Precise rpm at changing loads

[Herman the German]

???

I got an old 3 1/2 inch disk drive and of course I want to use some parts of it. The most interesting part however is the small stepper motor (bipolar). Connecting it to the shaft of another motor will produce voltage output on the two output coils which can be converted to digital and be used for precise rpm measurement (like in a frequency counter) Using DC-motor to drill PCBs (sometimes five in a pack) the rpm drops considerably while the drill works itself through the FR4. My idea is to compensate the drop vai this stepper. The signals of the stepper can be compared with the signal of the selector and then trigger the PWM drive for wider pulses or a higher frequency or both.

Any idea to start with?

Kind regards

HtG