Emerson Exchange 365

Brian,
Good questions! I hope that I can clarify this topic. A few things to note before we get into the details. Note that the OUT limits can be set outside OUT_SCALE values(higher that the 100%EU or lower than the 0%EU). Second, my explanation will refer to the value of the OUT in the units and scale of the OUT_SCALE. So, the action of the ARW "impact" is only determined by the value of the OUT in relation to the ARW high or low limit AND the direction the PID action is currently moving the OUT. The basic action of the ARW feature is explained fairly well in Books on Line (BOL). If the OUT is higher than the ARW_HI_LIMIT, less than the OUT_HI_LIMIT, AND the PID action is moving the OUT lower, then the RESET time is reduced by a factor of 16 (makes it faster). Conversely, if the OUT is lower than the ARW_LO_LIMIT, more than the OUT_LO_LIMIT, AND the PID action is moving the OUT higher, then the RESET time is reduced by a factor of 16 (makes it faster). So, if the ARW limits are "inside" the OUT_LIMITS, there is potential ARW action. A small caveat is that there is still some potential ARW action when ARW limits are set exactly at the OUT limits when the OUT is at one of the OUT limits and the PID is trying to bring the OUT back inside the ARW and OUT limits.

A common use of this feature was for the case of a valve with only an I/P or with a pneumatic positioners. The problem was that you weren't sure if the valve was closed at 0% OUT or open at 100% OUT. So, you might set the OUT limits at -5% and 105%, and set the ARW limits at 0% and 100%. Thus, if the controller wanted to close the valve it would move its OUT to -5%, but when it was time to start opening the valve, the RESET time was sped up by a factor of 16 moving the OUT from -5% to 0 quickly and then at normal speed when above 0%. And opposite this on the high end. With the use of smart valve positioners, that can be set to remove all pressure to the actuator when the OUT is near 0% or apply full pressure when the OUT is near 100%, this feature is not needed. There are a few other uses of this but the new DeltaV PIDPlus feature call "RECOVERY_FLTR" (FRSIPID_OPTS) does a better job than ARW.

So, I recommend that the ARW feature be turned completely off. To do this, the ARW limits need to be OUTSIDE the OUT limits (in units of the OUT_SCALE) or where I think someone might want to move the OUT limits!

Hope this helps!
James

In reply to Brian Hrankowsky:

Hey Brian,

Sure, the OUT limits can be -10% to 110% of the OUT_SCALE  Try it!  But don't let that confuse the issue, it just slightly extends the OUT range.  Note that the OUT_SCALE is used to convert %OUT to OUT in EU's of the OUT_SCALE.

PIDPlus, including the Recovery Filer started in either Version 9.3 or 10.3.  See BOL.

This picture of the DeltaV PID (without Derivative) might help understand the positional PID algorithm.

Only Gain and Derivative are active on the "internal OUT" when the OUT is limited (and derivative is temporary).  FFWD may impact the internal OUT when the external OUT Is limited, I'll have to test this and get back to you!  This means integral is turned OFF and there is no "integral windup".

Think of all this working with in the OUT limits which are -10% to 110% of OUT_SCALE.  So, OUT_SCALE is related to the limits.

Dynamic Reset Limiting is "external reset feedback".  It stops or slows down the integral action if the external feedback on the BKCAL_IN stops moving, or moves slower than normal.

I recall that old versions of BOL said the DeltaV PID "is a digital implementation of" the Laplace equation of the PID function.  This didn't mean it was a velocity algorithm, simply digital vs. analog implementation.  It is hard to tell from each manufacturer's documentation whether it is positional or velocity but DeltaV has always been positional.

I have attached an article by Shinskey on external reset feed back and the positional PID algorithm.

Hope this helps!

James

external-reset Shinskey.pdf