Digital Temperature Controller

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Hey G'day Mr Spam,

Should be worth about $20 after curry decontamination ???

They're made by a third-string Japanese company (Toho) anyway.

Best regards, Spehro Pefhany

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Does anyone know of a 'good' digital controller. We use the CNi3244 from Omega,

The problem is that once the controller gets to the digital bounds of the set point 0.1 degree C, The controller stops changing and the temperature drifts around until it hits the top or bottom of the 0.1 C range. We've got one experiment where this is a bit marginal.

George H.

Flimsy, flakey, inaccurate junk.

John

Is it continuous/proportional or a bang-bang (relay or SSR) output stage?

John

I presume it's time-proportioning output? I presume you've run the AT but not fiddled with the parameters? Sounds like a quantization thing. Is the temperature actually drifting or just the display?

on,

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The output is PWM, (~1 Hz.) I feed it through a low pass, and an amp to a 50 ohm heater.

I'd just do our own analog PID thing, but unless we redo a front panel, it's all got to fit into the ~ 1" by 2" cutout. At least it's some industrial standard size so that I could 'in theory' buy something from another vendor. But I don't want to start testing everyones digital controller.

George H.

on,

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AT =3D auto tune? I ran that but, didn't like the answers. We put in our own parameters. (Crank up the gain until it just oscillates and record the time constant etc.)

The signal is light transmission through an Rubidium reference cell. It's a strong function of temperature. I've got plots of the transmission versus time... The signal drifts around randomly between two extremes. If you wathc the temperature display it sits at (say)

50.0 C and then goes to 50.1 for a bit... and then maybe down to 49.9 So it regulates nicely. The system has a time constant of ~8 minutes.

I've always assumed that it's a quantization thing. It's one of those problems that is annoying, but not bad enough for me to try and fix it.

George H.

 

You've got a pretty small cutout (presumably 48mm x 24mm face).

Usually the best ones are 1/8 or 1/4-DIN (96mm x 48mm face, or 96mm x

96mm). Look for a 10Hz or higher sample rate and high accuracy specs, they'll be a proxy for better overall performance. I would get one with an analog output if you can.

Most applications could care less about a degree or two, let alone a tenth. You need something a bit above the bottom end.

ition,

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Thanks Spehro, I'm pretty much stuck with the panel cut-out. 1/32 size. I guess I was hoping for a recommendation of some good manufacturer. (But don't worry about it.. it's not a big deal.) Googling "PID temperature controller" +"1/32" gets 86,000 hits. Nothing obviously better than what I've got in the first few pages. (Dwyer, Watlow, Cal controls... and other even cheaper stuff.)

Now maybe if I could add the right amount of noise to the temperature signal (1/2 the LSB), I could trick the Omega into tighter control. Easier said than done,

George H.