Design and Experimental Demonstration of Rapid, Precise End-Point Control of a Wrist Carried by a Very Flexible Manipulator

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 essen tial in such a system. However, the closed-loop control band width of a robot manipulator is still physically limited ulti mately by its structural flexibility, as the end effector and the actuator are separated. Basically, there is a wave propaga tion 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 funda mentally the robot's performance. However, dynamic interac tion between the minimanipulator and the structural flexibil ity 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 mo tion 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 ofpercus sion 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 ma neuvers of several demanding kinds were demonstrated ex perimentally. The end-effector position control bandwidth achieved was more than eight times the frequency of the beam's first mode, and accuracy was excellent. 1. Only the case of a one-degree-of-freedom wrist is analyzed here. The corresponding characteristic in a multidimensional case will be more complicated because of the nonlinear coupling effect.