Petri net based design and analysis of logic controllers.
Poster at the NSF Workshop on Logic Control for Manufacturing Systems, Ann Arbor, MI (USA), June 26-27, 2000.
- To design and analyze a logic controller there are basically two approaches, model-based and non-model-based. Model-based approaches can be further grouped into two types, synthesis and verification. The synthesis approach is the one most closely related to Control Theory. Here a model of the uncontrolled process is build and based on formal specifications a controller is derived mathematically. The verification approach takes a model of the uncontrolled process and a model of the controller. The combination of both results in a model of the controlled process. Using this combined model formal specifications can be verified mathematically. Non-model-based approaches stem from the area of Software Engineering. They measure or verify certain properties of a piece of software under minimal or no assumptions about its environment. This approach is well suited for logic controllers because of two main reasons. First of all, the realization of a logic controller includes hard- and software. But under the assumption of standard hardware with well-defined functionality, the application is realized by the program of the control algorithm, i.e. software. Hence the quality and functionality of the controller depends mainly on the quality and functionality of that software. Furthermore, a model of the process under control is in most cases not readily available. Non-model-based analysis methods for logic control algorithms based on Signal Interpreted Petri Nets (SIPN) are presented. They include correctness analysis (unambiguity, determinism, ...) and the measurement of transparency. A control algorithm is said to be transparent if it is easy and clear to see what the controller does in a given situation and what it will do in the next step. And if it is possible to reinterpret the algorithm (i.e. the aim of the algorithm is easily recognizable). Because of a close relation between SIPN and Sequential Function Chart (SFC) according to IEC61131-3 standard the SIPN can easily be implemented on programmable logic controllers.
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