In recent years,multiple-load automatic guided vehicle(AGV)is increasingly used in the logistics transportation fields,owing to the advantages of smaller fleet size and fewer occurrences of traffic congestion.However,...In recent years,multiple-load automatic guided vehicle(AGV)is increasingly used in the logistics transportation fields,owing to the advantages of smaller fleet size and fewer occurrences of traffic congestion.However,one main challenge lies in the deadlock-avoidance for the dispatching process of a multiple-load AGV system.To prevent the system from falling into a deadlock,a strategy of keeping the number of jobs in the system(NJIS)at a low level is adopted in most existing literatures.It is noteworthy that a low-level NJIS will make the processing machine easier to be starved,thereby reducing the system efficiency unavoidably.The motivation of the paper is to develop a deadlock-avoidance dispatching method for a multiple-load AGV system operating at a high NJIS level.Firstly,the deadlock-avoidance dispatching method is devised by incorporating a deadlock-avoidance strategy into a dispatching procedure that contains four sub-problems.In this strategy,critical tasks are recognized according to the status of workstation buffers,and then temporarily forbidden to avoid potential deadlocks.Secondly,three multiattribute dispatching rules are designed for system efficiency,where both the traveling distance and the buffer status are taken into account.Finally,a simulation system is developed to evaluate the performance of the proposed deadlock-avoidance strategy and dispatching rules at different NJIS levels.The experimental results demonstrate that our deadlock-avoidance dispatching method can improve the system efficiency at a high NJIS level and the adaptability to various system settings,while still avoiding potential deadlocks.展开更多
A new method for topology optimization of truss-like structures with stress constraints under multiple-load cases(MLCs)is presented.A spatial truss-like material model with three families of orthotropic members is ado...A new method for topology optimization of truss-like structures with stress constraints under multiple-load cases(MLCs)is presented.A spatial truss-like material model with three families of orthotropic members is adopted,in which the three families of members along three orthotropic directions are embedded continuously in a weak matrix.The densities and directions of the three families of members at the nodes are taken as the design variables.An optimality criterion is suggested based on the concept of directional stiffness.First,under each single-load case(SLC),the truss-like structure is optimized as per the fully stressed criterion.Accordingly,the directional stiffness of the optimal structure under an SLC at every node is obtained.Next,the directional stiffness of the truss-like structure under MLCs is determined by ensuring that the directional stiffness is as similar as possible to the maximum directional stiffness of the optimal structure under every SLC along all directions.Finally,the directions and densities of the members in the optimal truss-like structures under MLCs are obtained by solving the eigenvalue problems of the coefficient matrix of the directional stiffness at every node.Two examples are presented to demonstrate the effectiveness and efficiency of the method.展开更多
Multiple-load carriers are widely introduced for material delivery in manufacturing systems.The real-time scheduling of multiple-load carriers is so complex that it deserves attention to pursue higher productivity and...Multiple-load carriers are widely introduced for material delivery in manufacturing systems.The real-time scheduling of multiple-load carriers is so complex that it deserves attention to pursue higher productivity and better system performance.In this paper,a support vector machine(SVM)-based real-time scheduling mechanism was proposed to tackle the scheduling problem of parts replenishment with multiple-load carriers in automobile assembly plants under dynamic environment.The SVM-based scheduling mechanism was trained first and then used to make the optimal real-time decisions between“wait”and“deliver”on the basis of real-time system states.An objective function considering throughput and delivery distances was established to evaluate the system performance.Moreover,a simulation model in eM-Plant software was developed to validate and compare the proposed SVM-based scheduling mechanism with the classic minimum batch size(MBS)heuristic.It simulated both the steady and dynamic environments which are characterized by the uncertainty of demands or scheduling criteria.The simulation results demonstrated that the SVM-based scheduling mechanism could dynamically make optimal real-time decisions for multiple-load carriers and outperform the MBS heuristic as well.展开更多
