Greetings Dwarfer1,
... the company is looking for input regarding the severity
please keep in mind that the severity of this problem could be quite erratic and intermittent in nature ... as Allen pointed out, the PID is being executed on an “each-and-every-scan” basis ... as long as the processor’s scan time stays relatively consistent (from scan to scan) then any adverse effects on your PID’s ability to control will be minimal ... but if the scan time fluctuates appreciably (sometimes longer – sometimes shorter) then the PID will react (basically – see disclaimer below) as follows:
for the Integral component ...
when the processor’s scan time is LONGer, the contribution from the PID’s Integral component will be DEcreased from its expected action ...
when the processor’s scan time is SHORTer, the contribution from the PID’s Integral component will be INcreased from its expected action ...
for the Derivative component ...
when the processor’s scan time is LONGer, the contribution from the PID’s Derivative component will be INcreased from its expected action ...
when the processor’s scan time is SHORTer, the contribution from the PID’s Derivative component will be DEcreased from its expected action ...
note that the effect of using “scan time” to trigger the PID (basically) causes the Integral action and the Derivative action to respond in OPPOSITE ways ... specifically when one action is INcreased, the other action is DEcreased ...
additionally ... IF you are using Derivative action, the “each-and-every-scan” execution will often adversely affect the Derivative component’s “filtering” action ... this usually results is an extremely “jittery” response from the Derivative which, in turn, often makes the CV very erratic in nature ...
the main thing to keep a watch on while the company is “looking for input regarding the severity” is to make sure that the testing takes into account the fact that the processor’s scan time might be highly unpredictable ... it is conceivable that the system could be monitored for a very long time without noticing any serious impact on the PID’s ability to control ... BUT ... there could be issues in your program which could suddenly either increase or decrease the processor’s scan time at some point in the future ... if that were to happen, then the system could suddenly react in a manner which the previous testing and monitoring had failed to predict ... specifically, a system configured in this manner could suddenly become “detuned” by (for one example) something as simple as just adding in some additional programming code to the existing program ... one “bad case” scenario: the programmer adding in the new code might have no reason to suspect that the PID loop suddenly needs to have its Integral and its Derivative settings “retuned” in order to give the same response as it did before the program change ... if no one notices the PID’s sudden “poor control” then bad things could certainly happen ...
now ... a disclaimer ... most of the information above is “general” in nature ... since we can’t examine your program or your process, the best that we can do is to offer this type of “basic” information and hope that it will be accurate enough to help you out ...
finally ... you said:
I would also add that in the Pharmaceutical area, I would always communicate control issues I see
I certainly didn’t mean to imply that you wouldn’t ... I was only trying to help you focus your thoughts on the real issues ... specifically:
1. this needs to be fixed ...
2. it needs to be fixed in the right way ...
after all, you did ask:
Can I leave it as is or should I Fix it.
we’re all pleased to see that you’re obviously taking the correct steps to handle this situation ... please let us know if there’s anything else that we can do to help ...