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1. Introduction
A supply chain is composed of some independent organizations that work with one another in an integrated system to add value to a product, organizations such as suppliers, logistic providers, manufacturers and distributors.
In recent years using multi-site manufacturing system (MSMS) is paid attention by various companies (Averbakh, 2010). In an MSMS, the final products are produced by multiple production centers in different geographical areas. Using a multi-site manufacturing system has some benefits rather than single-site manufacturing system. The benefits are as follows:
Reducing transportation cost: In MSMS, the suppliers are clustered based on their geographical zones and a production site is established in each cluster. Each supplier conveys the raw materials to its related manufacturing center. This reduces the total transportation costs.
Reducing risk: Various hazards such as natural hazards (flood, earthquake, tsunami, storm, etc.) and human made hazards (fire, strike, etc.) could interrupt the whole production. Using MSMS reduces the risk of interruption in production, because if production in one site is stopped, other sites may continue the production of final products.
Environmental benefits: Some geographical zones may have useful attributes such as lower labor costs. Certain benefits may be achieved by establishing a production site in these zones.
This article studies scheduling in the supply chain of a multi-site manufacturing system. It is assumed that some suppliers produce raw material or initial parts and convey them by a fleet of vehicles to some manufacturing sites. The objective function is to minimize the total delivery time required to deliver orders to the manufacturing sites. Figure 1 shows a supply chain with seven suppliers, four vehicles and two manufacturing centers.
First, a linear mixed integer model which considers production and transportation constraints is presented to minimize the total delivery time of orders. Then, a new genetic algorithm based on the reference group concept in sociology named Reference Group Genetic Algorithm (RGGA) is presented to solve the problem. Robert K. Merton is a sociologist that presents the concept of reference groups in society (Merton, 1957). He believed that people such as heroes or entertainment artists, whom he calls role models, influence other members in every society (Calhoun, 2010). In other words, people in society try to imitate their role models. We use this...