Saturday, March 5, 2011

Simplify Sequential Process Using Grafcet





The programming task of the sequential process was simplified using the Grafcet. At the HMI level the operator can visualize the Grafcet diagram and in event of a malfunction of the machine the cause can be easily determined. The actions and the transitions were also programmed using a graphical language: Ladder. For each rang the programmer can see the state of any Boolean object, and the value of the numerical objects. The program is similar with a wiring diagram and is very effective for logical expressions.

This program can be implemented using the state diagrams as well but the programming and the debugging task would be very difficult because the whole sequential process would have to be implemented using a specific logic. The debugging of such program would be very difficult. The method proposed is also based on using some general subroutines that would be used in several sequences. Developing separate subroutines for each action is often a cause of the programming bugs. The program uses knowledge acquisition for the process application algorithms that allow the operator to input corrections without the program modification

The increasing complexity of the automatic systems imposes the use of high level programming languages such as Grafcet. The use of Grafcet facilitates implementing of the sequential command. The case study presented in this article is used to demonstrate that the programming task is simplified because the sequential processing was implemented using Grafcet. In the case presented in this article each working mode was treated in separate graphs. The debugging is greatly simplified because the state of the chart can be visualized isolating the problems. The debugging part can be treated as modular task. The same program would be more complex if the sequential task would be described using a “classical” method such as state diagrams.



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