Wide-Speed Autopilot System for a Swimming Hexapod Robot

For underwater swimming robots, which use the unconventional method of oscillating flippers for propulsion and control, being able to move stably at various velocities is challenging. This stable motion facilitates navigation, avoids blurring the images taken by a camera motion, and enables longterm observations of specific locations. Previous experiments with our swimming robot Aqua have shown that its autopilot system must adapt the control parameters as a function of speed. The reason is that the dynamics of both the robot and the thrusting system vary widely as a function of the overall velocity of the robot. In this paper, we present the results of manually tuning a stable autopilot system for this Aqua swimming robot. We employed a well-known technique called gain scheduling to allow for stable operation for velocities ranging from 0 to 40 cm/s, in real open sea conditions. Thus, our platform is now suitable for vision-based navigation in low light conditions as well as for extended observation through station-keeping. The results presented here are also a proof-of-concept that agile and reactive autonomous hovering is possible for flipper-based propulsion system.