Vibration analysis

Hi,

My transmitter has not arrived yet so I have been trying to get deeper understanding about brushless motor and prop vibrations.

I did not have any success using phone for vibration monitoring so I constructed a stand with two independent sensors:

1) Analog ADXL001-70BEZ

2) piezo-electric 

I also made optical blade sensor and counter to be able to trigger scope on full rotation.

In the scope screenshot

yellow - adxl001

blue - piezo

upper digital - optical blade sensor

lower digital - upper digital with counter / 2

When looking in the scope I find it interesting that vibration period is 180 deg of the propeller rotation and not 360 as one would expect. The yellow and blue graphs are quite different but the trend is the same. I would expect if adxl001 signal was put through low pass filter it would look similar to piezo.

If I make prop intentionally way out of balance (screenshot 2) then it does come out in the 360 deg period but if the prop is more or less balanced then the 180 deg period is predominant.

Has anybody done something similar? Do you have more insight as of what is the reason behind this?

--

Janis

vibro_stand.jpg

vibration_test.jpg

intentional_imbalance.jpg

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Replies

  • A wild guess...

    Balanced prop - vibrations created by downwash striking the two ends of your stand (180 degrees apart). Analogous to those created by prop wash striking the frame arms in a full airframe.

    Unbalanced prop - vibrations created by prop rotation itself overwhelm the first type.

    (?)

    George

    • You might better-isolate vibrations caused directly by motor/prop imbalance if you flipped your stand over (motor upside-down, prop right-side-up). Like a bottom-mounted rotor on a full airframe.

      Or try to narrow the stand.

      (?)

      George

      • Thanks. That's and interesting idea. I will try to do more tests..

        Janis

  • I changed 10x4.5 prop that was used in previous tests to CF 12x4 prop and did another test. 

    CF_prop_vibration.jpg

    https://storage.ning.com/topology/rest/1.0/file/get/3702026025?profile=original
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