An Improved Variable Spring Balance Position Impedance Control for a Complex Docking Structure

This paper presents a complex multi-peg-on-hole docking mechanism used to attach extravehicular experiment loads to a space station and analyzes the challenges of force compliance control for this mechanism. The influence caused by the spring term in impedance control on the complex multi-peg-on-hole docking is analyzed. The analysis shows that the spring term can avoid the posture adjusting error, at the same time it can also generate impedance force and make vibration during the docking process. Focusing on the effects caused by spring term, this paper expends the spring term of classical impedance control. The classical spring term with two input parameters which only considers the original terminal position and actual terminal position is expanded to three parameters. The real expected position is also considered. This third parameter cannot be predicted because of the random error, this paper proposes a prediction method based on the forgetting factor function. This method can predict and adjust this parameter during the whole docking process, so it decreases the impedance force caused by spring term effectively, meanwhile the positive effect by spring term remains. So the proposed method can achieve the docking task and limit contact force and vibration effectively. At last, on the basis of the existing space robot system and preliminary completed work, detailed docking task experiments were conducted. The experiment results demonstrated that the proposed control method was feasible.