Kinematic Modeling and Nonsingular Terminal Sliding Mode Control Method for a 2-DOF Parallel Robot Actuated by IPMC

Ionic polymer-metal composites (IPMCs) exhibit substantial application potential in underwater biomimetic robotics, attributed to their low driving voltage, rapid response, and superior flexibility. Nevertheless, the intrinsic bending deformation behavior of IPMCs typically results in arc-shaped trajectories at the actuator tip, precluding precise linear translational motion and thereby restricting their operational scope. To address this issue, this paper proposes a 2-degree-of-freedom (2-DOF) parallel robot structure actuated by IPMCs. First, an equivalent rigid-body kinematic model of the robot is established. By decoupling the motion of the end of the flexible active arm into the serial motion of the revolute pair and the prismatic pair, the 2-DOF linear translational motion of the end reference point is achieved, and the forward and inverse position solutions are derived to establish the mapping relationship between the end coordinates and the actuating voltage of the IPMC. Then, a nonsingular terminal sliding mode control (NTSMC) law based on the feedback of the end translational distance is proposed. This control law can calculate the driving voltage of the IPMC using the end-displacement information measured by the laser distance sensor and eliminate the singularity by introducing an integral sliding surface to make the tracking error converge to zero within a finite time. Experimental results show that the established kinematic model can accurately compute the mapping relationship between the end position and the driving voltage of IPMC; compared with PID control, the proposed NTSMC strategy can improve the positioning accuracy and trajectory tracking accuracy by up to 49% and 55%, respectively, while effectively enhancing system robustness.