Functional Design Specifications: The Building Blocks of Solid Systems

April 12, 2023
Though there are no specific rules on how long or detailed a functional description should be, it's author should keep in mind that it’s dependent on the user's needs and the equipment and application being used.

If I’ve heard it once I’ve heard it a thousand times: Documentation is the least favorite part of engineering. But it also may be one of the most important and is growing to be more so. This was made apparent recently during a webcast I participated in on the topic of how the services provided by control system integrators can advance a manufacturer’s competitiveness.

During the webcast, a listener asked: What are the tips for developing an efficient functional description about lessons learned on how to not go overboard? I think what the listener was really asking is: What is the delicate balance between too much information and too little information you have to achieve on a complex project?

The answer is simple, yet complex because…it depends.

It all depends on a host of factors from customer preference to subject matter expertise on the job to the complexity of the project and more. But in the end, there are some commonalities that must be achieved regardless of such factors.

So, let’s start in the beginning. A functional description, also sometimes referred to as a loop description or functional design specification is, in its most general form, an overall description of how a system works. This description seems basic when it comes to small engineering projects; but the more complex the project, the more important and detailed the functionals specs should be.

There is no hard and fast rule on how long or detailed a functional description should be. It’s dependent on the customer need and the equipment and application being used. There are important factors to consider in the beginning stages of a project, especially more complex projects, that will be fundamental to the success not only of the functional design and description, but to the project itself.

 Functional descriptions should deliver on the following three descriptive sections:

  • Define the Control Module—How does each piece of equipment interface?
  • Define the Equipment Module—How do multiple pieces of equipment integrate?
  • Define Procedure—When do I turn it on and activate it? It’s in this phase that the function of the system is really defined. Describing the flow of the process from A to Z is what’s most important to the operators who will run the system every day. 

When compiling this information, it’s important to know your audience to ensure comprehension. Often management or automation subject matter experts (SMEs) are the ones who are most aware of functional descriptions. But those who might benefit from them the most are often the operators. However, their need-to-know information can be very different based on their duties and levels of expertise. 

When it comes to functional descriptions, one way we like to train operators is to allude to the functional description, but focus training on the human machine interface (HMI) screen they will be using on a daily basis.

Knowing, recognizing and writing to your audience will help make sure the right information gets to the right people. Also, repeating some of the same information in a manner that is understood to all parties in a consistent manner—no matter the size of the project—can ensure success. Above all, remember to be brief and to the point.

Alex Winking is senior mechanical engineer at Huffman Engineering Inc., certified members of the Control System Integrators Association (CSIA). For more information about Huffman Engineering, visit its profile on the Industrial Automation Exchange.

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