[Racer26]

Yes they DO make a double solenoid valve in which energizing both sides of the coil moves the setup to a locked position, but I think we came to the conclusion that they were either far too expensive to be practical for our team (we're pretty low budget, $10,000 or less on average per year, including our registration), or that they were not FIRST-legal, and as such designed our system using FIRST parts.

The primitive system we used in 04 was exactly what you drew out. Our 07 system was much more advanced with the valving, and the software much more sophisticated. Not only were we using the valving to control it, but we had coded it with the position sensor such that any outside forces (other robots, game pieces, team members, quantum particles) causing a change in the position of the arm would induce the arm to automatically correct itself. We thought this to be important when trying to place a tube on the rack, that the robot needed to maintain position with the arm. It worked fairly well, and was among the fastest arms we saw at GTR.