Design and Analysis on a Plant-inspired Variable-stiffness Physiotherapy Actuator

Compared to traditional rigid physiotherapy actuators, pneumatic soft physiotherapy actuators exhibit high flexibility and adaptability, but the material surface tends to exhibit lower stiffness and the inherent hysteresis property of soft materials poses a greater challenge for control. Inspired by the morphological and behavioral properties of clover in the plant world, this paper designs a physiotherapy actuator with variable-stiffness layers based on a three-layer mortise-and-tenon structure of blocking particles. Aiming at the modeling and control problems caused by hysteresis characteristics, a compensatory control strategy based on Prandtl-Ishlinskii (P-I) modeling and fast terminal sliding mode is proposed. A P-I hysteresis model describing the motion characteristics of the system is established through experimental results, and the Levenberg-Marquardt (L-M) algorithm is used to identify the parameters of the system model. The simulation results show that the root-mean-square error of the proposed method is 3.82×10⁻² and 6.83×10⁻⁵ in step signal and triangle wave signal, respectively. Compared with PID control and adaptive control, the system tracking error is reduced, and most of the hysteresis effects are compensated effectively. The 3D printing technology is used to make casting molds for the preparation of physiotherapy actuators, and the performance test platforms of variable-stiffness structure, bending angle and physiotherapy strength are built. The experimental results show that the fabricated physiotherapy actuator can reach the predetermined bending angle under specific air pressure, and exhibit different physiotherapy strengths under different stiffness, with the maximum strength reaching 1.61 N.