# Towards Productive and Ergonomic Order Picking: Multi-Objective Modeling Approach
## 1 Introduction
### Decision Support Models for Storage Assignment
Researchers' recommendations are valid for a specific order picking environment: a single-aisle consisting of a single shelf at a time, each item is stocked in a unique location, fixed and equally dimensioned storage locations, six different vertical and horizontal storage levels (0.25, 0.58, 0.94, 1.30, 1.63, 1.96 m), the depot (I/O point) at one end of the shelf, a return routing policy, walking speed 1 m/s, male operator: 75 kg, load of 0.2 kg per picked item. As authors already discover, further analyses of the impact of order profiles, especially the number of items (mass) to be picked per line of an order, on the relative efficiency of different storage assignment policies are required. [PERSON] et al. [17] found out in the laboratory experiment that the mass of the handled box, the initial lifting height, and the final lowering height significantly affected the lifting, carrying, lowering, and returning task times, and as such should be considered in the development of time prediction methods. In both the field and the laboratory experiments, the workers carried the box faster as the box mass increased. Further, the walking velocity for returning without a box was lower than the velocity for carrying the box in the laboratory experiment. Surprisingly, in their laboratory experiment, the worker's height and weight did not significantly influence the task times. [PERSON] and [PERSON] [39] concluded similarly. That is why we think it makes sense to introduce several different masses and even sizes of loads into modeling. [PERSON] et al. [40] researched trade-off analysis between time-based function and ergonomics-based function. If [9] studied human factors through energy expenditure, ref. [40] studied workers' discomfort using the Borg CR-10 scale [41] based on self-reporting. The authors propose a unified methodology to quantify and balance two potentially conflicting criteria at order picking. The first is the short-term economic criterion of minimizing total order-picking time (considering a location in an aisle, shelf height, quantity of a specific item to be picked, volume). The second is a human wellbeing criterion of minimizing average discomfort ratings. The recommendations are similar to [9], tied to a specific warehouse environment: order-picking, \"pick-by-light\", zones, limited walking distances, retrieving time is dominant in the cycle time, items are picked from tones on shelves at three levels (from 0.2 to 1.90 m), the maximal mass of loads is 3 kg, and most picks involve single order lines. Numerical data were collected from WMS, and workers reported discomfort rate. The authors propose a simple heuristic that combines two criteria and the popularity of an item. The item should be preferably stored close to the depot and in the golden zone, whereby more frequent items are preferred for more favorable locations. The authors in [40] excluded the influence of mass from the model, which becomes less suitable for warehouses where heavier loads are manipulated. In their specific case, the results showed that volume is a better proxy than mass to handle complexities such as easiness to grab a product or ease retrieving a product from its location. The authors confirmed that picking outside the golden zone requires additional retrieving time. These approaches [9; 40] are valuable and beneficial but do not reveal how often an employee has been exposed to situations with medium or high risk for injuries like MSDs. The missing aspect of the health risk was included in the analytical model [29] to assess metabolic cost using the energy expenditure concept and critical postures using the Ovako Working Posture Analysis System (OWAS) index. The model is one of the first attempts to integrate health risks and covers different warehouse racks for storing pallets. It does not respond to the challenges of picking small boxes in various dimensions and masses from a multi-shelf warehouse rack and the cost viewpoint is missing.