Recent Services Discussions
Similar Posts
What are Good Signal Filtering Tips?
Tip on how filters can improve tieback model performance
PID Tuning Rules
Is Loop Tuning an Art or Science?
PID Controller Tuning Rules
Share

Filtering of changes in tuning parameters

It is my understanding that for a PID controller, changes to the tuning parameters are filtered using the Reset time as the time constant for the filter.  I am working on using Gain Scheduling and I am seeing some things that don't make sense.  Since there are a lot of things happening at one time, I'd like to get a better picture of what is happening to the tuning parameters that the PID module is using.  Is there anything comparable to working set point for tuning parameters?  That way I could see what parameter values the control module is actually using.  It would greatly help sorting out what I'm seeing.

Thanks!

Kent
  • James Beall
    Kent,
    The change to tuning parameters is not filtered. However, the change in OUTPUT due to a change in the GAIN with a non-zero error, is absorbed in a "temporary bias" so that the output does not change when the GAIN is change (and the error is non-zero). The "internal bias" is then diminished to 0 in a first order shape with the time constant of the RESET. If you are using the Gain Scheduling template, there can be an iterpolation of the tuning constants from one region to next using the DEADBAND that you configured. If you use the Detail Display shown in the PID_GAINSCHED module template, you can see the actual GAIN, RESET and RATE from that are being trasnferred to the PID block from the Gain Scheduler. The PCSD loop detail does not show the interpolated values though you can expose them in the module and see them with Control Studio On-line. What kind of things are you seeing that don't make sense?
    James
  • Kent
    I have noise in the system (which I am in the process of dealing with), that is causing the error to fluctuate enough that the parameters from the Gain Scheduler to jump around when I have a disturbance big enough to move things out to zone 2 on the schedule.  When I get this fluctuation, the system sometimes acts as if there was little or no reset in the tuning. 
     
    The first order of business is to deal with the noise.  Once I have that taken care of, I’ll watch and see if it acts more like I think that it should.
     
    Thank you for the help.
     
    Kent E Mitchell, P.E.
    92-H PRC
    Bartlesville, OK 74003-6670

    918-977-4654 office
    918-977-7520 fax
     
     
    Confidential Information - This email is for use by the intended recipient(s) and may contain privileged, confidential or copyrighted information.  Unauthorized use, copying or distribution of this e-mail, in whole or in part, is strictly prohibited.  If you should not be an intended recipient of this e-mail, please notify the sender by return e-mail and delete this e-mail.
    No Contract Formation - This e-mail does not constitute a contract or an offer or acceptance of an offer to enter into a contract.  Further, this e-mail may not be used to modify, supplement, novate, or waive any rights with respect to an existing contract or other binding commercial terms.
     
  • James Beall

    In reply to Kent:

    Kent,
    Yes, we have seen this impact when the process is noisy and/or the GAIN or RATE is being changed. I have seen this on other system's PID fucntion, not just DeltaV. It is caused by the non-symmetrical response of the changes in tuning. As you say, it has the appearance that the integral is turned off but that is not what is happening. If the noise is fast, you might be able to use a smaller filter time constant if you speed up the module execution (due to signal aliasing of fast noise with a slow sample rate).

    Another PID parameter setting that can cause this same sympton is improper ARW high and low limits on the PID block. As you probably know, the ARW limits are in EU's of the OUT_SCALE and in all but rare cases, should be set equal to the high and low OUT limits.
    James
  • Kent

    Thanks. I appreciate the help.

    Kent E Mitchell, P.E.

    92-H PRC

    Bartlesville, OK 74003-6670

    918-977-4654 office

    918-977-7520 fax

    ________________________________

  • James Beall

    In reply to Kent:

    Hope this helps! Let me know what you find out!
    James

This is the official online community site of the Emerson Global Users Exchange, a forum for the free exchange of non-proprietary information among the global user community of all Emerson Automation Solution's products and services. Our goal is to improve the efficiency and use of automation systems and solutions employed at members’ facilities by sharing our knowledge, experiences, and application information.

User Groups | World Areas | Community Guidelines | Legal Information | Privacy Policy | Contact Community Manager

Website translation provided by Website Translator

© 2015-2022 Emerson Global Users Exchange. All rights reserved.