# A Survey of the Literature on Order-Picking Systems by Combining Planning Problems
## 1 Introduction  
Manufacturing companies confront different challenges to fulfill customer orders and distinguish between them. Many manufacturing companies consider reducing and eliminating costs and improving productivity in their logistic system important improvement activities. On the other hand, the warehouse is one of the most critical in-house logistics components, playing a crucial role, among other activities, in the supply chain of any manufacturing company. To meet customer needs, the OPS, as part of the warehouse preparation process, has an important place in studies. Sixty percent of all productive activities and 55% of warehouse operating costs include order-picking processes [1]. In each supply system, there are parts-warehousing-related activities, such as receiving parts, warehousing, ordering, and shipping along product lines [2]. For reducing warehouse operating costs, in assembly lines, it is highly significant for the company to supply necessary materials within the factory to promote its competitive power. In a factory, materials should be provided with high efficiency and flexibility. The complexity of the material and parts transportation system will lead to an increase in production simultaneously. Researchers, in recent years, have been increasingly motivated to develop OPS enhancement policies [3]. Internal logistics aspects also need a great deal of attention due to the constraints and resources available to reduce costs and increase the flexibility of production schedule changes to meet customer demands by expanding the ability of line feedingsystems and offshore components. In this regard, using new methods of feeding materials and parts such as picking seem necessary [4]. Picking refers to some factors, including moving, picking, deploying, and packing parts according to the amount of order required to meet the production schedule [5]. Receiving materials and parts from the warehouse, preparing them, and moving them along product lines are the most strenuous activities in an internal logistic system. According to studies by [PERSON] et al. [5], order picking activities account for 65% of total operating costs in the internal logistics process. The order-pickings are based on the workforce and the flexibility of the workforce against changes due to existing constraints [6]. Other concepts used in the picking process are as follows: travel distance, travel time, layout, channel configuration, dimension and size of the picking, order batching, ordering of picking parts, minimizing travel distance, maximizing occupational health, picking list (required orders), stress, and psychological conditions [7]. The process of OPS is performed in different ways, as discussed below:  
1. To allocate the piece to the picker
2. Picker assignment for parts  
In the part assignment method for picking parts, the picker is carried, by a parts transporter or courier container, or he picks up the pieces himself [8]. In this method, the picking station is defined separately for each individual. The type of transfer operation is also proportional to the kind of concerned picking process. The part assignment method is also ergonomically beneficial to the operator [4]. On the other hand, the method of assigning a piece refers to time, when each part has its picker in a specific place [8]. Figure 1 shows the process map of the picking operation. As seen, picking begins with receiving parts from the warehouse. There are two different ways of warehousing: batching and picking operations. If the received items do not meet the quality standards and documentation required for the factory, they will be returned to the manufacturer's dock. Picking operations are performed either manually or automatically, using semi-robotic or robotic equipment. Each moving part to the picking area is also selected based on batching order and picking operations. New market developments encompassing e-commerce, globalization, increasing customer expectations, and new regulations have intensified the competition between warehouses.