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Old Jul 23, 2010, 10:38 AM
Ugly planes do not fly
_Sergey_'s Avatar
USA, GA, Alpharetta
Joined Mar 2007
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Inexpensive alternative to accelerometer

There is an inexpensive alternative for DIY accelerometers.

A little "theory" first

The accelerometer is a device that measures vibration. What else measures vibration? A microphone! The only catch here is most microphones measure change in air pressure. In our case we need to measure change of movement of a solid surface. And there is a special microphone that does exactly that - a contact microphone. It is based on a piezo crystal that picks up a vibration and converts it to electrical signal. Almost same principal cell phone accelerometers work on.

For $4.50 you can buy one on eBay, connect it to a laptop or desktop computer, and use sound recorder to get vibration levels.

I am going to buy it myself, do experiments with it and write a software to make balancing a simple and quick task. Looks like a fun weekend project!
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Old Jul 23, 2010, 02:52 PM
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iamzoli1's Avatar
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Originally Posted by jzuniga View Post
I downloaded "Vibration" on the app store on my iphone and it seems to be what I need?...
Z
thanks ill buy; is very cheap
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Old Jul 23, 2010, 11:55 PM
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United States, IN, Indianapolis
Joined Mar 2004
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dynamically balance EDF

I really like your method of balancing a EDF fan. I use two different pieces of equipment to balance with some good results.
One is http://www.bobsworkshop.co.uk/outdoo...d-fan-balancer Bob's ducted fan balancer- only a strobe light no readout .
Then I have a Digital Precision Vibration Meter Tester Analyzer- $200- It works pretty good -plus does other testing ,rpm,etc. --see picture below.

I just got the latest Droid X the other day. Can't wait to try this out using it as a EDF dynamically balancer. What a great idea.
Hope you fine time to do a U-tube demo.
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Old Jul 24, 2010, 01:45 PM
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United States, TX, Grand Prairie
Joined Nov 2005
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Quote:
Originally Posted by _Sergey_ View Post
There is an inexpensive alternative for DIY accelerometers.

A little "theory" first

The accelerometer is a device that measures vibration. What else measures vibration? A microphone! The only catch here is most microphones measure change in air pressure. In our case we need to measure change of movement of a solid surface. And there is a special microphone that does exactly that - a contact microphone. It is based on a piezo crystal that picks up a vibration and converts it to electrical signal. Almost same principal cell phone accelerometers work on.

For $4.50 you can buy one on eBay, connect it to a laptop or desktop computer, and use sound recorder to get vibration levels.

I am going to buy it myself, do experiments with it and write a software to make balancing a simple and quick task. Looks like a fun weekend project!
"....My God........it's full of stars..." (2010)

Doing the same...
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Old Jul 25, 2010, 07:09 PM
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Fubar69's Avatar
Gold Coast, Australia
Joined Sep 2004
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Well, I spent Friday mucking around with my EDF out of my Freewing F16. Its not the sort of unit you can leave in the aircraft to test so I made a test bench to mount it on and held it in a vice. The bench mount is very very sturdy and I was wondering if it was too sturdy as the fan still have wobbles at certain speeds due to it being held in place very rigidly.

The proof is in the flying though. I have friends who own Freewing SU34's with the same EDF/motor. I have less than half the noise that they do.

You may have to turn your volume up to hear it. Its not a low volume video, its a quiet jet!!
FreeWing F-16 EDF Radio Control Jet with 360 degree Thrust Vectoring System (2 min 34 sec)


Important Note: For those that watch this vid, you can see the engine hatch fall off part way through the video giving direct access to the EDF. Note that there is no change in the volume of the motor!! Proof that the engine really is quieter!
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Old Jul 25, 2010, 07:59 PM
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United States, WA, Gig Harbor
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Fubar,
that's amazing how quiet it is! What app did you use to do the "Sergey" method? Thanks
Z
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Old Jul 25, 2010, 08:04 PM
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Originally Posted by jzuniga View Post
Fubar,
that's amazing how quiet it is! What app did you use to do the "Sergey" method? Thanks
Z
I have a 3G iPhone running the "Vibration" app. It worked a treat. You can make things happen faster if you go into the apps settings (under iPhone/Settings/Vibration) and set the default email address.
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Old Jul 25, 2010, 08:19 PM
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Yeah, that's the same app I got.... I think I may have turned you on to that one? or someone on here asked about it a week back or so? I don't understand about the "default mail address?" Also, I have a hard case on my phone, when I do my vibration tests should I remove the case? I calibrated it with the case on? would this change anything, or should I recalibrate with the case removed and run the tests that way? Thanks
Z
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Old Jul 25, 2010, 08:40 PM
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Originally Posted by jzuniga View Post
Yeah, that's the same app I got.... I think I may have turned you on to that one? or someone on here asked about it a week back or so? I don't understand about the "default mail address?" Also, I have a hard case on my phone, when I do my vibration tests should I remove the case? I calibrated it with the case on? would this change anything, or should I recalibrate with the case removed and run the tests that way? Thanks
Z
I did a search and found the vibration app last week but also saw that someone here had recommended it also. There is another app called AccelGraph which you could use but it sends the data via the message body rather than the CSV which I like.

