mini dc generator

Circular encoders are available and should provide you all the information you require about rotating shafts (speed, direction, current position etc).

Any DC permanent magnet motor can be used as a generator. You may need to experiment a bit to find what gearing you need to spin it for a decent output voltage/current.

I'd suggest you go to a hobby shop & get a model car motor, & try it out. You'd need to smooth the output (with a series resistor/parallel capacitor), and regulate it to a suitable level for your sensor supply.

Obviously, if the rotation stopped, the output would stop - unless you included energy storage (a very high value capacitor or a rechargeable battery).

I'm wondering how mini it supposed to be. Generator given from any vehicle is affordable and simple decision. You need a voltage regulator. Backup battery PS wouldn't be an excessive option.