So, to get the ball rolling. Some thoughts I had while washing the pots (living the dream).
There's a few ways we can get feedback from the mirror position, but they have to be passive and not interferer with the mirror in little or no way.
There are (the ones I could think of):

  • Capacitance

given the current mirror and it's small size, would not hold well to added to that much added weight, also I'm sure normal air movement will add to much noise to the sensing.

  • Magnetic field polarity strength

Not even sure how this one would work, at all....

  • Light, IR/visible

I'm thinking either another laser (CD drive?) either bouncing off the back of the mirror (Will require the mirror to be aluminised both side or something to the same effect) or have two polarizing sheets along the axis of rotation, one being static on the support arm and the other being on the back of the mirror (this would be small).
At a guess the distance the peach mirror is from the printing layer is around 40cm(?) and with (what looks like) only a projection area of 5cm that's only going to move the mirror by 7deg... That's only a 12% sample range to fit 262,143 points in(18bit), which is would mean a stupidly sensitive sensor (ignoring noise), which brings me to my next point.

From my understanding to get a good reading to feedback you have to have twice the resolution, I'm not sure on the specs of the micro-controller being used but that's a big hurdle to tackle no matter that sensing approach. Maybe an additional micro-controller would be needed which serially feeds back.... Food for thought.