Emerson Exchange 365

In reply to TreyB:

Good questions! Let me explain one way in which this feature might be useful. Consider non-smart, pneumatic positioners which can have imprecise calibration. Though you know the valve should be fully closed at 0% controller output and fully open at 100% controller output, it was common to under/over drive the output signal to make sure the valve was closed. For example the out limits might be set at -5% and 105%. However, considering the valve was PROBABLY closed at 0% and open at 100%, you could set the ARW_LO_LIMIT at 0% and the ARW_HI_LIM at 100%. Thus, when the controller output was below 0% but the controller wanted to increase the output above 0%, it would increase the output with an 16 X sped up integral action (reset time/16) until the output was above 0% ( the ARW_LO_LIMIT). You can apply the same concept to the ARW_HI_LIMIT and OUT_HI_LIMIT. However, modern smart positioners have a feature such that if the controller output (input to positioner) fall below a preset limit, say 0.5%, the positioner will remove all pressure to the actuator. A similar concept for the near 100% controller output applies. So, the ARW feature is really not needed for modern smart positioners.

Another use is when the PROCESS can become saturated in a known way. An example might be the low flow anti-surge override controller. However, the new PIDPlus feature called "RECOVERY_FLTR" is a better solution.

So, I agree, the ARW feature is rarely, if ever, needed any more. Even if you set the ARW limits to the corresponding limits to the OUT limits there is some action that takes place. In order to completely disable this feature, you must set the ARW limits OUTSIDE of the OUT limits. If you want to make sure it doesn't become active, you should set them to -10% and 110% of the OUT_SCALE range. On earlier version of DeltaV (before 12.3??), to do this on a running system you will have to set the OUT limits to -10% and 110% of the OUT_SCALE, then set the ARW limits to -10% and 110% of the OUT_SCALE, and then set the OUT limits back where you want them. In off-line configuration you can set ARW limits to -10% and 110% of the OUT_SCALE regardless of where the OUT limits are set. My colleague Matt Stoner pointed out that if you don't want to use the ARW feature, you probably want to set the ARW limits as wide as possible so that you don't have to change them if you later change the OUT limits.

!!!!!!!!!!!!! Note that ARW limits and OUT limits are in EU's of the OUT_SCALE!!!!!!!!!!!!

As my colleague Mark Coughran pointed out, Reset Windup (aka integral windup) is prevented in DeltaV through the use of the "BKCAL" feature. If the secondary (slave) loop in a cascade arrangement becomes limited (SP or OUT), the BKCAL_OUT from the downstream PID block will let the primary (master) PID know that it is limited and its integral action will stop. Similarly, in an override application, the Control Selector block will let the all of the master controllers know whether they are selected or not. If the PID is NOT selected, its integral action will be turned off.

There is another feature that can additionally help avoid integral windup for the case where the secondary PID is not limited (yet) but its PV is not responding as fast as it normally does, is the FRSIPID_OPTS "Dynamic Reset Limiting" on the master loops combined with CONTROL_OPTS "Use PV for BKCAL_OUT" on the slave loop.

Hope this helps! Let me know if there are other questions on this topic!