Virtual sensing of load forces in hydraulic actuators using second- and higher-order sliding modes

Abstract External load forces are challenging for sensing or estimating in the hydraulic actuators. Once it is due to inconvenient instrumentation of the force sensors, especially on an open-end mechanical interface. The other way, the complex nonlinear system behavior aggravates reconstructing the system states in a robust and real-time suitable manner. This paper proposes a sensorless estimation of external load forces in standard hydraulic actuators by using a well-established equivalent output injection of the second-order sliding mode and also higher-order sliding mode differentiator. Only the basic inertial and frictional parameters are assumed to be known from an initial identification without external load. Afterwards, the robust exact differentiators are used in order to reconstruct the system states. Noisy signals of the cylinder chamber pressures and piston stroke are the single quantities available from the measurement. An experimental case study, accomplished on the setup of two hydraulic cylinders arranged and operated in antagonistic way, is provided. The force-cell on the rigid interface between both cylinders is used for reference measurements and evaluation of the estimation algorithms. Two estimation approaches, one of the 2nd and another of the 4th order, are assessed in performance and compared to each other along with discussion.