Sequential automation modeling tools — Petri nets

Since their first definition in 1962 by Carl Adam Petri, Petri nets have become a powerful modeling and analysis paradigm, both in the industrial world and in research laboratories. They are taught in engineering schools and universities, and in 2004 became an international standard (ISO/IEC-15909-1) for "high-level" aspects. They are now widely disseminated, and many industrial studies use this tool for the design and/or operation of automated systems.

The growing complexity of our production systems, particularly in manufacturing, has prompted a call from designers and users of discontinuous systems. The success of GRAFCET is due to this new need for a tool capable of expressing the two major characteristics of sequential systems: parallelism and synchronization.

Today, however, we know that the design and operation of manufacturing production systems, to take just one example, require models that are richer in information and more concise than GRAFCET, for the purposes of analysis, simulation and control.

The design or modification of an industrial plant can be summarized in four phases:

• the specification of its component functions and their interactions;

• analysis or validation of the description obtained;

• simulation, which completes our knowledge of the planned system and enables us to design and evaluate its performance;

• operation and maintenance.

Each of these phases relies on the use of a model, and therefore a language. All too often, the modeling tools used only apply to one or other of these phases. As a result, the transition from one stage to the next, or the backtracking that is often necessary in this design approach, leads to a loss of knowledge and the introduction of errors and ambiguities that have been resolved in the previous phase. The design of systems, which must increasingly be able to adapt easily to production requirements, calls for the use of models common to the different stages in the life of an industrial application. Petri nets can play this role.