PLC5/40E PID PV & SP setup

Must the setpoint & process variable be 0-4095 in the PID instruction?

Thanks for your time & patience.

no ... once you've set the range of your "setpoint scaling" to match the range of your field input transmitter, then you can enter the setpoint in "engineering units" (for example: degrees, gallons, etc.) ...

which type of control block are you using for the PID? ... is it N-type (integer) or PD-type? ... it makes a difference on where you enter the "setpoint scaling" - but the ideas are the same ...

tip: suppose that you set your setpoint scaling for 0 to 100% ... that COULD be a problem ... with "integer limitations" your operator would never be able to enter something like 23.5% ... the best he would be able to do would be 23% or 24% (round off) ... sometimes things like that make a difference when you're deciding how to scale the setpoint ...

and welcome to the forum ...

I'm glad I found your site, it's very helpful. I'm using an integer type PID instruction.

I have "no" selected in the input scaling block.

the picture below might help ... this is an example of using a field transmitter with a range from 0 to 500 degsF ... the setpoint of 125 indicates a setting of 125 degsF ...



note that your input module (usually an 1771-IFE) needs to provide a "full scale" range of 0 to 4095 into the PV location of the PID ... if that's not right, then the quality of the PID control will suffer ...

that might seem to contradict what I said earlier ... you originally asked:

Must the setpoint & process variable be 0-4095 in the PID instruction?

Click to expand...

once again, the PID needs to see an input signal of 0 to 4095 arriving at the PV address (the address that you entered on the PID rung) ... that MUST happen or you're asking for trouble ... now if you decide to use the "setpoint" scaling feature (as shown in the picture above) then you will be able to read the "Scaled PV Value" from "inside" the PID setup screen ... you'll find that value in the middle column - third from the bottom ... in other words the PID is able to "scale" the PV signal for you ... but that should be done internally by the PID - and NOT before the input signal is applied to the PID's PV location ...

finally ... I'd recommend that you zip your .RSP file and attach it to your next post ... there might be some other issues that we could help you with if you're relatively new to PID with the PLC-5 ...