CASCADE CONTROL WITH FRICTION COMPENSATION BASED ON ARTIFICIAL NEURAL NETWORK FOR A HYDRAULIC ACTUATOR

ABSTRACT This paper addresses the friction compensation in hydraulic actuators using an artificial neural network combined with a suitable control technique. The proposed neural network is trained off-line and allows calculate an estimative of the friction force on-line very quickly based on the hydraulic force and on the cylinder velocity. The estimated friction force is introduced directly in the force line of the system using a cascade controller, in which the hydraulic actuator is interpreted as two interconnected subsystems: a mechanical one driven by a hydraulic one. The convergence properties of the closed loop system are established using the Lyapunov method. Experimental results validate the main theoretical results of the proposed strategy. NOMENCLATURE INTRODUCTION Hydraulic actuators provide high force, stiffness and durability suitable to various applications, and there is a growing demand for such actuators to operate with improved precision and repeatability. Unfortunately, these actuators present some undesirable characteristics, namely, lightly damped dynamics and highly non-linear behavior introduced by both, pressure dynamics and friction, among others. In a typical hydraulic actuator, the movement of the piston, and hydraulic fluid are subject to friction. The friction is generated by the contact between both the rod and the seal, and the piston seal and the cylinder, and the viscous effects of the hydraulic fluid. This friction affects the controllability, accuracy, and repeatability of the actuator. The lightly damped dynamics and the non-linear flow and friction dynamics complicate the controller design for high performance closed loop applications. The simple classical controllers cannot overcome the bandwidth limitation caused by the lightly damped open loop poles position. The use of a linear controller is limited by the non-linear behavior. Due to these control difficulties, a combination of non-linear control techniques offers good theoretical and experimental results. One way to combine control techniques is to use the backstepping method. Another way to do it is based in interpreting the hydraulic actuator as two interconnected subsystems: a mechanical subsystem driven by a hydraulic one. In this paper is chosen the second method in which the main idea is to promote a fast loop in the hydraulic subsystem in order to generate hydraulic forces that allow the mechanical subsystem to track the desired trajectory. This idea was formalized in [1,2] taking into account an error during the hydraulic subsystem trajectory tracking and by presenting a stability proof of the whole interconnected system. The resulting controller is referred as cascade controller. The experimental and theoretical results employing this controller have demonstrated that its closed loop performance overcomes the performance obtained with classical controllers, with respect to the lightly damped dynamics and to the non-linear pressure dynamics. In this paper is shown that the cascade controller allows compensate the friction dynamics in