In logic control design, production sequences are typically expressed as a series of informal specifications to be translated into a formal model. The present paper introduces a novel Petri net based methodology conceived as an incremental process, which progressively adds sub-nets to a partial model until all specifications have been included. A formal verification of the model is performed at every stage, so that specifications which are incongruent with the current model and can jeopardize its correctness can be detected and rejected or modified. No modeling constraints are imposed to the design process, as is the case with classical methodologies, which are generally not well suited to represent behaviors characterized by complex alternative-parallel relationships between operations. An algorithm is provided for the automatic synthesis of complex connection structures in the model, allowing for greater flexibility and complexity in the design process.