Robotic fish with tunable stiffness soft tail based on bionic spinal muscle system

Changes in muscle state in vertebrates directly influence their movement properties, offering inspiration for research on bionic robotic fish. This paper presents an untethered robotic fish equipped with a novel soft tail, which mimics the ability of real fish to modulate tail stiffness in real time. Inspired by the musculoskeletal system of organisms, the soft tail presented in this work primarily utilizes electrorheological fluid (ERF) and features a bionic structure mainly composed of a printed circuit board (PCB) with microfibers attached. Under the influence of a strong electric field, both the stiffness and damping of the tail increase simultaneously, thereby altering the hydrodynamic characteristics of the tail. Experiments demonstrate that the maximum increase in bending stiffness reaches 12% at a 5 kV voltage input. In dynamic tests, both thrust and swimming speed exhibit a strong correlation with stiffness. Under certain amplitudes and frequencies of the tailbeat, the swimming performance is effectively improved. Furthermore, active stiffness modulation facilitates progressive convergence toward the optimal Strouhal number regime observed in real fish. This study offers a novel approach to construct bionic robotic fish and highlights the effectiveness of stiffness adjustment in enhancing swimming performance.