Hydraulic Control Unit

The hydraulic control unit (HCU) contains the valve block assembly, two accumulators, and pump motor assembly.

Valve Block Assembly:  The valve block assembly contains a total of 12 valves: 10 solenoid valves with four inlet valves and four outlet valves, and 2 traction control valves and 2 hydraulic shuttle valves. The inlet valves are spring-loaded in the open position and the outlet valves are spring loaded in the closed position. During an anti-lock stop, these valves are cycled to maintain the proper slip ratio for each wheel. If the CAB detects slip, the inlet valve is closed to prevent any further pressure increase. If necessary, the outlet valve is opened to release the pressure to the accumulators until the wheel is no longer slipping. Once the wheel is no longer slipping, the outlet valve is closed and the inlet valve is opened to reapply pressure. If the wheel is decelerating within its predetermined limits (proper slip ratio), both valves will close to hold the pressure constant. The valves are also cycled during a traction control event.

Pump Motor Assembly:  The pump motor assembly provides the extra amount of fluid needed during anti-lock braking. The pump is supplied fluid that is released to the accumulators when the outlet valve is opened during and anti-lock stop. The pump is also used to drain the accumulator circuits after the anti-lock stop is complete. The pump is operated by an integral electric motor. This motor is controlled by the CAB. The CAB will turn on the pump motor when an anti-lock stop is detected. The pump continues to run during the anti-lock stop and is turned off about 3-5 seconds after the stop is complete. The pump mechanism consists of 2 opposing pistons operated by an eccentric cam. One piston supplies the primary hydraulic circuit. The opposing piston supplies secondary hydraulic circuit. In operation, one piston draws fluid from the accumulators. The opposing piston pumps fluid to the valve body solenoids. The CAB monitors the pump motor operation internally.

Accumulators:  The 3.0 mm accumulators provide temporary fluid storage during an anti-lock stop and are drained by the pump/motor.