Tunable Stiffness Caudal Peduncle Leads to Higher Swimming Speed Without Extra Energy

Tuning body stiffness like fish to improve swimming efficiency and speed has been adopted by many fish-inspired robotics. However, it is unknown whether the energy saved from improved efficiency can compensate for the energy consumption brought by tuning stiffness itself. To explore this issue, we develop a robotic fish with a tunable stiffness caudal peduncle (TSCP), simultaneously allowing for untethered swimming and online tunable stiffness, and conduct a series of tests. We first apply interchangeable caudal peduncles to our robot to explore the effect of a tunable stiffness mechanism and determine the optimal tunable stiffness interval. The results show that tunable stiffness can significantly improve the response of robot at different frequencies. Then we develop the TSCP by embedding shape memory alloy wire into a silicone matrix. TSCP can adjust the stiffness in real time through current and increase the initial stiffness by up to 57.4%. More importantly, we incorporate the cost of tunable stiffness into the total cost of transport compared to previous robots for the first time. The cost of maintaining medium and maximum stiffness accounts for 8.72% and 17.87% of the total cost of transport, respectively. As a result, TSCP increases the swimming speed by up to 35.5% and reduces the Strouhal number by up to 21.9% at high frequencies without extra power. This study supports the necessity of applying tunable stiffness to robotic fish and offers a novel idea for online tunable stiffness mechanism.