CTLS Low Select

Question: How does the  CTLS block Limits the output of the non selected PID block ?  It seems like the output of the non selected PID block is somehow fixed by the BKCALC of the selector block.

Status:

• I have  a very simple low select CTLS which selects between a PIC and a FIC.  
•  Output value of the my FIC is %16 and the PC is %47.  
• Output of my CTLS block is of course %16
• %16 is entering the BKCALC of my PC and FC 

What I want to know:  

• Although the Setpoint of my PC is 2000 and the current PV is at 1500, the output of the PC is not ramping up towards %100.  It seems like it somehow has been fixed to 47.  it moves very slightly but I expected to move a lot faster.  Does it have anything to do with the CTLS affecting the PC block?  I don't see any tracking logic on the PC. The Action and Gain of my PC is correct.

Regards,

Learner

learner

• 21 Jan 2016 12:24 AM
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3 Replies

• Matt Stoner

  

  • 21 Jan 2016 3:14 AM

  Learner,
  
  Yes the Control Selector Block is sending a BKCAL status to the non selected loop that it is not selected and this is what is not allowing the PC loop output to increase. It won't allow the not selected loop to keep increasing the output as it will never get selected by going higher. If the PC loop does become the selected loop, the output will increate as you expect.
  
  Regards,
  
  Matt

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• Former Member

  

  • 21 Jan 2016 10:18 AM

  In reply to Matt Stoner:

  Learner,
  
  I think the BKCAL status provides an anti-reset windup function so that the pressure controller doesn't just keep on winding itself open, as the actual selected output from the CTLS module is limited to the output by the FIC. If both the FIC and PIC were to increase output, the CTLS module would take which ever one was increasing the slowest (as it would be the lowest value at somepoint) and keep the system in control.
  
  Regards
  
  Col

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• Andre Dicaire

  

  • 21 Jan 2016 1:30 PM

  In reply to Former Member:

  The back cake status tells the PID to disable reset and uses the bkcal_in value for the integral term. The OUT value of this PID therefore responding only to the error with proportional action. It is more than anti reset windup.
  
  The out value will respond to change and as the error approaches 0, the OUT value will approach the CTS output for a bump less cross over if the PID error goes the other way.

  Andre Dicaire

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