David, a couple of things to verify. When I configure a proportional-only controller, I set the control algorithm to PI instead of PID. The integral time should be set to 0.0. The proportional weighting should be set to 1.0. The execution time should be set to a reasonable number (0.5 - 1.0 sec).

When you do a step-change in AUTO mode, the PV should approach the new SP but not reach it (offset). Are you in simulation? Or connected to a live process? How much does the PV drift? Is the output changing? By how much?

Lastly, how did you determine the gain? In P-only control, if the gain is too high, the output can start to cycle or become unstable.

Hello Scott,

It's a simulation, no live process. I have chosen PI instead of PID. Everything else is set as you have described.The gain is set to 1.0 (have also tried 0.2). Problem is that the MV keeps creeping up until output reaches 100%. I don't get why as this kind of behavior is exclusive to integral control.

Instead, with a proportional controller I would expect the MV to remain constant as long as PV, SP and the disturbance variable remains the same - it doesn't however.

I have tried to create a SUBTRACT (SP-PV) and CALCULATION block (Kp*error+feedforward bias) and this works exactly as intended. However, I would prefer to use the PID compact block as this block outputs in percent and therefore I don't need to do all the number crunching myself.

Any ideas on what is wrong with my PID controller?

Scott, I tried creating a new project with a new PID controller and now it actually seems to work - no clue why. However, in the example I have created my SP is 500 and my PV scales from 0-1000. However, for some reason the output only changes between a PV 400 and 500 (although perfectly linear). Below 400 and my MV is 100. Above 500 and my MV is 0.

Why is that? What am I doing wrong?

Thanks

One thing to try with the tuning constants is to upload them from the PLC when you are online (can be done via the PID block configuration page. Make sure the values are what you wanted to operate with. It is possible you could have modified the parameters but never downloaded them to the PID block.

For your second question, how did you set up you input scaling in the PID block configuration? By engineering units or by percentage (_PER)?

I am actually having trouble again with deactivating the integral control from the PID.
Here is a video of my current setup:

Update: Notice 0:27 seconds into my video when I click on "monitor all" and the parameters reverts back to default. I just tried updating the parameters in "monitor all" mode and for some reason it resulted in the parameters now being saved properly into the PID.. same procedure for "process value limits" and now it works perfectly. Also, wasn't allowed to change the "units" under controller type (not even in 'monitor all' mode), but when I clicked on "go offline" in Step 7 the parameter seemed to update properly.

Clearly something is wrong the way I am saving stuff.

Also, in the same setup you see in the video:

if:
SP: 100
PV: 50
then MV = 50

if:
SP: 100
PV: 25
then MV = 75

so far so good, but then if:
SP: 200
PV: 50
then MV = 100

Why is that? Shouldn't MV be 75 (200-50=75%)??

Sorry if the questions seems very basic but I am merely an electrician apprentice trying to wrap my head around this stuff.

I much appreciate your help, Scott.