If I have this right, you have:
a motor with 6-facet mirror spinning at ~ 10,000 rpm
roughly 160rps * 6 facets = 960 facets/sec or roughly 1mS/facet
To beam UV onto a PCB at, say, 600DPI over 6 inches = 3600 dots in 1mS
which is approx. 0.27uS/dot.
I think you will have to really accurately govern the motor speed to something which matches data coming out of the processor. i.e. Figure out a UV modulating pulse stream out of the CPU at around .27uS/dot, the actual granularity will depend on the CPU clock etc.
Then work out what motor speed will match that 'dot rate'
To trigger the dot line generation you'll need an opto on the bed - i.e. next to the PCB. At this position you only need it to be repeatable to 1/2 a dot or so ( bit under 1/1000th of an inch ). Trying to measure this on the motor itself will be trying to measure an angle of only:
Each facet draws 600dpi over 6" = 3600 dots, 6 facets = roughly 6 x 3600 = 21,600 in one rev
Rotational accuracy reqd is 360 degrees / 21,600 = very very small angle
Yours,
TonyWilk