1520 measure voltage normalized?

[kayotik0987]

I'm using 1520's to replace some old signal conditioning amplifiers in a wind tunnel. Our sensors include pressure transducers and load cells on a force balance. They act like strain gages in that they require an excitation voltage, and produce a small return voltage that needs to be gained up to acheive good resolution for the old A/D converter.

I've been able to hook up and read a pressure transducer with the new system, but am confused by the reading that I receive. I'm using the DAQmx system to run everything, and when I set the AI.GAIN parameter to a valid gain setting, it doesn't seem to do much. If I set the gain to a high value, like 100, it clips the signal that I'm reading. It seems like the signal is normalized to someth
ing, but I made sure not to use the excitation for scaling.

Any ideas on what is happening? Is the hardware already doing what I'm trying to manually do?

Thanks for any help!

-Warren

[Robert M]

Hello Warren,

This discussion appears to address some of the same issues you are facing. The related links in the discussion may also help.

If none of these suggestions help, please post the software and versions you are using, the applicable portions of your code, and any other information that may help, and I�ll be happy to look further into it.

Have a nice day!

Robert M
Applications Engineer
National Instruments

[kayotik0987]

I'm using NI-DAQ 7 and LabView Express 7. Here is the code that I've written so far. My hardware includes a PCI-6052E connected to an SCXI-1001 chassis holding two SCXI-1520's. I've been testing with a single pressure transducer from Validyne.

I don't mind if I need to let the program set the gain based on the input limits, but I want to understand exactly what is being done in the hardware and software so that I know what relevance the value read by the DAQmx read vi has. If it is normalized or scaled somehow I'd like to know so that my units will make sense through the rest of the code.

I'd already read through some of the threads on using the 1520 for these types of measurements and it included using a custom scale. I wanted to write a s
ensor calibration vi later to do this, so the custom scale that I called 1520 voltage is just a linear scale of slope = 1.

I think that's all the information that would be needed to arrive at the same point that I am.

Thanks for your help!

-Warren

[Robert M]

Warren,

What are the input range values you are using for your global channel? What reading are you actually getting out of mxRead that don�t seem reasonable? I�m looking further into this and want to find out exactly what happens in the hardware and software.

Thanks.

Robert M
Applications Engineer
National Instruments

[kayotik0987]

I have been using the mx create channel to make my channels, and then setting up the parameters based on a config file. So, I haven't been using any global channels, though I am considering moving to this technique. I've varied my input limits from + or - 0.1 on up to + or - 10, and no matter what these settings are at, I always get a value in the + or - 0.1 range.

The parameter AI.GAIN is also a mystery when it comes to it's effect. When it is set to 1 and input limits set to 0.01, it's value ranges between + or - 0.05 before clipping (very strange). When gain is 10 and input limits set to 0.1, it ranges between +&- 0.1 and then clips. With gain at 100 and input limits at 1.0, the same result as for 10. Then, with gain set
to 1000 and input limits at 10.0, I get values between +&- 0.01.

I've got an updated version that I'll attach.

Thanks for helping me out.

-Warren

[Robert_Jackson]

Warren,
I am glad to see you using DAQmx and I also was impressed with the code you attached. One thing that has become a little different with DAQmx is how gain and scaling is applied. The AI Config.VI is the traditional way to set the gain on a DAQ board, with this setup a user would set the input limits and then LabVIEW calculated the gain. In the case where you set the input limits to 0.01. LabVIEW would calculate the largest gain possible. It is worth noting for input limits you need a reasonable minimum and maximum value. You are using a powerful feature of DAQmx with the property node to programmatically set the AI.Gain. This is a good way to do things, 1.) If the DAQ board supports the gains you are using, 2.) If
you are not in another place resetting those gains by also providing input limits.

I looked through you code and noticed you have a DAQmx Create Channels VI with a minimum and maximum input not connected inside the DAQ Create Channels.vi. I see you are doing this so you can have less code for creating multiple channels. Also on the VI above you are setting both the gain and the Input limits and dividing by 100. I wonder if you just set the input limits what the response would be.

I hope this all clarifies a little about what is happening. I recommend using LabVIEW to set the gains by specifying input limits. I also would refer to the following KBs to clarify more about available gains.

Linked KB about Gain Settings

P.S. Another interesting fact I saw was that you are setting a lowpass filter. I wonder how this affects the application.
Regards,
Robert Jackson