Optimal Order Picking System In A Distribution Center

The total cost of social logistics in China is approximate 18% of GDP, significantly higher than the costs of the United States, Japan and other developed countries, which are estimated to be about 11%. So reducing logistics cost is of great benefit and potential. The current market characteristics of multi-items and small-quantities make turning over goods through the distribution center an important measure to reduce the costs of transport, storage, sorting and distribution. In the distribution center, the picking costs account for 60% of the total cost while the picking time occupies 35% of the total operation time. Storage Location Assignment Problem (SLAP) is a key factor that affects the efficiency of picking operation in reserve area and Complex Automated Sorting System (CASS) has been widely used for the picking operation in picking area. Therefore, research on SLAP and CASS is of great importance in increasing the working efficiency, improving service level and reducing logistics costs.Existing storage location assignment methods are mainly based on average storage features such as Turnover Frequency (TF) or Cube-per-Order Index (COI),regardless of the storage time of each load unit. Duration of Stay (DOS) based storage location strategy can come up with optimal solution. However, there is little research on it. This paper concentrates on mathematical model and heuristic algorithm of storage location assignment and provides a solid foundation for future study on storage location assignment problem and improvement of picking efficiency.The overall performance of single CASS depends on the combination of dispensers. Current combination methodology, which is based on EIQ analysis and ABC classification, does not take into account system performance and cost. Therefore, a new quantitative method of determining the optimal combination of dispensers is proposed in this paper. Combinatorial optimization model is established and then solved. Since there is no effective design and planning method for system configuration and task scheduling of multi-CASS-task problem currently, this paper establishes a corresponding optimization model based on th study of single CASS for multi-CASS picking system. And this provides new method as well as foundation for the research on multi-CASS picking system.Major work and innovation of this paper are as follows:(1) DOS-based storage location assignment optimization model and static greedy storage location assignment method are analyzed and summarized. According to the practical applications in storage location assignment problem, this paper comes up with a DOS-based dynamic storage location assignment method and presents concrete implementation steps as well as determination of the involved items parameters, storage travel time, actual and virtual DOS list. In the proposed method, load units that have overlaps in storage time with inbound load units are first forecasted and load units sequence, involved in the storage assignment, is obtained; then storage location is allocated based on the storage and rerieval time of each load unit. This approach is consistent with characteristics generated with access task in the practical application and can be easily applied to practice because of the low complexity in calculating the time.(2) To verify whether the DOS-based dynamic storage location assignment method is effective, a simulation program for the storage location assignment problem is developed based on dynamic storage location assignment, nearest open location, class-based assignment and dedicated assignment. The program uses Client/Serve (C/S) architecture. The server-side uses database to save the simulation data and several store procedures to realize the storage assignment algorithm and task maintenance; the client-side uses object-oriented design method to realize the simulation calculation and graphical display. The simulation program, the hierarchical structure of which is very clear, has good maintainability and openness. Simulation animation, which is intuitionistic and visual, is a representation of real warehouse operation process. Design of the simulation plan, which is based on the ABC phenomenon and the economic order quantity model, makes the simulation program representative and the simulation result persuasive strongly. Simulation results show that compared with the nearest open location, class-based assignment and dedicated assignment, in the DOS-based dynamic storage assignment method, the average one-way travel time is reduced by 8.32%,9.29% and 12.24% separately and the overall average is reduced by 9.95%. At the same time, the change of arrival sequence has little effects on the average one-way travel time, which fully demonstrates the effectiveness of the DOS-based dynamic storage assignment method.(3) Aiming at solving the problem of combinatorial optimization for dispensers, this paper analyzes the relationship of the sorting time, the sorting cost and dispensers'combination. An optimization model is developed to minimize the sorting unit cost. Moreover, the formulas of fixed cost, mini-variable cost and mini-soring time are deduced from the device characteristics, which simplifies the solution of the model. The results show that as the horizontal dispensers'quantity in the sorting system increases, the equipment cost and space cost per unit of time increase linearly, fixed staffing cost remain constant, personnel cost and sorting time gradually reduce with time, the increased or decreased degree is related with the number of associated horizontal dispensers. The sorting unit cost decreases at the first and then increases. The lowest point, which corresponds to the number and cost of horizontal dispensers, is the optimal combination of dispensers and smallest unit of sorting cost. This method avoids the subjectivity of the EIQ analysis and ABC classification and can be used to guide the choice of CASS dispensers combination effectively.(4) In order to solve the multi-CASS system configuration and task-scheduling problem, this paper establishes an optimization model by minimizing the total sorting cost and proposes a backtracking algorithm based on the single fixed cost of CASS, the mini-variable cost and mini-sorting time formulas. According to the characteristics of the problem, this paper rationally designs the branch structure and bound function, effectively narrows the space of understanding and reduces the computational complexity and computation time. An application example shows that the total cost mainly depends on the total fixed cost and less depends on the total variable cost. Total fixed cost mainly depends on the sets number of the sorting system. Therefore, by increasing the number of horizontal dispensers and reducing the sets number of the sorting systems, the total sorting cost can be reduced in the sorting system planning.