Hydraulic Control Unit
HYDRAULIC CONTROL UNIT
The hydraulic control unit is a fluid pressure control assembly which consists of an electric motor, plunger pump, plunger piston, housing, solenoid valve and a relay. The solenoid valve moves to three positions, as shown in figure. When it is moved to position "1" (current "OFF"), the master cylinder and brake system are connected to each other. With the valve moved to position "2" (part of current flow "ON"), all passages are closed. At position "3" (full-current flow "ON"), master cylinder ports are closed and the line between the hydraulic unit reservoir is connected to the brake system
As wheel deceleration exceeds the specified deceleration setting during braking, the solenoid valve moves to position "2" to hold brake fluid pressure. Further deceleration increases moves the solenoid valve to position "3" 50 that brake fluid pressure is released and, at the same time, the motor starts to drive the plunger pump. When wheel deceleration stops, the solenoid valve is held at position "2" to maintain brake fluid pressure.
1) During normal braking (Electric current is OFF)
When the brake pedal is depressed, fluid pressure is generated in the master cylinder and is routed to each wheel cylinder [FR (Front right), FL (Front left), and RR (Rear right)] passing through each solenoid valve [FR, FL, and R (Rear)], causing the brake to work. Pressurized fluid passes through the port of plunger piston, activating the RL (Rear left) wheel cylinder
Releasing the brake pedal allows fluid pressure in the reverse direction.
2) Pressure "decrease" action with ABS in operation (Electric current is fully ON)
If one wheel shows signs of locking, solenoid valves are controlled under the state of decompression (full electric current) by the ABS control module, and the inlet port of each solenoid valve is closed while the outlet port is opened. Brake fluid from the FL, FR, and RR wheel cylinders is released through the outlet port to the reservoir, which decreases the braking force. The unequal fluid pressure in the pressure chamber at the right side of the plunger piston results in the reduction of the fluid pressure in the RR wheel cylinder by the R (Rear) solenoid valve, which energizes the plunger piston to move to the left. Upon closing the pressure chamber port on the left side, the side chamber of the RL wheel cylinder is decompressed.
When the solenoid valve is controlled at full electric current, the ABS control module simultaneously sends an electric current to the motor relay thus driving the motor. Brake fluid in the reservoir is pumped to the damper chamber of the plunger piston by the plunger pump where it dampens pulsating pressure and then returns to the master cylinder (at this time, the brake pedal feels as if it is pressed back).
3) Pressure "hold" action with ABS in operation (Halt electric current)
When the fluid pressure in the wheel cylinders is optimally reduced (or increased), the solenoid valve is maintained in a "holding state" (half electric current) in which both inlet and outlet port are closed. Fluid pressure in the FL, FR, and RR wheel cylinder sides from the solenoid valve is held in this controlled state. Thus, the fluid pressure generated in the wheel cylinder side from the solenoid valve corresponds to a force depressing the brake pedal.
Since fluid pressure in the RL wheel cylinder is controlled by the plunger piston, it is kept equal to the pressure in the RR wheel cylinder.
4) Pressure "increase" action with ABS in operation (Electric current is OFF)
As soon as wheel lock is avoided, electric current in the solenoid valve is cut off as instructed by the electronic controller.
Thus, fluid pressure becomes constant from the master cylinder through the FL, FR, and RR wheel cylinders.
The fluid pressure in the master cylinder generated by depressing the brake pedal can pass directly to the FL, FR, and RR wheel cylinders and reenergize the braking force.
When an electric current in the R solenoid valve cuts oft, the fluid entering the plunger piston pressure chamber on the right side is compressed. Then, from an equalized position, the plunger piston moves to the right side reducing the capacity of the pressure chamber on the left side (compressed). Thus, the pressure in the RL wheel cylinder increases. Further movement of the plunger piston opens the plunger piston port, and the fluid pressure of the master cylinder is directly applied (at this time, the brake pedal feels as if it is being pulled in).