Design and Experimental Study of a Robotic Tuna with Shell-like Tensegrity Joints

We developed an untethered robotic tuna featuring tensegrity joints for the purposes of simplifying the design procedure, reserving enough internal space, reducing the frictional loss of structures and generating a relatively smooth fish body wave. To achieve these objectives, a novel shell-like tensegrity joint was introduced, paired with a single-motor multiple-joint driving mechanism. The morphology matching design method of the tensegrity joint was proposed to fit the streamlined fish body, where the deflection angles of each joint were predetermined to generate the specific body waveform. Stiffness analysis shows that the tensegrity joint could function equivalently to a traditional rotational joint, given certain geometric conditions. Based on the fabricated robotic tuna prototype, extensive free-swimming experiments were performed to optimize its swimming performance by varying key parameters, including the caudal fin‘s shape, flexibility and rotational stiffness and joint deflection angles. The results reveal that the robotic tuna achieved the highest swimming speed of 1.31 body lengths per second (BL/s) at a driving frequency of 2.4 Hz, and the maximum stride length increased to 0.81 BL/cycle at 1 Hz, demonstrating the effectiveness of the proposed design scheme. This study provides valuable insight for developing high-performance bio-inspired autonomous underwater vehicles.