Simultaneous Position and Stiffness Distribution Control of Elastic Redundant Closed-Loop Mechanism

Abstract

The present paper discusses a new design of robotic manipulator having redundant closed-loop mechanism with elastic passive joints. The proposed design is aiming to be utilized for developing robots that are capable to work in complicated and unpredictable environments such as natural environments, disaster sites or daily human living space. In order to deal with various contact conditions between surrounding objects and robot, effective approach called flexibility control is figured out. This methodology includes planning and optimization of force application not only on single considered point on an object but also its distribution among certain contact area. This paper defines the ability to optimize stiffness distribution at a number of contact points as “flexibility” and proposes elastic closed-loop mechanism which has a serial chain of revolute joints with torsion coil springs anchored at two ends to form a closed-loop, as a lightweight and supple hyper redundant mechanism. Output stiffness is formulated based on the minimization of potential energy, the balancing of internal force and the velocity constraint to construct a closed-loop mechanism. Joint input to obtain both the desired stiffness distribution and desired output position simultaneously is derived from partial derivative of the output stiffness and compensation by a learning control scheme. Motion control experiments with a 10R elastic closed-loop robot demonstrate the effectiveness of the proposed control scheme.