One of the factors that effects the fidelity of the neutral
buoyancy simulation is the balance of the vehicle. If the center of
mass and the center of buoyancy are not aligned, the thrusters must
continuously apply a moment to the vehicle to compensate for the
moment generated by the misalignment. This can attenuate the thruster
control authority that is required to accomplish other vehicle
objectives, hence it is beneficial to use the RBCs to compensate for
these buoyancy effects. The vehicle can move the RBCs to shift the
center of mass to match with the center of buoyancy.
The vehicle is typically balanced at the beginning of each water test to compensate for buoyancy modifications due to minor hardware changes such as filing on board scuba tanks with air. The algorithm currently used to automatically balance the vehicle with the RBCs uses one of the attitude control laws to hold in the x-y plane (for balancing along the x and y axes) or the x-z plane (for balance along the x-z axes). While the vehicle is holding attitude, the auto balance algorithm averages the applied torque about the axes in the attitude hold plane for n (currently 5) seconds. This filters out the high frequency effects due to closing the servo loop. At the end of that time period, new RBC set points are chosen along each of the two axes in the plane proportional to the bias torque about the other axis in the plane.
For example if the vehicle is auto balancing in the x-y plane, the
vehicle holds attitude with the x-y plane level, and the z axis
pointing down. Every 5 seconds a new x RBC position is chosen
proportional to the bias torque about the y-axis, and vice versa.
Note: Another method of using the RBCs and VBC to balance the vehicle would be to use them to implement the buoyancy compensation portion of the PD + adaptive NL compensation controller. This approach was not explored for this thesis.