Passive Compliance from Robot Limbs and Its Usefulness in Robotic Automation

Abstract

Robots have been traditionally used as positioning devices without much regard to external forces experienced by the tool. This has limited further potential applications of robots in automation. Most tasks that remain to be automated require constrained robot motion and/or involve work done by the robot on the environment. Such tasks require both force and position control. The ability to control the end-effector compliance is critical to successful force and position control tasks. Although the end-effector compliance can be actively controlled through the joint flexibilities provided by the joint servos or by active force sensing, the usefulness of having the minimum passive compliance in addition to active compliance control can improve performance. In surface following, for example, it is necessary to make the end-point of a robot have the right compliance to prevent jamming. The usefulness of passive compliance has been demonstrated by the use of compliance-devices on the robot end-effector such as the Remote Center Compliance. The natural compliance inherent in light weight and flexible robot structures, however, can be exploited to provide the necessary passive compliance required. In this paper we present a novel framework for computing the end-effector compliance from the compliance offered by the limbs of a serial robot. The emphasis is on the explanation of the passive end-effector compliance resulting from these structures, and particular attention is given to the use of these results in the selection of the type of robot for a particular task. We show examples of end-effector compliances as functions of joint configurations for the SCARA- and PUMA-type robots. The joint-configuration dependent end-effector compliance can be used to select the desired robot pose for the performance of a robotic task.