Basic Stamp Temperature Controller - Ideally PID

Bob

I need to heat up dye and other chemicals for a home-brew annodizing line.· Basically pots of liquid sitting on top of hot plates.· The temperatures range form 100 degrees Farenheit up to approximately 180 degrees Farenheit.· Is it practical to build a system around a Basic Stamp that would monitor the temperature of the stainless steel pot and then adjust the hot plate accordingly?· Further, has anyone ever implimented a Proportional Integral Derivative algorithm on a Basic Stamp?· I understand from my research that PID is the "hot" setup.

Any thoughts or direction would be greatly appreciated.

Bob in Florida

Larry

Well I know a guy who brews great beer and uses a Stamp to conroll heaters and pumps to heat the mash and wort.. Same temp ranges as you propose.

The only real problem is that you will need to control line voltage with solid state relays or something similar. If you do it wrong, You die or burn your house down.

IF you don't have knowledge in this area, your insurance company would appreciate it ifyou used a licenced electrician.

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Bean

There is a PID implementation in the "Industial Control" text.

http://www.parallax.com/dl/docs/books/edu/ic.pdf

From the sounds of your project, PID may be overkill. But should definately work.

Bean

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Chris Savage

I agree with Bean.· PID doesn't seem necessary for your requirements, although you could use it if you want.· Really you need to monitor temperature and cut power to the hot plate when the desired temperature is reached.· Once it falls below it (Or some threshold) you re-energize the hot plate.· If you use an SSR (Solid State Relay) you could cycle the power continuously like that without any issues and maintain a very close temperature range.


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Chris Savage
Parallax Tech Support
csavage@parallax.com

Bob

Thanks for the information guys, the Industrial Control text looks like the right place to start. I suspect that you're right regarding PID being overkill. However, I'm trying to stretch myself technically so if I can collect the PID components (hardware control) for a reasonable price I'll probably go this route.

I'll keep you posted on progress and thanks for the quick response.

Bob

Chris Savage

Bob,

·· On the same token I don't want to discourage you from learning about PID.· The speed at which the temperature rises and falls on the Hot Plate will ultimately dictate whether you need PID (Or just some portions of it).· If the Plate rises too fast it may overshoot your target temperature even after the power has been killed, since it will still be rising.· On the other hand, this is likely to settle.· It will basically involve some testing on your part to determine necessity.

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Chris Savage
Parallax Tech Support
csavage@parallax.com

Cogburn

I was wondering·how one could use a pid algorithm with a ssr to supply power to a hotplate since it is basically an on-off device?· Doesn't pid require the use of current adjustments on the mains to be effective?

steve_b

Well, I guess you can think of it like PWM (Pulse Width Modulation)....it's about changing the dutycycle of the power supplied to the device.
We use it to speed up motors and to slow them down....

One could....switch an SSR enough to regulate the power applied to a heat plate. Problem would be that the SSR would get "exercised" (as opposed to exorcised) more often and would really just decrease it's lifespan. One might look at Mosfets (or similar) to switch power to a hotplate.

You're steps may not be as fine, as the hotplate takes time+n to get to temperature (dependant on ambient temp)....so you adjust your PID task accordingly.

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Steve

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rockin_rick

Larry said...
Well I know a guy who brews great beer and uses a Stamp to conroll heaters and pumps to heat the mash and wort.. Same temp ranges as you propose.

There is an article VERY similar to this in the June 2006 issue of Circuit Cellar. The author states that, in his situation and setup, heating the fluids (thermal mass) accurately did, in fact, need PID. Without PID, the system oscillated.

PID or no PID probably depends on the amount of thermal mass (how much fluid needs to be heated) and how accurately you need to hit and maintain the set point.

Rick

Kevin Wood

Parallax has a project showcased in the 2006 catalog where they built a temperature controller to regulate solder temperature in a solder pot. You might be able to get some design ideas from it.

Chris Savage

Kevin,

·· The Solder Pot controller does not use PID, as for our purposes it wasn't needed.· It would be interesting to implement, but in that scenario very little improvement would be gained by it.· It works using differential gap, much like the soldering stations we use.· You will see the LEDs on both turning on/off sometimes quite rapdily, but they maintain temperature well.· The Solder Pot is controller by a 50A SSR so switching it on/off isn't a big deal.· The only thing that happens as a result of no PID control is that when the pot initially comes up to temperature it overshoots by about 5 degrees or so.· At 425 degrees this isn't a big deal so we don't worry about it.· I hope this information helps.

As a side-note, if I ever get the chance to update the code I may well implement PID into the system.


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Chris Savage
Parallax Tech Support
csavage@parallax.com