3 term control for temperature loop
3 term control for temperature loop
(OP)
I'm working on a gas fired heater using a fixed air flow, fixed process gas flow and variable fuel gas flow which is controlling the exit temperature of the process gas.
The default PID settings are Gain 50, Integral Time 5 Derivative rate 0. The manufacturers state that the heater is designed for a higher process flow than at present, however, these still seem pretty rare values. The fuel gas control valve is a ball valve. The exit temperature oscillates around 10 degrees centigrade each side of the set point (300), almost like 2 step control.
Could it be that what it says is the Gain is actually the Prop Band? Plus, how do I know if the Integral time is in repeats/min or mins/repeat?
Any ideas apart from trial and error on this one??
The default PID settings are Gain 50, Integral Time 5 Derivative rate 0. The manufacturers state that the heater is designed for a higher process flow than at present, however, these still seem pretty rare values. The fuel gas control valve is a ball valve. The exit temperature oscillates around 10 degrees centigrade each side of the set point (300), almost like 2 step control.
Could it be that what it says is the Gain is actually the Prop Band? Plus, how do I know if the Integral time is in repeats/min or mins/repeat?
Any ideas apart from trial and error on this one??





RE: 3 term control for temperature loop
Gunnar Englund
www.gke.org
RE: 3 term control for temperature loop
RE: 3 term control for temperature loop
You can also use Ziegler Nicholls, and can then adjust in a more scientific manner. If you don't know what it is, just do a web search and you will find a good description of it.
RE: 3 term control for temperature loop
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If we learn from our mistakes,
I'm getting a great education!
RE: 3 term control for temperature loop
RE: 3 term control for temperature loop
The OP asked: "Could it be that what it says is the Gain is actually the Prop Band? Plus, how do I know if the Integral time is in repeats/min or mins/repeat?"
To be able to anser these questions, it is necessary to know manufacturer - and perhaps also model. P-band or Gain? Reset or integration time? It's anyone's guess as long as you do not know what controller you are talking about.
It really does matter, you see.
Gunnar Englund
www.gke.org
RE: 3 term control for temperature loop
I don't agree at all. Gain is the reciprocal of Proportional band, regardless of who made the controller, (likewise Reset versus integral).
True there are differences in PID algorithms used by differing suppliers, but these are relatively minor.
Anyway, if the loop is managing to control, albeit with a 10 Degree oscillation, then it is working to a degree and I think it would not do this if the gain was as drastically wrong as being the reciprocal of what it should be.
khan101 is right about needing to tune the controller.
However even if it were perfectly tuned, such problems as stiction in the valve could still cause oscillation.
Francis
www.controldraw.co.uk
RE: 3 term control for temperature loop
The OP specifically asked how he could know if it is Gain or Proportional Band etc that has been set on the controller. To know that, you have to know how this particular controller manufacturer usually does it.
The fact that PB = 1/Gain is text-book knowledge and the OP also shows that he understands that.
So, I still maintain that a direct contact with the manufaturer is the best way to go. Or find the manual. On the web or in a book-shelf.
Gunnar Englund
www.gke.org
RE: 3 term control for temperature loop
Gunnar Englund
www.gke.org
RE: 3 term control for temperature loop
But I think I know what you mean, ie that it might be worth seeing the controller manual to know what they mean by gain - except if they mean PB it would normally say so on the physical (or software) controller Faceplate.
Cheers
RE: 3 term control for temperature loop
Gunnar Englund
www.gke.org
RE: 3 term control for temperature loop
If you have a characterized ball valve you would have much better control than possible with a standard API 6D type ball valve.
John
RE: 3 term control for temperature loop
This is a good book on Parctical Process Control:
http:
I suggest to create some step responses of the system and derive the proper P and I values from the curve. Yes that's famous Ziechler & Nichols approach.
Cheers,
CARF
www.carf-engineering.com
RE: 3 term control for temperature loop
Using a feedforward loop will resolve the problem: Measure the parameter that varies that causes you to have to modulate the fuel flow, then control the signal to the fuel flow valve as a function of this variable. In your description you said that the fuel flow is the ONLY variable, but if this was true your valve would never have to modulate unless the fuel composition was changing.