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Loop-powered device to NI 9265 module?

Hello folks,

 

I have a big pneumatic control valve with a controller that's loop powered. It's controlled with a 4-20 mA signal. It has terminals for positive, negative, and ground. I want to send mA signals to the controller with my NI 9265 module (plugged into a cDAQ chassis), the mA signals are converted by the controller to valve position information.

 

Please forgive me if this project seems simple, I don't have an electrical engineering background, and in fact, I hooked the valve up to the NI 9265 and cDAQ a few months ago, quickly wrote a DAQmx VI code in LabVIEW and controlled the valve just fine in under 30 minutes. The valve was at my workstation, not mounted to any piping or anything, I was just testing it out at the time.

 

The difference between then and now is that I've now installed the valve in its desired location on some piping up in a ceiling in a building far away. I've run signal cables from the valve to the spot near there where I don't yet have a computer assembled and set up. I do however have my cDAQ chassis there, and it's powered on with the NI 9265 installed. I was hoping to test the valve now before we start adding more hardware, making it more difficult to access.

 

My question is, does this NI 9265 only complete the circuits once it's recognized by software, or should I be able to power the valve on now without a computer? If I should be able to power it on now, perhaps my connections aren't right.

 

For reference, wiring a power supply directly to the signal cables powers on the valve, it's only when I connect the cables to the NI 9265 that something doesn't seem to be right. The NI 9265 is powered with a 24 VDC power supply. I also have a secondary power supply (12 VDC) that I am denoting below with "SUPPLY."

 

I've done 3-4 different tests and none seem to power on the valve. In all cases, the NI 9265 was powered by its own 24 VDC supply.

 

[Test 1] VALVE (+) --- NI 9265 (AO0) --- NI 9265 (COM) --- VALVE (-)

[Test 2] VALVE (+) --- SUPPLY (+) --- SUPPLY (-) --- NI 9265 (AO0) --- NI 9265 (COM) --- VALVE (-)

[Test 3] VALVE (+) --- NI 9265 (AO0) --- NI 9265 (COM) --- SUPPLY (+) --- SUPPLY (-) --- VALVE (-)

[Test 4] SUPPLY (+) --- NI 9265 (AO0) --- NI 9265 (COM) --- VALVE (+) --- VALVE (-) --- SUPPLY (-)

 

I know it wasn't this difficult a few months ago, maybe I'm missing something and just having a bad day. Are any of these tests right?

 

I also have an NI 9208 in the same chassis that's powering a bunch of different loop-powered sensors. The sensors turn on and work just fine without the cDAQ connected to a computer, which made me think I could do the same with the NI 9265.

 

Thanks for the help and patience!

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Hey,

 

First of all, we always reccomend that our cDAQ line be connected to a computer when in use, as it relies on commands and configuration from the computer to function properly. If you need a product that can function independantly of a computer, I would reccomend our cRIO line.

 

That being said, the difference in behavior you are seeing between the 9265 and 9208 is most likely due to the type of input. I assume you are using a configurable analog output on the 9265. This will require programming from a computer in order to properly set up the output. On the 9208, the only output on the card are Vsup pins for external power routing. These may be able to funtion properly with no communication from a computer, since they are just a supply and not an analog output. 

 

Hopefully this clears things up!

Matt W.
Applications Engineer
National Instruments
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