An extraction method of control flow from ladder diagram by judgement of transfer frequency

In sequential controllers of plants, the ladder diagram is one of the major programming languages. However, it is difficult to understand its control sequences and to maintain them because ladder diagrams do not clearly represent the sequence of plant behavior. The Sequential Function Chart (SFC), which is based on the Petri net, has recently spread as a powerful graphical description of sequential logic. SFC can represent clearly the sequence of control programs. Therefore, a method to translate ladder diagrams to SFC is required. A control sequence extraction method is proposed. In this method, a tentative SFC is created by searching all possible states by using transitions in the ladder diagram. However, the tentative SFC description is complicated because of the existence of non-order control, which does not depend on a fixed sequence, for example manual control. Therefore, the sequential flow of plant behavior in the tentative SFC is extracted according to the difference of transfer frequency between order control and non-order control. Through an application of the proposed method to a practical plant model, it is confirmed that the adequate sequence which represents the original plant behaviour is extracted from the ladder diagram.