Why can't I get a PID to tune without having an offset from setpoint?

[KerryLib]

I am using the basic PID algorithm supplied in the PID toolkit and after calculating the terms as described in the manual, my system controls quite well. However, there is about a 2-3 degree offset below setpoint on my heater.

I am just starting to use the toolkit and have it controlling a small heater on a piece of tubing, with a thermocouple on the inside wall.

I have turned up the proportional gain and gotten the offset to get smaller, as well as increasing the integral action. Ultimately the system has such high gains that it becomes unstable and begins oscillating wildly.

I have tried to leave the system alone for a long period of time and the integral term never seems t
o push the temperature the last little bit to the setpoint.

Is there anybody familiar with the toolkit and PID algorithms who could make suggestions about what I might try to get something a little better?

I am using Fieldpoint TC-120 modules to read the temperature and controlling the heater with a PWM-520 module. I have the duty cycle of the PWM module set to 1000 ms, and am running the PID loop at 500 ms. (maybe that is affecting my control)

Thank you for your help

Kerry Libberton
TDA Research
Wheat Ridge CO

[Aaron_G]

Kerry,

First of all, let me point out some timing discrepancies. The PID algorithim is running every 500 ms, the period of the pulse on the PWM module is every 1000 ms, and (what may be overlooked at the moment) is the all channel update rate of a FP-TC-120 is 800 ms. The all channel update rate is the rate at which the A/D converter on a FieldPoint module is updating the registers for all channels of the module.

In terms of the fact that the PID never quite makes it all the way, I would suggest looking at the actual readings coming back from the FieldPoint modules, and how they are configured. Make sure that the FP-TC-120 module is correctly congifured to use the correct thermocouple type and make sure it is set to Internal Cold Junction Compensation. If the bank has a l
arge number of FieldPoint units, it is recommend that the FP-TC-120 be placed next to low power modules (I.E. not next to FP-1600's or FP-RLY-420's or other high power modules).

[KerryLib]

Thanks for the suggestions.

I have adjusted the timing. The readings from the modules were all set correctly. I do have the internal cold junction active, etc.

It turns out that the iteration connection was not hooked up and as a result the algorithm to calculate the integral and derivative action was deactivated.

Thanks again
Kerry

[gagir]

Hi,

I have encountered the same phenomenon.

Increasing the derivative action seems to dynamize the system so it will
reach the setpoint.

It is still strange to me, since all temperature controllers I use don't
need any derivative action.

Regards,

gagir

"KerryLib" wrote in message
news:50650000000800000015160000-983677784000@quiq.com...
> I am using the basic PID algorithm supplied in the PID toolkit and
> after calculating the terms as described in the manual, my system
> controls quite well. However, there is about a 2-3 degree offset
> below setpoint on my heater.
>
> I am just starting to use the toolkit and have it controlling a small
> heater on a piece of tubing, with a thermocouple on the inside wall.
>
> I have turned up the proportional gain an
d gotten the offset to get
> smaller, as well as increasing the integral action. Ultimately the
> system has such high gains that it becomes unstable and begins
> oscillating wildly.
>
> I have tried to leave the system alone for a long period of time and
> the integral term never seems to push the temperature the last little
> bit to the setpoint.
>
> Is there anybody familiar with the toolkit and PID algorithms who
> could make suggestions about what I might try to get something a
> little better?
>
> I am using Fieldpoint TC-120 modules to read the temperature and
> controlling the heater with a PWM-520 module. I have the duty cycle
> of the PWM module set to 1000 ms, and am running the PID loop at 500
> ms. (maybe that is affecting my control)
>
> Thank you for your help
>
> Kerry Libberton
> TDA Research
> Wheat Ridge CO

[KerryLib]

Thanks for your comments.

I was just not seeing any response when I adjusted the integral and derivative terms. It turns out that I had not correctly wired the iteration connection to the PID.vi. After re reading the manual a couple times, and inspecting the example VI's, I got it straightened out.

Kerry