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Overhead cranes are essentially a kind of complex underactuated nonlinear mechanical systems, and it is a challenge to design an anti-swing controller for overhead cranes, hence, many people design controllers of overhead cranes ignoring the changing of the rope length and other parameters. In this study, we propose a multi-sliding mode controller to solve the problem of anti-swing and positioning controller for an overhead crane with rope length variations. This controller can provide a simultaneous trolley-position regulation, sway suppression, and load hoisting control. Also, this paper adopts a new method to decrease the chattering of the control input. Stability analysis of the proposed controller is given in the paper. The proposed controller does not rely on the accurate knowledge of overhead crane system model, and it is simple and easy to apply, and robust to disturbances. The simulation results show the good performance of the proposed controller.

Compared with a traditional single-container overhead crane, the double-container overhead crane is a type of new efficient container stevedoring machinery, however, it is actually a complicated nonlinear system. In this brief, both the kinematic and dynamic characteristics concerning the double-container crane are investigated, and then an accurate dynamic model of the underactuated double-container overhead crane is established by utilizing Euler-Lagrange's method. Not only is the variation of the rope and payloads, but also some uncertainties such as mechanical frictions and air-resistance in the environment are taken into account in modeling. Simulation results show the validity of the established double-container overhead crane model.

In an inductively coupled power transfer (ICPT) system, a primary capacitance is usually used to minimize the VA rating of power supply, and a secondary capacitance is adopted to maximize the power transfer. The changes of the secondary capacitance because of environmental temperature or other factors will alter the resonant frequency of the secondary side, and thus the power transfer capability is varied. In this study, the power transfer capability of ICPT systems is investigated in detail when the secondary capacitance changes. A resonant frequency control strategy is suggested to keep the system operation frequency to follow closely the secondary resonant frequency when the secondary capacitance varies. This new strategy is based on the synergic action of the system operation frequency and the primary capacitance. The performance of three frequency control methods, that is the resonant frequency control, the traditional variable frequency control, and the fixed frequency control, are compared when the secondary capacitance varies. A distinguish feature of a resonant frequency control is that the power transfer is an approximately linear function of the secondary capacitance. This linear characteristics facilitate a voltage control strategy being used to keep the load power unchanging when the secondary capacitance varies. A traditional viewpoint in an ICPT system is that maximizing of output power is achieved when the system operation frequency is equal to the secondary resonant frequency. Numerical simulations show that the power output by a resonant frequency control do not always dominate the power output from other two frequency control methods. This phenomenon suggests that above traditional viewpoint is a misconception, and should be reexamined. The power transfer capability of ICPT systems is an intricate function of the operational frequency, and thus resonant operations of the secondary side do not always guarantee maximizing the power transfer. A suitable control mechanism, such as the resonant frequency control, the variable frequency control, or the fixed frequency control, should be designed carefully according to the practical engineering requirement when the secondary capacitance varies.