DeltaV Insight is Emerson’s next generation control monitoring and tuning application. It was introduced in DeltaV Version 9.3 replacing DeltaV Inspect and DeltaV Tune in earlier versions. The control performance monitoring piece of DeltaV Insight monitors how well your control loops are performing. It does this by constantly monitoring for control loops that fall into the following 4 categories.
· Incorrect Modes
· Control Limited
· High Variability
· Questionable Inputs
This blog will focus on the first of these categories; Incorrect Modes.
If your site is anything like mine, as soon as you begin enabling control loops for monitoring in DeltaV Insight, you will begin to see loops that are being flagged as having an incorrect mode. Any loop whose “Actual” Mode is not equal to the “Normal” mode will be flagged. In many cases, sites have not even made an attempt to set the “Normal” mode.
The default “Normal” mode in the PID function block is AUTO. If the control loop is the secondary loop of a cascade strategy, then the normal mode needs to be changed to CAS or RCAS. Some control loops normally operate in manual and their normal modes will need to be set accordingly.
Changing Normal Mode Values
The normal mode value is an attribute of the MODE parameter for any function block that supports mode. It can be set from DeltaV Explorer or DeltaV Control Studio. The allowable values area any of the current permitted and operator selectable modes. This means that IMAN and LO cannot be sued for normal modes.
The normal mode can be changed online from DeltaV Insight. Drill down to a single function block in the Insight tree and select the Summary tab. From there you can make an online change to both the target mode and the normal mode. Unfortunately, because of the way that this change is written to the change log, it cannot be uploaded into the configuration database. To make the change permanent you will need to make the change in Control Studio or DeltaV Explorer.
Dealing with Multiple Normal Modes
Sometimes a control loop can have more than one normal mode. In this case, you will need to add some logic to programmatically modify the normal mode values based on the criteria that you define. Some of the situations that I have found where this is required are listed below.
1. Loops that are only intermittently used. When not in use, the loop will be in MAN with an output of 0.0%. When it is in use, the loop should be operated in AUTO. You can use the controller output value as the trigger to change the normal mode. To do so, just insert the following code into a CALC block inside the module. The value of OUT that you use as the trigger condition can be set to any appropriate value.
(* Set Normal mode = MAN if actual mode is MAN and output < 10% *)
IF ‘^PID1/MODE.ACTUAL’ = MAN AND ‘^PID1/OUT.CV < 10 THEN
IF ‘^PID1/MODE.NORMAL’ != MAN THEN
‘^PID1/MODE.NORMAL’ := MAN;
ENDIF;
ENDIF;
(* Set Normal mode = AUTO if actual mode is MAN and output > 10% or actual mode is AUTO*)
IF (‘^PID1/MODE.ACTUAL’ = MAN AND ‘^PID1/OUT.CV >= 10) OR ‘^PID1/MODE.ACTUAL’ = AUTO THEN
IF ‘^PID1/MODE.NORMAL’ != AUTO THEN
‘^PID1/MODE.NORMAL’ := AUTO;
ENDIF;
ENDIF;
2. Loops that have multiple valid normal modes, for example, AUTO and CAS. In this case you could use code similar to that above to set the normal mode to AUTO if the actual mode is AUTO and set the normal mode to CAS if the ACTUAL mode is CAS.
Dealing with IMAN and LO
There are two modes, IMAN (Initializing Manual) and LO (Local Override) that are not operator selectable and cannot be set as the normal mode. The actual mode of the primary loop of a cascade control strategy will go to IMAN if the actual mode of the secondary loop is not CAS. This will trigger an incorrect mode in DeltaV Insight for both the primary loop and the secondary loop. If this is sometimes a normal operating condition, you could set the normal mode of the secondary loop to AUTO but there is no way to keep the IMAN in the primary loop from being flagged in DeltaV Insight.
The same is true for LO mode. Any loop in LO mode will be flagged by DeltaV Insight regardless of the target mode. The only workaround I have found is to redesign the control logic to use something other than the tracking function to manipulate the loop output. One option would be to use the ROUT mode to set the loop output.
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