Fuzzy position control of hydraulic robots with valve deadbands

Abstract

The development of a real-time fuzzy-logic controller for a class of industrial hydraulic robots is described. The main element of the controller is a PD-type fuzzy control technique which utilizes a simple set of membership functions and rules to meet the basic control requirements of such robots. The controller, although effective, is shown to produce steady-state errors. The steady-state error in a step input response is mainly due to the hydraulic valve deadband in which the control action is not effective. The steady-state position error in a ramp input response is due to both valve deadband and the nature of the controller, since zero position and velocity errors would result in the controller closing the valve and stopping the motion. Conceptually simple actions are then identified to be applied in parallel with the above PD-type fuzzy controller in order to eliminate the steady-state errors. In particular, two methods are developed. In one method, a control term proportional to the integral of the error is calculated and added to the output from the main controller. In the second method, the control measure of the steady-state error obtained from the PD-type fuzzy controller is used as an offset. Both actions are activated only in the region of fuzzy zero velocity error. The performances of the techniques developed in this paper are evaluated experimentally. The experiments are performed on an instrumented Unimate MK-II industrial hydraulic robot which has been retrofitted as a research robot.