Admittance-Based Output Feedback Fuzzy Switching Control for PAM-Driven Parallel Robots via Nonsingular Terminal Sliding Mode

As a kind of soft actuator with inherent compliance, pneumatic artificial muscles (PAMs) have great application potential in robots. However, some challenging issues, such as high nonlinearities, sensor noises, and external disturbances, inevitably bring enormous difficulties to the accurate control of PAM-driven robots. To this end, this paper proposes an adaptive output feedback fuzzy switching control method for switched-form PAM-driven parallel robot systems, utilizing admittance models to rebuild compliant trajectories. Specifically, based on the nonrecursive high-order sliding mode (HOSM) differentiators with fixed-time convergence, unmeasurable velocity signals can be reconstructed to eliminate the adverse effects of measurement noises, decreasing the time delay of feedback signals. Moreover, a soft switching strategy is designed to flexibly adjust the switching weights and intervals of fuzzy structures, maintaining smooth control commands. Further, by introducing a nonsingular terminal sliding manifold, tracking errors can rapidly converge to a small neighborhood around the origins within a finite time, and all closed-loop variables are proved to be bounded through the Lyapunov stability theory. Finally, several groups of experiments are carried out on a self-built PAM-driven parallel robot to verify the effectiveness of the suggested method. Note to Practitioners—More compliant requirements need to be satisfied when facing an increasing number of human-robot interaction tasks. Naturally inheriting flexibility, pneumatic artificial muscles (PAMs) can achieve positioning/tracking control with various merits, including large actuated forces, low friction, clean power, etc. Once injecting a large amount of compressed gas, the overall stiffness of the system will significantly increase; hence, it is still an open challenge to combine compliant methods with high-efficiency tracking control for PAM-driven robots. To address this problem, based on Backlash-like hysteresis models, the whole switching dynamics of the PAM-driven parallel robots can be fully described, considering more comprehensive modeling elements. Then, by using admittance models, the closed-loop control is endowed with adjustable flexibility. Together with rapid velocity estimation, a nonsingular terminal sliding mode with finite-time convergence and a switching fuzzy update law are both introduced to further enhance the tracking performance and response speed. This combination gives the first solution to simultaneously improve the interaction compliance, response speed, and control accuracy for this kind of soft actuator-driven robots, which provides potential applications for practitioners.