PID not responding

[ahoover]

I have setup a simple PID conf by editing one of the Unitronix examples, but cannot get it to ouput a CV, no matter what I try.

Am using an analog thermocouple signal for PV, and CV should be ouput to a VFD. I wish to get this PID working at least partially before installing the PLC on the project, and therefore have this setup on my desk.

I tryed manually doing auto-scan, by changing tempatures on the thermocouple, but can not get the CV to change from 0. Any pointers?

[Joe Tauser]

I'm assuming you've got a PID run block in your program and you've verified that you have power to it. If it's in a subroutine, make sure you're calling the sub. Otherwise, look at the value of the status word of the PID block and then look in the Help at what the values mean to see if that will give some direction.

Post your program so we can look at it if you're still having trouble.

Joe T.

[HigHTech]

Check too the MB for direct or indirect type. A lot of times no CV is related to this MB.

[ahoover]

I have verified everything you mentioned. The PID status is 12. When I try using the FB auto-tune, PID status is 1, but again CV never changes.

I have posted the program along with the output from the PID server log that I used to try auto tuning.

Thank You

V130_PID_Test.vlp

[HigHTech]

I found some important things that need to be check:

1.- I think the main problem is that you are configuring the PID direct from your net 1. You need to put a direct contact of SB2. Remember this direct contact (of SB2) acts like a positive transition contact. So you actually are configuring constantly your PID.

2.- Don´t write power up values to MI2,3 and 4. That way every time you turn on your PLC will overwrite the Autotune values (you lost your autotune).

3.- You need to select SET or RESET on MB 0. If you are working with a direct system (MB = 1) if your init your PLC you will lost the 1 and then your system will not work again untill you set again that MB. So the best is to choose from power up value.

You can copy and paste the full ladder of PID 1 autotune example for V120 and paste to V130 to test the PID function.

After learning how it works you will need to make some changes to this basic program to a full operational PID program. For example you are going to need a reset button to abort an incomplete autotune. From the expample the only way is to make a reset init to restart.

Good luck.

[Damian]

I have verified everything you mentioned. The PID status is 12. When I try using the FB auto-tune, PID status is 1, but again CV never changes.

I have posted the program along with the output from the PID server log that I used to try auto tuning.

Thank You

I think the answer to your problem may be simply that you have tied the PID config block directly to the rail and are executing a re-config every scan.

Use and SB2 contact between the config block and the rail and let us know how you make out.

[HigHTech]

I answer Yesterday but something happened with my post. I recommend the SB2 on the config too.

I also recommended:

1) Do not write power up values to MI 2, 3 and 4 cause it will reset you PID auto tune values.

2) you need to write a power up value to MB 0.

[ahoover]

Thanks a lot for your helpful information. The SB2 direct contact was the key to the problem. Once I have the PID in my real program, I will need to do an auto-tune, but this gets me started.

Thanks again.