internal excitation stability NI 9219 USB

[DAQx]

Hello,

I'm new to LabVIEW and would definitely appreciate any insight from you all. I am trying to record data from a load cell with an NI 9219. Does anyone know how stable the excitation voltage is from say an NI 9219? My waveform outputs have irregular oscillations when with no loading (and with loading).

I don't think it's electrical interference since I've tried it in two different rooms. I was thinking an unstable input could be the cause. I believe my code is correct, simply using the DAQ assistant to output to a waveform (5 Hz sampling rate).

I need to check on the load cell's required excitation voltage, but how would the signal output respond to an under powered excitation voltage?

Am I missing another possibility?

Thanks for any insight you can share.

Attached: Image of Waveformfor 10 mins

*Y-axis is not scaled, it is whatever the output values are. (What units are they?)

[mudson]

Hello DAQx,

The graph you provided shows an awfully small signal (+-40uV), but that may be the normal output range for the transducer you're using.  Could you provide the make and model of the load cell in this system?  

Also check to make sure that, in your program, you have specified the required excitation voltage for the transducer.

Best regards,

[DAQx]

Hello Matthew,

Thanks for getting back to me so promptly.

I'm using 3 old load cells by United Calibration Corp. (500 lbs, 1,000 lbs, 20,000 lbs) I gave them a call and found out that they're all 10V excitation and 2mV/V. For that waveform I used the 1K load cell just sitting on a desk (Model# 1K T/C). I saw the NI 9219 cannot host external excitation and has a max internal excitation of 2.5V. Would that 2.5V excitation degrade the output signal from the load cell to have such an odd output? I was testing the other day and even after loading in 20, 40, 60, and 80 lbs, the waveform would show a step when the load was first applied but then ease back toward the oscillating as if nothing happened.

I have an NI 6212 also that I plan to connect to an encoder, so tried using the "Custom Voltage with Excitation" setting with a 10V external excitation supply. I'm not too sure how to continue with the particular DAQ Assistant settings though. I was taking a look at this thread http://forums.ni.com/t5/Multifunction-DAQ/Misread-voltages-in-task-7-35V-max-reading-on-10-V-signal/.... Would it be correct to say the Signal Input would be from 0mV to 20mV? Also the Terminal Configuration options are a bit confusing to me (Seen in this screen shot

http://forums.ni.com/ni/attachments/ni/250/42132/1/pic_3.jpg), how could I go about selecting the best option for me?

Is the NI 9212 is still salvagable for this project, or should I just keep trying with the NI 6212?

I appreciate your help Matthew.

Thanks,

Jon

[mudson]

Hello DAQx,

You are right that the 9219 cannot be used with an external excitation source.  Given the 10V excitation requirement of the load cells, you will not be able to use the 9219.  I don't have a schematic of the load cells, but the excitation signal is typically used to power an amplifier.  From a "just multiplying stuff" point of view, one would expect output signals with a dynamic range of 0 - 5mV when the load cells were provided with 2.5V of excitation (2mV/V).  However, from your description of the behavior you were getting from the load cell when it was supplied with 2.5V excitation, it sounds like you will not be able to get the cell to function properly without the specified excitation voltage.

The 6212 does have the ability to generate a 10V DC signal.  However, to make sure that the signal is generated at the same time as the measurements you wish to make, you may need to move away from the DAQ Assistant and use the more granular DAQmx nodes in LabVIEW — getting DAQ Assistants to execute in parallel can be frustrating.  Learn 10 Functions in DAQmx is a pretty decent primer on how to get started using them.  You will need two tasks, one that generates the 10V excitation signal, and another that performs the measurements.

The terminal configuration options for the 6212 would be differential, referenced single ended, and non-referenced single ended.  Our Field Wiring and Noise Considerations for Analog Signals provides a good explanation of how to setup each of those measurement types, and when to use them.

Best regards.

[DAQx]

Matthew,

Thanks for confirming about the 9219.

I wasn't sure how to get the 10V output from the 6212. Someone provided a unit to supply the 10V and scale the output from 0-10V. Now I can simply scale the voltage reading for the loads. I am definitely checking out that DAQmx link though, as it looks very helpful for future VIs.

Yes, I was looking around at the Field Wiring & Noise Considerations for Analog Signals page before and it was very helpful to determine which to use. Thanks!

Regards,

Jon