I was considering building a variable speed controller for a "universal" (DC) motor with proper feedback.
Of course an encoder is the most straightforward solution. However, is it possible to read RPM by the noise in the current waveform? Mechanically building a mounted encoder and adding durable wiring from the motor to the controller is very problematic and there is more than enough reason to avoid it.
I've worked with DSP chips. Implementing the scheme on a controller is not going to be the hard part. The question is the scheme itself.
This'll be a power line 120v rectified and the waveform to a high voltage DC motor switched on/off by MOSFET, synchronous to the initial AC of course. So there will already be huge harmonics of the original 120Hz going off into higher frequencies, which can hopefully be filtered out.
Is there a scheme out there people have used? Project page maybe?