Modelling battery constraints in discrete event automated guided vehicle simulations

This paper investigates a commonly omitted aspect of Automated Guided Vehicle (AGV) simulation—battery modelling. Battery usage is frequently overlooked in simulation for a variety of reasons. First, it is incorrectly believed to have minimal impact on system operation. Second, many analysts do not have an understanding of how batteries power AGVs. Third, battery analysis requires additional input information and varies for different AGV vendors. This paper contains several methods that can be used to account for the impact of various battery usage schemes on AGV simulations. These methods range from general philosophies to detailed model-specific coding examples.     

Operational control of a bidirectional automated guided vehicle system

Bidirectional AGV systems are intuitively more attractive than unidirectional systems in terms of efficiency because of shorter travel distances. An efficient algorithm for finding conflict-free shortest-time routes for automated guided vehicles moving in a bidirectional flow path network has been proposed in a previous paper. This paper is a sequel and describes a conservative myopic strategy to coordinate the movements of vehicles in a bidirectional AGV system. Simulation results are presented to compare the performances of unidirectional and bidirectional systems.     

A framework for the selection of idle vehicle home locations in an automated guided vehicle system

In the study of automated guided vehicles systems (AGVS), one assumption often made in the management of vehicles is the existence of either home locations or a circulatory loop to buffer or hold idle vehicles. A review of AGVS literature reveals that different researchers pursue one idle vehicle management policy or the other. In some studies, idle vehicles are assumed to circulate in some loops in the system while others assume they are routed to some dwell point locations where they are held until they are reassigned. When the dwell point option is implemented, different strategies for implementation are also available. However, what is generally lacking in references to dwell point strategies are the techniques for selecting the optimal or best dwell points for a given facility. In this study, the problem of dwell point or home location specification is addressed. The problem was modelled mathematically to determine the optimal home locations. The emphasis in the paper is on a unidirectional AGVS guide path. The criterion of optimization employed is the minimization of the system response time to vehicle demand points when dispatched from home locations. Solution methodologies are presented and example problems solved to illustrate the applications of the models presented.     

Comparison of dynamic routeing techniques for automated guided vehicle system

An automated Guided Vehicle (AGV) System with a limited number of vehicles cannot perform all of the transportation requests on demand. When a capacitated AGV system is used, then it is possible for the material handling system to become the bottleneck. There are many factors that affect the efficiency of an AGV system including how vehicles are routed and schedules. This paper presents a new dynamic approach to AGV routing. Simulation is used to study the effectiveness of this approach.     

激光导引AGV的自动引导系统设计

 激光导引技术是目前AGV系统普遍采用的引导方法，柔性好和精确高是其突出特点．介绍了AGV的基本概念、导引方式、车体结构和运动控制方式，重点介绍了激光扫描车体方位计算原理并结合路径轨迹推算导向法，设计了激光导引AGV的自动引导系统的结构和软件功能。     

Design of material transportation system for tandem automated guided vehicle systems

A new design concept of materialtransportation system is proposed on the basis of tandem automated guided vehicle (AGV) systems. The first design stage is to find the one and only one optimal transfer point for each traffic zone. All in-process parts to be moved into and out of a zone must go through the specified transfer point and parts can be moved directly to their destinations. The second design stage is to link all transfer points as a 'transportation centre' by several bidirectional tracks. This design concept can simplify material transportation system and provides accurate estimation of traffic load in a zone.     

A neural algorithm for finding the shortest flow path for an automated guided vehicle system

The automated guided vehicle (AGV)system is emerging as the dominant technology to maximize the flexibility of material handling, while increasing the overall productivity of manufacturing operations. This paper presents a new way of finding the shortest flow path for an AGV system on a specific routing structure. An optimal solution of the system is determined by using an approach based on the Hopfield neural network with the simulated annealing (SA) procedure. In other words, the proposed approach reduces the total cost of an AGV delivery path from one workstation to another on the shop floor. By changing the temperature of the two-stage SA, a solution can be found that avoids potential collisions between AGVs. Both the flow path and the potential collision, which are major problems in AGV systems, may be solved simultaneously by the proposed neural network approach. Other advantages offered by the proposed method are its simplicity compared with operations research (OR)methods and a decreased number of needed AGVs. The performance of the approach is also investigated.     

Guide path design and location of load pick-up/drop-off points for an automated guided vehicle system

The guide path layout for an automated guided vehicle system (AGVS) is a critical component in the overall design of a flexible manufacturing system (FMS) that utilizes AGVs for materials handling. Not only does it affect the total distance travelled by the vehicles but it also affects vehicle requirements and space utilization. In this study, the problem of selecting the guide path as well as the location of pick-up and drop-off points for outward and inward bound parts to departments is addressed. The problem is modelled and solved as a linear integer program with the objective of minimizing the total distance travelled.     

