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FAQ >> Active Sway Suppression Techniques of a Gantry Crane System

Active Sway Suppression Techniques of a Gantry Crane System

The main purpose of controlling a gantry crane is transporting the load as fast as possible without causing any excessive sway at the final position. Research on the control methods that will eliminate sway angle of gantry crane systems has found a great deal of interest for many years. Active sway angle control of gantry crane consists of artificially generating sources that absorb the energy caused by the unwanted sway angle of the rope in order to cancel or reduce their effect on the overall system. Lueg in 1930 (Lueg, 1930), is among the first who used active vibration control in order to cancel noise vibration. The requirement of precise cart position control of gantry crane implies that residual sway of the system should be zero or near zero. Over the years, investigations have been carried out to devise efficient approaches to reduce the sway of gantry crane. The considered sway control schemes can be divided into two main categories: feed-forward control and feedback control techniques. Feed-forward techniques for sway suppression involve developing the control input through consideration of the physical and swaying properties of the system, so that system sways at dominant response modes are reduced. This method does not require additional sensors or actuators and does not account for changes in the system once the input is developed. On the other hand, feedback-control techniques use measurement and estimations of the system states to reduce sways. Feedback controllers can be designed to be robust to parameter uncertainty. For gantry crane, feed-forward and feedback control techniques are used for sway suppression and cart position control respectively. Various attempts in controlling gantry cranes system based on feed-forward control schemes were proposed. For example, open loop time optimal strategies were applied to the crane by many researchers such as discussed in (Manson, 1992; Auernig and Troger, 1987). They came out with poor results because feed-forward strategy is sensitive to the system parameters (e.g. rope length) and could not compensate for wind disturbances. Another feed-forward control strategy is input shaping (Karnopp et. al., 1992; Teo et. al., 1998; Singhose et. al., 1997). Input shaping is implemented in real time by convolving the command signal with an impulse sequence. The process has the effect of placing zeros at the locations of the flexible poles of the original system. An IIR filtering technique related to input shaping has been proposed for controlling suspended payloads (Feddema, 1993). Input shaping has been shown to be effective for controlling oscillation of gantry cranes when the load does not undergo hoisting (Noakes and Jansen, 1992; Singer et. al., 1997). Experimental results also indicate that shaped commands can be of benefit when the load is hoisted during the motion (Kress et. al. 1994). On the other hand, feedback control which is well known to be less sensitive to disturbances and parameter variations (Belanger, 1995) is also adopted for controlling the gantry crane system. Recent work on gantry crane control system was presented by (Omar, 2003). The author had proposed proportional-derivative PD controllers for both position and anti-sway controls. Furthermore, a fuzzybased intelligent gantry crane system has been proposed (Wahyudi and Jalani, 2005). The proposed fuzzy logic controllers consist of position as well as anti-sway controllers. However, most of the feedback control system proposed needs sensors for measuring the cart position as well as the load sway angle. In addition, designing the sway angle measurement of the real gantry crane system, in particular, is not an easy task since there is a hoisting mechanism. This paper presents investigations of anti-sway angle control approach in order to eliminate the effect of disturbances applied to the gantry crane system. A simulation environment is developed within Simulink and Matlab for evaluation of the control strategies. In this work, the dynamic model of the gantry crane system is derived using the Euler-Lagrange formulation. To demonstrate the effectiveness of the proposed control strategy, the disturbances effect is applied at the hoisting rope of the gantry crane. This is then extended to develop a feedback control strategy for sway angle reduction and disturbances rejection. Two feedback control strategies which are Delayed feedback signal and PD-type fuzzy logic controller are developed in this simulation work. Performances of each controller are examined in terms of sway angle suppression, disturbances rejection, time response specifications and input force. Finally, a comparative assessment of the impact of each controller on the system performance is presented and discussed. Investigations into sway suppression of a gantry crane system with disturbances effect using the DFS and PD-type fuzzy logic controller have been presented. Performances of the controller are examined in terms of sway suppression, disturbances cancellation, sway angle response specifications and input force. The results demonstrated that the effect of the disturbances in the system can successfully be handled by both DFS and PD-type fuzzy logic controller. A significant level of reduction in the system sway has been achieved with the DFS controller as compared to the PD-type fuzzy logic controller. By using the PD-type fuzzy logic controller, the speed of the response is slightly faster and exhibits lower overshoot as compared to the DFS controller. The results show that both controller required fast control action with high magnitude of positive forces to reject the disturbance effect.

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