A Stable Control Strategy for Industrial Robots with External Feedback Loop

Abstract

The inner/low-level control loop of most industrial robotic manipulators is protected from any modification by a closed control architecture. The only way to specify joint inputs to them is through position or velocity commands. Furthermore, the inner controller is unknown/uncertain, as it is not revealed to the user. This makes it very difficult to determine the configuration of the inner controller, including the structure and values of the control gains. As a result, integrating external sensory feedback systems into a closed architecture system becomes a difficult task because the interaction of the external feedback loop with the inner control loop can affect the stability of the overall system. In this paper, a stable control strategy is proposed to generate the joint velocity commands for industrial robotic manipulators with uncertain closed control architecture. Based on the proposed method, external feedback controllers can be added regardless of the inner control loop configuration. The proposed external (or outer) control loop approach provides a high degree of design flexibility by enabling smooth implementation of various modern control applications on industrial robots. Unlike previous studies, the proposed control method is not limited to a particular configuration of the controller in the inner loop, nor to the structure of its control gains. The proposed controller design is validated on the UR 5e industrial manipulator.