Optical will be the simplest to set up, and will have lowest influence on the measurand (the fan speed in this instance).
You could either place a peice of retro-reflective tape on the blade
and measure from one side or use the blade to break the beam.
The tape allows you to increase the received light from the blade /
shaft surface by increasing the reflectivity, you only get one pulse
for each piece of reflective tape so normally one pulse per revolution
(rev). If you use the blade to break the beam you get a pulse for each
blade, which will be a higher frequency than one pulse per rev, this
then has to be processed to arrive at the shaft RPM.
You can use an infra red diode as found in typical remote controllers
for T.Vs or a visible LED. An old ball style mouse also has suitable
sensors inside if you are strapped for cash. The infra red sensor will
be less sensitive to ambient light variation if you select the
appropriate sensor. These devices are available from most electronics
suppliers, watch the wavelength specification to determine if the
sensor is suitable for the light source, if the fan is rotating at high
speed (thousands of RPM) watch the maximum switching frequency
available from the receiver device.
For very low speeds or very high speeds make sure you have sufficient
counts available if you are using a counter to time events. In either
case you may need to ensure that any conditioning circuitry switches
reliably at the anicipated operating frequencys, you could test the
conditioning circuits with a signal generator or just hope they will
switch cleanly.
Essentially the conditioning circuit (or the receiver) can be done
using a single transistor say a BC109C or other transistor device with a
couple of biasing resistors. A general pupose op-amp will also suffice
if correctly connected.
Test it BEFORE connecting it to your expensive computer and data aquisition hardware. You will find plenty of circuits on the net for this kind of stuff, a search on google will yield a few hundred.
Message Edité par Conseils le 11-20-2005 10:19 PM