PID instruction parameter fault

[kurt.brinker 0]

I recently put in several PID loops to regulate a few pumps. When i downloaded the program, it faulted and i got the following error. "an invalid value is being used for a PID instruction parameter" What are possible causes for this? I take the readings from 3 rtd's and use the CPT function to average them. I then write that value directly to the PV of the PID. My setpoint comes from the HMI and is rescaled from 0-99 to 0-16383. I use several SCP's to rescale my cv and such in order to display it properly but i don't know where my error is. I am sure its a common error between the pids but i can't find out where. Any ideas on what causes this? SLC 5/05 Edited 30 Jan 2007 by kurt.brinker

[Ken Moore 23]

Is your PV scaled also? What does the PID error byte indicate?

[rpraveenkum 5]

Set the loop update time in Pid setup screen more then zero value even the PID is selected in STI mode

[kurt.brinker 0]

my pv is not scaled. that may be the problem. It will always be coming in bewteen 20-60 degrees but i am now thinking i need to find the range on the rtd's and then rescale it to 0-16383 and make that my PV input. correct? I can take the average value and rescale that correct?

[Ken Moore 23]

From the help files: Process Variable PV (Not editable, for display only) This is the scaled value of the Process Variable (the analog input). Without scaling, the range of this value is 0–16383. Otherwise, the range is minimum scaled (word 8) to maximum scaled (word 7). Rockwell Software 2005

[kurt.brinker 0]

since an rtd works off of resistance, there can't be a range right? I mean, other than the physical limitations of the component and wiring. Our room will never see below 0 and never more than 150. Can i scale that input 0-150 and then rescale it 0-16383 and use it as my proces variable? here is one of the 5 pid loops i have. The only difference is the amount of rtd's. this one has three and some of the other have 6. N11:25 is the setpoint from the HMI, N11:28 is a manual % setpoint for the VFD when the pid is in manual F8:15 is a rescaled CV to be displayed as a 0-100 on the PV F8:8 takes the actual VFD output and rescaled it so it can be shown in the PV as 0-100 lemme know if you have any other questions pid.RSS Edited 1 Feb 2007 by kurt.brinker

[CTSoft 0]

Hi,Mr.Kurt, Past project I'm used SLC505 too. Here attach the PID logic for your reference. You may find out more information you need on this logic. PID_for_reference.RSS

[kurt.brinker 0]

read through your program and i understand most of it. i DID find a range for my rtd's. I have it scaled now and i am about to try it out. One question. on my rtd input card, the input is set for engineering units. If the temperature is 50 degrees, it comes in as 500. I multiply this x .1 to get my readable value. I noticed i have an option for engineering unitsx10 as an option for the input. will this give me a 50 degree input or a 5000. Edited 1 Feb 2007 by kurt.brinker

[Ken Moore 23]

If you select the x10 option, then the reading is in whole degrees, if not selected, you get 1 decimal place.

[CTSoft 0]

What Ken said is correct. refer to manual 1746-NR4 ,chapter 5 page 9. you will get more clear picture.

[kurt.brinker 0]

I scaled my pv and it still doesn't work. My setpoint comes straight from the HMI and is rescaled like it needs to be. All of my PID setpoints are valid

[Ken Moore 23]

What do you mean by "still not working"? Still getting errors, or not controlling? I noticed in your screen shot, that the PID is in manual.

[kurt.brinker 0]

apparently my last 4 pid loops didn't get their parameters set. I always looked at the first one which was correct but i looked at the fault code and it led me to the second pid loop and when i opened up the setup screen it was blank!. everything is great now. Thanks!

[Ken Moore 23]

Thanks for following up. It's not often that the original requester responds with the final solution.