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PID
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Multiplication of PID out with another PID out before passing to AO
Hi,
I have configured a PID control module with AI1 and AO1 for Input and output respectively, let us consider it as PID1.
I Have kept a multiplication block between my PID1 out and AO1's CAS_IN. In the Multiplication Block my PID1 out will be multiplied with another PID blocks output lets say PID2 (PID 2 out scale is 0.95 to 1.05) as per process requirement.
The BKCAL_OUT of AO1 Block is wired to BKCAL_IN of my PID1
In this case if my PID1 generates an output of 20 and my PID 2 output is 100% (ie 1.05 as per out scale) my value at CAS_IN of A01 will be 21,So my PID1 BKCAL_IN will get 21.
Is this configuration correct? Normally PID out and AO blocks BKCAL_OUT will be same so what will happen in this case? Hope you understand what Im trying to do please suggest is there is any better way.
Thanks,
Neeraj S
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DeltaV #DCS
configuration
PID
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gamella
9 Dec 2020 4:04 PM
You should better consider using a RATIO block instead of MULTIPLICATION block to properly propagate BKCAL. RATIO OUT is SP * IN_1 * Gain (of ratio block). So OUT from PID1 will be wired into RATIO CAS_IN and RATIO OUT wired into PID2 CAS_IN.
Both, IN_1 or Gain are the factors to multiply PID1 OUT before it reaches PID2 CAS_IN. Normally IN_1 is used to provide a calculated factor and Gain to adjust PID1 OUT scale and PID2 PV Scale. Do not forget to set GoodNonCascade on IN_1 if it will have a fixed value.
Scaling/Limits must be properly configured on RATIO block.
PID2 BKCAL_OUT wired to RATIO BKCAL_IN and RATIO BKCAL_OUT wired into PID1 BKCAL_IN.
Matt Forbis
9 Dec 2020 6:00 PM
In addition to what gamella has said (which is all good advice), if you choose to go at it with custom blocks, you should also do the inverse calculation for the BkCal so the PID block BKCAL_IN sees the same value on it's OUT in normal cases. Also, keep it mind with some of the simpler blocks, such as the multiply block, the status is set to the lowest of the inputs. GoodNonCascade has a lower value if you wire in a constant for example and this will cause issues with the BkCal also. In cases like these, it's usually better to use a Calc block and explicitely pass status from the correct input and output. You can use the same calc block for both directions, similiar to a splitter block.
Neeraj S
9 Dec 2020 6:18 PM
In reply to
gamella
:
Hi gamella thanks for your reply.
As per process design my PID2 will be working in auto mode with a local setpoint given. Basically it is two different loops and its multiplied output should be passed to AO block.
James Beall
9 Dec 2020 6:39 PM
Good info from Gamella and Tim! My preference would be to use the Ratio block. This is one of the applications that is made handle. It can be a bit tricky to configure but it will handle the tracking, modes, etc.
Neeraj S
9 Dec 2020 6:41 PM
In reply to
Matt Forbis
:
Hey Matt , Thank you for your reply.
Can you please break it down for me. What do you mean by good non cascade has a lower value? How do I utilize calc block here , I mean what would my expression look like. I really want to understand the concept you mentioned here.
James Beall
9 Dec 2020 6:50 PM
In reply to
James Beall
:
I meant Matt! That's what happens when you get old! Sorry Matt!
Neeraj S
10 Dec 2020 7:25 AM
Hi everyone, I'm uploading a schematic of what I'm trying to achieve so that you have a clear understanding. Please visit the link for image.
https://ibb.co/1vBStYY
gamella
10 Dec 2020 11:21 AM
In reply to
Neeraj S
:
link not working
Neeraj S
10 Dec 2020 11:36 AM
In reply to
gamella
:
Hi gamella,
I have uploaded the picture directly along with the question. Please check now
gamella
10 Dec 2020 12:31 PM
Ok, thanks.
Not sure because FX3762 function is unknown but I will use a ratio where RATIO is slave of TIC3781 and FX3762 OPUT is wired into RATIO IN_1. Control valve is then slave of ratio block.
IN_1 does not require BKCAL wired so I think this could be best approach as long as FX3762 OUTPUT seems to be a small value that will not require strict bumpless transfer precaution.
Neeraj S
11 Dec 2020 5:45 AM
In reply to
gamella
:
Hi Gamella,
Function of FX3762 is nothing but to scale TIC-3782 OUT to required range (.95 to 1.05). I didn't use it, i directly used the range in pid out scale.
I have implemented your recommended configuration and I think it is much much better and easy to understand, thank you so much.