AUV Buoyancy Control with Hard and Soft Actuators

Autonomous underwater vehicles (AUVs) find many applications in oceanography, environmental research, and inspection and maintenance of subsea energy assets. Subsea Resident AUVs remain in subsea for extended periods of time which could last for several months. Maintaining depth using traditional hard actuators (HAs) is very energy expensive. Mimicking aquatic creatures, it is shown in this paper that HAs and proposed soft actuators (SAs) can collaborate in a novel way. This collaboration can stabilize AUVs at any desired depth with minimum energy consumption at steady state. This is demonstrated using a laboratory AUV for which a nonlinear dynamic model was developed that uses experimentally validated system parameters. The AUV uses HAs to quickly reach any desired depth, while SAs generate volume change to adjust the system's buoyancy to maintain neutral buoyancy at the desired depth. In the neutral buoyancy state, the HAs shut off while the SAs stabilize and maintain the depth with virtually zero energy consumption. A control algorithm architecture is developed to manage the HA and SA collaboration. The HAs use a proportional controller with a dead-band, while the SAs use a proportional-derivative-acceleration (PDA) feedback controller. The ability of both types of actuators to mitigate disturbance forces are explored and analyzed. Simulation results show that SAs alone can reject small disturbances while using both SAs and HAs in collaboration can reject large disturbances. Simulation results demonstrate that combining traditional HAs with SAs leads to dynamic performance and very low energy consumption capabilities that cannot be achieved by either one alone.