基金supported by the National Natural Science Foundation of China(Nos.52005427,61973154)the National Defense Basic Scientific Research Program of China(No.JCKY2018605C004)+1 种基金the Natural Science Research Project of Jiangsu Higher Education Institutions(Nos.19KJB510013,18KJA460009)the Foundation of Graduate Innovation Center in Nanjing University of Aeronautics and Astronautics(No.KFJJ20190516)。
文摘In recent years,multiple-load automatic guided vehicle(AGV)is increasingly used in the logistics transportation fields,owing to the advantages of smaller fleet size and fewer occurrences of traffic congestion.However,one main challenge lies in the deadlock-avoidance for the dispatching process of a multiple-load AGV system.To prevent the system from falling into a deadlock,a strategy of keeping the number of jobs in the system(NJIS)at a low level is adopted in most existing literatures.It is noteworthy that a low-level NJIS will make the processing machine easier to be starved,thereby reducing the system efficiency unavoidably.The motivation of the paper is to develop a deadlock-avoidance dispatching method for a multiple-load AGV system operating at a high NJIS level.Firstly,the deadlock-avoidance dispatching method is devised by incorporating a deadlock-avoidance strategy into a dispatching procedure that contains four sub-problems.In this strategy,critical tasks are recognized according to the status of workstation buffers,and then temporarily forbidden to avoid potential deadlocks.Secondly,three multiattribute dispatching rules are designed for system efficiency,where both the traveling distance and the buffer status are taken into account.Finally,a simulation system is developed to evaluate the performance of the proposed deadlock-avoidance strategy and dispatching rules at different NJIS levels.The experimental results demonstrate that our deadlock-avoidance dispatching method can improve the system efficiency at a high NJIS level and the adaptability to various system settings,while still avoiding potential deadlocks.
基金The research reported in this paper was financially supported by the Natural Science Foundation of China(No.11572131)the Subsidized Project for Postgraduates’Innovative Fund in Scientific Research of Huaqiao University(No.17011086002).
文摘A new method for topology optimization of truss-like structures with stress constraints under multiple-load cases(MLCs)is presented.A spatial truss-like material model with three families of orthotropic members is adopted,in which the three families of members along three orthotropic directions are embedded continuously in a weak matrix.The densities and directions of the three families of members at the nodes are taken as the design variables.An optimality criterion is suggested based on the concept of directional stiffness.First,under each single-load case(SLC),the truss-like structure is optimized as per the fully stressed criterion.Accordingly,the directional stiffness of the optimal structure under an SLC at every node is obtained.Next,the directional stiffness of the truss-like structure under MLCs is determined by ensuring that the directional stiffness is as similar as possible to the maximum directional stiffness of the optimal structure under every SLC along all directions.Finally,the directions and densities of the members in the optimal truss-like structures under MLCs are obtained by solving the eigenvalue problems of the coefficient matrix of the directional stiffness at every node.Two examples are presented to demonstrate the effectiveness and efficiency of the method.
基金the National Natural Science Foundation of China(Grant No.71471135).
文摘Multiple-load carriers are widely introduced for material delivery in manufacturing systems.The real-time scheduling of multiple-load carriers is so complex that it deserves attention to pursue higher productivity and better system performance.In this paper,a support vector machine(SVM)-based real-time scheduling mechanism was proposed to tackle the scheduling problem of parts replenishment with multiple-load carriers in automobile assembly plants under dynamic environment.The SVM-based scheduling mechanism was trained first and then used to make the optimal real-time decisions between“wait”and“deliver”on the basis of real-time system states.An objective function considering throughput and delivery distances was established to evaluate the system performance.Moreover,a simulation model in eM-Plant software was developed to validate and compare the proposed SVM-based scheduling mechanism with the classic minimum batch size(MBS)heuristic.It simulated both the steady and dynamic environments which are characterized by the uncertainty of demands or scheduling criteria.The simulation results demonstrated that the SVM-based scheduling mechanism could dynamically make optimal real-time decisions for multiple-load carriers and outperform the MBS heuristic as well.
