Kiln overshoot problems

[thiamant]

Anyone else has a small Nabertherm kiln that could share with me the PID parameters he's using? I'm having overshoot problems, I don't know if they are critical or it's normal, but I thought something weird was going on:

In this picture you can check an overshoot of 8ºC. I had to turn off the kiln because I was afraid it could go up even more.

In this case I had an overshoot of 5ºC, but the kiln couldn't react fast enough and kept overfiring for 30 minutes:

 

I also had some problems with drop & hold segments:

 

Again, the power curve (in pink) goes up again too fast before reaching the setpoint, and even after that it goes up only to 60% when it's already underfiring by 5 degrees.

I purchased this kiln with an upgraded controller (to have more segments), and I'm afraid that the PID settings were not corrected for this smaller kiln (60L), but were left as the standard big kiln settings. I tried to contact Nabertherm but it's like talking with a wall. So if someone has a 60L Nabertherm kiln and can tell me what PID parameters he's using, that would be great. 

Maybe I'm just mad and this happens all the time? I started suspecting when I had to manually skip the segment because it would enter an infinite loop (setpoint was go down to 1160 and hold, kiln went to 1162, then up to 1165, then down to 1162, then up to 1165 again, and so on). This doesn't look normal to me.

 

Adrian. 

[thiamant]

Here is another case with a 9ºC overshoot, again I had to turn the kiln off:

 

[neilestrick]

I can't help you with specific PID settings and you may already know this, but what could be happening is that the controller isn't taking into account that the elements will continue to radiate heat once they cycle off. So the controller needs to cycle them off before the set point in order to avoid overshooting. At low temps and fast ramps this will be more of an issue than at high temps.

[thiamant]

1 hour ago, neilestrick said:

I can't help you with specific PID settings and you may already know this, but what could be happening is that the controller isn't taking into account that the elements will continue to radiate heat once they cycle off. So the controller needs to cycle them off before the set point in order to avoid overshooting. At low temps and fast ramps this will be more of an issue than at high temps.

Yes this is directly related with the PID parameters. Overshooting can be fixed by adjusting the parameters, but the problem here is that the default parameters don't make any sense for this type of kiln. The upgraded version of the controller wasn't adjusted. I'm still trying to make Nabertherm understand that 10ºC overshoot isn't something "acceptable".

[PeterH]

4 hours ago, neilestrick said:

I can't help you with specific PID settings and you may already know this, but what could be happening is that the controller isn't taking into account that the elements will continue to radiate heat once they cycle off. So the controller needs to cycle them off before the set point in order to avoid overshooting. At low temps and fast ramps this will be more of an issue than at high temps.

For their Lab Furnaces Nabatherm seem to recommend "smoothing" the ramp.
http://data.dt-shop.com/fileadmin/media/ga/43073_ga_enu.pdf

Maybe you could achieve a similar effect by adding an extra ramp?