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Motivation Programmable Logic Controllers (PLC), a computing device invented by Richard E. Morley in 1968, have been widely ud in industry including manufacturing systems, transportation systems, chemical process facilities, and many others. At that time, the PLC replaced the hardwired logic with soft-wired logic or so-called relay ladder logic (RLL), a programming language visually rembling the hardwired logic, and reduced thereby the configuration time from 6 months down to 6 days [Moody and Morley, 1999].
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Although PC bad control has started to come into place, PLC bad control will remain the technique to which the majority of industrial applications will adhere due to its higher performance, lower price, and superior reliability in harsh environments. Moreover, according to a study on the PLC market of Frost and Sullivan [1995], an increa of the annual sales volume to 15 million PLCs per year with the hardware value of more than 8 billion US dollars has been predicted, though the prices of computing hardware is steadily dropping. The inventor of the PLC, Richard E Morley, fairly considers the PLC market as a 5-billion industry at the prent time.
Though PLCs are widely ud in industrial practice, the programming of PLC bad control systems is still very much relying on trial-and-error. Alike software engineering, PLC software design is facing the software dilemma or crisis in a similar way. Morley himlf emphasized this aspect most forcefully by indicating [Moody and Morley, 1999, P110]: deathworm
If hous were built like software projects, a single woodpecker could destroy civilization.”
英语六级成绩查询我在学外语Particularly, practical problems in PLC programming are to eliminate software bugs and to reduce the maintenance costs of old ladder logic programs. Though the hardware costs of PLCs are dropping continuously, reducing the scan time of the ladder logic is still an issue in industry so that low-cost PLCs can be ud.
In general, the productivity in generating PLC is far behind compared to other domains, for instance, VLSI design, where efficient computer aided design tools are in practice. Existent software engineering methodologies are not necessarily applicable to the PLC bad software design becau PLC-programming requires a simultaneous consideratio
n of hardware and software. The software design becomes, thereby, more and more the major cost driver. In many industrial design projects, more than SO0/a of the manpower allocated for the control system design and installation is scheduled for testing and debugging PLC programs [Rockwell, 1999].
In addition, current PLC bad control systems are not properly designed to support the growing demand for flexibility and reconfigurability of manufacturing systems. A further problem, impelling the need for a systematic design methodology, is the increasing software complexity in large-scale projects.
1.2 Objective and Significance of the Thesis
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The objective of this thesis is to develop a systematic software design methodology for PLC operated automation systems. The design methodology involves high-level description bad on state transition models that treat automation control systems as discrete event systems, a stepwi design process, and t of design rules providing guidance and measurements to achieve a successful design. The tangible outcome of thi
美少女的谎言第三季13s rearch is to find a way to reduce the uncertainty in managing the control software development process, that is, reducing programming and debugging time and their variation, increasing flexibility of the automation systems, and enabling software reusability through modularity. The goal is to overcome shortcomings of current programming strategies that are bad on the experience of the individual software developer.
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A systematic approach to designing PLC software can overcome deficiencies in the traditional way of programming manufacturing control systems, and can have wide ramifications in veral industrial applications. Automation control systems are modeled by formal languages or, equivalently, by state machines. Formal reprentations provide a high-level description of the behavior of the system to be controlled. State machines can be analytically evaluated as to whether or not they meet the desired goals. Secondly, a state machine description provides a structured reprentation to convey the logical requirements and constraints such as detailed safety rules. Thirdly, well-defined control systems design outcomes are conducive to automatic code generation- An ability to prod
uce control software executable on commercial distinct logic controllers can reduce programming lead-time and labor cost. In particular, the thesis is relevant with respect to the following aspects. 道具英文
Customer-Driven Manufacturing
In modern manufacturing, systems are characterized by product and process innovation, become customer-driven and thus have to respond quickly to changing system requirements. A major challenge is therefore to provide enabling technologies that can economically reconfigure automation control systems in respon to changing needs and new opportunities. Design and operational knowledge can be reud in real-time, therefore, giving a significant competitive edge in industrial practice.
Higher Degree of Design Automation and Software Quality
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Studies have shown that programming methodologies in automation systems have not been able to match rapid increa in u of computing resources. For instance, the progr
英文歌下载amming of PLCs still relies on a conventional programming style with ladder logic diagrams. As a result, the delays and resources in programming are a major stumbling stone for the progress of manufacturing industry. Testing and debugging may consume over 50% of the manpower allocated for the PLC program design. Standards [IEC 60848, 1999; IEC-61131-3, 1993; IEC 61499, 1998; ISO 15745-1, 1999] have been formed to fix and disminate state-of-the-art design methods, but they normally cannot participate in advancing the knowledge of efficient program and system design.
