Ying-Chin Ho

Solving cell formation problems in a manufacturing environment with flexible processing and routeing capabilities

In this paper, we investigate cell formation problems in a manufacturing system with flexible processing and routeing capabilities. Flexible processing means that a part can be manufactured by any one of several process combinations. Flexible routeing means that even when the processes are fixed, the part can traverse any one of several routes—each offering the same processes. The cell formation problems for these types of manufacturing systems are different from traditional problems. Traditional cell formation problems are for manufacturing systems with fixed processes and routes where one knows what machines will be used for manufacturing the parts. However, this information will not be available in the flexible systems discussed here, since a part can be completed by any one of the many different combinations of machines. In other words, it is possible to manufacture two identical parts using different sets of machines. Traditional cell formation techniques rely on knowing the specific machine usage in...

Machine layout with a linear single-row flow path in an automated manufacturing system

This paper investigates the machine layout problem with a linear single-row flow path within an automated manufacturing system. Traditional machine layout approaches can produce inappropriate layout designs because they often do not consider flow path characteristics, such as flow path configuration and feasible flow path direction. This paper investigates the effects of such flow path characteristics on machine layouts. A two-phase layout procedure that combines flow line analysis with simulated annealing is proposed. The procedure can be employed using a variety of evaluation criteria (minimize total flow distance, maximize number of in-sequence movements, and so on). Different layout procedures can be obtained by varying the flow line analysis method and temperature reduction strategy employed for simulated annealing. Experiments are performed to test the performance of different layout procedures for different combinations of flow path characteristics and quantities of machines. The experimental results provide vital information on selecting appropriate flow line analysis methods and temperature reduction strategies for different layout problems. Furthermore, the results will help designers determine appropriate evaluation criteria for different layout problems.

Order-batching methods for an order-picking warehouse with two cross aisles

Order batching is one of the methods used in warehouses to minimize the travel distance of pickers. In this paper, we focus on developing order-batching methods for an order-picking warehouse with two cross aisles and an I/O point at one of its corners. Each of these methods is made up of one seed-order selection rule and one accompanying-order selection rule. Eleven seed-order selection rules and 14 accompanying-order selection rules are studied here. These rules include those newly proposed by us and those by others. Rules proposed by others have been shown to perform well in minimizing the travel distance of pickers. They are included here for the comparison purpose. Unlike previous studies that only focus on developing aisle or location-based rules, this study also develops rules that are distance- or area-based. In addition, two different route-planning methods and two different aisle-picking-frequency distributions are considered in this paper. This studys objective is to investigate not only the performance of seed-order selection rules and accompanying-order selection rules, but also the mutual effects between route-planning methods, aisle-picking-frequency distributions, seed-order selection rules, and accompanying-order selection rules on their performance. The result of this study shows that some of the newly proposed rules outperform those from other studies. It also shows that seed-order selection rules and accompanying-order selection rules significantly affect each others performance. Lastly, the performance rankings of seed-order selection rules and accompanying-order selection rules are affected by aisle-picking-frequency distributions, but not by route-planning methods.

A study on order-batching methods of order-picking in a distribution centre with two cross-aisles

One common practice in order picking is order batching, in which items of two or more orders are picked together in one picking trip. Order batching can reduce the total order-picking travel distance if orders with similar picking locations are batched together and picked in the same order-picking trip. In this paper, the performance of different order-batching methods that are made up of one seed-order selection rule and one accompanying-order selection rule is investigated. A seed-order selection rule selects the first order (i.e. the seed order) in an order batch, while an accompanying-order selection rule selects the rest of orders (i.e. the accompanying orders) to be added to the order batch. In this paper, one investigates the performance of nine seed-order selection rules and 10 accompanying-order selection rules under two different route-planning methods and two different aisle-picking-frequency distributions. The problem environment is a distribution centres warehouse which has an I/O point at one of its corners and two cross-aisles—one front cross-aisle and one back cross-aisle. One wants to understand not only the performance of every seed-order selection rule and every accompanying-order selection rule, but also their combined performance. The effects of route-planning methods and aisle-picking-frequency distributions on the performance of seed-order selection rules and accompanying-order selection rules are also investigated. Different random problems were generated and tested for this purpose. It is hoped that the knowledge learned from this study can benefit practitioners in distribution centres with order-batching operations.

