Putting the final touches on RUTUs refrigerator and freezer. I want to use
digital thermostats but the Carel units are $150 each.

In my overflowing junk box I have a pair of 12VDC CN132 Omron process
controllers with type T thermocouples. These things are a lot smarter than
a plain old thermostat but I wonder if they would work? What worries me is
that as the box gets close to the target temperature the PID logic might
start trying to turn the compressors on and off as if they were heating
elements.

--
Glenn Ashmore

I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at: http://www.rutuonline.com
Shameless Commercial Division: http://www.spade-anchor-us.com

On Mon, 3 Jan 2005 18:42:29 -0500, the renowned "Glenn Ashmore"
wrote:

Putting the final touches on RUTUs refrigerator and freezer. I want to use
digital thermostats but the Carel units are $150 each.

In my overflowing junk box I have a pair of 12VDC CN132 Omron process
controllers with type T thermocouples. These things are a lot smarter than
a plain old thermostat but I wonder if they would work? What worries me is
that as the box gets close to the target temperature the PID logic might
start trying to turn the compressors on and off as if they were heating
elements.

Compressors need a timer that limits the on time and off time to a
minimum (typically something like 2 minutes each). Most process
controls don't have that feature, but you could look. That kind of
timer supplied externally will also fool with the integral control-
it's a form of dead time- so the controller would have to be
significantly detuned for stability.


Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com

That's what I was afraid of. These things have an 80 second maximum cycle
limiter in their programming but these things are still to smart for their
own good I guess. I really don't need them sneaking up on the set point.

Trash: Stuff you throw out.
Junk: Stuff you put in the attic until you figure out what to do with it.

--
Glenn Ashmore

I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at: http://www.rutuonline.com
Shameless Commercial Division: http://www.spade-anchor-us.com

"Spehro Pefhany" wrote in message
...
On Mon, 3 Jan 2005 18:42:29 -0500, the renowned "Glenn Ashmore"
wrote:

Putting the final touches on RUTUs refrigerator and freezer. I want to
use
digital thermostats but the Carel units are $150 each.

In my overflowing junk box I have a pair of 12VDC CN132 Omron process
controllers with type T thermocouples. These things are a lot smarter
than
a plain old thermostat but I wonder if they would work? What worries me
is
that as the box gets close to the target temperature the PID logic might
start trying to turn the compressors on and off as if they were heating
elements.

Compressors need a timer that limits the on time and off time to a
minimum (typically something like 2 minutes each). Most process
controls don't have that feature, but you could look. That kind of
timer supplied externally will also fool with the integral control-
it's a form of dead time- so the controller would have to be
significantly detuned for stability.


Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
Info for manufacturers:
http://www.trexon.com
Embedded software/hardware/analog Info for designers:
http://www.speff.com

On Mon, 3 Jan 2005 18:42:29 -0500, "Glenn Ashmore"
wrote:

Putting the final touches on RUTUs refrigerator and freezer. I want to use
digital thermostats but the Carel units are $150 each.

In my overflowing junk box I have a pair of 12VDC CN132 Omron process
controllers with type T thermocouples. These things are a lot smarter than
a plain old thermostat but I wonder if they would work? What worries me is
that as the box gets close to the target temperature the PID logic might
start trying to turn the compressors on and off as if they were heating
elements.

See if one of the available output modes is simple on-off with
deadband. You for sure don't want time proportioning. However, if
you're stuck with time proportioning PID, then set the proportional as
low as possible (or gain as high as possible), integral and derivative
to 0. This will make it act like an on-off controller, but without
deadband.

As a matter of fact, this will even work with an analog output
(0V-10V) if you can get a gain of about 100,000 to 1,000000. You just
have to use the output to drive a relay.

Deadband is a lot nicer, because there's no chance of flipping back
and forth right around setpoint. I'd guess a deadband of 5-10 deg.
would be fine if available.

