Underwater Buoyancy and Depth Control Using Reversible PEM Fuel Cells

A variable buoyancy device (VBS) operated by a reversible fuel cell (RFC) with feedback depth control is developed. The system varies its buoyancy by inflating or deflating a balloon via gases produced by electrolysis or consumed through fuel cells. The system has advantages in the perspective of energy efficiency since some of the energy used for the electrolysis process is recaptured by the fuel cell. Furthermore, it is noiseless and compact, hence, facilitating smooth integration with other underwater robots requiring buoyancy control. A PDA (Proportional-Derivative-Acceleration) feedback controller is designed to regulate the generation and consumption of gases to position and stabilize the device at a certain depth. The model describing the motion dynamics of the device with bounded gas generation/consumption rates is used to evaluate the range of motion before instability. Then, a minimum jerk trajectory planner (TP) is used to generate an optimal path for the device to move between two known depths. Finally, the effectiveness of the controller is confirmed with experiments. The real-time experiment shows that the controller can track the trajectory correctly with no overshoot and stabilize the VBS at various depths. The device can achieve fine depth control with the depth resolution of 0.1 m, which makes its application promising in bio-inspired underwater robots.