Design of an automated guided vehicle-based material handling system for a flexible manufacturing system

Automated guided vehicle (AGV)-based material handling systems (MHSs), which are widely used in several flexible manufacturing system (FMS) installations, require a number of decisions to be made. These include the number of vehicles required, the track layout, traffic pattern along the AGV tracks, and solving traffic control problems. This paper addresses the key issues involved in the design and operation of AGV-based material handling systems for an FMS. The problems arising from multi-vehicle systems are analysed, and strategies for resolving them are examined using analytical and simulation models.     

Virtual vehicle in the connecting transport automated material-handling system (AMHS)

In the connecting transport system, the time spent waiting for an empty vehicle is eliminated effectively and the work-in-process level can be reduced. Four different fixed vehicle types were used to carry out the wafers moving task. Thus, the automated material-handling system for a 300-mm wafer fab can be one of the three methods above or any mixture of these three connecting transport methods. However, no one method outperformed the others in all operational scenarios. A new concept of the virtual vehicle in the connecting transport automated material-handling system is proposed that can improve the task of moving wafers between bays and within each bay by a single system with interconnected lines. In this virtual vehicle environment, when the moving request is initiated, the wafers are assigned to the appropriate vehicle that can be changed to the vehicle type. Discrete-event simulation models are developed in eM-Plant? and compared with the virtual vehicle through the fixed vehicle in the connecting transport automated material-handling system. Using the virtual vehicle, the simulation results show that the performance measures such as throughput, transport time, 95% transport time and waiting time can be improved for different flow rates of the environment.     

Analysis of automated guided vehicle configurations in flexible manufacturing systems

Automated guided vehicle (AGV) systems complement the operation of flexible manufacturing systems (FMS) by providing integrated automated material handling that capitalizes on the system's flexibility. Previous research considering AGV systems for use in FMS installations has focused on complex control strategies to reduce the congestion problem often encountered in these systems. Recently, attention has been given to tandem system configurations that reduce congestion and simplify system control. The present study uses the simulation methodology to compare the performance of three AGV configurations under a variety of experimental conditions. The results indicate that system size, load/unload time, and machine failure rate factors have significant impacts on the operation of the systems considered. In general, with respect to due date performance, it is recommended to use the traditional configuration in small systems while using the tandem/loop configuration in larger systems. Furthermore, it is shown that the addition of the loop to the tandem configuration mitigates the sensitivity of the tandem configuration to the load/unload time factor as well as significantly improving its performance under high load/unload times. Thus, if tandem configuration is desired to reduce congestion and simplify system control, investments must be made to directly reduce the load/unload times or to construct a loop to avoid the load/unload time penalty.     

Real-time deadlock-free control strategy for single multi-load automated guided vehicle on a job shop manufacturing system

An unmanned automated job shop manufacturing system with a single multi-load automated guided vehicle, which traverses around a single-loop guidepath, is considered in this work. This type of shop design is often used as an independent sector of some complex AGV layouts, such as tandem, segmented bi-directional single-loop and divided configurations. The type of multi-load vehicle is a good alternative against using more single-load vehicles to serve a higher transportation demand. To an unmanned automated manufacturing system, the management of finite system resources, e.g. finite input/output queuing space and transporting carriers, plays a vital role in avoiding system deadlocks and machine blockages. The proposed control strategy for a single multi-load vehicle uses global shop real-time information to achieve the objectives: avoid shop deadlocks caused by inappropriate job movement as well as satisfy the system transport requirement. The efficiency of the proposed vehicle control strategy and the other two expanded strategies under various parameter designs are verified by computer simulation.     

Deadlock avoidance in an automated guidance vehicle system using a coloured Petri net model

This paper addresses the problem of deadlock control in automated guided vehicle (AGV) systems for automated manufacturing systems with unidirectinal guided paths. First, a Petri net (PN) model was developed for the problem. Then, by using the PN model developed, the condition for deadlock-free operation in AGV system and a control law are presented. To avoid deadlocks in AGV systems by this law, one needs only to observe the state of the system and check the number of free spaces available in some of the circuits. It is estimated that the proposed control law is simple enough to be used in the real-time control of contemporary system configuration. Three examples are used to show the application and efficiency of the proposed control law.     

Modelling an automated storage and retrieval system using Petri nets

We investigated the feasibility of using stochastic Petri nets (SPNs) to describe the behaviours of automated storage/retrieval systems as well as to evaluate the performance of different control policies of such systems. SPNs have been studied in the past 2 decades, and have become a powerful tool in modelling concurrent and distributed systems that exhibit synchronization and contention of shared resources. It appeared that some important characteristics of AS/RS could be well modelled with SPNs. The SPN is a graph-based tool suitable for modelling systems (manufacturing, computer, biological, etc.). It can be used to model a system at different levels of abstraction. In this study, the efficiency, control rules, bay assignment, and many other performance issues associated with AS/RS have been studied in detail using SPN models. SPNs are shown to be equivalent to the stochastic processes in which embedded Markov chains can be recognized. In addition, a stochastic Petri net package, SPNP, was adopted to solve the reachability trees from which an embedded Markov chain was generated. Finally, the performance of those systems of our interest was evaluated. The unique features and flexibility of SPNs are presented. Furthermore, the SPN models for different control algorithms of an AS/RS are discussed with examples.     

