 This paper was not presented at any IFAC meeting. This paper was recommended for publication in revised form by Associate Editor L.-C. Fu under the direction of Editor M. Araki. * Corresponding author. Tel.: #61-7-3327-4464; fax: #61-7-3327- 4455. E-mail address: adrian@cat.csiro.au (A. Bonchis).  Supported by the Centre for Mining Technology and Equipment, Australia. Automatica 37 (2001) 589}595 Brief Paper Variable structure methods in hydraulic servo systems control  A. Bonchis*, P. I. Corke, D. C. Rye, Q. P. Ha CSIRO Manufacturing Science and Technology, P.O. Box 883, Kenmore Qld 4069, Australia Australian Centre for Field Robotics, The University of Sydney, NSW 2006, Australia Received 2 August 1999; revised 1 August 2000; received in "nal form 16 October 2000 Abstract In the general framework of hydraulic servo systems, this paper addresses the problem of position control in the presence of important friction nonlinearities. The accent falls on the variable structure methodology, as we try to use its intrinsic robustness properties. Several friction observers, including the one based on a variable structure approach, were incorporated and tested in an acceleration feedback control. Next, we present a novel implementation of a variable structure controller, which lumps friction and load as an external disturbance. Results of extensive experimental testing encourage the use of variable structure methods in a class of highly nonlinear hydraulic servo systems.  2001 Elsevier Science Ltd. All rights reserved. Keywords: Variable structure; Friction compensation; Observers; Robust control; Hydraulic servo systems 1. Introduction In hydraulic servo systems friction is an important source of nonlinearity, considerably diminishing the force or torque available at the actuators. In addition, at motion reversal and low velocity, a host of dynamic e!ects have been observed, and appropriate friction mod- els have been developed (Armstrong-He`louvry, Dupont, & Canudas de Wit, 1994). Increasing the positioning accuracy in such systems requires adequate measures to alleviate the adverse e!ects of friction. One of the most common ways is to provide the controller with quantita- tive information on friction, achieving what is commonly referred to as model-based friction compensation. As direct measurement of friction is not possible, two op- tions are models based on experimental friction identi- "cation or the use of nonlinear friction observers. For the system at hand, experimental friction identi"cation re- sulted in a pressure-dependent model capable of describ- ing friction over the entire velocity range (Bonchis, Corke, & Rye, 1999). Nonlinear reduced-order friction observers require at least position and external force measurements (Friedland & Mentzelopoulou, 1992; Tafazoli, de Silva, & Lawrence, 1995). In order to in- crease robustness of estimates, an observer based on a variable structure systems approach was suggested (Ha, Bonchis, Rye, & Durrant-Whyte, 2000). Its application to position control for hydraulic servo systems is high- lighted in this paper. In electrical servo systems, friction compensation is a straightforward technique, due to the proportionality between the control current and the output torque. This is hardly the situation in their hydraulic counterparts. Acceleration feedback has been used in order to achieve friction compensation in such systems (Tafazoli, de Silva, & Lawrence, 1998), on the ground that the estimated acceleration bears friction information. The use of friction models or compensation is not a requisite condition for improving the positioning per- formance of the system. In essence, any robust control technique should provide a solution to the problem. We will focus here on a variable structure control with sliding mode, which proved its potential in electro-hydraulic servo systems (Hwang & Lan, 1994; Fung, Wang, Yang, & Huang, 1997). Most results have been reported for systems operated by hydraulic motors, while our case deals with an asymmetric hydraulic cylinder. The ap- proach of Slotine and Sastry (1983) combined with a fuzzy logic reasoning reported by Ha (1997) was used in 0005-1098/01/$ - see front matter  2001 Elsevier Science Ltd. All rights reserved. PII: S 0 0 0 5 - 1 0 9 8 ( 0 0 ) 0 0 1 9 2 - 8