My spindle speed controller broke and it seemed a good opportunity to learn more about phase control of mains voltage, and closed loop feedback control.
I found a great writeup of a very similar project by Klass.
The controller consists of:
- an RPM sensor (an opto emitter and receiver pair) mounted in the router body
- a TRIAC to control the router speed
- a zero cross detector so we can do phase control
- an Arduino to compare the target RPM with the real RPM and make changes using PID
You can get all the schematics, order boards and find the BOM over at the project page on hackaday.io.