In: Zhou, M.C.: Petri Nets in Flexible and Agile Automation, pages 265-304. Dordrecht, NL: Kluwer Academic Publishers, 1995.
Abstract: Design methods for sequence controllers play a prominent role in advancing industrial automation. The increasing complexity and varying needs of modern discrete manufacturing systems have challenged the traditional design methods such as the use of ladder logic diagrams (LLDs) for programmable logic controllers. The methodologies based on research results in computer science have recently received growing attention by academic researchers and industrial engineers in order to design flexible, reusable, and maintainable control software. Particularly, Petri nets (PNs) are emerging as a very important tool to provide an integrated solution for modeling, analysis, simulation, and control of industrial automated systems. However, in order to establish PNs as alternative to LLDs there is a need for benchmark studies to formally compare them. The contribution of this chapter is twofold. First, certain criteria are identified to compare LLDs and PNs in designing sequence controllers subject to the changing control requirements. The comparison is performed through a practical system after introducing `real-time Petri nets' for discrete-event control purpose. Secondly, some analytical formulas and a methodology are developed to estimate the number of basic elements used in the PN and LLD designs prior to their constructions. The results will be useful for researchers and engineers to design control systems for complex industrial automated systems.