Frequency sweep with peak frequency

Question

Hi guys, I need some help here. I´m using a NI 9215 to do a frequency sweep from 30 to 90 Hz.
So far so good. Now I need a resulting resonance frequency at highest voltage.
I tried a max peak frequency VI but couldn´t get it to work.
Any idea what to do?
mfg, Andi
ok, here is my sweep.vi for better understanding
Message Edited by andinew on 08-11-2009 04:14 AM
Attachments:
sweep.vi ‏216 KB
create_log_frequencies.vi ‏15 KB

Answer 1

Hi Andi,
one thought first off: looking at your "create_log_frequencies.vi", I realized that it generates linear frequency hops, not logarithmic ones...
But on to your question: to find the frequency with the highest amplitude (this will be your resonace, I guess), you will have to look at the entire signal - your "sweep.vi" code iterates the measurement and interpretation five times per second, independantly of the speed of the sine sweep.
My advice would be to generate one waveform containing the whole sweep run, and writing this sweep block for block with a constant sample rate onto the output. You could then read back the response with blocks of the same size as your output, do the single tone measurement on this block and store the resulting frequency and amplitude in an array preserved trough a shift register or simply into two auto-indexing tunnels. The latter is okay as you don't need a "while loop" in the measurement loop (you know how long the analog output loop runs - why not use this information to configure a "for loop").
After your analog input "for loop" has finished, search your amplitude array for the maximum - then use the found element's index to index the frequencies array and return the corresponding resonance frequency.
A good starting point could be the example I wrote a couple of month ago: it runs continuously the same "sweep" (you'd have to adjust the waveform, I know) and reads back the stimulus and the response (wire the output to your probe and parallely to ai0, your response signal will be connected to ai1). You can then insert tone measurements etc.  at the marked section...
When trying to open the example, you will be prompted to find a VI called "_Get Terminal Name....vi" - you can find this VI at the following location: C:\Programme\National Instruments\LabVIEW 8.5\examples\DAQmx\_Utility\_Utility.llb
As an afterthought: the "phase difference" you are calculating between every other sample of the analog measurement array is a bit confusing: if oyu need the phase difference between the original signal and the stimulus response, you will need to wire the stimulus to a second input and compare the phases of the two separate signals...
Best regards,
Sebastian
Message Edited by SFK on 08-12-2009 04:12 PM
Attachments:
Multi-Function-Synch AI-AO mit Signalgeneration.vi ‏91 KB