A functional description for the IEC61131 program of an automation project is a necessary and vital document. You wouldn’t build a house without first creating plans to specify your requirements and communicate them to the builders, so why would you try to automate your process without a “plan”. A functional description (FD) is a comprehensive “plan” describing how a process will function and how the IEC61131 program will control the process. The FD is used to communicate process operation and functional requirements between the programmers, designers, builders and owners of the automation plant.

Do you want to mitigate you automation risks?
Automation projects are inherently high risk and seem to nearly always end up over budget and behind schedule. Unfortunately an FD document can only be as good as the information provided. We have seen too many projects were the original scope and process documentation was inadequate and every time this resulted in cost variations and project delays.

What is the solution?
It is important that as many people as possible review the FD, at least then if an automation concept is wrong it wasn’t because of the Automation Engineers misinterpretation of the process. There is also nothing better than the written word to cause people to focus on specific concepts that previously may have only received a fleeting glance.

AIEglobal always uses a top down object oriented design methology to describe the functional software modules called Function Blocks. Our experience has been that design errors in the FD often only require changes to how the Function Blocks are used, and not to the Function Blocks themselves. These changes are usually only minor.

An example of a Function Block to control a variable speed drive (VSD) is presented below. The FD would give an overall operational description, describe the inputs and outputs, and document the alarms generated by the controlled VSD. The FD would then reference this Function Block when describing the higher process functionality.


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