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How to obtain Power Spectral Density plots using DaqView's Scope...

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How to obtain Power Spectral Density plots using DaqView's Scope


Question:
I have DaqView and a signal-conditioned accelerometer with a gain of 10 mV / g. I ultimately need to obtain Power Spectral Density plots in (g's)2 / Hz [RMS]. I don't see: (1) where to enter this sensor gain or (2) how to obtain these plots using DaqView. Please advise on what I need to obtain these plots.

Solution: 

DaqView comes with limited frequency domain analysis. PSD (Power Spectral Density) plotting is one of the few functions that DaqView provides for the frequency domain.

For DaqView, you need to scale the data using the 'Units' column of the configuration sheet, which by default is for 'V' (volts). When that cell is selected, there is an option in the selection box for 'mx+b' which is linear transformation from volts to engineering units. Assuming you have a 10 mV / g accelerometer, this is the same as 100 g's / V; and thus the proper scaling factor for the 'm' of 'mx+b'.

Note that if you want acceleration in m/s2 (meters per second squared), you need to convert acceleration from g's using the standard gravity constant for Earth being 9.81 (m/s2) / g:


m = [100 g / V] * [9.81 (m/s2) / g] = 981 (m/s2) / V

The following is an example screen shot for having DaqView convert accelerometer readings from volts to g's:

You can get plots of either current acquisition data or previously saved binary data using the DaqView scope. The scope window is obtained with the far left icon of the main DaqView window.

Note, to have multiple channels on any given plot, the units label (as defined above) must be the same for every included channel.

Adding a PSD to the scope requires the scope selection as pictured below:

The far right icon at the top of each plot is how you get to plot properties pages. An example set of settings for a PSD in the properties page of the above selected plot type could possibly be:

Generally, you want powers of 2 data points per frame for efficient frequency domain analysis. These units will be g2/Hz due to data being in g’s, and nature of a PSD plot. Another good rule of thumb for determining scan rate is to multiply desired maximum analysis frequency by 2.56. (Nyquist factor) So, to have a 1000 Hz frequency range, this data should be taken at 2560 Hz. Note the bigger the block size (the longer the acquisition), the finer the resulting spectral resolution will be.

You must also drag and drop the appropriate channels into the plot area as you would a time domain plot. The image below was taken with a Personal Daq/3000 using a scan rate of 2560 Hz, the settings you see above, and 'Hanning' chosen in the 'Window' tab above:

The next button to the left is how you could copy the PSD data in ASCII format to the clip board. The above was also zoomed to 1000 Hz as DaqView will, by default, give you the entire possible analysis range, which would be half of your scan rate. Note that by following the fore mentioned rules (powers of two for scans, even analysis frequencies, and a 2.56 factor for scan rate), you'll find that the resulting frequency bins will consequently be at nice round increments of frequency.

This kind of feature is an advanced feature that has recently been added to DaqView. DaqView has historically been a general purpose software providing basic time domain data acquisition. eZ-Analyst, which we will now discuss, is more specific for this type of application and thus  is easier to use for this purpose. However, the scope feature [within DaqView] is an inexpensive solution for limited frequency domain analysis.



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Article ID: 50347

Last Modified:5/21/2012 1:56:30 PM

Article has been viewed 6,267 times.

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