| Bridge-type grab ship unloader is widely used in bulk terminal. In process of working, the grab will sway consequentially caused by acceleration and deceleration of the trolley and other disturbances such as wind, because the grab and trolley are linked by wire ropes. Grab sway should be effectively controlled for it causes increase of operation time and additional dynamic load on structural body of the crane. Grab-sway control is the key technology of realization of automation and high performance of the crane, and has been a hot topic in recent years.Based on bridge-type grab ship unloader with electrical differential driving of four drums, in this article, the characteristics of grab-sway control problem was studied, the mechanism of grab-sway generation and control was revealed and then was verified by the simulation model of grab-trolley system built by ADAMS. The main research contents and innovations of this article are as follows:Analysis of grab-sway control problem. The grab-trolley driving system and operation process of grab were analyzed, and the controllability and control objective of all components of grab-sway were studied. Then characteristics of grab-sway control problem were promoted compared with classical closed loop control problem: 1) the control objective is that the controlled variable achieves a certain value at a certain time, and the controlled variable has no reference value in the control process. 2) the interaction between control variable and controlled variable has long time delay. 3) the function between control variable and control objective is unknown. For the reasons above, it's difficult to solve grab-sway control problem by classic closed loop control theory.Study on mechanism of grab-sway generation. Analytical model of grab sway was built based on moving simple pendulum. And then the grab-sway response was gotten by solving the differential equation with subsection solution. Based on the results, grab-sway mechanism was revealed. Grab will not sway but move synchronistically with trolley if trolley stops accelerating and start to move at constant speed just at the time of integer periods of grab-sway motion; otherwise, the asynchrony between grab and trolley will be maintained.Study on mechanism of grab-sway control. Method of trolley acceleration mode was promoted for grab-sway control. Trolley motion parameters were solved under maximum velocity and maximum acceleration constraint, when trolley accelerating by three times. Control effectives were compared by different acceleration ratio. And then four grab-sway control mechanisms were promoted-"waiting mechanism","synchronization mechanism","half-period mechanism"and"integer-period mechanism", and characteristics and effectives of the four mechanisms were compared.Build of simulation-model of grab-trolley system, and verification of mechanisms above based on the model. Numerical model of grab-trolley system was built. The modeling of wire rope was studied with emphasis. Four wire ropes with certain position relation were built. And equivalent model of single wire rope for four wire ropes was built for dynamic analyze with hoist. Analytic solving method of grab-sway response and mechanisms of grab-sway generation and control were verified, based on numerical simulation. Finally, closed loop grab-sway control system based on variable prediction was promoted, and main modules of the system were discussed.
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