A review of design and control of automated guided vehicle systems <s> Suggestions for further research <s> Most current applications of automated guided vehicle systems (AGVS) in manufacturing shop environments employ uni-directional guide-paths for vehicle routing despite the fact that bi-directional vehicles exist. In this paper comparisons and issues regarding unidirectional and bi-directional flows are presented. Also presented is a model of a bi-directional traffic flow guide-path. The effect of the traffic flow pattern on the shop throughput is demonstrated and compared to that of a uni-directional flow system of an equivalent facility. The model is implemented using computer simulation. <s> BIB001 </s> A review of design and control of automated guided vehicle systems <s> Suggestions for further research <s> Abstract An alternative to increasing automated guided vehicle (AGV) fleet size to improve vehicle availability for material transport requests is to introduce vehicles with multiple load-carrying capacities. Aside from alleviating communication and congestion problems, the use of multiple-load carriers can potentially reduce the deadhead or unproductive time of vehicles. This paper discusses several key issues related to multiple-load AGV systems and demonstrates the potential benefits of using them. Alternative vehicle dispatching strategies for multiple-load AGVs are discussed, stressing that different considerations are involved in dispatching empty and partially loaded vehicles. The effectiveness of single-load and two-load AGV systems is compared using simulation. The results show that, with multiple-load vehicles, the sensitivity of AGV system performance to guidepath layout is decreased, and the job input rate, which the AGV system can effectively handle without becoming a bottleneck for the overall system, is significantly increased. The relative effectiveness of selected control strategies is also investigated. <s> BIB002 </s> A review of design and control of automated guided vehicle systems <s> Suggestions for further research <s> The importance and benefits of using multiple-load carriers in advanced manufacturing systems has been well documented in recent studies. This study assumes a manufacturing system arranged around a single loop serviced by a single multiple-load carrier. The study presents a {0-1} integer programming model for optimal schedule design, if complete knowledge is available, using a finite planning horizon. In order to solve larger dynamic problems where jobs keep on arriving, two heuristic algorithms are developed, each using a different look-ahead period. The ability of these algorithms is analyzed and compared to the simple First-Encountered-First-Served (FEFS) rule using 31 randomly generated problems. Finally, the impact of increasing carrier capacity is studied. <s> BIB003 </s> A review of design and control of automated guided vehicle systems <s> Suggestions for further research <s> In previous papers, we used a simulation model to look at how different dispatching rules can be classified, and at the effects on the performance of internal transport if the release time of loads can be forecasted using a virtual release time. We concluded that centralized rules outperform decentralized rules and that the performance of internal transport can be improved using virtual release times. <s> BIB004 </s> A review of design and control of automated guided vehicle systems <s> Suggestions for further research <s> The importance and benefits of using multiple-load carriers in advanced manufacturing systems has been well documented in recent studies. A Dynamic Scheduling Algorithm (DSA) for multiple-load carriers was developed and analysed in the present study using a manufacturing system arranged around a single loop serviced by several multiple-load carriers. In order to evaluate the DSA performance, criteria measures such as throughput, cycle time, WIP and carrier utilization were chosen and compared through simulation with those achieved by three other scheduling rules, Least Work remaining (LWKR), Maximum Utilization (MAX_UT) and First Come First Serve (FCFS). The simulation results also serve to analyse the effect that the carrier fleet size has on the above criteria measures when total carrying capacity is kept constant. <s> BIB005 </s> A review of design and control of automated guided vehicle systems <s> Suggestions for further research <s> Agent technology has been considered as an important approach for developing distributed intelligent manufacturing systems. A number of researchers have attempted to apply agent technology to manufacturing enterprise integration, supply chain management, manufacturing planning, scheduling and control, materials handling, and holonic manufacturing systems. This paper gives a brief survey of some related projects in this area, and discusses some key issues in developing agent-based manufacturing systems such as agent technology for enterprise integration and supply chain management, agent encapsulation, system architectures, dynamic system reconfiguration, learning, design and manufacturability assessments, distributed dynamic scheduling, integration of planning and scheduling, concurrent scheduling and execution, factory control structures, potential tools and standards for developing agent-based manufacturing systems. An extensive annotated bibliography is provided. <s> BIB006
Several future research subjects have been proposed in the previous sections. Here we repeat the most promising future research directions. • In section 2, we have seen three main types of guide-path systems. It is important to know when and where we should choose a specific guide-path system. In addition, the bidirectional guide-path system has not received much attention, in spite of the fact that it can improve the system performance BIB001 ). • For estimating the required number of vehicles, obtaining a better estimation of the vehicle empty travel and time lost caused by vehicle interference under some good dispatching rules is still an important issue. • BIB002 show that using multi-load vehicles improves the system throughput. In literature, there are only few studies considering multi-load AGV systems BIB003 BIB005 BIB004 . Multi-load vehicles are expensive, but can lead to a substantial throughput increase, depending on the quality of the scheduling system. This leaves ample room for future research. • Smart AGVs become more and more popular in practice. They change the importance of some design and implementation issues in AGV systems. For example, obtaining an optimal guide-path system is less crucial, but the flexibility of layout and system adaptation for future demands might be more important. In addition, agent-like smart AGVs use decentralized control in a more efficient way than just applying FEFS dispatching. Still, there are very few studies related to designing and dispatching smart AGV systems, which clearly gives an important research direction. • Intelligent manufacturing systems (IMS) using intelligent machines and smart AGVs become increasingly important in practice. In such systems, machines and AGVs can act as intelligent agents. These agents have advanced capabilities such as self-learning lead to adaptiveness and responsiveness capabilities of IMS. Holonic manufacturing system (HMS) is an example of such IMS BIB006 . However, more research is still needed to improve performance of IMS and smart AGV agents. We have pointed out several fruitful issues for future research. These issues certainly have an important impact on improving the AGV system's performance. Besides these issues, researchers should pay attention to other issues such as battery management, positioning idle vehicles and conflict resolution as well.