Electronics > Projects, Designs, and Technical Stuff

Help me think about controlling 5 heaters with 3 RTDs

(1/3) > >>

tmadness:
I have been thrown a interesting problem: I have to control 5 heating elements with inputs from 3 RTDs. See, normally when the number of RTDs == number of heating elements, the power going to the heaters can be set using an independent PID loop for each pair. But I've been given a device with 3 heating elements directly underneath 3 RTDs, and 2 extra heating elements connected to a common substrate. I've drawn a diagram hopefully clarifying this. Heaters are in orange (H1-H5) and RTDs in purple (RTD1-RTD3), the container is just a "container" for whatever is being heated.

Now, how do I think about how much power I ought to send to the two side elements? I do not have the ability to change the RTDs or the elements. All the containers are set to the same temperature.
 

ledtester:
Hmm... do you need to use H4 and H5 for this application?

Also, do you expect the temperatures to differ a lot between the heaters?

It seems that H4 and H5 would heat up the entire space - i.e. affect all of the RTDs. How about using the "max" of the RTDs to control H4 and H5. So if all of the RTDs are below the target temp H4 and H5 are utilized. I'm assuming that there is one single target temperature for all of the heaters.

tmadness:
Yup all the containers are set to the same temperature, Ive added that detail to the main post. H4 and H5 are important in keeping even temperature on the whole container, ie no cold spots.
The Issue I see with using max is that there may be a race/oscillating condition.

NiHaoMike:
Control the outer heaters based on the duty cycles of the inner 3, trying to keep those inner 3 at close to a 50% duty cycle. You'll probably have to use a much slower loop to control them compared to the inner heaters.

dmendesf:
Double oven OCXOs usually set the outer oven to a slight lower temp than the desired temp and the inner closes the gap. The idea is to cancel the constant temp difference with the outer oven and the derivative with the inner. Maybe you can work out something like that.

Navigation

[0] Message Index

[#] Next page

There was an error while thanking
Thanking...
Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod