use of temperature controller for non-temperature applications

[amazing azza]

Hello friends, I have a temperature controller left over from a previous project and I want to put it to good use. I would like to use it as a constant pressure controller (on a water distribution pump). This is a 4-20 mA input/output model, so the input can seemingly be arbitrary. I can set the the 4 and 20 mA mappings during setup. The pressure transmitter, on its part, outputs 4-20 mA too. I plan to use PID mode to drive 4-20 mA controller output (sent to VFD) and use one of the alarms as an 'on' switch for the pump.

So is the only drawback I will encounter is the units painted on the box being C/F instead of bar/psi or am I missing something deeper?

 

[itsmoked]

Probably.
Would seem to me that most VFDs can do all that by themselves. If you already have a sensor that puts out 4~20 then the VFD should be able to do everything. There's probably a parameter that can require the input to reach a certain level before the motor is commanded on.

Let us know how it works out!

Keith Cress
kcress -

http://www.flaminsystems.com

 

[danw2]

Controllers targeted at temperature control make some assumptions about temp control process characteristics which become apparent when you use the autotune feature. I've found that temp controllers do not autotune flow loops, at all.

It was years ago, but a particular controller targeted for the plastics/packaging electric heat apps (a relatively slow response) and its proporational band setting maxed out 100 (gain = 1). It was installed in gas fired dryer with a massive burner, given the load size, with an incredibly fast response and its proportional band setting could not raised enough to slug down the response for stable control.

But I suspect that most modern controllers have very wide adjustment ranges and are not limited like that particular model was.

 

[IRstuff]

I would imagine that a company selling a general purpose PID controller would market it as such, and not limit itself.

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[xnuke]

Temperature controllers often have slow sample rates that are okay for slow processes like temperature control, but are unsuitable for fast processes like pressure and flow control, even when sampling as fast as they can. I second itsmoked - if the VFD can do PID, use it.

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[amazing azza]

Thank you guys for informative responses! I reviewed the VFD docs (Toshiba VF-S15), and while it can be driven from a 4-20 mA external input, it does not appear to have PID functionality.

The controller claims 150 ms/scan sampling rate for the 4-20 mA input, is that too slow?