Design and Control of an Untethered Robotic Tuna Based on a Hydraulic Soft Actuator

In this study, a hydraulic autonomous soft robotic tuna (HasorTuna) was developed from the perspective of bridging technology and physiology. HasorTuna processes a muscle-inspired hydraulic soft actuator. A double-cylinder plunger pump driven by servos supplies alternating pressure to the driving units on both sides of the soft actuator. Furthermore, a Lagrangian dynamic robotic fish model with a continuum fishtail and a passive flexible joint was developed to explore propulsive performance, and it was validated by extensive experiments and simulations. Prompted by the driving differentiation characteristics of fish muscle, we performed experiments to study how the activation duty cycle of the driving units affects the swimming performance. The results show that the low duty cycles are more suitable at low tailbeat frequencies, whereas high duty cycles gradually became advantageous with the increase in frequency. This finding is not only consistent with the known habits of fish and but can also be used to decrease the cost of transport of robotic fish. HasorTuna demonstrated a speed of 0.84 body lengths per second and a cost of transport of 11 J kg$^{-1}$ m$^{-1}$. Finally, the three-dimensional swimming capability of HasorTuna was verified via turning and diving–floating tests. Results from this study provide a valuable reference for bioinspired research of aquatic mechanical design and locomotion control.