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J Tiers
09-29-2011, 08:34 AM
Apparently the iphone has a vibration sensor, and a person on an email group suggested using a particular app to track down lathe vibrations.

He stated the sample rate as 100 Hz, which would make the highest normal signal frequency 50 Hz.

Obviously to track down vibrations at a motor RPM rate, that sample rate is totally inadequate. Either crazy aliasing would occur, OR the

Does anyone know the actual internal sample rate?

It might be variable with sample time, which is about 10 sec at 100Hz.

Many low cost integrated circuit vibration sensors are themselves limited to frequencies of 500 to 2000 Hz, so it makes sense to have a low bandwidth. And the use in an iphone would be to figure out the orientation of the device, or maybe for "wii" type game apps, neither of which need anything over 50 hz.

I found this which seems to confirm the information, but may not be the only app. (why would there be two competing ones?)

http://www.macresearch.org/showcase-vibration-turns-your-iphone-spectrum-analyzer

dp
09-29-2011, 10:44 AM
Here is the chipset info for recent versions of the iPhone.
http://www.memsinvestorjournal.com/2010/12/motion-sensing-in-the-iphone-4-mems-accelerometer.html

The motion sensor has a 400Hz limit.

Edit: Whoops - that is the earlier version. The current version can sample at 1 kHz.

ikdor
09-29-2011, 03:56 PM
Well, with 1kHz you can measure the main vibration mode up to 30kHz theoretically. 10kHz is a more realistic number as the Nyquist frequency is not a practical upper limit in the real world.

You need to divide this frequency by half the number of rolling elements in the bearing to be able to analyse that mode.

So it does seem quite feasible to do basic vibration analysis from your smartphone.

Igor

J Tiers
09-29-2011, 10:08 PM
Except that the app limits to 50 Hz, all by itself.

and the A/D rate allowed to the sensor may limit the frequency well below the sensor limit.

unless that is upped a bit, it can't even find 60Hz vib, let alone 120hz torque ripple, 3450 rpm vibration, etc.

It looks "almost" useful for machinery, probably Ok for many other things

lazlo
09-30-2011, 10:31 AM
The iPhone 5 (and iPad3) will be officially launched on Tuesday (4 months late) -- pretty safe assumption that the latest accelerometer has a higher sample rate.