【正文】 or crisis in a similar way. Morley himself emphasized this aspect most forcefully by indicating [Moody and Morley, 1999, p. 110]: `If houses were built like software projects, a single woodpecker could destroy civilization.” 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 PLC are dropping continuously, reducing the scan time of the ladder logic is still an issue in industry so that lowcost PLC can be used. 2 In general, the productivity in generating PLC is far behind pared to other domains, for instance, VLSI design, where efficient puter aided design tools are in practice. Existent software engineering methodologies are not necessarily applicable to the PLC based software design because PLCprogramming requires a simultaneous consideration of hardware and software. The software design bees, 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 based control systems are not properly designed to support the growing demand for flexibility and manufacturing systems. A further problem, impelling the need for a systematic design methodology, is the increasing software plexity in largescale projects. Objective and Significance of the Thesis The objective of this thesis is to develop a systematic software design methodology for PLC operated automation systems. The design methodology involves highlevel description based on state transition models that treat automation control systems as discrete event systems, a stepwise design process, and set of design rules providing guidance and measurements to achieve a successful design. The tangible oute of this research 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 overe shortings of current programming strategies that are based on the experience of the individual software developer. A systematic approach to designing PLC software can overe deficiencies in the traditional way of programming manufacturing control systems, and can have wide ramifications in several industrial applications. Automation control systems are modeled by formal languages or, equivalently, by state machines. Formal representations provide a highlevel description of the behavior of the system to be controlled. State machines can be analytically evaluated as to whether or not they 3 meet the desired goals. Secondly, a state machine description provides a structured representation to convey the logical requirements and constraints such as detailed safety rules. Thirdly, welldefined control systems design outes are conducive to automatic code generation An ability to produce control software executable on mercial distinct logic controllers can reduce programming leadtime and labor cost. In particular, the thesis is relevant with respect to the following aspects. CustomerDriven Manufacturing In modern manufacturing, systems are characterized by product and process innovation, bee customerdriven 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 response to changing needs and new opportunities. Design and operational knowledge can be reused in realtime, therefore, giving a significant petitive edge in industrial practice. Higher Degree of Design Automation and Software Quality Studies have shown that programming methodologies in automation systems have not been able to match rapid increase in use of puting resources. For instance, the programming of PLC still relies on a conventional programming style with ladder logic diagrams. As a