Modelling and control of vertical oscillation in overhead cranes

Overhead cranes are widely used in industry for lifting and moving various loads. Right before lifting a heavy load from the ground the crane bridge slightly bends, which makes the load oscillate vertically during the lifting. Vertical oscillations may introduce unnecessary stress to the crane metal structures and hoisting machinery, and thus reduce the lifetime of the crane. Furthermore, vertical oscillations make the load handling and positioning more difficult, thus decreasing the performance of operations. Nevertheless, study of vertical oscillations in overhead cranes has been neglected in literature. In this paper, a model for analyzing vertical oscillations and for control design is developed. Oscillation control, based on rope force measurement, is developed to decrease maximal forces and to reduce the time of oscillation. The resulting controller is tested with a real overhead crane having nominal load of five tons.