AN INCREMENTAL APPROACH TO LOGIC-CONTROLLER DESIGN WITH PETRI NETS

A conceptual and practical methodology is proposed to design logic control systems with Petri nets. In particular, the specification formalism used in the paper has been defined at the Politecnico di Milano and is based on a special version of Petri nets, tailored to represent logic controllers. The main objectives of this methodology is to reduce the design effort due to the complexity of control problems and to provide design criteria that perfectly fit the fundamental engineering approach to complexity: the incremental design. The methodology proposed is based on concepts coming from a number of theorems stated and proved in the paper. The designer is then provided with particularly powerful basic net structures, implementing single control tasks, and with particularly useful connections to coordinate and synchronize these tasks. So the designer can face and solve simple control problems and, successively, aggregate separate designs to form a more complex structure. Thanks to the theorems stated, simple rules are provided that guarantee the preservation, after the merging of previous designs, of extremely important net properties, like boundedness, cyclicness, and lifeness. To do so, it is sufficient to analyze an opportune graph based on the connections among tasks.