Hydraulic variable stiffness mechanism for swimming locomotion optimization of soft robotic fish

In this paper, we describe the development of a soft robotic fish with Hydraulic Variable Stiffness (HVS), a simple and light mechanism that actively controls the tail stiffness. Modal analysis model using PRBM was developed to predict the tendency of optimal tail stiffness which maximizes the forward swimming speed in response to driving frequency. The forward swimming locomotion experiment demonstrated that the optimal tail stiffness which maximizes swimming speed exists for each driving frequency, and that optimal stiffness increases as the frequency increases. The turning locomotion experiment showed that, as the tail stiffness decreases, the turning radius decreases and the turning angular velocity increases significantly. By controlling the tail stiffness to its optimal for specific locomotion, the robot can increase its swimming speed by 118% on average, reduce its turning radius to almost a quarter, and increase its turning angular velocity by almost three times. These results demonstrate that the HVS soft robotic fish can optimize its swimming locomotion more effectively compared to conventional robots by optimizing its tail stiffness depending on its required swimming situation with only one actuator to oscillate the tail using the HVS device.