| "Countries are what based on people; while food is what matters to people". Grain is an important commodity relating to the national economy, the people's livelihood and even the national economic security. The grain underproduction or total crop failures caused by natural disasters have occured frequently. Emergencies have tremendous negative impacts on people's work and life; simultaneously forward an ordeal on the grain security and emergency system to every country in the world. Sufficient food supply is not only the premise of emergency rescue, but also the safeguard of post-disaster reconstruction. A full-featured grain logistics network can ensure grain supply and transportation dispatch, thus reduce the service pressure of government. Therefore, it has theory value and practical significance that network structure and path optimization problem of food emergency system are studied.In this paper, grain emergency logistics is taken as research object. The main task is shown as follows:First of all, the node, distribution level and network construction are analyzed according to the characteristics of grain logistics. The number, location decision and evaluation principle of the grain logistics base are discussed. The extension, classify and solving algorithm of Vehicle Routing Problem (VRP) have described.Second, the existing problems are summarized through researching the present situation of grain supply and demand, grain logistics' infrastructure. Constructing some grain logistics bases is proposed in the nationwide in order to improve grain logistics system and form regional grain emergency logistics network. Two systems of evaluation are separately constructed to satisfy with the future economy development and disaster relief, on the basis of researches on related factors of grain logistics base site selection. Then comprehensive evaluation of 31 regions in our country is done with Analytic Hierarchy Process. At last, the location program of the grain logistics base is obtained.Once more, considering the characteristics of the grain emergency logistics, the grain emergency logistics' Vehicle Routing Problem (VRP) could be boiled down to Vehicle Routing Problem with Time Deadlines (VRPTD) with single cargo, fully loaded, no inventory limit, aiming at total transportation cost minimizing. The grain emergency logistics VRPTD model is established which aiming at maximum satisfaction through the correlation function of traffic congestion and time. Then, the appropriate heuristic algorithm to solve VRPTD model is selected between Genetic Algorithms (GA) and Ant Colony Optimization (ACO). This paper selects ACO which has better performance of solving speed, optimal solution quality and algorithm stability, etc compared with GA under the same conditions.Finally, the mechanism and mathematical model of Max-Min Ant Colony Optimization (MMACO) and ACO are researched. The solution steps of VRPTD are presented through modifying the transition probability, the selection strategy and the pheromone update strategy. Two-stage method and improved ACO are used to solve the case respectively. By comparing the routing optimization results obtained, the solution of modified ACO is more reasonable. |
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