Software quality measures to determine the diagnosability of PLC applications

Abstract

In case of failures in a PLC controlled process the problem of diagnosis arises. Today this is often a manual task. Whenever the failure in the controlled system is not directly related to an actuator, its sources have to be tracked back through the PLC program to the corresponding sensors. This process can be very time consuming. The presented work aims at providing a means for determining this effort and to assist the diagnosis process. To do so, software quality measures are studied. The evaluation of these measures on a program shows the most complex parts and hence gives hints on possible improvements. During the calculation of the measures graphical representations of some dependency relations are derived. These can be used as an aid for the engineer in the actual diagnosis process. Two case studies illustrate the presented approach.

On the Analysis of PLC Programs: Software Quality and Code Dynamics

“…There, the complexity of solutions to the same problem described with different PLC languages is measured and compared. A systemic method dedicated through the modeling of PLC system architecture and PLC features as components was proposed in [12] and [13]. This model is used for the construction of verification model.…”

“…There are many measures which are normally used nowadays to determine the quality of software were applied to investigate the software quality of the PLC programs as Presented in [13]. Most measures presented concentrate more on the program's structure than on the contents.…”

“…This slice is then converted into a model in a type of dependency graph [14]. This graph corresponds to the data flow graph, which additionally edges for paths of all conditions to directly controlled instructions whose execution depends directly on the evaluation of the considered condition inserted [13]- [15]. This provides information on cause-effect relationships in an observed section of the program.…”

Calculating Software Metrics for Ladder Logic

“…Various software metrics have existed since the 1970s, and they have even been used to assess the complexity of manufacturing control architectures based on software and information flow (Phukan 2005), but very few have been used to evaluate PLC code. There is a desire to see practical tools to achieve this (Frey 2002) and recently a Java program has been built that analyzes the metrics of an Instruction List PLC program (Younis 2007).…”