I have a number of small 'high speed' (5000-10000rpm max) "RC car" style hobby motors and I am interested to sense and control their speed with an Uno. Or even possibly higher speed for drone motors.
I was thinking either magnetic or optical. The problem is the shaft is not accessible in my application. I think it's possible to magnetize the shaft enough to read it with a Hall sensor (or put a flat on it), but haven't seen it or tried to do it.
So then I thought, you should be able to read the EMF from the motor itself - from the coils and magnets. Has anyone tried to do this or know of good references for motor speed sensing like this, or with an 'antenna'? Maybe you can sense high frequency voltage/current variation on the power wires themselves???