When we speak of automatism, it is very difficult to make our ideas understood and it is even more difficult for the other person to understand us. Suppose you ask an integrating company to design an automatic machine to carry out a certain process. Your idea should be made known to a person from that company. How will you get that person to get your idea? In the integrating company, the idea must be transmitted and shared by others (mechanical, electrical, electronic, control specialists) who must work as a team to design their machine.
How long did the information regarding your machine have to travel? Would the last person to know receive the information as clear and precise as you provided it? In the circulation of information, there will surely be many doubts and ambiguities for the manufacture of the machine, which will undoubtedly cause it not to do exactly what you need.
In order for the machine to function as you requested, it is necessary to be in continuous communication in order to clarify the doubts that may arise in its elaboration and to make the necessary modifications for its correct operation. However, how can we make the design specifications of an automation known clearly and precisely without doubts and avoiding ambiguities? The answer is simple and it is not new, it has been used in Europe for approximately twenty years and is currently being adopted by several companies in America and consists of the GRAFCET (Graphe Fonctionnele de Commande Etape-Transition or Functional Control Chart of Stages and Transitions).
The GRAFCET is a document and a clear description tool, precise, without omissions or ambiguities that indicates the purpose and operation of the team to perform. This tool is understandable to all the people involved regardless of their training: user, designer, supplier, operator.
It is currently a standard: " Etablissement des diagrammes fonctionnels pour systèmes de commande " (IEC 848), which allows the description of the function and behavior of the control system, by means of a graphical representation independent of the technological implementation and that allows to have:
- A graphical model Independent representation of the technology to be used Easy to modify, adjust and add Easy to read and understand how it works A standardized document
A simple example:
The machine in diagram A drills two holes in one part
- Cylinder 1.0 serves to hold the workpiece Cylinder 2.0 raises and lowers the hole Cylinder 3.0 moves the table to the second drilling position Cylinder 4.0 ejects the workpiece
We can describe the operation of the machine by means of a GRAFCET Level 1, as shown in diagram B. It can be clearly understood which is the sequence of operation that the machine must carry out in its process, this graphic (together with the sketch of the machine) will help us so that all the people involved can know exactly what we require and there are no ambiguities.
Another important thing is that we have already started the documentation of the machine and part of the operation manual and in fact also the programming of the PLC (Programmable Logic Controller) or the control logic with relays because it is possible to deduce the logic equations from this diagram and therefore a very simple and easy to understand ladder diagram.
The advantages and virtues that we can obtain from GRAFCET are indeed many and if we implement it in our daily work we will realize that when talking about automatisms we can communicate very easily.
