Petri Nets and Ladder Logic for Fully-Automating and Programmable Logic Control of Semi-Automatic Machines and Systems

Abstract

Problem statement: Automating semi-automatic dynamic machines and complex systems, where some functionalities are already automated but the machine doesn’t run fully in automatic mode, represents another challenge to automating dynamic and manual complex systems. This study deals with a special case of this general problem, where the system’s semi-automatic and automatic functionalities run separately in a sequence with either automatic and/or semi-automatic functionality running at any point of time, but not both. Approach: Petri-nets can successfully represent the operations of both the semi-automated and fully-automated functionalities of such dynamic systems. Three roles for the programmable logic controllers were suggested: (a) Ignoring the presence of the semi-automatic part, (b) simulating the semi-automatic part and recognizing discrepancies and(c) simulating the semi-automatic functionalities and supporting it, where the last choice was most costly and most reliable. Results: This study presents a case study for a PVC mixing dynamic process to illustrate the three suggested controller design possibilities, where a Petri net model and related ladder logic program were developed to show these three controller design options. In this respect, the semi-automatic functionality is an intermediate step between the two automated functionalities, within which an important decision should be made to end the previous automatic step and to begin the next automatic step. Conclusion/Recommendations: The automation method that was introduced in this study is applicable to a large number of machines within industrial and mechanical systems that were built using older semi-automatic control systems. Various decision analyses {DA1, DA2, DA3} were shown to produce basic choices for such types of applications. Petri-nets and ladder logic in this respect can successfully represent the operations of both the semi-automated and fully-automated functionalities, where a Petri net model and the consequent ladder logic program, used to program the utilized programmable logic controller, are developed to show three possible controller designs, where the choice between three designs depend on the designers objective of cost, reliability and fault tolerance.