The effect of warehouse cross aisles on order picking efficiency

This paper investigates the effects of adding cross aisles to the layout of an order consolidation warehouse, with respect to order picking efficiency. Intuitively, cross aisles provide greater flexibility in the routing of order pickers, thus providing shorter order picking travel distances. However, this effect may be negated when the number of cross aisles becomes excessive, as the cross aisles themselves must be traversed in order to reach the required items. A shortest path pick sequencing model is developed which allows for any number of cross aisles in the warehouse. The optimal routing is computed for a large number of randomly generated picking requests, over a variety of warehouse layout and order picking parameters. The results are used to characterize the optimal number of warehouse cross aisles, as well as the conditions under which cross aisles generate the greatest benefit.     

Exploring the role of picker personality in predicting picking performance with pick by voice,pick to light and RF-terminal picking

Order pickers and individual differences between them could have a substantial impact on picking performance, but are largely ignored in studies on order picking. This paper explores the role of individual differences in picking performance with various picking tools (pick by voice, RF-terminal picking and pick to light) and methods (parallel, zone and dynamic zone picking). A unique realistic field experiment with 101 participants (academic students, vocational students and professional pickers) is employed to investigate the influence of individual differences, especially the Big Five personality traits, on picking performance in terms of productivity and quality. The results suggest that (PbV) performs better than RF-terminal picking, and that Neuroticism, Extraversion, Conscientiousness and the age of the picker play a significant role in predicting picking performance with voice and RF-terminals. Furthermore, achieving higher productivity appears to be possible without sacrificing quality. Managers can increase picking performance by incorporating the insights in assigning the right pickers to work with a particular picking tool or method, leading to increased picking performance and reduced warehousing costs.     

Robotics in order picking: evaluating warehouse layouts for pick,place, and transport vehicle routing systems

Motivated by recent technological advances in mobile robotics, this paper explores a novel approach for warehouse order picking. In particular, this work considers two types of commercially available mobile robots – one that can grasp items from a shelf (a picker) and another (a transporter) that can quickly deliver all items from the pick list to the packing station. A new vehicle routing problem is defined which seeks to minimise the time to deliver all items from a pick list to the packing station, a problem termed the pick, place, and transport vehicle routing problem. A mixed integer linear programming formulation is developed to answer three related research questions. First, what combination of picker and transport robots is required to obtain performance exceeding traditional human-based picking operations? Second, how should the composition of the robot fleet be altered to affect the greatest performance improvements? Finally, what are the impacts of warehouse layout designs when coordinated mobile robots are deployed? An extensive numerical analysis reveals that, (1) increasing the number of cross aisles decreases system performance; (2) centrally located packing stations improve system performance; and (3) the average distance from each pick location to the packing station and the average distance between pick locations are effective metrics for identifying specific fleet modifications that are likely to yield system improvements.     

The effects of picker-oriented operational factors on hand-off delay in a bucket brigade order picking system

OR Spectrum - During collaboration between neighboring pickers in a bucket brigade order picking, the downstream picker frequently experiences productivity loss in the form of a hand-off delay...     

Integrated order picking and vehicle routing with due dates

OR Spectrum - Supermarkets typically order their goods from a centrally located distribution center (warehouse). Each order that the warehouse receives is characterized by the requested...     

The use of bucket brigades in zone order picking systems

As the transactions through electronic commerce and TV home shopping increase, the warehouses often receive a large amount of small orders to be picked within tight time windows. One of the important warehousing activities is order picking, the process of retrieving a number of items from warehouse storage to meet a number of independent customer orders. This paper examines a new order picking method, bucket brigade order picking (BB picking). Bucket brigade is a way of coordinating workers who progressively perform a set of operations on a flow line. In the BB picking system, a worker performs operations on an order until the next worker downstream takes it over; then goes back to the previous worker upstream to take over a new order. We discuss distinct characteristics in order picking systems when bucket brigades are applied. We identify some efficiency losses under the BB picking and present a new BB picking protocol to improve the performance of order picking systems. The new BB picking is compared with the existing BB picking and zone picking through simulation experiments.     

Maverick picking: the impact of modifications in work schedules on manual order picking processes

Order picking, the process of retrieving items from their storage locations to fulfil customer orders, ranks among the most labour- and time-intensive processes in warehousing. Prior research in this area had a strong focus on the development of operating policies that increase the efficiency of manual order picking, for example by calculating optimal routes for the order pickers or by assigning products to storage locations. One aspect that poses a major challenge to many warehouse managers in practice has, curiously enough, remained largely unexplored by academic research: modifications in workflows (i.e. workplace deviance in a positive or negative sense) in order picking, which we define as ‘maverick picking’. The purpose of this paper is to characterise maverick picking and to study its causes, its forms of appearance and its potential impact on order picking performance. To gain insights into maverick picking, we first survey the literature to illustrate the state-of-knowledge of maverick picking. Subsequently, we report the results of a multi-case study on maverick picking and deduct a related content framework. The results of our case study support the proposition that maverick picking is highly relevant in practice and that it is a major determinant of order picking performance.     

