I often see the output parameter of PID function blocks wired into the SP of AO function blocks, with the AO function block pointing to an IO Reference.
I usually just point my PID block to the IO reference. Is there any advantage to doing it with an AO block? Doesn't that increase memory and processing time?
Thank you,
FK Jr.
In reply to Matt Stoner:
I've never worked on a DeltaV system with AMS so can't say anything about that. Apart from Fieldbus devices, I prefer to wire the PID straight from and to the I/O unless I need some further processing downstream of the PID such as split-range output, high/low selector or valve linearisation, in which case a separate AO block is required. (In upstream and offshore oil and gas, there is a premium on simplicity!)
Best regards,
Neil.
In reply to neilrbrown:
Other, quite unusual, reason to use an AO block after the PID is allowing manual control of the control valve after the PID is in TRACKING mode (i.e. set to 0% in a particular condition)
I would also add the default faceplates point to the PID block parameters. I've seen AO modules added to PID outputs without updating the module faceplates causing soom confusion.
In reply to Travis Neale:
in my opinion the main difference accept for scaling which is already discussed, is that an AO block should be used for instance in case of a Fieldbus because AO and PID do not have to be in the same instrument/valve. Also, AO block can also be put into AUTO mode and a valve can be operated as a hand control valve in that case. Also in the Online mode, it is easier to see the diagnostics.