Performance Optimization for Bionic Robotic Dolphin with Active Variable Stiffness Control

Abstract

Aquatic animals such as fish and cetaceans can actively modulate their body stiffness with muscle to achieve excellent swimming performance under different situations. However, it is still challenging for a robotic swimmer with bionic propulsion mode to dynamically adjust its body stiffness to improve the swimming speed due to the difficulties in designing an effective stiffness adjustment structure. In this paper, based on the special torque mode of a motor, we propose an active variable stiffness control method for a robotic dolphin to pursue better swimming speed. Different from a variable stiffness structure design, a torque control strategy for the caudal motor is employed to imitate the physical property of a torsion spring to act as the variable stiffness component. In addition, we also establish a dynamic model with the Lagrangian method to explore the variable stiffness mechanism. Extensive experiments have validated the dynamic model, and then the relationships between frequency and stiffness on swimming performance are presented. More importantly, through integrating the dynamic model and torque actuation mode-based variable stiffness mechanism, the online performance optimization scheme can be easily realized, providing valuable guidance in coordinating system parameters. Finally, experiments have demonstrated the stiffness adjustment capability of the caudal joint, validating the effectiveness of the proposed control method. The results also reveal that stiffness plays an essential role in swimming motion, and the active stiffness adjustment can significantly contribute to performance improvement in both speed and efficiency. Namely, with the adjustment of stiffness, the maximum speed of our robotic dolphin achieves up to 1.12 body length per second (BL/s) at 2.88 Hz increasing by 0.44 BL/s. Additionally, the efficiency is also improved by 37%. The conducted works will offer some new insights into the stiffness adjustment of robotic swimmers for better swimming performance.

Simulation Analysis of Frog-Inspired Take-Off Performance Based on Different Structural Models

“…The foot is long and flexible, and there are flippers between the toes. The musculoskeletal structure of the frog is shown in Figure 1 a [ 6 , 27 ]. It can be seen that the skeleton mainly includes the skull, scapula, spine, and pelvis.…”

“…The biological structure of natural organisms has gradually become reasonable over the course of long-term evolution, and the mode of movement has also demonstrated intelligence and adaptability [ 1 , 2 ]. The diversity of biological structures and their motions provides a steady stream of reference model samples for the bionic design and motion control of robotic structures [ 3 , 4 , 5 , 6 ]. Bionic jumping robots have gradually become a research focus in the field of robotics due to their unique advantages, which can show strong adaptability to different structural environments [ 7 , 8 , 9 , 10 ].…”

Design and Analysis of a Novel Bionic Tensegrity Robotic Fish with a Continuum Body

“…More importantly, body stiffness can be actively modulated to adapt to different fluid environments to show outstanding swimming motion [ 7 ]. Many investigations have indicated that the tunable stiffness mechanism is of great significance for the design of bionic underwater robots with high performance [ 8 , 9 , 10 ].…”