An Assessment of PLC Software Structure Suitability for the Support of Flexible Manufacturing Processes

Industrial control engineers continue to make extensive use of ladder logic for programming programmable logic controllers (PLCs) although the limitations of the language are well recognized. Evaluations of alternative approaches have been conducted in academia but industrial users have shown little enthusiasm for the recommendations. This work addresses this by conducting an experimental evaluation using a range of skilled and unskilled participants to establish which of four software design methodologies is best suited to the support of flexible manufacture in a small program through an experiment in which ldquoright first timerdquo rates and completion time are measured. Unlike earlier work which had an emphasis on program creation, this paper reports on process modification. The object modelling tool tested, Enterprise Controls, was found to be the most effective methodology under study with the highest ldquoright first timerdquo rate as well as demonstrating a 80% time saving over traditional relay ladder logic. This was closely matched by step-based ladder logic, provided the concept is used correctly. Areas for further work include the assessment of program development time during setup and control performance in use.