George Anwar

Low Velocity Friction Compensation and Feedforward Solution Based on Repetitive Control

One of the most significant sources of tracking error for an X-Y table is static friction, a nonlinear disturbance at low velocity. In traversing a circular profile, an X-Y table encounters zero velocity crossings at ninety degree intervals around the circle, leaving relatively large tracking errors referred to as “quadrant glitches.” To learn the control input which eliminates errors caused by stiction, repetitive control, a subclass of learning control, is employed. Experiments, conducted on the X-Y bed of a CNC milling machine, demonstrate that near-perfect tracking can be achieved in twelve cycles or less. Of particular interest is the velocity command input generated by repetitive control. The input contains feedforward information for developing perfect trajectories at low velocities.

Plug in repetitive control for industrial robotic manipulators

The implementation of plug-in repetitive control on the direct drive axis of a prototype GMF A500 robot is considered. Plug-in repetitive control was developed and implemented using an IBM AT. A multirate sampling algorithm was designed to reduce the memory requirements of repetitive control. A microcontroller board based on the Intel 8096 was designed so that plug-in repetitive control could be applied from a small unit embedded within the KAREL controller. In each application, experimental results show that the tracking error is smoothly absorbed in a few cycles.<<ETX>>

Application of nonlinear friction compensation to robot arm control

This paper deals with a design of a digital servo controller for a robot manipulator with mechanical nonlinearities. The experimental system is a D.C. motor driven manipulator arm. A model to predict the nonlinear behavior exhibited by the experimental system and a nonlinear controller with Coulomb friction compensator are developed and tested. The effectiveness of the nonlinear controller is certified by designing a linear tracking controller based on this scheme. Experimental and simulated responses are given.

Discrete time repetitive control for robot manipulators

The analysis and experimental implementation of a discrete-time repetitive control scheme is presented. The repetitive control structure is designed to be easily implemented on any system without modification to the existing controller. Simulation and experimental results show that the repetitive controller in conjunction with a computed-torque control or a simple proportional-derivative control law achieves good tracking performance when the desired trajectory is periodic and the period is known.<<ETX>>

Model reference adaptive control of a two axis direct drive manipulator arm

The model reference adaptive control of a two axis direct drive manipulator arm is presented. The two axes exhibit a significant dynamic interaction. The model reference adaptive controller is utilized in the velocity loop to adaptively decouple the dynamic interaction and to linearize the dynamics. The outer loop controller for positioning and tracking is designed based on the linear decoupled dynamics. The adaptive velocity loop controller as well as position loop controller are implemented digitally. The use of a series-parallel, as well as a parallel reference model is suggested in the adaptive velocity loop controller. The stability problems arising from the straightforward digital implementation of the continuous time algorithm are analyzed and a modification in the digital algorithm is introduced which guarantees the asymptotic stability of the system. Simulation and experimental results show a consistently superior performance of the manipulator under adaptive control.

Experimental Study on Discrete Time Adaptive Control of an Industrial Robot Arm

Abstract This paper deals with an implementation of a discrete time adaptive controller for two axes of an industrial manipulator. The friction terms are shown to be detrimental to the parameter adaptation algorithm and to the stability of overall system unless they are properly included in the model. In this paper, the effect of friction is minimized by a cancellation technique based on off-line estimation of friction terms. The primary purpose of the adaptive controller is to compensate for inertia variations due to payload changes and other disturbances.