Hydraulic Control Assembly - Antilock Brakes: Description and Operation

MODULATOR COMPONENTS
Modulators and solenoid valves for each wheel are integrated in the modulator unit.
- The modulators for front and rear brakes are of independent construction.
- The solenoid valve features quick response (5 ms or less).
- The inlet and outlet valves are integrated in the solenoid valve unit.

Fluid Flow During Base Braking:

MODES OF OPERATION
Base Braking Mode
In ordinary brake operations, the cut-off out valve in the modulator is open to transmit the hydraulic pressure from the master cylinder to the brake calipers via the chamber A and the chamber B. The chamber C is connected to the reservoir through the outlet valve which is normally open. It is also connected to the hydraulic pressure source (pump, accumulator, pressure switch, etc.) via the inlet valve which is normally closed. The chamber D serves as an air chamber. Under these conditions, the pressures of the chambers C and D are maintained at about the atmospheric pressure, permitting regular braking operations.

Switching Modes
If brake inputs (force exerted on brake pedal) are excessively large and a possibility of wheel locking occurs, the control unit operates the solenoid valve, closing the outlet valve and opening the inlet valve. As a result, the high pressure is directed into chamber C, the piston is pushed upward, causing the slide piston to move upward and the cut-off valve to close.

Anti-Lock Braking Mode
As the cut-off valve closes, the flow from the master cylinder to the caliper is interrupted, the volume of chamber B, which is connected to the caliper, increases, and the fluid pressure in the caliper declines. When both of the two valves, inlet and outlet, are closed (when only the outlet valve is activated) the pressure in the caliper is maintained constant.

Return To Base Braking Mode
When the possibility of wheel locking ceases, it is necessary to restore the pressure in the caliper. The solenoid valve is therefore turned off (outlet valve open, inlet valve closed).

Fluid Flow During ABS Assisted Braking:

SLIDE PISTON FUNCTION
When the vehicle is used on rough roads where the tires sometimes lose adhesion, the anti-lock brake system may function excessively, causing a large volume of brake fluid to flow into the chamber C. As this occurs, the piston is moved excessively, resulting in an abnormal loss of pressure in the chamber B. In order to overcome this problem, the slide piston is kept in the proper position by the spring force to avoid a negative pressure in the chamber B.

KICKBACK
When anti-lock brake system is functioning, the piston moves upward, the volume of chamber B increases, and the fluid pressure on the caliper side is reduced. At the same time, the volume of chamber A is reduced and the brake fluid is returned to the master cylinder. When the brake fluid is pushed back to the master cylinder, the driver can feel the functioning of the anti-lock brake system because the brake pedal is kicked back.