A hybrid approach for concurrent layout design of cells and their flow paths in a tree configuration

The layout design of multiple-cell automated manufacturing systems includes cell layout design and flow path layout design. Traditional layout methods often treat these two as separate problems and the sequence for solving them is usually cell layout first and flow path layout later. However, approaches of these kinds have one major drawback, that is, they may produce cell layouts that are awkward or difficult for designers to conduct flow path layouts, or cell layouts that do not turn out to be as good as expected after flow path layouts have been performed. Other drawbacks of traditional layout methods include irregular shapes of cells, inaccurate calculations of flow distances, etc. This paper addresses the layout problem of cells and their connecting flow paths in a tree configuration. The proposed layout procedure is designed to avoid the aforementioned drawbacks of traditional layout methods by emphasizing concurrent layout design of cells and flow paths. It combines a search algorithm and mathematical programming models. The search algorithm has a backtracking procedure that allows one to explore alternative layouts, while the mathematical programming models help one obtain accurate layouts of cells and flow paths. The proposed layout procedure also interacts with designers and allows designers to include their qualitative consideration into the layout design. As a result, one can obtain more accurate and good-quality layouts with the proposed layout procedure.

A machine-to-loop assignment and layout design methodology for tandem AGV systems with multiple-load vehicles

In this paper, we propose a design methodology for tandem Automated Guided Vehicle (AGV) systems with multiple-load vehicles. Our goal is to devise a design methodology that can achieve the following objectives in multiple-load tandem AGV designs. The first objective is to achieve the workload-balance between vehicles of different loops. The second objective is to minimize the inter-loop flow. The final objective is to minimize the flow distance. To achieve these objectives, the following problems are studied. First, we conduct simulations to study the relationship between a vehicles load-carrying capacity and work capacity. This relationship is then considered in the second problem, i.e. the problem of determining the machine content of each loop. A two-stage machine-to-loop assignment method is proposed for solving this problem. At the first stage, an initial machine-to-loop assignment is generated. This initial assignment is then improved at the second stage using Simulated Annealing (SA). The third problem is the problem of determining the layout of machines in each loop. A flow-line design method and SA are adopted for solving this problem. The fourth problem is the layout problem of guide-path loops. To solve this problem, a famous layout method found in the literature is adopted. The last problem includes the problem of determining the best orientation of each loop and the problem of finding the best places to set up transfer stations between adjacent loops. A mathematical programming model that can solve them simultaneously is proposed. Simulations are then conducted to verify the design, and to show that the proposed design methodology is indeed capable of producing good and satisfactory tandem AGV designs with multiple-load AGVs. Finally, it is our hope that the knowledge learned from this study can help us have a better understanding of multiple-load AGV systems and allow us to have successful implementations of similar systems in practice.

A dynamic-zone strategy for vehicle-collision prevention and load balancing in an AGV system with a single-loop guide path

Abstract One of the advances in computer-aided production is the application of computer control to automated material handling systems. Computer control is especially important to Automated Guide Vehicles (AGVs) since their control problems are more complicated than those of traditional automated material handling systems, e.g., conveyors. Of the many design and control issues of AGVs, an important one is the prevention of vehicle collision. Many traditional vehicle-collision prevention strategies are the so-called zone strategies that divide guide paths into several non-overlapping zones, and restrict the presence of at most one vehicle in any zone at any time. In this paper, the concept of a new vehicle-collision prevention strategy called “dynamic-zone strategy” will be introduced. Detailed procedures based on this new strategy will be proposed for an AGV system with a single-loop guide path. Traditional zone strategies are fixed-zone strategies in which the zone assigned to each vehicle cannot be changed and vehicles are not allowed to help each other. As a result, fixed-zone strategies often have the disadvantage of not being able to satisfy the transportation demand whenever there is a load imbalance between vehicles. The objective of this paper is to develop a strategy that not only can prevent the collision of vehicles but also can avoid the disadvantage of fixed-zone strategies. To accomplish this, the proposed strategy relies on two procedures — Zone Adjustment Procedure and Zone Assistance Procedure. With Zone Adjustment Procedure, the area of each zone will be changed according to the current production demand. On the other hand, with Zone Assistance Procedure, vehicles are allowed to help each other so that the workload of every vehicle is balanced all the time. The methods of these two procedures will be developed in this paper. In addition, a Simulated Annealing (SA)-based zone-division design method that can find near-optimal zone-division designs will also be proposed in this paper. Experiments are then conducted to show that the proposed dynamic-zone strategy and the proposed zone-division design method are indeed beneficial to the throughput performance of production systems.

