Engineering the control software development process

The variety and complexity of processes required to be controlled in modern manufacturing places heavy demands on PLCs. To meet these demands PLC manufacturers are producing increasingly more powerful PLCs which require more complex programming languages. These complex programming languages result in longer specialised training and familiarisation times. To overcome these problems, the PLC programming language standard IEC 1131-3 was created. The standard details the workings of languages which provide PLC users with a set of programming languages based around familiar PLC concepts, but with language constructs similar to those of current generation PC programming languages. This paper describes the work done in a TCS partnership between Elite Control Systems Ltd. and Napier University to examine the viability of using structured methods for developing PLC software. Object oriented methods have established themselves as the future of software development for high level language users. We discuss how the object modelling technique of Rumbaugh et al. (1991) [one of the principal object oriented methods used in the UK] can be adapted for the creation of PLC software, and how the resulting practices differ from traditional PLC software development. We then describe the improvements in terms of software quality, development time, development costs, and maintainability that we have observed by adopting these techniques.