Dynamic order batching in bucket brigade order picking systems with consecutive batch windows and non-identical pickers

The blocking mitigation mechanism contained in a batching and sequencing model improves order picking performance in bucket brigade order picking systems by adjusting the batch formation in a given batch window. The batch formation of a given batch window affects the transition to the next batch window and limits the batch formation’s structure in the next window. In addition, imbalance in picker capability increases the variation of batch formation while mitigating blocking delay. Our paper proposes a dynamic indexed batching model to smooth the transition between consecutive batch windows and give a better approximation of non-identical pickers’ capability. We develop a mixed integer programming solution for a dynamic indexed batching model in a bucket brigade order picking system (DIBMB). Simulation experiments show that the DIBMB smooths the transition between batch windows as measured in the delay experienced by the first batch in each batch window. Comparisons to the available batching models show that under DIBMB, the blocking delay gradually increases throughout the sequence of batches.     

A structured procedure for analysis and design of order pick systems

We present a structured procedure for order pick system (OPS) analysis and design that has been established on literature review and interviews with and presentations to OPS experts. In particular, we attempt to include the thinking processes that occur between OPS designers and owners. The design procedure and related issues are discussed in the order of input, selection, and evaluation stages.     

Storage policies and order picking strategies in U-shaped order-picking systems with a movable base

Prior research on storage policies and order-picking strategies in order-picking systems has concentrated on studying warehouses with parallel shelves that are separated by horizontal and/or vertical aisles. This paper analyses a special case of an order-picking system where the warehouse is divided into zones with shelves being arranged in the shape of a U in each of the zones. The paper assumes that the shelves of the order-picking system are made up of two rows of stillages that can be flexibly exchanged and that the base of the order-picking process can be moved within the aisle prior to the beginning of the order-picking process. We describe the order-picking system in a formal model and propose different storage location assignment policies whose efficiency is compared in a numerical study. The paper pays special attention to the practical applicability of the model and proposes heuristics that can be easily implemented in practice.     

Determining the number of zones in a pick-and-sort order picking system

In this study we consider a pick-and-sort order picking system, in which batches of orders are picked simultaneously from different (work) zones by a group of order pickers. After picking, the orders are transported by a conveyor to a next station for consolidation and packing. Packing can only occur when an order has been picked completely. For a given number of workers, each assigned to a single zone, a larger number of zones reduces pick time (since travel time reduces), but increases waiting time for completion at the packing stations, because more partial batches needing assembly arrive at the packing stations. Our aim is to determine the optimal number of zones such that the total (picking and packing) time to complete a batch is minimised. We solve this problem by optimally assigning items to pick routes in each zone. We illustrate the method with data taken from a distribution centre of one of the largest online retailers in The Netherlands.     

Human factors in order picking: a content analysis of the literature

Order picking (OP) is one of the most labour- and time-intensive processes in internal logistics. Over the last decades, researchers have developed various mathematical planning models that help to increase the efficiency of OP systems, for example, by optimising storage assignments or by specifying routes for the order pickers that minimise travel distance in the warehouse. Human characteristics that are often a major determinant of OP system performance have, however, widely been ignored in this stream of research. This paper systematically evaluates the literature on manual OP systems and conducts a content analysis to gain insights into how human factors (HF) have been considered and discussed in the scientific literature. The results of the analysis indicate that management-oriented efficiency criteria dominated prior research on OP, and that there is a clear lack of attention to HF in the design and management of OP systems. This poses an opportunity for research and design of manual OP systems.     

Realization of a short cycle time in warehouse replenishment and order picking

Today's competitive and dynamic environment forces companies to be highly responsive and customer oriented to survive and be successful. This paper considers an automated gantry picking complex with 16 pick zones and 16 replenishment zones. Currently, it operates on a 24-h cycle time. A short cycle time operation allows a warehouse manager to be more responsive, flexible and competitive. For example, it allows insertion of late arriving, but high priority, orders into the current cycle. However, to operate in a short cycle environment, the warehouse operations' planning and execution must be more dynamic, real-time and intelligent. To realize a short cycle time without loss of productivity, a new replenishment process logic for the gantry-picking complex is developed. The main idea of the new replenishment logic is to minimize the set-up time. Experimental results demonstrating the efficiency of the new approach are also presented.     

The value of integrating order picking and vehicle routing decisions in a B2C e-commerce environment

In B2C e-commerce sales, customers expect a fast and low-cost delivery. To be able to fulfil these customer expectations, both warehouse and distribution operations have to be performed in an efficient and effective way. Ideally, these two supply chain functions should be considered simultaneously in an integrated problem since they are interrelated. In this paper, a record-to-record travel algorithm is proposed to solve the integrated order picking-vehicle routing problem (I-OP-VRP). Experiments with both small-size and large-size instances are conducted. Furthermore, the integrated approach is compared with an approach in which both problems are solved sequentially. Results show that integration leads to increased service levels, i.e. it allows to shorten the time between placing an order and receiving the goods. On top, the integrated approach leads to costs savings of on average 1.8%. Thus, integration is indispensable for a fast and cost-efficient delivery of goods.     

