A direct compliance control for a parallel link arm. (2nd Report, The stability analysis for the arm with negative stiffness joints).

Abstract

In the Direct compliance Control (DCC) of robotic arms, the arm compliance which can be selected is limited when only the positive joint stiffness is allowed. This is drawback of the DCC, as the previous paper suggested. One way of removing this is to expand the adjusting ranges of the joint stiffness to negative values. However, this may cause unstable motion of the arm. This paper discusses the stability of the parallel link arm which includes joints with negative stiffness. Through potential energy analysis, the stability condition for the arm is derived, and it is suggested that a parallel link arm structure provides a great benefit in obtaining get stable control motions by robotic arms with negative stiffness joints. A planar parallel link arm with three variable stiffness joints was designed. Using the arm, two compliant motion tasks were carried out and the analytical results were confirmed.