I emerge from the depths once more to give you a linear-traveling CAD assembly. I had originally planned to publish this at the end of summer, but life got in the way - oops.
(PhotoView 360 has been deprecated, and I don’t have a Visualize license, so no pretty renders this time. Let me know if you have a Visualize license that I can borrow :])
This is a differential elevator: it has two degrees of freedom (a prismatic extension and a revolute shoulder pivot) but both motors driving it are fixed at the bottom. We achieve this via a differential mechanism: the left and right sides of the carriage are held up by two independent belts, driven by independent motors. The common-mode travel of the motors controls the rotation of the shoulder, while the differential-mode travel determines the position of the elevator. This gets big savings in weight and wiring complexity at the cost of increasing the difficulty of controlling and fabricating the robot.
The entire thing is held together by one giant continuous belt run - it’s 9.5m long. To assemble this, you’d have to splice the belt from a piece of 10m stock (which does exist on the internet). I think that’s the main impracticality of the design - if building a real differential elevator, ropes might be a saner choice.
Since the shoulder pivot requires a much larger gear ratio than the elevator itself, I added an extra gear stage inside of the carriage to increase the shoulder torque.
The big orange blob is a sample hub for mounting an arm made from a 1" diameter tube. It also contains a little track for an energy chain. In-season, I would probably make a special hub for whatever arm I build, so that I can keep all the motors and high-weight parts away from the tip of the arm.
View the CAD here (native + STEP): https://grabcad.com/library/a-differential-belt-in-tube-elevator-for-frc-1
Also, check out my design calculator here: DE design calculator - Google Sheets
The assembly was designed before this year’s game, so it’s a little too tall, but it’s not much trouble to alter the height of a few parts. It’s intended for a 5-lb 12" arm, so you’d have to build a very lightweight manipulator to use it. The total weight of the assembly (according to SW) is 13 lb.