A practical PID variable stiffness control and its enhancement for compliant force-tracking interactions with unknown environments

Compliant interaction control is a key technology for robots performing contact-rich manipulation tasks. The design of the compliant controller needs to consider the robot hardware because complex control algorithms may not be compatible with the hardware performance, especially for some industrial robots with low bandwidth sensors. This paper focuses on effective and easy-to-use compliant control algorithms for position/velocity-controlled robots. Inspired by human arm stiffness adaptation behavior, a novel variable target stiffness (NVTS) admittance control strategy is proposed for adaptive force tracking, in which a proportional integral derivative (PID) variable stiffness law is designed to update the stiffness coefficient of the admittance function by the force and position feedback. Meanwhile, its stability and force-tracking capability are theoretically proven. In addition, an impact compensator (Impc) is integrated into the NVTS controller to enhance its disturbance-suppression capability when the robot is subjected to strong vibration disturbances in complicated surface polishing tasks. The proposed controllers are validated through four groups of experimental tests using different robots and the corresponding results demonstrate that they have high-accuracy tracking capability and strong adaptability in unknown environments.