This paper discusses the utilization of the so-called twisting algorithm when applied in output feedback position control schemes for electro-hydraulic cylinder drives. The twisting controller was the first second order sliding controller ever introduced, and can structure-wise be considered a straight forward extension of the simplest first order sliding controller, that is, a relay controller. Such a controller may be implemented without the knowledge of system time constants etc., as opposed to the surface based first order sliding controllers which has been presented in numerous contributions in literature. This paper considers the twisting algorithm when applied directly for output feedback control, and with the design based on a reduced order model representation of an arbitrary valve driven hydraulic cylinder drive. The consequence of implementing such a controller with the well-known saturation-, or boundary layer method is discussed, and the control operation inside- and outside the boundary layer region is considered. Furthermore, the global stability of such a controller is discussed, with emphasis on possible local instability modes. Results demonstrate that the proposed output feedback controller may be successfully applied to hydraulic valve driven cylinder drives, with performance being on the level with a conventional surface based first order sliding mode controller.
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