Control Response Period for Foundation Fieldbus Control-in-the-Field Loops

A great question came in the Process Automation Usability Project forum asking why there is not more control in the field being done. If you’re unfamiliar with the term, it’s where a Foundation fieldbus (FF) transmitter, digital valve controller, or other instrument runs the control logic for the loop, instead of this being done in the automation system controller.

When I saw the question, I bounced it over to Emerson fieldbus consultant Dan Daugherty, whom you may recall from earlier fieldbus-related posts. Dan made a couple of great points, the first on segment design:

If you do control-in-controller, and you don’t care a whole lot about process segregation (only do it on a coarse, controller level), then you can skip all the concerns regarding segment design except for voltage drop and number of devices. Many engineering companies didn’t want to learn segment design, so they stuck with control-in-controller.

His second point dealt with the complexity of the control strategy used:

…some control strategies are complex enough they can’t be done on a single segment (prior to 2007) because not all the control functions they wanted to use were available in devices. That is no longer true with Emerson because we have ability to pick up functions in the H1 card, effectively filling out any gaps that used to exist in the control-in-field palette of functions.

His third point is that there are economic and performance advantages to using control in the field. In his response, he references a presentation that he and some colleagues are giving at the upcoming Sep 28-Oct 2 Emerson Exchange conference, Effects of Macrocycle Time and Sampling Rates on Control Loop Performance.

The team set up controlled tests in Emerson’s Marshalltown Flow Lab to compare performance between Foundation fieldbus and conventional 4-20mA analog loops for control response period, load frequency response, and setpoint step response.

Their tests showed that with FF control in the field, the control response period equaled the macrocycle and they could get 0.18 seconds, which is adequate for almost all loops. The exceptional loops that require faster response times, such as surge control and compressor lube oil, typically have dedicated controllers. Dan does however reference a 2008 Emerson Exchange presentation where field-based control was successfully performed in a compressor anti-surge application.

Another important finding is that there is even more reason to use control in the field as the number of loops on a fieldbus segment increases. The control response period is much greater when control is performed in the automation system controller than when control is executing in an FF device when they looked at 8 loops on a fieldbus segment.

If you’ll be joining us in Orlando at the Emerson Exchange, you may want to check out Dan and team’s presentation. One session will be Sep 29 at 11am and the second on Oct 1 at 10am. Visit the Personal Scheduler to plan the all sessions you want to see.

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