Measure frequency with NI 9205 and Labview

[Krisasdasd]

Hi all,

I am having a great deal of trouble measuring frequency with the CompactDaq NI-9172 chassis.  I am using the NI 9205 module with the signals hooked up in differential pairs to take the measurements.  I am looking to measure the frequencies of magnetic pickups and turbine flow meters.  I realize the NI 9205 is an AI +/- 10V module, so I tried using signal conditioners to convert from frequency to voltage.  I found the signal conditioners I was using to be extremely inaccurate.  I am now wondering if I can use programming in Labview to convert the signal.  I am not sure if this is possible, as I know VERY little about Labview.  I have tried the timing and transition VI in Labview with no luck.  Any help would be greatly appreciated.

Thank you,

Ryan

PS...I do not believe NI makes a module for the CompactDaq that directly measures frequency.  If I am wrong, please let me know of such a module.

[Efrain_G]

Ryan,

You are unfortunately right. National Instruments does not make a CompactDAQ that measures analog frequency. There are definitely cDAQ modules that can use counters to calculate digital frequency, but not analog. In this case what you need to do is to use LabVIEW to convert your signal from the time domain to the frequency domain so that you can extract the frequency information. You can begin by looking at the LabVIEW help for using the fft.vi:

http://zone.ni.com/reference/en-XX/help/371361F-01/lvanls/fft/

[johnsold]

Ryan,

If your analog device does hardware timed conversions, you can use the dt between samples to measure period and calculate frequency.  Assuming that the signals are not very noisy, find the index in the array of samples where the values cross a threshold (typically at the midpoint of the peak to peak range). The Threshold 1D Array function will give a interpolated result for rising signals. The difference in indexes of successive threshold crossings multiplied by dt is the period.

Lynn