After reading _Sergey_ 's article ( http://hobbykingvampire.wikia.com/wiki/Dynamic_fan_balancing ) I started thinking - this could be done with a microphone and soundcard logger. After a little bit of net searching I didn't really find anything along my lines of thought, rather I read a lot of articles with people talking about accelerometers and other expensive sensors. So in the sprirt of experimentation I thought lets try it.
I looked at microphones and nothing really seemed to fit the bill (super cheap!). Then I found a $3.90 mylar 27mm speaker and thought, well a speaker works as a microphone in reverse so let's try that. http://www.jaycar.co.nz/productView.asp?ID=AS
Next on the list was some decent logging software. Again nothing really came up trumps (free) until I came across "Scope". This is a brilliant PC soundcard oscilloscope coded by Christian Zeitnitz.
Scope is remarkable and can be downloaded from here: http://www.zeitnitz.de/
Christian/scope_en?mid=1022 . It includes a data logger that outpts in CSV, so can be used for _Sergey_'s methodology.
Here's a screenshot of scope >
The connections for the 'sensor' are simple. Take a 3.5mm jack plug with cable and solder the wires to the speaker. I also wired back-to-back diodes in parallel, to snub any inadvertent high voltages inputing to the soundcard (back-to-back diodes conduct/short-out at above 980mV).
I then taped some plastic to the diaphram of the speaker so that the diaphram would be in contact with the model. Finally I taped it all to stop it moving. Here's a photo:
The little speaker looks like this:
And here is a scope trace. Scope can save a trace screen shot as well as saving the data as CSV (did I mention how good this program is!):
To describe this trace. I taped a piece of metal to the motor rotating-hub and then started the motor. The big spike is where I ran it up to full-speed, then brought it back down in speed. At this point the metal weight flew off (it was only held on with tape) and this can be seen by the relatively low signal levels before I cut the motor. So I think its safe to say this works! Not bad for the price of a super-cheap mylar mini-speaker.
Feel free to add to this page if anybody else experiements.