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.