The structure of fermion density matrices. III. Long-range order

Generalizing C. N. Yang's discussion of the onset of superconductivity, a criterion is proposed for the existence of infinite or localized long-range order. Questions are raised concerning the spectrum of the reduced Hamiltonian and the statistics of Fermi pairons.     

A simulation-based comparison of two goods-to-person order picking systems in an online retail setting

Design and analysis of order picking systems continues to be an active topic of interest both in academia and practice, especially in light of the significant increase in online retail sales. In this paper, we examine two types of well-known, goods-to-person order picking systems, namely, a miniload system and a Kiva system. Using a simulation model, we compare the performance of the two systems on the basis of expected throughput and expected container retrieval times to process the same set of customer orders. We also discuss some of the advantages and limitations of the two systems.     

Experimental research on wear of conical pick interacting with coal-rock

For severe wear of a conical pick on tunneling and mining equipment, the influence of pick cutting types, structure and working angle parameters on pick wear is studied using an experimental apparatus for cutting coal-rock. The influence of pick wear on cutting performance is analyzed, which provides an experimental basis for reducing pick wear. The results indicate that the pick wears on only one side under asymmetrical cutting types, which increases the pick's working life. The larger the height of carbide tip and cone angle of pick tip are, the easier it is to protect the head face of the pick-body from interference with coal-rock and avoid carbide tip loss due to wear of the pick-body. In the range of research, the suitable height of the carbide tip is found to be 20 mm ~ 24 mm, and the cone angle of the pick tip should be approximately 80°. The wear area on the pick-body increases with the head face diameter of the pick-body, and a suitable range for the head face diameter is found to be 20 mm–22 mm. Pick wear decreases with cutting angle, but the cutting angle should be maintained at 45°–50°, considering the pick cutting load and coal-rock caving effect. For picks with an inclined angle, pick wear increases with incline angle, however, wear differences on the two sides of the pick form and increase gradually, which improves the pick's self-rotatory ability. The mean peak of pick cutting torque increases with wear degree, and the largest increase percentage was found to be 30%. Furthermore, a worn pick is unfavorable for the caving of coal-rock and the coal-rock fragment size of < 2 mm increases by 14% compared to a pick without wear.     

The effects of age and traffic density on street-crossing behavior

Past research has shown that road users accept shorter time gaps when the waiting time/number of vehicles they let pass before attempting to merge into the traffic increases. While elderly pedestrians are known to be an extremely vulnerable group of road users, very few studies dealt with the effect of environmental constraints and crossing complexity on this population's safety. The present study aimed at determining whether or not street-crossing decisions and behavior of younger and older pedestrians were differently affected by a traffic flow. In an interactive street-crossing task, we assessed whether mean time gap and crossing decisions depended on the position of the gap pedestrians selected into the traffic stream. Results revealed that irrespective of their age pedestrians accepted a smaller time gap when they chose the second interval of the traffic compared to the first one. Contrasting with previous hypotheses, this traffic-related behavior was not accompanied by an increase in the decisions risk. The findings also showed that the transition threshold from rejecting to accepting time gaps was shorter when the second interval was selected compared to the first one. This increment in task constraints might help younger and older pedestrians alike to perceive action possibilities more accurately and to be better attuned to traffic conditions by comparing gaps between each other. This opens an interesting perspective in the understanding and the training of the ability of elderly road users to remain accurate in their judgements.     

The method of caustics for mixed-mode crack loading with higher order term effects

The method of caustics or shadow-spot method for mixed-mode crack loading is investigated within the framework of Westergaard stress function analysis. The effect of higher order terms in the stress functions onto the shape and size of the caustic is studied for optically isotropic materials. The classical evaluation procedure is generalized and it is shown that the relationship between stress intensity factor and caustic diameter is of the general form K∞ D^(5/2)+n.     

The ballistic performance of narrow fabrics

Woven fabrics are widely used in soft body armour. The ballistic performance of a range of narrow fabrics was determined and compared to that of wider fabric panels. In contrast to conventional ballistic fabrics, it was found that the performance of narrow fabrics was highly sensitive to changes in fabric specification. Furthermore, it was shown that square weave fabrics may not necessarily be superior to unbalanced fabrics in narrow fabric form. It was also confirmed that narrow fabrics absorb more energy than wider fabric panels when undergoing direct ballistic impact, particularly when gripped in a two-sided configuration. Ways were sought of utilising these improvements in performance when the narrow fabrics are brought together to make full-width armour panels. Potentially, the observed improvements in performance could have been offset by the lines of weakness between adjacent fabric strips or by the weight of the structure required to grip on two sides. However, after investigating a number of potential solutions, significant improvements in the performance-to-weight ratios over simple fabric armour were achieved using an assembly of narrow fabrics attached to a lightweight composite frame.