Catapult releasing mechanism

[CAVBOTS]

Here is our issue, our catapult needs to be able to lower into the ball collecting position to scoop up the ball, and them we want to be able to, depending on where along the trajectory we want to relase, hold the arm at that angle, until we need to release. Some form of electric or pneumatic clutch would work, but what we need is recommendation on which ones have been tried and how well they work. Any thoughts??