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Custom scale inoperative in accelerometer task

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I am using a NI 9234 to measure acceleration. In the Acceleration task, I have fixed the sensitivity at 1000mV/g and I am trying to use a Custom Scale to correct for the actual sensitivity of the accelerometer(~10mV/g). The excitation is set to Internal and 2mA. The input is AC coupled. When I run the task, the scale has no effect on the output values. With 1G applied to the accelerometer, the output remains at ~.014G(pk) regardless of whether a Custom Scale is used or not. Is this normal behavior for an accelerometer task?

 

LabVIEW 2009 SP1

NI-DAQ v9.1.1

Phillip Neir
GM Engineering
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2mA is the lower end of most sensors supply current.... however should work.

When you say you apply 1G, do you mean 1G peak @ say 160Hz? If you apply 1G static you will see only a small signal while the sensor is moved but no static change since the AC coupling (and maybe even your sensor) don't allow static measurements.

A quick coarse check: fix your sensor on a speaker and place a small brass nut on top of the sensor (or nearby)  and apply something like 50 to 200 Hz. Slowly increase the amplitude and watch the sensor signal. At an acceleration slightly > 1G the small object will lift off and bounce back and you can see it in your signal (and even hear it :), but watching the signal is more sensitive )  

 

Greetings from Germany
Henrik

LV since v3.1

“ground” is a convenient fantasy

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Greetings Henrik,

Thanks for your reply. I think you misunderstood my problem. The accelerometer works fine. The NI 9234 works fine. In MAX, when I use the correct sensitivity, the output values are correct. In my application, I wish to make the task generic so that any accelerometer can be used. So, I set the sensitivity to 1000mV/g. The user will then use a custom scale to obtain the correct engineering units. My problem is that the Custom Scale has no effect on the output. I am using an accelerometer calibrator (1G @ 159Hz) coupled to the accelerometer as a reference. The output values remain the same with or without a Custom Scale applied. For example, without a Custom Scale, the output is 0.01G(rms). Using a Custom Scale with a slope of 100, the output is 0.01G(rms). The number should be 100 times larger. So, is this normal behavior for an accelerometer task or do I have a driver problem? Thanks, again.

 

I, too, am LabVIEW since v3.

Phillip Neir
GM Engineering
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Can you post a snippet of your code where you setup the scale?

 

Thanks,

 

Sean

Applications Engineering Specialist - Semiconductor Test
National Instruments
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Hi Sean,

I have attached a representative example of my code. The problem is not rooted there. When my application didn't behave as expected, I went straight to MAX and duplicated the task. The scaled values would change as expected when I changed the sensitivity. However, when I applied a scale with a slope of 100, it had no effect on the scaled values.

Phillip Neir
GM Engineering
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Can you verify the value of the reading in Volts, and in doing so, can you make the calculation to verify that it is the correct value?

 

Thanks,

 

Sean

Applications Engineering Specialist - Semiconductor Test
National Instruments
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Sean,

The basic accelerometer task operates and measures as expected. As I stated previously, I can set the sensitivity value and obtain correct magnitudes. In this case correctness doesn't matter. It is the application of the Custom Scale that is giving me the grief. The output magnitude should reflect the raw value and the math of the scale. If I have an output value of 1 with no scale applied, and if I then apply a scale that has a slope of 100, I expect to see the output increase to 100. I have done this and yet the output values do not change. The Custom Scale has no effect on the magnitude.

Phillip Neir
GM Engineering
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Can you post a screenshot of your task setup.  One thing to make sure is that your 'Signal Input Range' has values that you expect AFTER scaling.  If you look at the attached screenshot, I have a range from -300V to 300V because I am inputting a 2 V signal and scaling it by 100.  If you have this correct, there may be something else wrong with your task setup.

Applications Engineering Specialist - Semiconductor Test
National Instruments
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Sorry, I just realized I already had your code.  I did notice that your max/mins were +/- 5V.  This value should be the value after scaling.

Applications Engineering Specialist - Semiconductor Test
National Instruments
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If this is all set correctly, then I would update your drivers.  DAQmx 9.3 is the newest and should work.

 

Thanks,

 

Sean 

Applications Engineering Specialist - Semiconductor Test
National Instruments
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