I have a soft silicone case on my phone and removed it. I expect the hard case would be fine as it would not dampen the results like my soft one would. I did not calibrate my phone at all but still got good results. As it was a constant calibration setting, all results were of the same baseline.

My Vibration app settings were:
Sample Rate: 30Hz
Sample Delay: 0 sec
Acquisition Time: 8.53 seconds
Time Series Vertical Scale: 0.05

The only thing I couldnt get constant was the throttling up to full power and back down. I would like to make something to control this. I have an old servo tester I made years ago sitting in a draw. Putting an electronically controlled varistor on it instead of the knob it has now would allow for me to control the throttle as a constant and get more accurate readings.

With the CSV that I get from Vibration I open the file and remove columns B and D and use the Frequency data (below Amplitude data) which is 128 samples then put them into the excel spreadsheet.

The Default Email Address is when you capture your sample and hit the CSV button, you dont have to constantly add the email address. The app settings allows you to set it so that every time you hit CSV it is already in the TO: field.
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Old Jul 25, 2010, 09:04 PM
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Cool! Thanks Fubar! that helps... I'll try it soon as I get back home next week... Z
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Old Jul 25, 2010, 10:31 PM
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Gold Coast, Australia
Joined Sep 2004
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Quote:
Originally Posted by _Sergey_ View Post
There is an inexpensive alternative for DIY accelerometers.

A little "theory" first

The accelerometer is a device that measures vibration. What else measures vibration? A microphone! The only catch here is most microphones measure change in air pressure. In our case we need to measure change of movement of a solid surface. And there is a special microphone that does exactly that - a contact microphone. It is based on a piezo crystal that picks up a vibration and converts it to electrical signal. Almost same principal cell phone accelerometers work on.

For $4.50 you can buy one on eBay, connect it to a laptop or desktop computer, and use sound recorder to get vibration levels.

I am going to buy it myself, do experiments with it and write a software to make balancing a simple and quick task. Looks like a fun weekend project!
Dude, there may be an alternative cheap source for the piezo microphone. The piezo speakers in computers these days have that ability I am pretty sure. A site about making guitars out of cheap parts like old 20 litre oil cans for the body and thick fishing line as the frets, he used pc piezo speakers as the pickups.
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Old Jul 26, 2010, 06:55 PM
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Originally Posted by jzuniga View Post
Cool! Thanks Fubar! that helps... I'll try it soon as I get back home next week... Z
Hmmm, iPhone Vibration app has 256 samples for "Amplitude Data" and 128 samples for "Frequency Data" in the generated CSV file.

Using Amplitude vs Frequency data in Sergey's Excel spreadsheet gives greatly differing results when populating 2 separate graphs and comparing side by side.

Although I have a quiet unit I am wondering if I used the right data to measure and balance? Is it possible I could make it even better?
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Old Jul 26, 2010, 07:19 PM
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Ken Lapointe's Avatar
Narragansett, RI
Joined Oct 2000
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its probably amplitude versus time and its frequency content for that entire time window. They just do an FFT using the time series data collected.

30 times per second times 8.53 seconds = 256 samples

an FFT on 256 samples will dump 128 data points, 0 - 15hz wide

Note that the FFT will not be all that accurate since there is much noise content that goes above the 30 hz sampling rate and some of that noise will get back into the data set as a lower put incorrect frequency.
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Old Jul 26, 2010, 07:33 PM
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Gold Coast, Australia
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Originally Posted by Ken Lapointe View Post
its probably amplitude versus time and its frequency content for that entire time window. They just do an FFT using the time series data collected.

30 times per second times 8.53 seconds = 256 samples

an FFT on 256 samples will dump 128 data points, 0 - 15hz wide

Note that the FFT will not be all that accurate since there is much noise content that goes above the 30 hz sampling rate and some of that noise will get back into the data set as a lower put incorrect frequency.
Thanks Ken. Yep, I have 2 full sets of samples that I recorded. I have created 2 spreadsheets for Amplitude and 2 for Frequency and your advice is right on the mark.

Amplitude data shows the best balance point and looks like an offset wheel in the spider graph whereas the Frequency data looks more like a star (a sick one).

Note to all using Vibration App on iPhone: Use Amplitude data in CSV to populate spreadsheet!
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Old Jul 26, 2010, 09:55 PM
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Quicksdraw's Avatar
Union City, Ca
Joined Mar 2005
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I just did a new setup for an E-Flite F-86.

IMPRESSIVE results using the Vibration App on the iPod Touch.

My final position reads 1/10 of the highest measured position.

Sergey, thanks for a clear explanation of this process. This ROCKS
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