Adaptive output force tracking control of hydraulic cylinders

With respect to the output force control problem of the hydraulic cylinders, an adaptive control scheme is proposed by using direct output force measurements through loadcells. Due to the large and somewhat uncertain piston friction force, cylinder chamber pressure control with Coulombviscous friction compensation may not he sufficient enough to achieve a precise output force control. In the proposed approach, the output force error resulting from direct measurement is used to update the parameters of a novel friction model which includes not only the Coulomb-viscous friction force in sliding motion, hut also the output force dependent friction force in presliding motion. The experimental results achieved on a hydraulic setup comprised of two hydraulic cylinders in pull-pull configuration demonstrate a frequency response of ±ldB up to 20Hz. The excellent output force (joint torque) control performance implies the dynamic equivalency between the hydraulic actuation and the electrical actuation up to 10 ~ 20Hz.