Abstract: The "Four-Way" shuttle based order picking system has been widely used in recent years, which adopts the "Four-Way shuttle-Hoister-Roller" operation model and can effectively enhance order picking efficiency and shorten order picking time. At present, the optimization of order picking time has become a key factor and difficult problem to improve the efficiency of the four-way shuttle based order picking system. To shorten order picking time and enhance the efficiency of "Four-Way" shuttle picking system, an optimization model is proposed for batch order sequencing whose target is simplified to decrease the travel time of four-way shuttles between storage positions. Considering the characteristic of order sequencing problems when the order quantity is larger than 50, this model can be seen as NP-hard problem, which is hard to obtain the optimal solution within limited time. To attain a solution under an acceptable range within limited time a modified coupling degree greedy algorithm is developed to solve this problem, which is mainly based on order storage position coupling degree for determining the sequence of orders and SKUs so as to minimize the order picking time. The test results show that compared with traditional algorithms this algorithm can effectively cut down order picking time and increase the picking system's efficiency.

Key words: “four-way” shuttle system, part-to-picker, order sequencing, coupling degree, order picking time