Hi, guys, I made a Catapult this weekend, here are some pictures.
First, the left DC motor would rotate in slow speed to pull down the beam until the beam trigger the limit switch. While the limit switch is triggered, the left motor would stop rotate and the right one would start to rotate a little bit to lock the beam. After that, the left motor would rotate again to relax the blue line. Last, the right motor will rotate back, that would unlock the beam. The beam would launch out with great speed at this time.
I use infrared controller to control the on-off of DC motors.
I will upload a video here later 
There are still some where to improve. The Catapult is not very stable now. I use ping-pong as my ammunition, but the ball can not be launch to the same target at each time. And, the blue line would wrap itself after times (10-15) rotating.