Actuadores Electro-Hidráulicos (Ingles)

Control Engineering Practice 8 (2000) 1335}1345

Design and experimental evaluation of a robust force controller for an electro-hydraulic actuator via quantitative feedback theory
N. Niksefat, N. Sepehri*
Experimental Robotics and Teleoperation Laboratory, Department of Mechanical and Industrial Engineering, The University of Manitoba, Winnipeg, Manitoba, Canada R3T 5V6 Received 17 September1999; accepted 4 May 2000

Abstract This paper presents the design and experimental evaluation of an explicit force controller for a hydraulic actuator in the presence of signi"cant system uncertainties and nonlinearities. The nonlinear version of quantitative feedback theory (QFT) is employed to design a robust time-invariant controller. Two approaches are developed to identify lineartime-invariant equivalent model that can precisely represent the nonlinear plant, operating over a wide range. The "rst approach is based on experimental input}output measurements, obtained directly from the actual system. The second approach is model-based, and utilizes the general nonlinear mathematical model of a hydraulic actuator interacting with an uncertain environment. Given the equivalent models,a controller is then designed to satisfy a priori speci"ed tracking and stability speci"cations. The controller enjoys the simplicity of "xed-gain controllers while exhibiting robustness. Experimental tests are performed on a hydraulic actuator equipped with a low-cost proportional valve. The results show that the compensated system is not sensitive to the variation of parameters such asenvironmental sti!ness or supply pressure and can equally work well for various set-point forces. 2000 Elsevier Science Ltd. All rights reserved.
Keywords: Hydraulic actuators; Force control; Uncertain dynamic systems; Nonlinear systems; Robust control

1. Introduction Hydraulic systems are potential choices for modern industries due to their sti!ness, compactness and high payload capability. Theirapplication scope ranges from heavy-duty manipulators to precision machine tools. Hydraulic actuators are able to maintain their loading capacity inde"nitely, something that would usually cause excessive heat generation in electrical components (Alleyne, 1996). They are also advantageous for applications requiring environmental interactions because of their high force-to-weight ratio and fast responsetime. The utilization of hydraulic systems, however, is not without a cost. Hydraulic systems are complex and pose nonlinearities, which make the modelling and design of feedback controllers challenging. The nonlinearities are mainly due to servovalve #ow-pressure characteristics, ori"ce area openings, variations of #uid volume under compression and in part, to cavitation and seal friction.

*Corresponding author. Tel.: #1-204-474-6804; fax: #1-204-2757507. E-mail address: nariman@cc.umanitaba.ca (N. Sepehri).

Aside from the nonlinearities, hydraulic systems contain large extent of model uncertainties (Yao, Bu, Reedy & Chiu, 1999). The uncertainties can originate from #uctuation in supply pump pressure, variation of some hydraulic parameters such as bulk modulus and for forcecontrol tasks, changes in the environmental sti!ness. To overcome these di$culties much research has been undertaken. This paper addresses the problem of robust force control for hydraulic actuators that experience uncertainties in both the environment and the hydraulic functions. Force control in hydraulic actuators is a di$cult problem. Conventional PID controllers do not yield reasonable performanceover a wide range of operating conditions (Alleyne, Liu & Wright, 1998; Niksefat and Sepehri, 1999). Several researchers have, therefore, considered the use of adaptive and sliding mode control techniques. To name a few, Chen, Lee and Tseng (1990) designed a variable structure force controller for a single-rod hydraulic cylinder. Using position, velocity, acceleration, force and pressure...