This is just to post something that I've done in the past year with my robotics team. For those of you who are unfamiliar (Especially those who are still young), FIRST robotics is a league in which a group of engineers from NASA come up with a game defined by a set of rules. The robotics team then will construct a robot that will play that game best. One of the things that the robot could do is shoot foam balls up into a goal for points.
Our team decided to peruse this and came up with a flywheel design, something similar to so:
However, these launchers are not very consistent, and I wanted to make the machine more accurate. Being the lead programmer, I developed a solution. I made a self contained tachometer using a laser and a photo diode. You can see it working in this video:
Video of tachometer working.
I had it working so that the arduino put out the speed in revolutions per second. IIRC, our top speed was about 27 revolutions per second. Insanely fast for the size and mass of this wheel.
It's certainly not a high quality project, but I'd like to put this out there for anyone attempting to do the same thing.