Recent Control & Safety Systems Discussions
Similar Posts
How does PID Function Block handle bad status of feedforward variable?
SIS Bad Devicefailure status in the blocks
SIS NET Repeaters Link Status BAD
DeltaV workstation's network card bad status problem
VM Server Deltav Primary and Secondary bad status
Share

PID Block Bad PV Status

I am having issues with a PID block going to manual when the input PV status goes “BAD”, the block stays in manual until the PV status goes good then switches back to Auto.

I am trying to figure out a way to have it remain in Auto when the status goes bad but have not gotten anything to work yet. I have tried doing this which is from BOL:

“When the Uncertain if Limited option is selected, then as long as the input status is limited, the PV status will be set to Uncertain (even when the input status is Bad). Selecting this option and the Use Uncertain as Good option disables the mode from changing to Man under these condition when the target mode is Auto.”

But even with the above implemented it still switches to manual when the PV status goes bad.

 

The status is going bad as a flow meter gets over ranged 1-2 times a week as we are currently running right at the top end of the meter, we are working on upsizing the meter but until then I was wondering if there was anything I could do with the PID loop?

ditners
  • Andre Dicaire

    In reply to Matt Forbis:

    The AI channel sets status based on several things: NAMUR limits enabled, HART ERRORs not ignored, Signal out of range.

    The AI channel has short circuit protection to indicate the signal is BAD if the current is too high. This trip point is set above the maximum mA signal a transmitter would send when saturated. If the process is forcing the Transmitter output to its maximum, this should not set the Short circuit trip. You can very the channel status to see that it is not flagging short circuit.

    Next, the AI channel can have NAMUR limits set. Unlike the Overrange/Underrange, NAMUR sets the channel BAD. These limits are fixed, and if you enable this, the status of the input will be set BAD. In your case, you want to make sure that the NAMUR limits are not enabled as your application "normally" results in the process exceeding the transmitter's operational range.

    Then we have the HART ERRors. There is a PV Saturated error that is set if the transmitter gets to its maximum mA signal, meant to indicate the process is not being accurately reported. You can set this to be ignored. By default, in the DCS, these errors are not ignored. ( In SIS, these are set to Ignore. ) you may choose to ignore all HART errors to get started and re-enable the ones you want after you've confirmed this was part of your issue. I like to see the PV_Fixed alert as this means the transmitter is out of service and the AI channel is not connected to the process.

    First confirm that these configuration options are not enabled and the source of the BAD status. Now, if your Input only shows Limited, the Use UNCERTAIN as GOOD should allow the PID to remain in AUTO when the PV goes out of range and saturates the 4-20 mA. the Open Loop diagnostic will still be there, but should not be triggered on a saturated mA, just a short circuit.

    Andre Dicaire

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.