The Case for Control in the Field-based Control

John Rezabek closes his Surprise! Field-Based Control Beats DCS article with the thought:

The SP50 committee set out years ago to define and specify a robust, vendor-independent controls suite exploiting the intelligence of microprocessor-based field devices that would equal or exceed the “bulletproof” DCSs of the 90s. They succeeded.

The source of this conclusion is two recent studies, one conducted by Emerson fieldbus consultants Dan Daugherty, Mark Coughran, and Ferrill Ford. The other is by Kenexis Consulting‘s Ed Marszal, who applied a similar reliability model used with safety instrumented functions to Foundation fieldbus (FF) control loops running PID in the FF device. The common vernacular for this type of control is control-in-the-field (CIF).
I described and embedded Dan, Mark, and Ferrill’s Emerson Exchange presentation in a post, Dispelling a Rules of Thumb in Foundation Fieldbus Segment Sampling Rate. In John’s article, he summarizes this study:

One of the things revealed was that control in DCS did not benefit substantially from over-sampling, which involves running the macrocycle two or more times faster than the DCS PID block. The other notable result was that control-in-DCS could not approach CIF in performance, especially when macrocycles (the deterministic cycle time of all the function blocks on a fieldbus segment) were cranked down to as little as 150 milliseconds. CIF was distinctly better and had no problem matching the performance of fast, pure analog (4-20 mA) control.

Control in the field has the advantage of single loop integrity, diagnostics, and signal status across the fieldbus segment. For the reliability study comparing control in a DCS controller versus CIF for a simple PID loop, John quotes Ed’s findings:

Fieldbus is significantly better–mean time to fail (MTTF) of 48.2 [years] versus MTTF of 15.9 [years].

The numbers were rerun using field devices with the same reliability numbers and the CIF case again had greater MTTF. I sent Dan a link to the article and asked what thoughts he might have to share. He wrote:

It is good to see confirmation of our conclusions from independent researchers using different methods of analysis. In the tests ran in the Marshalltown flow lab, Mark Coughran, Ferrill Ford and I not only demonstrated the superior performance of FOUNDATION fieldbus deployed as Control-in-Field (CIF), but were able to show the relative contributions of the control algorithm update rate and the I/O sampling rate of the macrocycle. This is useful information for economical control system design to the necessary performance requirements. We were able to dispel the myths that inhibited the choice of FOUNDATION fieldbus due to unnecessary overdesign, which in turn leads to low density and higher cost segment design.

I fully concur with the conclusions that use of FOUNDATION fieldbus can lead to higher availability than one can get with point-to-point I/O. Without going into all the complicated algebra, the common sense view is that one H1 card for 16 loops (8 loops per segment x 2 segments per card) will have a lower failure rate than 4 I/O cards for the same number of loops. Then once one chooses to use redundant H1 cards and fieldbus power supplies, the availability is much, much higher. Clearly, FOUNDATION fieldbus is by far the most reliable way to deploy control. (I assume that by now, everyone understands individual devices and short circuits are isolated from the segment through the field connection hardware.)

Given these performance and reliability results, the case favoring this highly distributed control-in-the-field approach continues to build.

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