If you think about car tire balancing system, it is somehow as easy to
build up a device like that, involves some electronics and mechanics,
but I don't know if the effort worth to adjust just one rotor...

You can think about three magnetic pins glued (in a star shape, 120¡ one
apart from another) to the external cape of a bearing, the rotor shaft
goes inside this bearing.  Each magnetic pin is inserted in a coil, that
would convert the pin movement or vibration into an electric signal.  A
optosensor needed to be attached to the rotor, to identify exactly when
start each turn, somehow as a "home position".  

The output of each coil feeds an op-amp and goes to an ADC, so you have
three digital output information.  The final idea is to have the minimum
output digital value for all the 3 ADC's.  A time relation between the
peak ADC output and the "home position sensor" tells you where is the
unbalanced weight on the rotor. The 3 sensors are required to locate the
position by "triangulation" when the unbalanced weight is not in a
single position.  The ADC output level would tell you a relative
unbalanced weight.  

If you can use an oscilloscope and sync on the "home position" you would
need just one sensor, since you can actually "see" the unbalanced weight
as a wave form on scope, so position, intensity, etc.  Probably a magnet
attached to the bearing and a speaker close to it can do a nice cheap
sensor (the magnet doesn't rotate, just stay close to the speaker, but
will transmit the vibration).

The equipment used to balance tires (at least the old ones), used two
voltmeters indicators, the first one just indicate more or less voltage
according to the physical distance the "coil output pulse" was from the
"home position", while the second voltmeter indicates how big was the
"coil output pulse", so the weight required to balance it.  The last
step was the operator just position the mechanism by hand looking at a
scale to position the tire to the same "angle" showed by the first
voltmeter, and "hammer" a weight as said by the second.

Wagner.

"Cesar E. H. White" wrote:
> 
> Hi,
> 
> I want to make a electronic wight balancer for my R/C helicopter
> rotor, this must mark the right position for the "weight" and display
> the amount of weight to put on it.
> 
> The question is... there are some type of cheap pressure sensor for
> this?
> 
> Thanks in advance
> 
> Cesar