Simulation analysis of dispatching rules for an automated interbay material handling system in wafer fab

Here, the performance evaluation of a double-loop interbay automated material handling system (AMHS) in wafer fab was analysed by considering the effects of the dispatching rules. Discrete event simulation models based on SIMPLE++ were developed to implement the heuristic dispatching rules in such an AMHS system with a zone control scheme to avoid vehicle collision. The layout of an interbay system is a combination configuration in which the hallway contains double loops and the vehicles have double capacity. The results show that the dispatching rule has a significant impact on average transport time, waiting time, throughput and vehicle utilization. The combination of the shortest distance with nearest vehicle and the first encounter first served rule outperformed the other rules. Furthermore, the relationship between vehicle number and material flow rate by experimenting with a simulation model was investigated. The optimum combination of these two factors can be obtained by response surface methodology.     

Analytical model to estimate performances of autonomous vehicle storage and retrieval systems for product totes

In today's competitive scenario of increasingly faster deliveries and smaller order sizes, material-handling providers are progressively developing new solutions. A recent, automated material-handling technology for unit load storage and retrieval consists of an autonomous vehicle storage and retrieval system (AVS/RS). The present paper presents an analytical model to estimate the performances (the transaction cycle time and waiting times) of AVS/RS for product tote movement. The model is based on an open queuing network approach. The model effectiveness in performance estimation is validated through simulation.     

Effectiveness of vehicle reassignment in a large-scale overhead hoist transport system

This paper proposes an efficient vehicle reassignment dispatching rule and demonstrates the effectiveness of the rule using an overhead hoist transport (OHT) system for a semiconductor fabrication line. The OHT system has more than 150 OHT vehicles and its target vehicle utilization level is 70%. It allows direct delivery such that an inter-bay wafer movement can be accomplished by a single vehicle. Simulation analysis is used to compare the proposed rule with the shortest travel distance first (STDF) rule and existing reassignment-based rules. While STDF rule requires 170 vehicles for the target utilization level, the proposed vehicle reassignment rule requires only 161 vehicles. At the same time, the lead time and the variance of the lead time have been significantly reduced. The proposed rule also improves system performance compared to existing reassignment-based rules.     

An efficient cycle time model for autonomous vehicle storage and retrieval systems

A computationally efficient cycle time model for conceptualizing autonomous vehicle storage and retrieval systems and comparing their performance with crane-based automated storage and retrieval systems is presented. The model is based on an iterative computational scheme exploiting random storage assumptions and queuing model approximations. Relative to earlier models, the procedure scales up efficiently for large problems thereby enabling more extensive search of a design solution space. Simulation based validation studies suggest that model accuracy is adequate for system conceptualization. The procedure is demonstrated using realistically sized sample problems.     

Simulation-based regression analysis for the rack configuration of an autonomous vehicle storage and retrieval system

In this paper, a simulation-based regression analysis for the rack configuration of an autonomous vehicle storage and retrieval system (AVS/RS) is presented. The aim of this study is to develop mathematical functions for the rack configuration of an AVS/RS that reflects the relationship between the outputs (responses) and the input variables (factors) of the system under various scenarios. In the regression model, we consider five outputs: the average cycle time of storage and retrieval transactions, the average waiting time for vehicle transactions, the average waiting time of vehicles (transactions) for the lift, the average utilisation of vehicles and the average utilisation of the lifts. The input variables are the number of tiers, aisles and bays that determine the size of the warehouse. Thirty regression models are developed for six warehouse scenarios. The simulation model of the system is developed using ARENA 12.0 commercial software and the statistical analyses are completed using MINITAB statistical software. Two different approaches are used to fit the regression functions–stepwise regression and the best subsets. After obtaining the regression functions, we optimise them using the LINGO software. We apply the approach to a company that uses AVS/RS in France.     

