2.5.4 Buoyancy Control Unit (BCU)

The purpose of the BCU is the control of the buoyancy compensation system. It is responsible for the control of the 5 RBCs and the VBC. It receives buoyancy compensator position commands from the main processor, and then using a closed loop control system, moves or maintains the position of each BC to match the specified position. The actual BC positions are then returned to the main processor.

Figure 2-21 The Buoyancy Control Unit

The BCU receives buoyancy compensator position commands from the main processor via a RS422 serial interface. The 6811 LPU then transfers the appropriate position commands to the LM629 motion control chip associated with each BC. The LM629s send PWM drive signals to small H-bridge amplifiers located on the LM629 card which in turn drive the BC's. There is one exception in that the VBC is driven by an externally mounted PDU due to its high current demand. The actual BC positions are then calculated in the LM629s using encoder signals returned from the BC's. The LM629s then close the position loops by using a simple PID position tracking algorithm. The actual BC positions are returned to the main processor. Limit switches at the end of the travel of each BC interface to the 6811 through a custom built circuit board.
The BCU is located on the upper starboard side of the electronics module.