Iris F. A. Vis

Survey of Research in the Design and Control of Automated Guided Vehicle Systems

Automated guided vehicles (AGVs) are used for the internal and external transport of materials. Traditionally, AGVs were mostly used at manufacturing systems. Currently, AGVs are also used for repeating transportation tasks in other areas, such as warehouses, container terminals and external (underground) transportation systems. This paper discusses literature related to design and control issues of AGV systems at manufacturing, distribution, transshipment and transportation systems. It is concluded that most models can be applied for design problems at manufacturing centres. Some of these models and new models already proved to be successful in large AGV systems. In fact, new analytical and simulation models need to be developed for large AGV systems to overcome large computation times, NP-completeness, congestion, deadlocks and delays in the system and finite planning horizons. We specify more specific research perspectives in the design and control of AGV systems in distribution, transshipment and transportation systems.

A survey of literature on automated storage and retrieval systems

Automated Storage and Retrieval Systems (AS/RSs) are warehousing systems that are used for the storage and retrieval of products in both distribution and production environments. This paper provides an overview of literature from the past 30 years. A comprehensive explanation of the current state of the art in AS/RS design is provided for a range of issues such as system configuration, travel time estimation, storage assignment, dwell-point location, and request sequencing. The majority of the reviewed models and solution methods are applicable to static scheduling and design problems only. Requirements for AS/RSs are, however, increasingly of a more dynamic nature for which new models will need to be developed to overcome large computation times and finite planning horizons, and to improve system performance. Several other avenues for future research in the design and control of AS/RSs are also specified.

A model for warehouse layout

This paper describes an approach to determine a layout for the order picking area in warehouses, so that the average travel distance for the order pickers is minimized. We give analytical formulas that can be used to calculate the average length of an order picking route under two different routing policies. The optimal layout can be determined by using these formulas as the objective function in a nonlinear programming model. The optimal number of aisles in an order picking area appears to depend strongly on the required storage space and the pick list size.

Minimum Vehicle Fleet Size Under Time-Window Constraints at a Container Terminal

Products can be transported in containers from one port to another. At a container terminal these containers are transshipped from one mode of transportation to another. Cranes remove containers from a ship and put them at a certain time (i.e., release time) into a buffer area with limited capacity. A vehicle lifts a container from the buffer area before the buffer area is full (i.e., in due time) and transports the container from the buffer area to the storage area. At the storage area the container is placed in another buffer area. The advantage of using these buffer areas is the resultant decoupling of the unloading and transportation processes. We study the case in which each container has a time window [release time, due time] in which the transportation should start.The objective is to minimize the vehicle fleet size such that the transportation of each container starts within its time window. No literature has been found studying this relevant problem. We have developed an integer linear programming model to solve the problem of determining vehicle requirements under time-window constraints. We use simulation to validate the estimates of the vehicle fleet size by the analytical model. We test the ability of the model under various conditions. From these numerical experiments we conclude that the results of the analytical model are close to the results of the simulation model. Furthermore, we conclude that the analytical model performs well in the context of a container terminal.

Positioning of goods in a cross-docking environment

Cross docking is one of the options to reduce lead times and inventories and to improve customer response time in supply chains. Cross-docking centres are dynamic environments where products arrive, are regrouped, and leave the same day. In this paper we focus on the process of short-term storage of unit-loads in a cross-docking environment. The goal is to determine temporary storage locations for incoming unit loads such that the travel distances of the forklift trucks with these unit loads are minimised. We model this problem as a novel application of the minimum cost flow problem and show the applicability of the model for different types of layouts and priorities. We demonstrate both the efficiency and effectiveness of the method in the operational and design phase at cross-docking environments by applying it to practice-oriented examples. Furthermore, we show that the approach is superior to a commonly used heuristic method.

Sequencing Two Cooperating Automated Stacking Cranes in a Container Terminal

The containerized trade market is growing rapidly with the uprising of the Far East. Container ports worldwide should be responsive by developing tools to handle these massive volumes of containers in order to retain their level of competitiveness. One of the areas in a container terminal that is highly affected by the increase in the demand of containers is the stack. The stack is used to temporarily store containers upon further transport to their destination. This study is concerned with scheduling sea and landside storages and retrievals in a stack with two cooperating automated stacking cranes working in a single block. We present a mathematical model to minimize the makespan for both cranes. Both an algorithm to derive a lower bound for the makespan and a simulated-annealing based heuristic are proposed to efficiently solve the problem. Numerical experiments show that the solutions of the heuristic method are within 2% of the lower bound for large instances.

Scheduling of Container Storage and Retrieval

We consider the problem of scheduling the storage and retrieval of containers in the storage area of a container terminal. Some arcs in the underlying directed network must be visited; other arcs may be---but need not be---visited. We can, therefore, consider this problem to be a special case of the directed rural postman problem. We show that this problem can be reformulated as an asymmetric Steiner traveling salesman problem. This reformulation can be efficiently solved to optimality by a combination of optimal assignments in bipartite networks for parts of the problem and dynamic programming for the connections between those parts.

Performance analysis of berth configurations at container terminals

The containerized trade market has been growing rapidly since its introduction. The capacity of ships and the amount of containers being transshipped at container terminals increases significantly. Terminals should handle their operations efficiently to provide the necessary capacity and customer service. In designing a container terminal, terminal management has to consider the choice for a certain type of berth. In this paper, we compare by means of a simulation study the performance of traditional one-sided marginal berths and indented berths. An indented berth enables quay cranes to unload and load containers from both sides of the ship. As a result, more quay cranes can work on a single ship. As main performance measure in this comparison we use the total vessel operation time required to unload and load a ship. This time depends next to crane productivity also on the efficiency of the transportation and storage and retrieval processes in the terminal. We have performed a sensitivity analysis in which we also study the relation between the selection of an indented berth and other design and control issues in the terminal.

Optimising a general repair kit problem with a service constraint

Field services are a particular type of after-sales service performed at the customers location where technicians repair malfunctioning machines. The inventory decisions about which spare part types to take to the repair site and in what quantities is called the repair kit problem. This problem is characterized by an order-based performance measure since a customer is only satisfied when all required spare parts are available to fix the machine. As a result, the service level in the decision making process is defined as a job fill rate. In this paper we derive a closed-form expression for the expected service level and total costs for the repair kit problem in a general setting, where multiple units of each part type can be used in a multi-period problem. Such an all-or-nothing strategy is a new characteristic to investigate, but commonly used in practice. Namely, items are only taken from the inventory when all items to perform the repair are available in the right quantity. We develop a new algorithm to determine the contents of the repair kit both for a service and cost model while incorporating this new expression for the job fill rate. We show that the algorithm finds solutions which differ on average 0.2% from optimal costs. We perform a case study to test the performance of the algorithm in practice. Our approach results in service level improvements of more than 30% against similar holding costs.

Conceptualising inventory prepositioning in the humanitarian sector

Improved responsiveness to natural and man-made disasters is critical to saving lives and alleviating the suffering caused by such disasters. Emphasis on the design of the relief chain to reduce delivery time of relief inventory improves responsiveness. This is the essence of inventory pre-positioning (IPP). IPP is yet to be clearly defined; and the main factors affecting IPP decision-making need to be further discussed in the humanitarian literature. Quantitative factors such as costs usually take precedence over qualitative factors such as collaboration, infrastructure and others that can substantially impact IPP decisions. This can lead to ineffective decision-making in humanitarian organizations. This paper provides a definition of IPP by reviewing relevant literatures; and a conceptual framework of the main factors affecting IPP decision-making specific to the humanitarian sector, developed through desk reviews and observations.