A minimalistic walking fish robot twin based on the single actuator wave-like mechanism

Understanding terrestrial locomotion in walking fish species can unlock new insights into vertebrate evolution and inspire versatile robotic systems capable of traversing diverse environments. We introduce a novel, single-actuator continuum robot inspired by the terrestrial locomotion of the gray bichir (<i>Polypterus senegalus</i>), which employs a simple rotating helix to reproduce realistic undulatory movements. We hypothesized that a simplified robotic model with minimal actuation could accurately replicate the terrestrial locomotion patterns observed in<i>P. senegalus</i>. Using a 'robot-twin' methodology, we developed four helix configurations directly informed by the observed gait postures of real fish specimens and compared robotic performance and kinematics against biological data. We found that helix geometry significantly influenced both locomotion speed and lateral stability, with designs closely mimicking biological curvatures often exhibiting trade-offs between accuracy and performance. The fastest helix configuration produced the greatest lateral oscillation, whereas the most biologically accurate shape resulted in reduced locomotion efficiency. Additionally, integrating passive leg structures greatly enhanced stability, mirroring the biomechanical function of pectoral fins in the real fish. These findings underscore the value of minimalistic robotic designs in understanding fish-like locomotion and pave the way for future robotic platforms using reduced degrees of freedom.