Emerson Exchange 365

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Writing to an ISEL from a running phase

I'm very green and this is my first post, but I'm looking for guidance on the most efficient way to pick between two analog input blocks for use in a PID. The trigger for which AI signal to use depends on our tank configuration. The tank configuration is captured currently as an Operation recipe parameter. I thought to use an ISEL block then write to the OP_SELECT parameter indicating which AI to use depending on the recipe parameter. I appreciate your feedback. 

• AI block
• DeltaV #DCS
• PID
• ISEL

Erik Berry-Sellers

• 27 Aug 2019 11:45 PM
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3 Replies

• dmitriy sagadiev

  

  • 28 Aug 2019 4:05 PM

  I'd use the ISEL function block (FB) with the OOS and Auto modes enabled and HOT_BACKUP selection method. I'd use the Action FB containing expression for the ISEL1 FB. The expression would check whether the selected input has bad status ('^/AI1/OUT.ST' >= 64 expression check for good status) and switch to the good status input ('^/ISEL1/OP_SELECT.CV') and/or use the logic based on the recipe parameter mentioned.

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• Erik Berry-Sellers

  

  • 28 Aug 2019 4:35 PM

  In reply to dmitriy sagadiev:

  Thank you! To clarify, I do not want the ISEL function block to switch to the other input at any time other than when the phase instructs it explicitly. The HOT_BACKUP selection method indicates it would switch to the other input if the current input registered a bad status. I don't want to do this because only one of the probes will be inserted into the tank at a time, the other one will be recording ambient temperature in the room since it'll be hanging on the rack. Since I have to pick something for the selection method, would selecting HOT_BACKUP, then writing to the DISABLE_2 from the Action FB for when IN_1 is to be used and vice versa be sufficient?
  
  Also, I spoke with a colleague and could consider having the AI dynamically referenced and for the assignment to occur based on the recipe parameter.
  
  Thank you for your guidance, dmitriy.

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• dmitriy sagadiev

  

  • 28 Aug 2019 4:57 PM

  In reply to Erik Berry-Sellers:

  If one of the probes is exposed to ambient then the other should read one of the extremes (either colder or hotter than the ambient). The selection method can be specified accordingly. The Action FB will override the selection method based on its expression when triggered. Besides the two ISEL parameters I described, there is '^/ISEL1/SELECTED.CV' that provides selected input and status, which you may find useful.

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