International Journal of Robotics Research

Design and experimental demonstration of rapid, precise end-point control of a wrist carried by a very flexible manipulator

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This paper presents the very high performance achieved experimentally with a wrist carried on a very flexible arm and designed for extremely quick, precise pick-and-place tasks. There are substantial potential advantages to using robot manipulators that are very lightweight and flexible - advantages in power, quickness, and performance, as well as in weight. Control using end-point sensing is of course essential in such a system. However, the closed-loop control bandwidth of a robot manipulator is still physically limited ultimately by its structural flexibility, as the end effector and the actuator are separated. Basically, there is a wave propagation time delay. A minimanipulator can be added to the end of such a main robot arm to perform special tasks with high accuracy and bandwidth and thus to enhance significantly and fundamentally the robot's performance. However, dynamic interaction between the minimanipulator and the structural flexibility of the main robot arm may tend to destabilize the system, making the control design very difficult and sensitive to parameter variations. In the research described in this article, analyses were performed to study the dynamic interaction between the motion of a minimanipulator and the structural flexibility of the main robot arm that carries it. A general geometric relation between end effector/sensor location and the center of percussion was found, which assures that a simple controller design can be obtained that is insensitive to modeling uncertainty while achieving good stability and high performance. A plane-motion mechanical mini-macro manipulator system was designed and built, and with it fast, precise maneuvers of several demanding kinds were demonstrated experimentally. The end-effector position control bandwidth achieved was more than eight times the frequency of the beam's first mode, and accuracy was excellent.