On the robustness of fuzzy control of an overhead crane

This paper presents the robustness analysis of a fuzzy logic control (FLC) system of the crab of an overhead crane. The proposed control system has two independent feedback loops: the conventional proportional (P) regulator is used to stabilise crane velocity, while the fuzzy proportional-derivative (PD) controller has been designed to improve damping of the load angle changes during operation at the maximum travelling speed and after the crane stops. Robustness of the system is crucial because the system parameters change under normal operating conditions. Improved robustness of the proposed FLC system may be explained on the basis of the variable structure control theory. The fuzzy control of the nonlinear second order system is designed with a two-dimensional phase plane in mind. We show that the robustness results from the FLC property of driving the system into the sliding mode.