Enhancing the performance of automated guided vehicles through reliability, operation and maintenance assessment

Automated guided vehicles (AGVs), a type of unmanned moving robots that move along fixed routes or are directed by laser navigation systems, are increasingly used in modern society to improve efficiency and lower the cost of production. A fleet of AGVs operate together to form a fully automatic transport system, which is known as an AGV system. To date, their added value in efficiency improvement and cost reduction has been sufficiently explored via conducting in-depth research on route optimisation, system layout configuration, and traffic control. However, their safe application has not received sufficient attention although the failure of AGVs may significantly impact the operation and efficiency of the entire system. This issue becomes more markable today particularly in the light of the fact that the size of AGV systems is becoming much larger and their operating environment is becoming more complex than ever before. This motivates the research into AGV reliability, availability and maintenance issues in this thesis, which aims to answer the following four fundamental questions: (1) How could AGVs fail? (2) How is the reliability of individual AGVs in the system assessed? (3) How does a failed AGV affect the operation of the other AGVs and the performance of the whole system? (4) How can an optimal maintenance strategy for AGV systems be achieved? In order to answer these questions, the method for identifying the critical subsystems and actions of AGVs is studied first in this thesis. Then based on the research results, mathematical models are developed in Python to simulate AGV systems and assess their performance in different scenarios. In the research of this thesis, Failure Mode, Effects and Criticality Analysis (FMECA) was adopted first to analyse the failure modes and effects of individual AGV subsystems. The interactions of these subsystems were studied via performing Fault Tree Analysis (FTA). Then, a mathematical model was developed to simulate the operation of a single AGV with the aid of Petri Nets (PNs). Since most existing AGV systems in modern industries and warehouses consist of multiple AGVs that operate synchronously to perform specific tasks, it is necessary to investigate the interactions between different AGVs in the same system. To facilitate the research of multi-AGV systems, the model of a three-AGV system with unidirectional paths was considered. In the model, an advanced concept PN, namely Coloured Petri Net (CPN), was creatively used to describe the movements of the AGVs. Attributing to the application of CPN, not only the movements of the AGVs but also the various operation and maintenance activities of the AGV systems (for example, item delivery, corrective maintenance, periodic maintenance, etc.) can be readily simulated. Such a unique technique provides us with an effective tool to investigate larger-scale AGV systems. To investigate the reliability, efficiency and maintenance of dynamic AGV systems which consist of multiple single-load and multi-load AGVs traveling along different bidirectional routes in different missions, an AGV system consisting of 9 stations was simulated using the CPN methods. Moreover, the automatic recycling of failed AGVs is studied as well in order to further reduce human participation in the operation of AGV systems. Finally, the simulation results were used to optimise the design, operation and maintenance of multi-AGV systems with the consideration of the throughputs and corresponding costs of them.The research reported in this thesis contributes to the design, reliability, operation, and maintenance of large-scale AGV systems in the modern and rapidly changing world.</div

Selecting and scheduling of improvements in urban transportation networks using metaheuristics

Deciding which projects, alternatives and/or investments should be implemented is a complex and important topic not only in transportation engineering, but in management, operations research, and economics. If the project’s benefits or costs depend on which other project is realized, then the projects are interrelated. The evaluation method computes the costs of network flows determined with the Frank-Wolfe algorithm, which is modified to consider intersection flows and delays. Intersections are modelled with pseudo-links. The methods used for choosing the optimal schedule of project improvements are: Ant Colony Optimization, Simulated Annealing and Tabu Search. The heuristic that yields the best most quickly solution is Ant Colony Optimization and it is chosen for the sensitivity analysis. The results of the sensitivity analysis show how the changes in ACO parameters and the model parameters influence the behavior of the model and the algorithm

INNOVATIVE SIMULATION AND OPTIMIZATION STUDIES ON GRID SYSTEM FOR TRANSSHIPMENT TERMINAL

Ph.DDOCTOR OF PHILOSOPH

Virtual production system

To satisfy customer\u27s demands in today\u27s market, industry and academe have invested considerable effort to make production systems more efficient and competitive. The production systems that have been implemented and identified in industry have their own unique advantages under certain conditions. In practice, once a production system is adopted in a shop, the operation mode of the shop will remain the same over time. However, in a changing product mix environment, what a shop really needs is an adaptable production system to gain the best performance that is possible for the shop;The objective of this study is to develop a systematic procedure to construct a virtual production system that allows an existing shop to switch its operation from one mode to another without physical reconfiguration of the shop. The machines and the material handling system of the shop are logically reorganized into various patterns to obtain different versions of virtual production systems. Actually, a virtual production system exists as a set of information in a computer database. A reconfiguration of the data in the database leads to a corresponding logical reorganization of the physical system. Hence, on one hand, while the layout of a shop still remains the same, on the other hand, the operation mode of the shop is logically changeable over time;In this thesis, the performance of virtual production systems and other forms of production systems are examined and compared to one another using three different measures. The results obtained show that virtual production systems are superior to traditional production systems and are competitive with production systems that allow for the physical rearrangement of the machines as the product mix changes. However, when one considers the cost that can be incurred to physically reconfigure a shop and the fact that movable machines are not usually employed in most industries, virtual production system provides a feasible and reasonable solution to improve a shop\u27s performance in a dynamic changing product mix environment

Intelligent Control of Vehicle-Based Internal Transport Systems

“Intelligent control of vehicle-based internal transport (VBIT) systems” copes with real-time dispatching and scheduling of internal-transport vehicles, such as forklifts and guided vehicles. VBIT systems can be found in warehouses, distribution centers, manufacturing plants, airport and transshipment terminals. Using simulation of two realworld environments, dispatching rules described in literature and several newly introduced rules are compared on performance. The performance evaluation suggests that in environments where queue space is not a restriction, distance-based dispatching rules such as shortest-travel-distance-first outperform time-based dispatching rules such as modified-first-come-first-served and using load prearrival information has a significant positive impact on reducing the average load waiting time. Experimental results also reveal that multi-attribute dispatching rules combining distance and time aspects of vehicles and loads are robust to variations in working conditions. In addition, multi-attribute rules which take vehicle empty travel distance and vehicle requirement at a station into account perform very well in heavy-traffic VBIT systems such as baggage handling systems. Besides dispatching rules, the potential contribution of dynamic vehicle scheduling for VBIT systems is investigated. Experiments using simulation in combination with optimization show that when sufficient pre-arrival information is available a dynamic scheduling approach outperforms the dispatching approach. This thesis also evaluates the impact of guide-path layout, load arrival rate and variance, and the amount of load pre-arrival information on different vehicle control approaches (scheduling and dispatching). Based on experimental results, recommendations for selecting appropriate vehicle control approaches for specific situations are presented

Energy: A continuing bibliography with indexes, issue 20

A bibliography is presented which lists 1250 reports, articles, and other documents introduced into the NASA Scientific and Technical Information System from October 1, 1978 through December 31, 1978