Decentralized Control of Elastic Limbs in Closed Kinematic Chains

The generation of movements in closed kinematic chains as opposed to open kinematic chains is a challenging task because all participating joints have to be moved in a highly coordinated manner in order to avoid destructive tensions in the limb. In this paper we present a new decentral joint controller which uses low level interactions between a moving joint and its environment consisting of neighboring joints, the body and the surroundings the agent is placed in. This local joint controller is based on a Local Positive Velocity Feedback (LPVF) mechanism which exploits the elastic properties of the joint. The control strategy is inspired by biological findings in the walking system of stick insects. We will show that a closed kinematic chain consisting of several LPVF controlled joints, though lacking a central controller, can solve tasks which need a high level of inter-joint coordination. As an example, a planar manipulator turning a crank is presented. In a further step, the LPVF algorithm is extended to switched LPVF in order to improve the mechanical power conversion. The extended capabilities are proven in a second experiment in which a 3DoF test leg generates powerful stance movements. KEY WORDS—crank turning, walking, joint coordination, elastic joint, compliant motion, positive feedback, active compliance