> What are good methods for measuring rotation rate?
>
> I have a rotating device.  Currently I measure how fast it rotates
> using a one-pulse-per-revolution optical sensor, and then assume
> that it rotates at a fairly constant rate between pulses.
>
> Lately I wonder -- is it really rotating at a constant rate?
> Or are there significant angular accelerations and decelerations
> between pulses?
>
> I suppose some sort of rotary encoder (along with accurately timing
> the width of each pulse) could possibly work.

> It would be more convenient if I could bolt something to the outside,
> with a pattern ring about a foot in diameter glued to the stationary
> part and a sensor bolted to the rotating part. (Or vice versa).
> (Perhaps replacing, the one-pulse-per-revolution sensor I have now).

Construct a flat disk with identical evenly spaces slots around
the outside edge.  Install disk on shaft, hopefully with little
wobble or runout.  Put an LED on one side of disk, shining through
slots.  Put a photo diode on other side of disk.  As shaft rotates,
photo diode output is train of pulses.

Non contact, so limiting factors are response speed of photo diode
and computational ability of microprocessor (at some speed, it will
start to miss pulses -- upper bound determined by number of slots,
shaft speed, any hardware buffering, CPU speed, quality of software,
species of dead fish(*), etc).

(*) I hadn't seen it mentioned on PIClist in a while, so ... :-)

						Lee Jones

old PIClist 
output is train of pulses 
of 

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