LabVIEW
wrong time results in wrong frequency
If you look at your measurement file, you have lost the X0 and dX values (X0=0, dX=1), so this is why you get the X axis going to 125 when you read the file back in.
Looking at your code, I believe the error is in youre wire vi here (see the red box):
You may not be able to see it, but there is a coercion dot where you wire the waveform into the Y input. This strips the waveform of all information except for the Y values, including your X0 and dX values.
My first question is do you need to replace the time value to begin with? I do not have a DAQ card to test, so you may not even need to do what you are doing in the red box. Assumign you do, then you need to do one of two things. You have an array of waveforms coming out of your mean/RMS VI. If there is more than one element in the array, then do the following:
If you only have one element in the array, then you can do the following:
As I said, I do not have a DAQ card available to me to see exactly what your data looks like. If you make these changes and they do not work, post the new measurement files. Or, perhaps someone with access to a DAQ card will chime in.
Message Edited by Matthew Kelton on 12-21-2007 02:00 AM
Looking at your code, I believe the error is in youre wire vi here (see the red box):
You may not be able to see it, but there is a coercion dot where you wire the waveform into the Y input. This strips the waveform of all information except for the Y values, including your X0 and dX values.
My first question is do you need to replace the time value to begin with? I do not have a DAQ card to test, so you may not even need to do what you are doing in the red box. Assumign you do, then you need to do one of two things. You have an array of waveforms coming out of your mean/RMS VI. If there is more than one element in the array, then do the following:
If you only have one element in the array, then you can do the following:
As I said, I do not have a DAQ card available to me to see exactly what your data looks like. If you make these changes and they do not work, post the new measurement files. Or, perhaps someone with access to a DAQ card will chime in.
Message Edited by Matthew Kelton on 12-21-2007 02:00 AM
The issue is that You are only writing one entry to the file per loop, so the dt value is pretty meaningless. You can do something like this:
This will force your loop timing and then set the dt for the file to the dt of the loop.
Message Edited by Matthew Kelton on 12-24-2007 01:29 PM
This will force your loop timing and then set the dt for the file to the dt of the loop.
Message Edited by Matthew Kelton on 12-24-2007 01:29 PM
The problems you've been running into mainly lie in the fact that the Measurement File VIs have some severe limitations. This latest problem comes from the fact you are reading in more samples than you actually collect. You've set the VI to read 1000 samples, so you get all these zeros which give you the PSD error.
If you modify the wire.vi to be similar to below, then you're sample numbers stored in the measurement file will be correct. This works only in the case where you have one Y value (which is your case) or the dt in the waveform I use to get the Y values is the same as the dt I set (you don't have to worry about this). The downside to this is if your VI were to crash while it is running, you lose all data, as it doesn't get saved until the end.
If you modify your read.vi to use the original sample size, you will not get all those zeros:
Message Edited by Matthew Kelton on 12-24-2007 04:43 PM
If you modify the wire.vi to be similar to below, then you're sample numbers stored in the measurement file will be correct. This works only in the case where you have one Y value (which is your case) or the dt in the waveform I use to get the Y values is the same as the dt I set (you don't have to worry about this). The downside to this is if your VI were to crash while it is running, you lose all data, as it doesn't get saved until the end.
If you modify your read.vi to use the original sample size, you will not get all those zeros:
Message Edited by Matthew Kelton on 12-24-2007 04:43 PM
