Optimization Of Unidirectional Circulation-type Material Handling System With Shortcuts

Unidirectional circulation-type material handling system on a single loop is a typical and basic unit in a flexible manufacturing system(FMS) or logistics system. It is widely used in the automated material handling systems(AMHSs) in semi-conductor manufacturing. It is also the important cohesion of subsystems in automation integrated system from loading and unloading points to the in-out stocks of the warehouse. Superposition efficiency of the basic unit often determines the one of the whole system. In this system, a fleet of AGVs circulates along a loop unidirectionally. After AGVs load jobs, jobs are sent to process at a processing station set along the path, and then unload at an unloading station. AGVs continue to circulate until all jobs are processed. The AGVs can’t be overtaken except in the cross point. So it is difficult to avoid the interference between AGVs.The addition of shortcut in unidirectional circulation-type material handling system on a single loop can decrease the impossible waiting time, provide shorter route for material handling system and also improve the efficiency of the system. To increase the throughput and improve the performance of the system, this paper made a mathematical model of this system, analyzed the different layout(quantity and locations setting) of short-cut layout and then carried out the experiments under the condition of two basic scheduling rules. Under the conditions of different scheduling rules and different number of vehicles, shortcuts set in different places can impact the throughput and the interference of the system. According to the analysis and experiment, under the condition of setting one shortcut, when the shortcut is set between xS and x?1S, i.e. location ? ???5.04)12(?snx, the maximum throughput can be achieved and the system efficiency become optimal. Under the condition of setting more shortcuts, from the view point of decreasing the energy consumption(interference), when the only shortcut must be located in the middle of all the processing station, the interference time achieves the minimum however the throughput is lower. From the view point of increasing the throughput in the system, when the all shortcuts must be located in the middle of each processing station, the maximum throughput can be achieved however the interference time increased. The experimental results confirmed the validity of the theoretical analysis.Different scheduling rules impact the interference between AGVs and then have an important effect on the efficiency of the whole system. In order to decrease the interference and improve performance of a unidirectional circulation-type material handling system on a single loop with a shortcut, this paper mainly studied the interference and scheduling problem between agvs when setting one shortcut in the middle of the system. Under the condition of same processing time, this part optimizes the scheduling rules based on processing sequence regardless of the processing time. According to the actual situation of material handling system, the interference of two scheduling rules(Random rule and Order rule)were analyzed, furthermore an optimal scheduling rule under the interference cases—Exchange order rule was proposed. Under the condition of different processing time, this part optimizes the scheduling rules combined the processing sequence and the processing time. The first come first serve rule combined with random rule(FCFS-Random), order rule(FCFS-Order) and E-order rule(FCFS-Eorder), the shortest processing time rule combined with random rule(SPT-Random), order rule(SPT-Order) and E-order rule(SPT-Eorder) are proposed to analyze the unidirectional circulation-type material handling system on a single loop with shortcut. According to the experiments, results showed that the E-order rule was better than the Random rule and Order rule under the condition of same processing time. E-order rule can reduce more interference and improve the efficiency of the system handling. Under the condition of different processing time, the SPT-Eorder rule not only made congestion(interference time) decrease but also achieved the optimal performance.