Killer Bees - 2014

[apalrd]

Originally Posted by DampRobot

Any details on how you implemented your catapult? I'm interested in how you packaged the springs in what looked to be a really tiny space, and how you designed for an optimized shot/designed for adjustability of the shot.

I attached some pictures and here is a description.

The moving portion of the catapult itself is a right triangle (or very close, it's a 95 degree corner). One leg spans the length of the robot, and has several applied pieces which hold the ball, a cross bar for the end stop strap (a seatbelt strap), and a cross bar for the pull down strap (a high strength rope). The other leg has mounts for the springs, the hypotenuse is a stiffening member, and the axle is located on the right angle. There are two welded triangles spaced 2" apart with the pull down and end stop strap axles in between, as well as structural pieces.

The fixed portion makes a 'Z' (really an 'N') with a vertical member on the front and rear of the machine connected by a diagonal member, with additional stiffening members triangulating the corners.

There are 6 springs. 2 are 2" diameter and 4 are 1" diameter. The 2" springs have a 1" spring inside of them. All of the springs mount to the ends of the moving portion. 2 of the 1" springs are mounted directly to the fixed portion, the 2 2" and 2 1" springs inside are mounted to a 'banana bar' which is adjusted with a lead screw. The lead screw is driven via a CIM and Versaplanetary 3:1.

The Choo Choo is remote mounted via the orange cable. We learned that we could play with the choo choo geometry to flatten the motor torque as the catapult is pulled in (as the spring force will increase with pull in distance, but we can also decrease the lever arm of the second hump to equalize the torque). The choo choo is driven via a Dewalt gearbox and then 7.2:1 in additional gear and chain reduction (total of 86.4:1). We did this instead of a rachet as we wanted to be able to reverse the choo choo, but this did not end up working. The dewalt gearbox was still handy for packaging, but it's a giant PITA to make and I don't recommend it at all.

As for adjustability, we have 3 primary points of adjustment: end position, start position, and spring length.
End Position: There is a black seatbelt strap which controls the end stop. At the fixed end it is wrapped through a hex shaft which is secured from rotating by a wrench-looking piece (it is visible in the 3rd picture). By removing the wrench-looking piece and using a pair of 1/2" wrenches we can adjust the end position under load.

Start position: The eye bolt which secures the choo choo cable to the catapult arm can be adjusted to set the start position. We do this to calibrate the dwell time, among other things.

Spring length: This is directly adjusted via the spring adjuster lead screw. We can also adjust the 2 fixed springs via their eye bolts, and have several springs which we can swap for major adjustments.


http://imgur.com/a/iQznZ