Effects of actuation angle of spatially distributed control surfaces of a bio-robotic sea lion on turning performance

To expand the performance envelope of current unmanned underwater vehicles operating in near shore environments, researchers have increasingly turned to marine animals as models to develop bio-inspired robotic systems that leverage biological swimming strategies. In particular, California sea lions swim with great maneuverability in highly dynamic flow environments by coordinating multiple, spatially distributed control surfaces along their bodies. Despite significant progress in the development of bio-inspired robots, the individual and combined roles of different control surfaces and how their actuation affects turning of the robotic system remains under explored. In this study, a bio-inspired sea lion robot and its numerical model were used to understand how the actuation angle of control surfaces such as head, pelvis, fore flippers and hind flippers affected pitch and yaw turns. The turning performance of the bio-robotic platform was evaluated using turning radius, maximum angular velocities, and final orientation. Experimental and numerical results showed that actuating anterior control surfaces in combination with posterior control surfaces reduced turning radius and increased maximum angular velocity and final orientation relative to posterior-only actuation. Actuating fore flippers near the center of mass during pitch turns further enhanced turning performance by reducing lateral slip and producing tighter turns. Importantly, the results also revealed that maximum actuation of control surfaces did not always yield superior turning performance, as specific non-maximal head-pelvis actuation combinations produced better turns. These findings demonstrate that turning performance in bio-inspired, multi-body underwater systems depends on both the geometric location of control surfaces and their actuation angles. More broadly, the results suggest that actuation strategies should be tailored to the intended turning behavior, providing design and control guidance for future articulated underwater robots.