Abstract: The existing heavy-duty industrial robots are short of force control functionality, which makes them difficult to perform contact-operations such as deburring, chamfering, and polishing. A 3-DOF translational force-controlled end-effector featured with 3-P(UU)₂ parallel mechanism and gas springs inside is introduced, and the robust adaptive force tracking admittance control method is proposed to regulate the contact force in the operational space. It can be applied to the robotic grinding and polishing process of the lateral surfaces, holes and narrow structures of workpieces. Based on the modeling of the forward and inverse kinematics of the parallel mechanism, and the dynamics of the pneumoelectric actuator, the robust adaptive force tracking admittance controller is designed. Experimental results show that the 3-DOF translational force-controlled end-effector based on robust adaptive admittance control tracks force fast and precisely, with the steady-state force error ‒4.5×10^‒4 N and the rise time 19.27 ms. The settling time 116.0 ms and overshoot 57.5% demonstrates good impact resistance and vibration absorption characteristics. Furthermore, the standard deviation and average peak of the contact force during the movement in a plane and cylindrical surface are 0.143 N and 0.694 N, respectively, indicating the adaptability to the environmental displacement error and stiffness variation under grinding tools made of different materials. The add-on force-controlled end-effector presented can improve the quality of the advanced robotic contact operations and expand the applications of the industrial robots.

Key words: force tracking, end-effector, pneumoelectric actuation, adaptive control, hybrid admittance control