Robust, High-Performance Control for Robotic Manipulators

NASA 's Jet Propulsion Laboratory, Pasadena, California

An improved control scheme for robotic manipulators results from an alternative approach to the use of both model-based and performance-based control techniques. This is a robust, high-performance adaptive control scheme that combines the advantages and overcomes the disadvantages of both types of techniques. Model-based control techniques like the Computed-Torque method are based on the cancellation, by the controller, of nonlinear terms in the mathematical model of the dynamics of the manipulator. Such techniques require that the model be accurate, that all the parameters of the model be known accurately, and that the model be computed in real time at the servo control sampling rate. These requirements are often not met in practice. Performance based (adaptive) techniques like the direct-adaptive-control method attempt to overcome the failure to satisfy these requirements by adjustment of the controller gains in re al time on the basis of the tracking performance of the manipulator. This eliminates the need for the model and for the computation of its parameters. Thus, a fast adaptation can be achieved. However, adaptive controllers do not take advantage of any known part of the dynamics. Furthermore, rates of adaptation that are set high enough for rapid response to variations in the manipulator dynamics or payload can cause instabilities through the excitation of unmodeled dynamics. The new control system includes a feedforward controller and a separate feedback controller. The feedforward controller is model-based and contains any known part of the dynamics of the manipulator that can be used to produce a nominal control signal. The feedback controller is performance-based; it compensates for any unknown dynamics and uncertainties or variations in the parameters of the manipulator and/or the payload. The feedback controller is a simple adaptive proportional/integral/derivative controller that generates an adaptive control signal to complement the nominal feedforward signal. The feedback laws are simple, enabling fast implementation of the servo control loop.

To counteract potential instabilities and assure robustness in the presence of unmodeled dynamics and disturbances, decay terms are included in the integral adaptation laws. A version of the new control scheme was tested by using it in the control computer of a commercial robot (see figure). The robot was found to track the commanded trajectory closely, even when the mathematical model of the dynamics was not used and the feedforward controller was eliminated. The results also showed that the control scheme is not sensitive to the configuration of the robot arm, torque disturbances, or the desired trajectory.

More details can be found in:

Seraji, H.: "Robust high-performance control for robotic manipulatorsS; Proc. IEEE Intern. Conf. on Robotics and Automation, Scottsdale, AZ, May 1990, Vol. 3, pp. 1663-1669.

Point of Contact:
Homayoun Seraji,
Mail Stop 198-219
Jet Propulsion Laboratory
4800 Oak Grove Drive
Pasadena, CA 91109
seraji@telerobotics.jpl.nasa.gov

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Last updated: May 10, 1996