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Increase to close

What the different between  select IO option (increase to close) in PID block  and select the increase to close from AO block  

6 Replies

  • Functionally they are exactly the same but this option is only used when an Output DST is assigned to the PID. So you would only use the option on PID if there is no AO block and Output DST is assigned to PID.
  • In reply to Matt Stoner:

    So we can assign the DST for output on PID (IO_IN) so how can the output signal go to control valve or output element ,without AO charm
  • In reply to Hassan.hameed:

    Hassan, The AO block is in the Control Module and is assigned the CHARM DST. You can assigned the CHARM DST directly to the PID IO_OUT and forgo the AO Block in the module. You still have the CHARM that physically drives the 4-20 ma Signal

    Andre Dicaire

  • In reply to Andre Dicaire:

    Thanks for answer
    So the issue that facing is the AO was increase to close and the faceplate for the PID control module take the control valve out parameter from PID function block out so that led to confuse to operator because he see the valve is normal work 0% is close and 100% is open but in the site is versa,please advise
  • In reply to Hassan.hameed:

    The signal direction often gets confused with the control Loop action (Direct or Reverse). When you add the PID and AO blocks both having the same output option, which do you use.

    First, why use an AO block with a PID block? Mainly this follows the Foundation Fieldbus model, where the PID block must connect to an AO block to drive the End Device. With conventional/HART signals, the same approach is typically used. If there are intermediate blocks like a Splitter between the PID and AO blocks, as for split range controls, then the AO blocks are needed. As a general guideline, having the AI and AO blocks abstracts the IO links from the primary control block. I prefer to have the AI and AO blocks rather than directly reference the PID to the IO channels.

    The Increase to Close Option is used to align the control signal with the position. It simply inverts the 4-20 mA signal so that when you command the valve to go to close, you enter 0 %, but the AO Block tells the output to go to 20 mA. The failure position of the valve, such as fail Open, is a process design decision. The location of the valve in the process will impact how the PID has to behave (Direct/Reverse). SOmetimes we see two wrongs provided stable control. The user needs reverse action on the PID but has DIrect acting set and the Valve Increase to close is on. Changing one of the settings results in a saturated output. Two wrongs make a right.

    For the Operator, the PID should be clear: 0% is closed, 100% is open. Always. This is achieved by the INcrease to Close option, and is only set in one location, which is the Function block to which the AO channel is assigned. IF there is a PID and AO block, the PID is always Increase to open (Increase to close is not selected). At the AO block, with the block in Manual or Auto, set the 0% signal and valve should be closed. IF it is full Open, the AO block setting is wrong. ONce the AO block is aligned with the proper position of the valve, the rest of the control can be aligned with the desired valve position. If the PID is saturating, i.e. not controlling the process towards SetPoint, Do not adjust the PIC Increase to Close. Rather align the PID action with the Direct/Reverse option.

    Andre Dicaire

  • In reply to Andre Dicaire:

    Just be aware that the AO/OUT has the inversion done for increase to close which might explain what the Operator is seeing. Use PV instead of OUT for the AO block because it is NOT inverted. If you don't have IO_READBACK configured the PV will be Implied Valve Position, when configured and reading from the field it will be Actual Valve Position.

    Also Man mode use is not recommended by the operator for AO because of this inversion as entering 0 would actually be 100%. Use Auto mode with SPs for "manually" changing the OUT at the AO block.