An Analysis of Automated Storage and Retrieval Systems in Manufacturing Assembly Lines

An automated storage/retrieval system is a material handling system that can be used to pick and to deliver materials in a direct access fashion. An M/G/1 queueing model is used to study the system performance under a first-come-first-served dispatching rule. More detailed studies for different dispatching rules have been done by simulation. A number of cases with different hardware characteristics and workloads have been investigated. Conclusion is drawn based upon the results obtained from analysis as well as numerical experiments.     

Conceptualizing tools for autonomous vehicle storage and retrieval systems

Autonomous vehicle storage and retrieval systems utilize rail-guided vehicles moving in rectilinear paths within and between the aisles of unit load storage racks. Vertical vehicle movement is provided by lifts installed at fixed locations along the rack periphery. As an alternative to traditional automated storage and retrieval systems, autonomous vehicle systems enable users to match vehicle fleet size and the number of lifts to the level of transactions demand in a storage system. Analytical conceptualizing tools based on the features of autonomous vehicle systems are proposed for modelling expected performance as a function of key system attributes including storage capacity, rack configuration and fleet size. The models are demonstrated for a sample problem and compared with analytical conceptualizing tools used for automated storage and retrieval systems.     

Solution methods for the mathematical models of single-loop AGV systems

Guide path simplification can potentially reduce the complexity inherent in conventional, multi-loop automated guided vehicle systems (AGVs). A single-loop configuration is one alternative. A procedure for designing single-loop AGV systems, the OSL method was presented in a previous paper. In this paper, we suggest faster and more efficient methods for solving the two mathematical models in the OSL procedure. The first model called the valid single-loop problem (VSLP) is used to determine an initial single loop for the procedure. The method suggested is a heuristic procedure that starts from a loop around one of the departments and keeps adding departments to the loop until a valid single loop is constructed. The second model called the single-loop station location problem (SLSLP) is used to determine the location of the pick-up and delivery stations along a given loop. The method suggested converts the mixed integer formulation into a linear formulation.     

A multi-attribute dispatching rule for automated guided vehicle systems

This paper considers the dispatching problem associated with operations of automated guided vehicles (AGVs). A multi-attribute dispatching rule for dispatching of an AGV is developed and evaluated. The multi-attribute rule, using the additive weighting method, considers three system attributes concurrently: the remaining space in the outgoing buffer of a workstation, the distance between an idle AGV and a workstation with a job waiting for the vehicle to be serviced, and the remaining space in the input buffer of the destination workstation of a job. A neural network approach is used to obtain dynamically adjusting attribute weights based on the current status of the manufacturing system. Simulation analysis of a job shop is used to compare the multi-attribute dispatching rule with dynamically adjusting attribute weights to the same dispatching rule with fixed attribute weights and to several single attribute rules. Results show that the multi-attribute dispatching rule with the ability to adapt attribute weights to job shop operational conditions provides a better balance among the performance measures used in the study.     

Interaction between dispatching and next station selection rules in a dedicated flexible manufacturing system

The interaction between nine dispatching and four next station selection rules in a relatively large dedicated FMS is investigated. The FMS contains 16 workstations with local buffers, nine load/unload stations, and produces six different part types. A simulation model is used, and analysed as a steady-state model. Flowtime is taken as the main criterion. It is found that WINQ (select the station whose input buffer contains the smallest amount of work) dominates, performing significantly better than the other next station selection rules considered across all dispatching rules, with few significant differences between dispatching rules when combined with WINQ. SIO/TOT (select the job with the smallest ratio obtained by dividing the processing time of the imminent operation by the total processing time for the part) performs marginally better than the other dispatching rules, particularly SIO (select the job with the shortest imminent processing time). Reasons for when a next station selection rule is more important than a dispatching rule, and vice versa, are discussed.     

