First three harmonic analysis

[pbsalunkhe]

Hi Mark

The code is working fine. However there is one more doubt.
I have placed 6 pressure sensors circumferentially around a circular duct. There is a fan rotating at 1800rpm (30 Hz) inside the duct. The fan has 11 blades. So the blade passing frequency will be 11*30=330 Hz. When I use the option "1D array of scalers-automatic" in the "convert from dynamic data" then it gives high peak at 33 Hz i. e frequency reduces by one tenth of actual. When I use other options in "convert from dynamic data", it gives correct plot showing high peak at 330Hz (i e blade passing frequency).

I want to see the high peak at rotor frequency i.e at 30 Hz and not at the blade passing frequency. I used the lowpass filters but didn't helped much. Can you please suggest the way to do it?

Thanks

Pramod

[Sappster]

Hi Pramod,

If you don't have a sensor on the rotation of the entire fan then you will just have to divide the frequency readings by the number of blades to get that measurement. You could seperate the waveform components to get the frequency component and divide it by the number of blades. I explained on the first page how to seperate the waveform components.

[pbsalunkhe]

Hi Mark

The other way to divide the frequency reading is multiply the x-axis by 1/11. This can be done by
x-axis>>formatting>>scales>>multiplier>>0.909090.
Is it correct?

Thanks

Pramod

[Sappster]

Hi Pramod,

You can do that to change the scale of the graph, but you are only correcting the graph so if the data changes to a different frequency range, the graph won't reflect the change.

[pbsalunkhe]

Hi Mark

I tried to change the frequency scale as you suggested. But it is changing the y-axis value instead of x-axis. Other thing is when I use "1D array of scalers-automatic" option in "Convert from dynamic data", the frequncy scale reduces from 330 Hz to 33 Hz as I said in previous post.  When I use options other than "1D array of scalers-automatic", the code is showing broken wires. But last time it was running well with these options.
The fan frequency is 30 Hz and currently the code is showing 33 Hz(reduced freq. from 330Hz). Can you please tell how to change the frequency scale?

Thanks
Pramod

[Sappster]

Hi Pramod,

You might be better off leaving the data in the waveform type, so that the timebase information isn't lost. Some of the prior examples we handled the waveform types, you might want to use techniques similar to methods we tried earlier. I would recommend using the convert from dynamic datatype with the wveform type selected and then correct the rest of the code to use those datatypes.

Let me know if you can't figure out how to handle the waveform types. Maybe we can break down your ultimate goal and then re-work your code to fit your needs.

[pbsalunkhe]

Hi Mark

I tried with the techniques that we used earlier, but it is taking only "1d array of scalers-automatic". However the maximum frequency it showing is 500 Hz instead of 5000 Hz. I am using 10 kHz sampling frequency. As I said earlier, the blade passing frequency 330 Hz has been shifted to 33 Hz. If we divide blade passing frequency by no. of blades we get rotor frequency which is 30 Hz. I need the plot should show the peak at 30 Hz but currently it is showing at 33 Hz.
Is there any other way to scale the x-axis or to average the data of 6 channels?

Thanks

Pramod

[Sappster]

Hi Pramod,

Here is an example that should fix your issue with the x-scale. If you have more questions let me know.

Also, you might find the LabVIEW training classes beneficial. They will teach you methods for manipulating data in LabVIEW and commonly used programming practices. If you have any questions about the training you can look here:

http://www.ni.com/training/labview.htm

[Sappster]

Hi Pramod,

I couldn't convert the VI to 7.1 so here is a screenshot:

Message Edited by Sappster on 07-11-2008 11:39 AM

Message Edited by Sappster on 07-11-2008 11:40 AM

[pbsalunkhe]

Hi Mark

Thank you for your reply. Now I am getting the 330Hz frequency using this code. I have attached the LabView code. But I am not able to divide the x-scale value by number of blades i.e 11.  I can do this by multiplying the x-scale value. Is there any other way to do this?

There is another doubt with the specific harmonic that we have discussed earlier. I have attached the code for specific harmonic. In the left most "Build waveform" function, the "dt" value was placed as the inverse of sampling frequency. In the DAQ Assistant there are 6 channels. Will I have to repeatedly write this code for 6 times to get the averaged first three specific harmonic?
The specific harmonics were seen to be of zigzag type even during no flow condition. Also, the x-axis time scale is not coming as per the real time scale. I am attaching the data for sp. harmonic. Do I need to modify the code?

Thank you

Pramod