Pulse Measurement problem

[Dave Crabbe]

I'm trying to use the Pulse Measurement function in LabVIEW 7 to measure the period of a periodic wave. Eventually I want to measure the frequency of any wave I put into my DAQ card, but for now I am simulating a sine wave signal of 10Hz freq and going between 0 and 4 units. 1000 samples / sec. I expect to simply hook the pulse measurement canned function to the signal and run it; but I get the following error: "Error 20308 .. the waveform did not cross the mid reference level enough times to perform the measurement"

Guess I am pretty new to LabVIEW.. anyone know what will correct this? (am using all defaults .. that is I have set no input values)

Dave Crabbe, dave.crabbe@nscc.ca

[Bob_Y.]

You are setting your simulated sine wave to 1000 samples per second. Have you set the number of samples to something more than 100? To ensure that you get at least one complete cycle, you should set the number of samples to put out more than a single cycle. I usually try to get over 2 complete cycles, but it should work with about 1.5.

[Dave Crabbe]

Yes, as I've specified, I am trying 1000 samples per second.. I've tried from 100 to about 5000, I think.. very easy to duplicate, just put a simulated sine wave, and then try any of the timing measurement or pulse measurements VIs and set the freg to less than 15 hz or lower .. seems the measurement VI times out if it doesn't see a cycle within a certain time frame (absolute). It doesn't seem related to samples .. I don't know ..

Dave

[Bob_Y.]

Could you please attach a VI that shows the problem? It works fine for me. As long as I have about 2 cycles (at least 3 edges), I can get down to .1Hz or less.

[Yuto Shinagawa]

Not sure if you've already solved the problem or not, but I'll post a
reply anyway. Ironically, I came across your post on google when I
encountered the same problem with Labview just yesterday; I'm trying
to make an optical tachometer for a propeller. Anyway, aside from
increasing your sampling interval or increasing the frequency of your
generated waveform, you can just chose to ignore the error message
that would otherwise interrupt your run by simply creating an
indicator for the 'error out' port. Doing this won't hault the
program and prompt you whether or not to continue. If you want to go
one step further, you can unbundle the error cluster output, see if
the error status is true or false (true if it returns an error; false
otherw
ise), and if true, return infinity as the period. The 'select'
function comes in handy for this.


Regards,

Yuto Shinagawa


Keywords: Error -20308 occurred at "ma_ptmFetch.vi" when called from
"Pulse Measurements 1 chan.vi The waveform did not cross the mid
reference level enough times to perform this measurement. Check the
signal length, reference levels, and ref level units.


Dave Crabbe wrote in message news:<506500000005000000858D0100-1079395200000@exchange.ni.com>...
> Yes, as I've specified, I am trying 1000 samples per second.. I've
> tried from 100 to about 5000, I think.. very easy to duplicate, just
> put a simulated sine wave, and then try any of the timing measurement
> or pulse measurements VIs and set the freg to less than 15 hz or lower
> . seems the measurement VI times out if it doesn't see a cycle within
> a certain time frame (absolute). It doesn't seem related to samples ..
> I don't know ..
>
> Dave

[Dave Crabbe]

Yuto;

Excellent suggestion. Thanks for your post. Wonder if the LabVIEW team have an address for 'bugs' or enhancements next time around .. hope they are monitoring this.

dave

[Queentest]

Hello Bob, could you plz tell that the unit of the output period of Pulse Measurements.vi, millisecond or second? Thanks