A dispatching method for automated guided vehicles by using a bidding concept

A dispatching method is suggested for automated guided vehicles by using an auction algorithm. The dispatching method in this study is different from traditional dispatching rules in that it looks into the future for an efficient assignment of delivery tasks to vehicles and also in that multiple tasks are matched with multiple vehicles. The dispatching method in this study is distributed in the sense that the dispatching decisions are made through communication among related vehicles and machines. The theoretical rationale behind the distributed dispatching method is also discussed. Through a simulation study, the performance of the method is compared with that of a popular dispatching rule. RID="*" ID="*" The research was financially supported by the Sasakawa Scientific Research Grant from The Japan Science Society. The original version of the simulation program is provided by Professor Jae Yeon Kim at Dong Yang University, Korea. Correspondence to: J. K. Lim     

TRIP-BASED MATERIAL HANDLING SYSTEMS: THROUGHPUT CAPACITY ANALYSIS

In this paper we present a general-purpose analytical model to compute die approximate throughput capacity of a trip-based material handling system used in a manufacturing setting. A wide variety of handling systems, including freight elevators, cranes, microload automated storage/retrieval (AS/R) systems, industrial lift trucks, and automated guided vehicle (AGV) systems can be modeled as trip-based handling systems. To our knowledge, this model is one of the few analytical models that explicitly considers an empty device dispatching rule. The model is first developed for a single-device system (such as a crane) and subsequently, with a simple modification, it is extended to multiple-device systems (such as lift trucks and AGVs). Using this model one can rapidly evaluate a wide range of handling and layout alternatives for given flow data. Hence, die model would be most effective when used early in the design phase to narrow down die set of alternative handling systems and configurations prior to simulation.     

Development of a framework for the design of autonomous vehicle storage and retrieval systems

In today’s competitive environment with increasingly faster deliveries and smaller order sizes, material handling providers are progressively developing new solutions. A more recent development in automated material-handling technology for unit load storage and retrieval is the autonomous vehicle storage and retrieval system (AVS/RS). The paper investigates the main design trade-offs for this new solution using simulation, and proposes a comprehensive design framework. Using data from a recently implemented AVS/RS, the application of the proposed framework is presented and the key design differences between the two types of AVS/RS configuration (i.e. tier-captive versus tier-to-tier) are identified.     

A discrete-time queueing network approach to performance evaluation of autonomous vehicle storage and retrieval systems

In this paper, we present a method for performance evaluation of autonomous vehicle storage and retrieval systems (AVS/RSs) with tier-captive single-aisle vehicles. A discrete-time open queueing network approach is applied. The data obtained from the evaluation of the lift and vehicle movements can be used directly as input for the general discrete service time distributions of the queueing network. Furthermore, the approach allows for the computation of the retrieval transaction time distribution as well as of the distribution of the number of transactions waiting to be stored. Consequently, not only expected values and variances but also quantiles of the performance measures can be obtained. Comparison to discrete-event simulation quantifies approximation errors resulting from the decomposition approach in the discrete-time domain. Moreover, the errors obtained by the discrete-time approach are compared to the errors obtained using a continuous-time open queueing network approach. Finally, it will be outlined how the model can be used for designing AVS/RSs according to given system requirements, such as storage capacity, throughput, height and length of the system as well as the 95% quantile of the retrieval transaction time.     

On-line dispatching rules for vehicle-based internal transport systems

On-line vehicle dispatching rules are widely used in many facilities such as warehouses and manufacturing facilities to control vehicles’ movements. Single-attribute dispatching rules, which dispatch vehicles based on only one parameter, are usually used. However, multi-attribute dispatching rules prove to be better in general. In this paper, we study the impact of reassigning moving vehicles on some good dispatching rules, both single- and multi-attribute, in the literature. Results suggest that reassigning moving-to-park vehicles has a significant positive effect on reducing the average load waiting time. We evaluate the dispatching rules’ performance using the experimental design of a real-life case study. The performance criteria are: minimizing the average load waiting time, keeping the maximum load waiting time as short as possible and utilizing better vehicles. The results show that the combined dispatching rules which integrates multi-attribute dispatching and vehicle reassignment yields the best performance overall.     