Zone design and control for vehicle collision prevention and load balancing in a zone control AGV system

Because of their routing flexibility, automated guided vehicles (AGVs) have been used in many manufacturing systems, especially those in which parts with diverse and complex processing routes are made. In recent years, there have been many studies on AGV-related problems. One of them is preventing the collision of vehicles. Some traditional vehicle collision prevention strategies are zone strategies that divide guide paths into several non-overlapping zones and restrict the presence of only one vehicle in any zone at any time. Traditional zone strategies are fixed zone strategies in which the area of a zone is fixed and vehicles are not allowed to help each other. Because of these restrictions, fixed zone strategies cannot always satisfy the systems transportation demand if there is a load imbalance between vehicles of different zones. In this paper, a new zone strategy is proposed for a zone control AGV system with a network guide path. The proposed strategy is a dynamic zone strategy, which is different from a fixed zone strategy. It relies on two methods - zone partition design and dynamic zone control - to prevent the collision of vehicles and to maintain the load balance between the vehicles of different zones. The zone partition design defines a relationship coefficient, which measures both the distance relationship and the flow relationship between workstations, and uses it to find an initial zone partition design. We then improve this initial design by an SA (Simulated Annealing) based improvement procedure to achieve a better load balance result. The dynamic zone control uses two methods - zone repartition and load sharing - to ensure that vehicle collision can be prevented and the systems load balance can be maintained when the system is in operation. Simulation experiments were conducted to understand the performance of the proposed strategy. The simulation results show that the proposed strategy outperform the fixed zone strategy in throughput, WIP inventory, and flow time. The results also show that the proposed strategy is able to adapt to any changes (in the system) that cause the load imbalance problem between zones.

A simulation study on the performance of pickup-dispatching rules for multiple-load AGVs

In this paper, the pickup-dispatching problem of multiple-load AGVs (automated guided vehicles) is studied. This problem is defined in the multiple-load control process proposed by Ho and Chien [Ho, Y. C., & Chien, S. H. (2004). A simulation study on the performance of delivery-dispatching rules for multiple-load AGVs. In E. Kozan (Ed.), Proceedings of abstracts and papers (On CD-ROM) of the 5th Asia-Pacific industrial engineering and management systems conference and the 7th Asia-Pacific division meeting of the international foundation of production research (pp. 18.1.1-18.1.15). Brisbane: APIEMS]. Their control process identifies four problems faced by a multiple-load AGV. These problems are task-determination, delivery-dispatching, pickup-dispatching and load-selection. This paper focuses on the third problem. For this problem, nine pickup-dispatching rules are proposed and studied. The first, second and fourth problems are not the main focus of this study, thus only one task-determination rule, one delivery-dispatching rule and two load-selection rules are adopted for them. The objective of this study is twofold. First, to understand the performance of the proposed rules in different performance measures, e.g., the systems throughput, the mean flow time of parts (MFTP) and the mean tardiness of parts (MTP). Second, the effects that the proposed rules have on each others performance are investigated. Computer simulations are used to achieve these objectives. The experimental results reveal a rule that dispatches vehicles to the machine with the greatest output queue length is the best in all performance measures. Also, distance-based or due-time-based rules do not perform as well as queue-based rules. It is also found that the performance of pickup-dispatching rules is affected by different load-selections rules.

A simulation study on the performance of task-determination rules and delivery-dispatching rules for multiple-load AGVs

In this paper, the control problem of multiple-load automated guided vehicles (AGVs) is studied. A control process that identifies four problems faced by multiple-load AGVs is proposed. The first problem is the task-determination problem, in which a multiple-load AGV determines whether its next task is a pickup task or a delivery task. The second problem is the delivery-dispatching problem, in which a multiple-load AGV determines which delivery point it should visit next if its next task is a delivery task. The third problem is the pickup-dispatching problem, in which a multiple-load AGV determines which pickup point it should visit next if its next task is a pickup task. Finally, the fourth problem is the load-selection problem, which requires a multiple-load AGV to determine which load it should pick up from the output queue of a pickup point. This paper focuses on the first and second problems. Different task-determination rules and delivery-dispatching rules are proposed for these two problems. For the problems that are not the main focus of this study, rules found in the literature or real systems are adopted in this study. The objective of this study is twofold. First, we need to understand how well the proposed rules will perform in different performance measures, e.g. the systems throughput and the mean lateness of parts. Second, we need to understand the mutual effects that different types of rules have on each other, so that the best combination of rules can be identified. Computer simulations were conducted to test the performance of the proposed rules. It is hoped the knowledge learned from this study can be beneficial to real multiple-load AGV systems similar to the one studied here.