Heavy traffic analysis of a single vehicle loop in an automated storage and retrieval system

We consider an automated storage/retrieval system in which cargo moves between the storage/retrieval machines and the system entrance/exit stations through a single automated vehicle loop. Past studies indicated that the cargo waiting time in the loop is affected by the dispatching rules, which govern the sequence of the cargo to be handled. In this paper, we show that the loop configuration, which has received little research attention, also has a big impact on the cargo waiting time. When the first-come-first-served dispatching rule is used, we derive the relationship between the number of stations and the ratio of the average cargo retrieval time to the average cargo storage time. When the first-encountered-first-served dispatching rule is used, we show that even the arrangement of the input channel and the output channel of a station can have significant impact on the cargo waiting time. Furthermore, we derive a formula for the vehicle visit rate for each station under heavy traffic conditions. This formula helps to explain the phenomenon that the waiting times at different stations can be very different even when the loop is symmetrically designed and the cargo arrival rates to the stations are similar. In addition to analytical models, we use simulations to evaluate the performance of different loop configurations. Our research suggests that a substantial improvement can be achieved by making proper adjustments to the loop configuration.     

A construction algorithm for designing guide paths of automated guided vehicle systems

A construction algorithm is suggested to design guide path networks for automated guided vehicle systems (AGVSs). This study uses the total travel time, including waiting and interference time, of vehicles as the decision criteria for determining the direction of path segments on unidirectional guide path layouts. The Q-learning technique is applied to estimate the travel times of vehicles on path segments. Computational experiments were performed to evaluate the performance of the proposed algorithm. The simulation results showed that the proposed algorithm is superior to Kim & Tanchoco's (1993) algorithm in terms of the average travel time, the interference time and the number of deliveries.     

A model for the design of zone control automated guided vehicle systems

An analytical modelling strategy is proposed as a screening device for use prior to the simulation phase in the design of zone control automated guided vehicle systems (AGVS). The proposed model predicts the effects of the major AGVS design variables on system-performance measures including maximum throughput capacity and risk factors associated with shop locking. This is done through a process which specifically evaluates the system operating dynamics associated with vehicle fleet size, guidepath layout, workstation storage capacity, routeing, and, to a less precise extent, vehicle dispatching. Operating dynamics are defined to include vehicle blocking resulting from guidepath contention and variation of in-process storage levels. The objective of the analytical model is to reduce the extent of simulation modelling necessary to design a zone-control AGVS for a given materials-handling workload. The model is illustrated for a sample AGVS with variation in the fleet size, workstation storage capacity and dispatching rule demonstrated.     

Cycle counting in a JIT environment using V-A-T focusing

As machine tools become more versatile, single-stage multimachine systems (SSMS) have emerged in which all the operations of a part are performed on a single machine. In this system, parts stay on a machine all through their operations while tools are delivered to the machines as requested. The performance of the system is then restricted by the tooling conditions. This paper addresses the problem of how many tools should be maintained in an SSMS under a tool sharing environment. An open queueing network-based approach is proposed which not only determines tool requirement levels but also predicts the tool waiting time given the tooling capacity. The tool life and tool delivery are taken into consideration in determining the tool requirement levels. A simulation study is performed to evaluate the proposed approach.     

A simulation study of multiple-load-carrying automated guided vehicles in a flexible manufacturing system

Although the technology exists for more advanced applications of automated guided vehicles in flexible manufacturing systems, the current employment of these vehicles in material handling generally subscribes to a simple mode of operation : single-load-carrying capacity for each vehicle and unidirectional traffic on each route of the system. Through a simulation programme, this study investigates the effect of several key factors related to the automated guided vehicles on the overall performance of a flexible manufacturing system. These are the number of pallets allowed in the system, the number of vehicles used and the carrying capacity of each and the input and output queue capacities of the machining stations; finally bidirectional traffic is allowed in some routes. The results show that there is a strong interaction among these factors and reveal their combined effects on the throughput from a small flexible manufacturing system. Upon the user's request, the simulation program also provides the animated colour graphics of the system to view developments concurrently under the selected decision values through time.     

A multi-attribute selection of automated guided vehicle using the AHP/M-GRA technique

The objective of this research is to develop and implement a new methodology for multi-attribute selection of automated guided vehicle (AGV) for the material handling. The present paper proposes a modified grey relational analysis (M-GRA) method combined with analytical hierarchy approach (AHP). AHP method is used to assign the relative importance between AGV selection attributes and M-GRA method is applied to determine AGV selection utility index. A proposed AHP/M-GRA technique is more suitable for the decision-making in the presence of vagueness. The methodology is illustrated by means of an example. The study was followed by the validation and the sensitivity analysis of the AHP/M-GRA technique. The results of the study build confidence in suitability of AHP/M-GRA technique for selection of AGV alternatives.