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Old Sep 21, 2003, 05:06 AM
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Nicke's Avatar
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Free program to measure rpm using sound?

I saw someone mention that there are free programs that use the soundcard to determine prop rpm, but no mention of where to find them.

So does anyone know if such programs exist, and where to find them?
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Old Sep 21, 2003, 07:44 AM
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mmormota's Avatar
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Re: Free program to measure rpm using sound?

Quote:
Originally posted by Nicke
I saw someone mention that there are free programs that use the soundcard to determine prop rpm, but no mention of where to find them.

So does anyone know if such programs exist, and where to find them?
You can use your soundcard az a digital storage oscilloscope.
There are free programs on the net, a useful one:
http://polly.phys.msu.su/~zeld/oscill.html

To use this program to measure rpm, you need a measuring head to convert the spin of the prop to electrical signal.

Possibilities:

magnetic method: a small piece of magnet (5*5*1 NdFeB or similar, size not critical) glued to the center of the prop (cellux etc), and a coil is the sensor (about 5 cm diameter, 10-20 turns, not critical), the soundcard input connected to the coil pins

optical method: a phototransistor in series with a 1k resistor and a battery, the soundcard input connected to the resistor pins

With the scope program you can check easily the new windings on a CDRom motor too: spin the motor with a drill and measure the signal on the coils, any shorts or asymmetries are clearly visible... In this case the voltage is too high to connect directly, you need 2 resistors to lower the signal (1k, 100k)
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Old Sep 21, 2003, 10:59 AM
agriebel's Avatar
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I see radio shack has the photoresistors. Now I need to know what type of battery and how to wire the mess together. I assume that you aim the photoresistor at the spinning prop.
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Old Sep 21, 2003, 12:55 PM
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Nicke's Avatar
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Thanks for the link!

I was about to hack my old microphone to get the connector, but thought i should try it as is first. Since the blades generate pressure when they pass, i thought why not try?

So i held the microphone just behind the prop (1/4-1/2 inch), and ran the program. the waveform was very clear, and using the fourier transform i found that the rpm of my cd motor was 12240 with a gunther prop, which seemed reasonable.
I also tried my miniac with 4,2x2 prop on 3 lipolys, and according to this method it spins at 25800 rpm, which again seems reasonable.

So - no need to start building sensors, if you have a microphone!!!
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Old Sep 21, 2003, 01:20 PM
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Cool, I hadn't thought of that!
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Old Sep 21, 2003, 01:34 PM
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Which control is the fourier transform button?
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Old Sep 21, 2003, 01:46 PM
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Quote:
Originally posted by Nicke
So i held the microphone just behind the prop (1/4-1/2 inch), and ran the program. the waveform was very clear, and using the fourier transform i found that the rpm of my cd motor was 12240 with a gunther prop
So - no need to start building sensors, if you have a microphone!!!
Nice idea!
Don't forget to divide the rpm with the number of blades.
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Old Sep 21, 2003, 10:33 PM
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Quote:
Originally posted by agriebel
Which control is the fourier transform button?
The one with the colorful stripes.
To get a numerical value, move the cursor to the maximum, and watch the frequency at the bottom of the window.
Multiply the result with 30 in case of a 2 blade prop, or with 20 at 3 blades.
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Old Sep 21, 2003, 11:23 PM
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could you tell me all of the buttons to depress to get the correct reading
(a screenshot would do) and also which number do you use for the calculations?
F? or I?

phil
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Old Sep 23, 2003, 04:42 AM
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Nicke's Avatar
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I'll see if I can make some instructions when i get home from work. It's quite simple.
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Old Sep 24, 2003, 03:15 AM
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Any success on the screenshot or instructions?

phil
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Old Sep 24, 2003, 11:28 AM
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Nicke's Avatar
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Oh sorry. totally forgot when i got home.
Anyway, I made some shots now. In both images, the white cross is where the cursor was. It disappears when taking screenshot...

To measure RPM:
Press the "play" button. Turn on the motor, and hold the microphone a few mm behind the prop, about half the blade out. While doing this, press the "pause" button.
You the have to play around with the T and Y1 slider to see the curve nicely. Note the distance between two peaks, with one in between as you can see in this case it reads 14.99 ms. To get rpm; rpm = 60*1000/14,99 = 4003 rpm. However, this is not very precise. This is just to verify that the reading in the fourier transform seems reasonable. SO now:
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Old Sep 24, 2003, 11:33 AM
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Nicke's Avatar
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Press the fourier button (the color striped one). Again, play around with the T and Y1 sliders to get a clear curve. Usually, prop rpm turns out as the highest peak, also with the lowest frequency (the one most to the left).
Put the cursor at that peak, and read the frequency in the status bar (127,1Hz). Calculate RPM = 30*F = 127,1*30 = 3813 RPM, which is close to the 4003 rpm, but probably more precise since it uses more data.
That formula is for two bladed props.
3 blades: RPM = 20*F
4 blades: RPM = 15*F
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Old Sep 24, 2003, 11:36 AM
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Great, thanks much...
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Old Sep 24, 2003, 12:00 PM
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Happy|Harry's Avatar
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Cool thanks
now all i need is to finish my thrust measurement rig and i can get the full
range of readings

phil
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