Pete Keillor

Well, I found a "Users Guide" but it reads like it was translated from
Japanese to English by an Italian and proof read by a Polish physist.. Very
hard to make heads or tails of it. I do see an on/off- proportional
timecycle setting. Also see Proportional band/Gain or Hysteresis, Intergral
time/Reset and Derivative time/Rate parameters but no indication of the
choices.

Think I will hook one up to a peltier chip and play around with it. At
least that way I will not be giving a $300 compressor a nervous breakdown.
:-).

--
Glenn Ashmore

I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at: http://www.rutuonline.com
Shameless Commercial Division: http://www.spade-anchor-us.com

"Peter T. Keillor III" wrote in message
...
On Mon, 3 Jan 2005 18:42:29 -0500, "Glenn Ashmore"
wrote:

Putting the final touches on RUTUs refrigerator and freezer. I want to
use
digital thermostats but the Carel units are $150 each.

In my overflowing junk box I have a pair of 12VDC CN132 Omron process
controllers with type T thermocouples. These things are a lot smarter
than
a plain old thermostat but I wonder if they would work? What worries me
is
that as the box gets close to the target temperature the PID logic might
start trying to turn the compressors on and off as if they were heating
elements.

See if one of the available output modes is simple on-off with
deadband. You for sure don't want time proportioning. However, if
you're stuck with time proportioning PID, then set the proportional as
low as possible (or gain as high as possible), integral and derivative
to 0. This will make it act like an on-off controller, but without
deadband.

As a matter of fact, this will even work with an analog output
(0V-10V) if you can get a gain of about 100,000 to 1,000000. You just
have to use the output to drive a relay.

Deadband is a lot nicer, because there's no chance of flipping back
and forth right around setpoint. I'd guess a deadband of 5-10 deg.
would be fine if available.

Pete Keillor

"Glenn Ashmore" wrote in message
news:J%pCd.9570$EG1.6293@lakeread04...
Well, I found a "Users Guide" but it reads like it was translated from
Japanese to English by an Italian and proof read by a Polish physist..

Hey! I know the work of that team. They wrote the manual for my Chinese
lathe. Best bit of Chinglish I've read for years. Best part is that they
keep referring to the operator as "the jockey" Oh, maybe that explains why
the cabinet is so short!

Tom

On Mon, 3 Jan 2005 18:42:29 -0500, "Glenn Ashmore"
wrote:

Putting the final touches on RUTUs refrigerator and freezer. I want to use
digital thermostats but the Carel units are $150 each.

In my overflowing junk box I have a pair of 12VDC CN132 Omron process
controllers with type T thermocouples. These things are a lot smarter than
a plain old thermostat but I wonder if they would work? What worries me is
that as the box gets close to the target temperature the PID logic might
start trying to turn the compressors on and off as if they were heating
elements.

You've answered your own question, Glenn. Compressors need minimum
cycle times so PID control doesn't fit. Why use electronics dumbed
down to function as a thermostat rather than just using a good
thermostat?

Contact life in a marine situation perhaps? Omega, Watlow, Honeywell
and other electronic controllers that accept thermocouple inputs do
have bang-bang conrol modes. I don't know about Omron but I'd expect
they do as well.

On Tue, 4 Jan 2005 00:41:01 -0500, "Glenn Ashmore"
wrote:

Well, I found a "Users Guide" but it reads like it was translated from
Japanese to English by an Italian and proof read by a Polish physist.. Very
hard to make heads or tails of it. I do see an on/off- proportional
timecycle setting. Also see Proportional band/Gain or Hysteresis, Intergral
time/Reset and Derivative time/Rate parameters but no indication of the
choices.

Think I will hook one up to a peltier chip and play around with it. At
least that way I will not be giving a $300 compressor a nervous breakdown.
:-).

Hysteresis = deadband. The point about requiring minimum cycle times
by the previous poster sounded like he knew what he was talking about.
Might be able to rig that with Agastats or something, but it'd be
getting too complex compared to just finding the right controller.
Deadband set high enough would do similar, depending on heat capacity
of fridge and cooling capacity, but not as positive.

Pete Keillor