A new adaptive control scheme for direct control of manipulator end-effector to achieve trajectory tracking in Cartesian space is developed in this paper. The control structure is obtained from linear multivariable theory and is composed of simple feedforward and feedback controllers and an auxiliary input. The direct adaptation laws are derived from model reference adaptive control theory and are not based on parameter estimation of the robot model. The utilization of feedforward control and the inclusion of auxiliary input are novel features of the present scheme and result in improved dynamic performance over existing adaptive control schemes. The adaptive controller does not require the complex mathematical model of the robot dynamics or any knowledge of the robot parameters or the payload; and is computationally fast for on-line implementation with high sampling rates.
I n t r o d u c t i o nAlthough end-effector control is the ultimate goal of any robot control system, direct control of the end-effector motion in Cartesian space has not attracted much attention. Conventionally, task description is expressed in terms of a sequence of end-effector coordinates in the Cartesian space. This information is transformed through inverse kinematics to a series of angular positions in the joint space. End-effector control is then accomplished indirectly by controlling the joint angles which are related to the end-effector coordinates through forward kinematics. This indirect approach to end-effector control is both computationally inefficient and unduly complicated due to degeneracy of joint angles corresponding to a given endeffector condition. Furthermore, negligible errors in the joint angles may result in noticeable errors in the end-effector coordinates, depending on the manipulator geometry.Most existing methods for robot control in the joint space fall into the two major categories of "global linearization" and "adaptive control." In recent papers [1,2], Khatib has extended the global linearization method to the direct control of end-effector motion in the Cartesian space. This method is based on cancelling out the nonlinearities of the robot model by means of a nonlinear feedback controller and requires exact knowledge of the complex dynamic model of the robot. The method also needs accurate values of the robot parameters and the payload. When perfect cancellations of the robot nonlinearities is not achieved due to imperfect modeling or inaccurate parameter values, dynamic performance of the robot may be degraded and a complicated stability analysis is necessitated and this situation may ever1 lead to instability of the closedloop system [3 -41.Adaptive control offers an appealing solution to the robot control problem. In adaptive robot control methods, neither the complex mathematical model of the robot dynamics nor any knowledge of the robot's dynamic parameters or ,the payload are required in generating the control action. Adaptive control methods in general fall into two distinct categories; namely, indire...