In discrete part manufacturing industries, and increasingly in process industries the device used for control is the Programmable Logic Controller (PLC). Today, the PLC is capable of handling considerably more I/O, has much more computing power, and is capable of more high level functions than when it was introduced two decades ago. These enhancements to the PLC have led to its use in increasingly complex applications.; This thesis begins with an analysis of the major sources of complexity with respect to PLC based control projects. This is followed by a brief description of the five common attributes of complex systems and the basic concepts of the object-oriented paradigm.; The remainder of the thesis shows how an object library for industrial control applications can be defined. The application domain is divided into fundamental abstractions, process abstractions, movers and positioners. A state-transition object is provided to create new abstractions which can be added to the library.; A material handling example is used to illustrate how an application can be implemented using these object oriented concepts. The last section briefly describes the benefits that were realized by using these concepts in the implementation of the control system for a large scale waste water treatment plant. (Abstract shortened by UMI.)
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