FUZZY POSITION CONTROL OF HYDRAULIC ROBOTS WITH VALVE DEADBANDS

The development of a real-time fuzzy-logic controller for a class of i ndustrial hydraulic robots is described. The main element of the contr oller is a PD-type fuzzy control technique which utilizes a simple set of membership functions and rules to meet the basic control requireme nts of such robots. The controller, although effective, is shown to pr oduce steady-state errors. The steady-state error in a step input resp onse is mainly due to the hydraulic valve deadband in which the contro l action is not effective. The steady-state position error in a ramp i nput response is due to both valve deadband and the nature of the cont roller, since zero position and velocity errors would result in the co ntroller closing the valve and stopping the motion. Conceptually simpl e actions are then identified to be applied in parallel with the above PD-type fuzzy controller in order to eliminate the steady-state error s. 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 con trol measure of the steady-state error obtained from the PD-type fuzzy controller is used as an offset. Both actions are activated only in t he region of fuzzy zero velocity error. The performances of the techni ques developed in this paper are evaluated experimentally. The experim ents are performed on an instrumented Unimate MK-II industrial hydraul ic robot which has been retrofitted as a research robot.