Use of second-order sliding mode observer for low-accuracy sensing in hydraulic machines

Low-accuracy sensing is very common for the large hydraulic machines and does not allow for directly measuring the relative velocity which can be, otherwise, required for the control and monitoring purposes. This paper provides a case study of designing the second-order sliding mode observer based on the super-twisting robust exact differentiator. The nominal part of the system dynamics is derived from the simple available system measurements and incorporated into the observer structure. Parasitic by-effects, arising from the sensor sampling, quantization, and non-modeled distortions due to mechanical sensor interface, are shown as the main causes of hampering the final (steady-state) convergence of the observer states. Two cases – a continuous chirp excitation and a sequence of the short square pulses –are demonstrated for the open-loop motion experiments performed on a hydraulic crane, for which an accurate estimation of the motion system states is obtained.