Modular design and implementation of a logic control system for a batch process

This paper addresses the problems of the design and implementation of the sequential and logic control functions of a multi-recipe chemical batch process. A hierarchical control structure is adopted, with one supervisor module co-ordinating many independent slave controllers. Specific rules are given for the construction of the supervisor, from the definition of recipes to the implementation of resource usage and deadlock prevention constraints. Particular care must be paid in the definition of events during the various steps of the control system development: distinct event sets are employed for the communication between supervisor and controllers and between these and the actual plant. As for representation models, Petri nets are used for the supervisor description, whereas the slave controllers are represented in Grafcet/SFC. Finally, the implementation of the control structure is discussed, showing that it is possible to write the control code in an IEC1131-compliant language for programmable controllers, like Ladder Diagrams, in an automatic way. The results shown in the paper are to be interpreted in a design perspective aimed at real-size industrial applications.