Transparency analysis of Petri net based logic controllers-a measure for software quality in automation

Control algorithms are a special type of application oriented software which clearly should meet the quality criteria defined by the ISO/IEC9126 standard. It is shown how these criteria are related to two basic properties of control algorithms: correctness and transparency. This contribution concentrates on the analysis of transparency. An algorithm is said to be correct if it fulfils several formal criteria as for example determinism. It is said to be transparent if it is 'easy and clear to see' what the controller does at the moment and what it will do in the next steps. A number of criteria for transparency are given. These criteria cover different aspects such as number of comments, directionality, and I/O-behavior. They are combined in a weighted sum to an automatically computable metric. The analysis is based on the signal interpreted Petri net (SIPN). It is shown how the transparency properties of the SIPN can be determined using the SIPN itself and its reachability graph. Because of a close relation between SIPN and the sequential function chart (SFC) according to IEC 1131-3 standard the analysis is easily extended to SFC-nets.