NI 9264 Voltage Drop

[Shazlan]

Hi all,

I have a CompactRIO with NI 9264 module, connected to this Mass Flow Controller: http://www.omega.com/pptst/FMA2600_FVL2600.html. Acquiring the flowrate from the MFC is not a problem. However, I'm having problem in controlling the MFC using NI 9264 module. Basically, when the module is not connected to anything, I'm reading the correct voltage out from the module - correct means, the same as what the program gives. However, it is connected to the MFC, the voltage drops down to 0.4V and this is regardless of what voltage command that I give. I was told that this may be due to impedance mismatch between the module. Is this true? If yes, can anybody please elaborate further? More importantly, how do I resolve this? Will putting an additional resistor in series or in parallel to the circuit work?

Thanks for your time and advices.

Shazlan

[crossrulz]

You flat out have the wrong module.  It looks like your MFC wants a 4 - 20mA signal.  The 9264 can only output up to 4mA.  What you should be using is the 9265.

[Henrik_Volkers]

If I got it rigth, the MFC supports a RS232 to read the flow and to write the setpoint. , so no need to go the analog way......

Since the manual point out a 50k pot to set the control voltage, the 9264 should be able to drive the control-voltage. Unless you have a voltage input ...  ontherwise the input is a 250Ohm resistor to ground, and  would need, depending on the control needs (setpoint accuracy) a 9265  , or a small voltage buffer/follower or a voltage in -> current out device you can buy or build.

A voltage follower could be as simple as a single transistor .. assisted by two resistors 😉

So please specify which MFC version you use (the link doesn't work for me) and provide a shematic how you did the wirering.

[Shazlan]

Hi Crossrulz,

The 4-20mA is an option, which we did not take. The MFC comes standard with 0-5V control input signal and this is how I would hope to control the MFC. I'm attaching the datasheet for your reference.

[Shazlan]

Hi Henrik,

Thank you for taking the time to respond. I've attached the datasheet to my previous reply and I'm attaching the manual in this reply, for other people's reference. We are using FMA-2600 the standard version (no optional selection selected). On wiring, based from our understanding of the explanation, we just need to connect the positive pin of the 9264 to pin 4 of the connector and the COM on 9264 to pin 8 (Ground). Voltage input is being connected to another C-Series module and using the same method (Pin 6 Output signal to positive on the module and Pin 8 ground to COM). Can you please help to clarify what do you mean when you say "250Ohm resistor to ground..."? 

I agree that ideally, RS-232 is the way to go. However, the only serial port on the cRIO is connected to another instrument and we do not plan to buy another 232 module. 

Thank you again.

Shazlan

[crossrulz]

I couldn't find a spec on how much current the 5V input needed.  I recommend pulling out a DMM and measuring the resistance.  Since your voltage is dropping to 0.4V, I would expect somewhere around 0.4V/4mA = 1350 Ohms.  Not unreasonable for some pull-down circuits I have seen.

But it is sounding like a simple voltage follower amplifier should do the job for you.