Loop Performance and Sharing What’s Learned

by | Jan 14, 2009 | Event, Services, Consulting & Training

Jim Cahill

Chief Blogger, Editor

When I struggle with a technical issue, whether it is hardware, software or just something I’m trying to understand better, I usually start with Google. I’ll add as many relevant keywords as I can think of, and add “howto” as one more keyword. The search often returns amazing results, usually from individual’s blog or a response in a forum, describing exactly how to solve the issue.

The web offers so many ways, not just blogs and forums, to share your interests and expertise. These many ways were the focus of a presentation Deb Franke and I gave at last year’s Emerson Exchange.

At the ModelingAndControl.com blog, Greg McMillan shares his wisdom every week. This week’s post, What Have I Learned – Einstein and the Ultimate Limits for Loop Performance is a perfect example. If you’re a process control engineer and you’re not already subscribed to the blog’s RSS feed, I recommend you do.

This post offers straightforward guidance, like:

The absolute limit to feedback control system performance is the total dead time in the loop, which is the summation of all the final element, process, measurement, I/O, and controller execution time delays. A feedback control system cannot correct for something it hasn’t seen yet and hasn’t been able to change yet in the process…

Greg references an on-line eBook, Funny you should Ask a Process Control Engineer where you can find more information to support this guidance. Greg has numerous eBooks, application notes, lectures, and articles available on the Modeling and Control Blog.

Another example Greg offers is that advanced process control (APC) also cannot violate this absolute limit. He writes:

Many of the early APC algorithms significantly increased the loop deadtime (See “Advanced Control Algorithms- Beware of False Prophecies in the Funny Thing E-book). While model predictive control (MPC) can potentially help dead time dominant systems, the original execution time (e.g. 1 minute) of separate MPC software packages was so large their applicability was restricted to slow processes. With the advent of the MPC embedded in the DCS, the execution time can be as fast as 1 second which means MPC can be applicable to all but the fastest processes (e.g. liquid pressure control and furnace pressure loops).

In all of Greg’s guidance, he provides links where you can get more detail. It’s like a self-directed, university-level course for process control engineers. All one needs is the quiet and focus to take it in and absorb it.

I’m sure you have some specialized knowledge for which you’re known. If you’re the type of person who believes you’ll get back far more than you give, consider using some of the tools we mention in our Emerson Exchange presentation like Google Reader, Delicious, LinkedIn, Twitter, etc. to share this knowledge for the next person searching for answers.

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The opinions expressed here are the personal opinions of the authors. Content published here is not read or approved by Emerson before it is posted and does not necessarily represent the views and opinions of Emerson.