Performance analysis of automatic storage/retrieval systems by stochastic modelling

This study considers an automated storage/retrieval system that performs either single or dual commands and analyses it as a queuing system with two waiting spaces and one server. It is assumed that the storage and retrieval commands arrive at the system according to Poisson processes with different rates and that the service times of single and dual commands are distributed differently. The steady-state probability distribution of the number of commands in the system at the service completion epochs is derived; subsequently, the semi-Markov process is employed to obtain the distribution at an arbitrary time. Several performance measures are derived and include the expected number of commands in the system, the expected waiting time, the probability that an arbitrary command cannot enter the system (the ‘blocking probability’) and the utilization of the storage/retrieval machine.     

多色集合在仓配一体化仓库货位分配建模中的应用

目的为了提高仓配一体化仓库的货物存储以及货物周转效率,对仓库货位分配进行研究。方法以西北地区某快消品仓配一体化配送中心仓库为背景,应用多色集合理论建立仓配一体化仓库中货位信息模型,通过实际案例对建立的模型进行仿真实验,得出仓库中货位的最优分区。结果对仓库在货物就近原则放置和货物分区管理下车辆的等待时间进行仿真,限定在100 d的时间里,得出仓库在运用多色集合理论分区的情况下装卸车辆等待时间相比之前未分区时间下降了20.84%。结论运用多色集合理论有效提高了城市快消品仓配一体化中货位优化的效率。     

A DISPATCHING ALGORITHM FOR MULTIPLE-LOAD AGVS USING A FUZZY DECISION-MAKING METHOD IN A JOB SHOP ENVIRONMENT

This paper proposes a dispatching algorithm for automated guided vehicles (AGV) with multiple-load capacity operating in a job shop environment. At each dispatching point, the algorithm prioritizes all move requests based on the fuzzy urgency which is determined by the fuzzy multi-criteria decision-making method. Then, the algorithm selects a set of requests under the restriction of target dispatching time interval. Through simulation, the performance of the proposed algorithm is compared with several dispatching algorithms in terms of system throughput in a hypothetical job shop environment.     

The value of the shortest loop covering all work centers in a manufacturing facility layout

In this study we develop mathematical models to design circular material flow systems. We first develop a tight formulation to find the shortest loop covering all work centers within a manufacturing facility layout. The shortest loop is an attractive solution for most types of conveyors and power-and-free systems, where the length of the flow path is the major driver of the total cost. We develop a primal as well as a dual graph formulation and discuss their one-to-one correspondence in node-edge as well as in connectivity constraints. Our solution times outperform other optimization models available for the facility layout shortest loop design problem. We then approach trip-based material handling, such as automated guided vehicle systems, where the total loaded and empty trip distance is the major driver of the total cost. The problem in these systems evolves into concurrent design of the loop, pickup and dropoff station, and the empty vehicle dispatching policies. On the foundation of the shortest loop model, we propose a decomposition heuristic for design of trip-based flow systems. Computational results indicate that the heuristic provides high quality and robust solutions.     

Optimal routing in an automated storage/retrieval system with dedicated storage

We address the sequencing of requests in an automated storage/retrieval system with dedicated storage. We consider the block sequencing approach, where a set of storage and retrieval requests is given beforehand and no new requests come in during operation. The objective for this static problem is to find a route of minimal total travel time in which all storage and retrieval requests may be performed. The problem of sequencing a list of retrievals is equivalent to the Traveling Salesman Problem (TSP), and thus NP-hard in general. We show that the special case of sequencing under the dedicated storage policy can be solved in polynomial time. The results apply to systems with arbitrary positions of the input and output stations. Tlus generalizes the models in the literature, where only combined input/output stations are considered. Furthermore we identify a single command area in the rack. At the end we evaluate the model against heuristic procedures.