Getting the Converted PID Values Right

I received an interesting story from Emerson’s James Beall about a process manufacturer with a process filled with many fermentors. They were being converted over from Provox control systems to DeltaV systems. pH control was a key part of the plant’s fermentation process. The first two fermentors were converted over but were now cycling. The DeltaV system was managing to keep the pH within specification but the trend charts showed excessive cycling.

Emerson’s Mark Coughran, whom you may recall from earlier posts, remotely connected the site’s system to investigate and solve the issues the manufacturer faced. Here’s a view of the pH and flow trends for a fermentor under control of each system as found:

pH Control comparison

As you can see, the tuning appears to be out of whack in the DeltaV system.

The issue was that the Provox proportional (P or gain), integral (I or reset), and derivative (D or rate) values of the PID controllers were simply converted to the corresponding units of the DeltaV tuning constant. The Provox PID controller users the Series form of PID, with PI action on error, D action on PV. The DeltaV PID controller is selectable between Series and Standard forms—but defaults to the Standard PID algorithm. Here’s a picture of the algorithms’ differences:

Series PID / Standard PID Forms

Also, the integral and derivative terms have different parameter units. The integral or reset action is in repeats per minute in Provox and seconds per repeat in the DeltaV system. The derivative term is in minutes in Provox and seconds in the DeltaV system.

James provided these equations to adjust the PID values between the systems. If you choose the Series Form for the DeltaV PID algorithms, the conversion equations are:

  • Gain (%out/%pv) = Provox Gain %out/%pv
  • Reset (sec) = 60/Provox Reset(rep/min)
  • Rate (sec) = 60 x Provox Rate (min)
  • PV_Filter (sec) = 60 x Provox PV_Filter (min)

If you keep the Standard form default PID algorithm, and the derivative/rate term is zero, then use the same conversions as above. If the derivative/rate term in non-zero, first convert to series tuning—same as above. Then, convert to Standard form:

  • DeltaV Std. Gain = Gain x (Reset + Rate)/Reset
  • DeltaV Std. Reset = Reset + Rate
  • DeltaV Std. Rate = (Reset x Rate)/(Reset + Rate)

James noted that it is also important to pay close attention to the anti-reset windup limits (ARW). In DeltaV, the values are in engineering units, not percentages.

The good news is that once the converted values were applied, the fermentor’s trends looked like what they had been under Provox—”almost flat line.” Of course another option would have been to change the DeltaV PID Form to “Series”. This must be done either by downloading the module or performing an on-line change with the PID block’s mode temporarily set to “Out of Service”.

No matter what your control system modernization path may be, make sure you check the PID algorithm forms in the old and new system as well as any changes in units. Hopefully sharing James’ findings in this post will help someone facing similar issues.

Update: I’ve updated the post to reflect Mark Coughran’s work connecting in from Austin to identify and solve these issues at the manufacturer’s plant.

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7 Comments on "Getting the Converted PID Values Right"

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Jim Cahill
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Control Talk blog’s Greg McMillan expanded on these points into a 14 point checklist that I wanted to share: (1) For cascade control set the output scale of the primary PID in engineering units of the PV scale of the secondary loop (PRoVOX and AC2output scale are 0-100%). (2) For cascade control set the low and high output limits in engineering units (DeltaV default is 0-100%). (3) Set the ARW limits to match the output limits using same units as Output limits unless there is some special need for ARW limits to be set otherwise due to pneumatic positioner and… Read more »
Jim Cahill
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Greg expands on these tips in a Control Talk blog post, Checklist for PID Migration Tips- http://community.controlglobal.com/content/checklist-pid-migration-tips

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[…] standard (ideal or ISA), and parallel. We highlighted the first two in an earlier post, Getting the Converted PID Values Right. The parallel form is not often applied in process control […]

Rafael Araujo
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Hi Jim, how are you? Congratulations for the blog, it is really nice. May you help me with a question? When using the Delta V Standard PID form, the manual mention P(s) and D(s) as the terms which the proportional and derivative actions are applied, but it does not explicit tells the P(s) and D(s) equation. Normally, when using a standard PID form, we use P(s)=[(betta)*R(s) – Y(s)], R(s) = reference and Y(s)=process variable. For D(s), we usually have D(s)=[(gamma)*R(s) – Y(s)]. Thus, when betta = 1 and gamma = 1, P(s) and D(s) become E(s) (error) and we have… Read more »
Jim Cahill
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Hi Rafael, Thanks for your kind words and question! Let me check with some colleagues and see if they can help clarify.

Jim Cahill
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Hi Rafael, I connected with James Beall and wanted to share this back with you: What version of DeltaV are you using? I think the equation you are showing is in the older Books-On_Line (BOL) (I found it in one of my older presentations!). I agree with you, this equation is not very clear! As you suggest, there are terms that are not defined. However, I believe your definitions of P(s) and D(s) are correct for this equation also. The DeltaV PID is a positional implementation that uses a positive feedback filter with a time constant equal to the Reset… Read more »
Jim Cahill
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Hi Rafael, I received one more update from James. He had some time to show how the DeltaV PID Laplace equation reduces to the familiar Standard PID equation when Beta, Gamma are both set to 1.0 (or the Structure “P, I and D on Error”) is chosen. This also assumes that the KNL (non-linear gain) is set to 1.0 (or not activated).

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