At first I thought, that I add an optical transponder what will measure the rotational speed.
I went further than this. Even I designed a new plastic rotor with an integrated encoder ring.
Yesterday I started to play with the MOSFET based driver for switching on and of the spindle. During this something flashed to my mind:
The motor itself is a highly inductive load, in addition this is a brushed DC one. When there is variation in the magnetic field when the brush switch from one winding to an other there is a massive fluctuation in the current. The frequency of this is proportional to the rotational speed of the motor.
If this is true, that can be measured. I picked a fan (because placing the CNC and the oscilloscope into close proximity needs some transportation), a PSU, a serial resistor and an oscilloscope:
And the result:
The result even more dramatic:
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