Thanks for your answer Richard.

I agree with everything you say, but still appearing harder than I can do.

About the ruler, which type of sensor I could use to see so small bars?

Again, you are right, I did given little information. Its because I dont have much requirements, I just want to create a laser XY Scanner using old HDD coils, I'm not caring very much for ultra high speed or precision, if I can draw something on a wall would be ok (at least by now).

Thanks a lot for remembering about the stepper motor =D
It worked well for the intent of that project, which was to be controlled by midi notes.
But is a little slow, and I having trouble for receive midi data from a Yamaha keyboard.
here a youtube video of it working http://www.youtube.com/watch?v=YbRyCPqETCE

Stepper motor for this application will have very very low resolution.