In 2011, we created a very cool lift system to elevate our grabber from the ground to the height of the scoring pegs. It's a bit different from 254's, as it incorporates a constant force spring motor to counterbalance the system (we preferred not to use exposed constant force springs, as if they snap it can be very scary and unsafe). By doing this, we were able to drive the system with a very small motor, but still have a very fast lift system. Video (go to 1:07):
https://www.youtube.com/watch?v=UzZJO0pQafY
The three stage system was nearly perfectly counterbalanced with a constant force spring motor assembly from SDP-SI (
https://sdp-si.com/eStore/Catalog/Group/580), as mentioned by DampRobot somewhere back in the thread. The line from the constant force spring motor was looped over a pulley on the second stage and connected to the third stage to counteract the weight of the manipulator. Even without the motor attempting to move the elevator, the system would stay put wherever it was, which greatly simplified the programming of the system.
The first stationary stage has a motor mounted to it (an RS-550, as it only needed to move the system, and didn't have to handle the weight of the manipulator), which turns a chain run that is connected to the second stage. The chain moves and it pulls the second stage with it.
The third stage (grabber) is slaved to the second stage very simply. A line is connected to the top of the third stage, runs over a pulley at the top of the second stage, and is connected to the bottom of the first stage (base frame member of the robot). When the second stage moves up, the third stage moves up with relation to the second stage as a result.
If you want more information, I can provide some. Also, our CAD is online at frc-designs if you want to take a closer look for yourself.