Thermal control system design

[Robert Wolcott]

I am designing a closed loop thermal control unit that would like to be very
accurate (+/- .2 degrees C). I plan to use a PID controller with an RTD
sensor and a small tape style heater.

Most PID controllers only provide a signal out and do not directly power the
heating element. If I had a PID controller providing a low voltage signal
(mV), what sort of intermediate circuit would I need to power a 10-50 watt
heating element? My main goal is not necessarily accuracy, but stability.

Thanks,
Bob

 

[John Popelish]

Robert Wolcott wrote:

John,

The controller I had in mind was an Omron unit and I believe it had an
current output instead of the voltage output I had stated. The temperature
I am looking for is 30-40 degrees C. Do you think a thermistor would be
better for that range?

That is an ideal temperature range for many thermistor sensors. You
will probably need a transmitter (signal conditioner) to convert the
variable resistance of the thermistor to the standard 4-20 ma
controller input. If you pick a transmitter that allows a narrow
range of resistance to be expanded to the full input signal range, you
will increase the resolution, though the stability and calibration
issues remain.
some examples that you can mine for key words for searches:
http://www.ametherm.com/ntc_thermistor_probes/welcome.htm
http://www.betatherm.com/
http://www.rtie.com/ntc/probe.htm

--
John Popelish

 

[Rob]

"Robert Wolcott" <rwolcott23@yahoo.com> wrote in message
news:ILmdnXiOYNbTMnXcRVn-pw@comcast.com...

I am designing a closed loop thermal control unit that would like to be
very
accurate (+/- .2 degrees C). I plan to use a PID controller with an RTD
sensor and a small tape style heater.

Most PID controllers only provide a signal out and do not directly power
the
heating element. If I had a PID controller providing a low voltage signal
(mV), what sort of intermediate circuit would I need to power a 10-50 watt

Robert, have a look at the stuff from wavelength electronics, it may give
you some ideas - there is not a lot in some of the controllers. I used one
of their small PID units for controlling a Peltier, had to add extra
heatsinking. It provided direct drive of the load and had analog in & out
signals as well.

www.wavelengthelectronics.com

rob

heating element? My main goal is not necessarily accuracy, but stability.

Thanks,
Bob

 

Maybe I'm just not familiar with it, but why would you want to use a
PID controller for a thermal process? A thermal system is much more
complicated than a simple spring/mass/damper system.

I would assume a PD controller with an experimentally obtained offset
would be adequate.

Dave

 

[Tim Wescott]

dave.harper@gmail.com wrote:

Maybe I'm just not familiar with it, but why would you want to use a
PID controller for a thermal process? A thermal system is much more
complicated than a simple spring/mass/damper system.

I would assume a PD controller with an experimentally obtained offset
would be adequate.

Dave

(A) because you can buy off-the-shelf PID controllers.

(B) because everyone knows how to tune a PID (well, sorta).
(C) because PID is often good enough.
(D) because if PD is good, PID is better and no harder (see A).

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

 

[Tim Wescott]

Robert Wolcott wrote:

John,

The controller I had in mind was an Omron unit and I believe it had an
current output instead of the voltage output I had stated. The temperature
I am looking for is 30-40 degrees C. Do you think a thermistor would be
better for that range?

Thanks,
Bob


Surely Omron has one with relay output? That's a very very common

output for temperature control.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com