PID and digital control efficiencies

Most proportional-integral-derivative (PID) applications use digital controllers, though some still use analog. Digital control can offer additional process control system efficiencies.
By CFE Media November 14, 2016

Digital controllers have some advantages over analog controllers for proportional-integral-derivative (PID) applications. James F. Beall, principal process control consultant, Industry Solutions Group, Emerson Process Management, explained how digital control may offer additional process control system efficiencies by answering five questions. 

Digital PID controllers

Q: For PID applications, about how much analog control is still in place?

Beall: Based on looking around in the plants where I have control projects and asking a few customers, I would say there are less than 5% analog controllers in plants.

Q: What are the main advantages of using digital controllers over analog in PID applications?

Beall: There are many useful options in digital controllers that are unavailable in analog controllers, such as variable tuning; selectable PID structure [P and D on process variable (PV) or error] and form; gap control; error squared; tuning algorithms; accurate tuning settings; and others.

Q: Are there PID applications where analog has advantages over digital?

Beall: I have heard that a digital control controller must have input to output response time less than 100 milliseconds to match the speed of an analog controller. So, for loops that require a very fast response, the analog controller is faster than most digital controllers.

Q: When transitioning from analog to digital PID-type controllers, what are the special considerations, if any, for selection, setup, operation, optimization, or modifications over time?

Beall: The first step is to make sure you PROPERLY convert the current tuning in the analog controller to the digital controller.

Also, ensure you don’t configure a lot of new alarms just because it is easy to do so.

Finally, investigate the new features of the digital controller to find better ways to duplicate or improve previous functionality.

Q: Beyond performance, what are some other advantages of digital controllers for PID applications?

Beall: Other advantages include ease of collecting data from the controller and easier troubleshooting the controller action.

Edited by Mark T. Hoske, content manager, CFE Media, Control Engineering, mhoske@cfemedia.com

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