Hydraulic Control Unit

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

Valve Block Assembly:  The valve block assembly contains 6 valves with three inlet valves and three outlet 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 channel. The CAB monitors wheel speeds. If the CAB detects a wheel deceleration that is disproportionate to the other wheels, it will close the inlet valve to that wheel. This prevents any increase in fluid pressure. If the wheel continues to decelerate disproportionately, the CAB opens the outlet valve for that wheel to release fluid pressure from that channel. The released fluid is routed to the accumulators. When the wheel speed is no longer disproportionate to the other wheels, the inlet valve will return to its normally open position and the outlet valve will return to the normally closed position.

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 an 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 turns on the motor when an anti-lock stop is detected. The pump continues to run during the anti-lock stop and is turned off approximately 3-5 seconds after the stop is complete. The CAB monitors the pump/motor operation internally.

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