Sensor Design Documents
Capacitive Sensor Design
The basics of the design are to have 2 plate on the same axis as the voice coil, so as one gets larger, the other one gets smaller, then to differentially measure a change in voltage of the circuit for the two capacitors.
After talking to the ME team, it was decided that the plate could be up to 5mm across, and then taking into account that there will be more then 1mm of travel distance up or down, came up with a 4mm coverage at the zero point, this gives a total capacitance of about 160pF, this will change as the two plates shift up and down so the coverage of the two will change depending on the position of the mirror, since one end will be attached to the mirror the other end will be fixed to the mount.
So an approximation made was that the plates would not be moving closer or farther apart from each other, due to the short range of motion (of +/- 2 degrees).
As for the circuitry it seems to not work well in simulation, but will be testing it out in laboratory settings within the week. But it is basically a Wheatstone Bridge using capacitors instead of resistors, and with the right choices of capacitors, the output is almost directly proportional to the change in capacitance.