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

Question

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

Answer 1

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:[email protected]..
> 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