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Inertially Aided Robotics
SYSTEM DYNAMICS INTERNATIONAL INC HUNTSVILLE AL
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For most of the current robot manipulators, control of its end- effector, position and orientation is done by controlling joint angles as shown in Figure 1. Each joint is controlled by a local joint servo. Angular position sensors are installed at manipulator joints to measure joint angles. For a desired end-effector position and orientation, inverse kinematics is used to generate command signals in joint coordinates. These signals become the reference inputs to local joint servos. Such control scheme may be called joint sensor based manipulator control. Note that although local feedback exists in each joint servo, there is no feedback to compare the actual end-effector state with respect to the reference state. End-effector control without feedback suffers from two major shortcomings. The first is the effect of arm compliance on the control, and the second is poor robustness. The compliance is caused by the physical nonrigidness of the manipulator and by the insufficient stiffness of the joint servos. The compliance effect causes two problems. The first is the bending andor drooping of manipulator arms caused by loading and by the weight of arms. This affects the accuracy of positioning the end-effector. The second problem is the existence of bending modes in the manipulators dynamical characteristics, making an accurate and steady control of the end-effector difficult. Current methods to cope with the problems of compliance is to adopt large size arm cross-sections, resulting in a bulky manipulator.
APPROVED FOR PUBLIC RELEASE