves / Closed for Outlet valves) The System will light the relevant warning lamp at the same time to inform the driver the vehicle has either an ABS or an ABS and EBD failure. In this condition, the conventional hydraulic brake circuit will still function, allowing brake pressure to reach all wheels. It should be noted that with an ABS fault it will be possible to lock the wheels and in the Case of an EBD failure the brake force proportioning between front and rear will no longer function and hence rear wheel lock is possible before front.
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