Mechanical intelligence facilitates limbless locomotion in cluttered aquatic environments

While open-water limbless locomotion has been well-studied robotically and biologically, less research has been devoted to principles of limbless swimming in cluttered aquatic environments. Motivated by the remarkable control scheme simplification that mechanical intelligence (MI, purely passive, mechanically controlled body-environment interactions) offers limbless systems in highly damped environments [Wang et al. 2023], we hypothesize that MI could also play a role in cluttered aquatic locomotion. Our robophysical limbless model (L = 45cm), whose actuation is inspired by organism muscle actuation morphology, generates shape changes via programable directional compliance, generated by bilateral cable actuation and characterized by a parameter G. G=0 is a tightly controlled shape and increasing G leads to larger potential deviations from commanded shapes. During locomotion in a shallow water tank with uniformly spaced obstacles (9cm diameter posts, hexagonal pattern, 25cm spacing), for intermediate G, the robot’s performance was insensitive to wave amplitudes and spatial frequencies. Lower G control resulted in the robot bouncing off obstacles or jamming. Unlike in terrestrial locomotion in which kinematic efficiency was independent of temporal frequency, in this inertial regime, the robot moved most effectively (0.51 BL/cycle) at lower undulation frequencies (below 0.1 Hz). Thus, MI can be utilized for effective open-loop locomotion in cluttered inertial regimes.