most heat loops (and other slow systems) will work fine with an update time of 0.50 second ... they'll also work fine with something faster - but they won't usually work any "better" ...
theoretically speaking, your “10-minute-per-cycle” system would probably do “ok” with a loop update as slow as 30 seconds ... but ... if you’re using any of the PID’s built-in scaling features, etc. for an operator’s display, then the display will appear “sluggish” and people will tend to think that it’s “hanging up” ... I usually shoot for 0.50 seconds unless there’s a VALID reason to change from that ...
just a tip: you mentioned that you’re more familiar with using an SLC for PID control ... do you realize that the PID rung in ControlLogix will usually need a “trigger” condition to tell the PID when to calculate? ... secret handshake: this feature is “built-in” to the SLC’s PID instruction ... but you have to provide the “clock/trigger” action yourself with the ControlLogix (and incidentally with the PLC-5 also) ... you can do it by setting up a “periodic task” - or by using a timer’s Done bit, etc. ... but you MUST do it for optimum results ...
click here for an old thread that might be helpful on the “loop update time” issue ... (and just in case you’re wondering, the forum went through an upgrade a couple of years ago the “attachments” are all still there - but they’ve all been shifted to the bottom of the posts) ...
you said:
I am getting a good amount of lag in the PID from responding and was wondering if I should change the loop update time
IF (that’s a big IF) everything (especially the “trigger/clock” action) is set up correctly, then changing the setting for the “loop update time” probably is NOT going to fix your “response” problem ... more to the point, if you DO change the “loop update time” and it DOES affect the loop’s response, then that USUALLY indicates that you haven’t set the “trigger/clock” action up correctly ... the previous statement assumes (gosh I hate that word) that you’re only changing the “loop update time” in reasonable amounts ... something in the range of 0.10 to 30 seconds sounds “reasonable” for the loop that you’ve been describing ...
going further,
click here for a thread that might help with these ideas ... be sure to continue reading all the way through the post about “Bubba’s paycheck” ... most of my students find that particular analogy to be the most helpful way of understanding the “big ticket” items that we’re dealing with here ...
survival tip: why don’t you ZIP and post your .ACD file and let us take a look at how the PID is set up? ... that would probably save you a LOT of time and effort ...
good luck with your project ...
