This one really deserves some actual documentation I guess. I spent quite a lot of time making it, streching my limits quite a bit, and I was very pleased with the result.
However, it seems I’ve misplaced my schematics of the curcuit and all pictures of it. I’ll see if I can scrounge something from the mess that is my desk (and my brain for that matter).
I can provide a small overview at the moment at least. I had a cheapo router running in my cnc, with an rpm span of 11k to 32k. The torque at 11k was low enough for me to be able to stop it with my fingers. This was, of course, horrible for cnc applications. I needed something with high torque at revs around 4k and up.
I did this by reading the rpm with an IR diode and phototransistor with a painted part of the shaft. I then used this info to control the speed through a TRIAC with an arduino running some PID magic. The end result was very impressive, I could get almost full torque at revs as low as 4-5k and have it keep a stable speed no matter what materials I went through. The PID values took a lot of fiddling to get right. The only drawback is of course that these routers are cooled by a fan attached to the drive shaft, meaning it gets really hot when running at low revs for too long.