Weird Catapult


Back a few weeks ago at Bay Area maker fair, I saw this very clever catapult system on an an FTC robot. I knew I had seen it somewhere before and went to work that evening. After a little while I finally found it. Here is the video: https://m.youtube.com/watch?v=92IbHU0Z76I (0:23 seconds)

However, I was wondering how they were able to adjust the catapult to create their variable shots? Hopefully, someone can enlighten me. Thanks in advance!

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Changed the end stops.

Can you elaborate on what you mean by this? If I am interpreting what you say correctly, I am fairly certain that this is impossible for this design. The design depends on the the two catapult stages being able to straighten out and release the spring tension. A simple hard stop would not allow this to happen and the catapult motor could not be rotated enough to rewind the system. I am probably missing something very simple that would explain how this system works though!

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Your choo-choo linkage doesn’t have to completely straighten out to rewind. Or at least, 125’s didn’t. In fact, due to the construction of the mechanism using the linkage to hard stop could result in damage to the gearbox. There’s enough links in the gearbox that you should be able to wind it even if it starts a little cocked.

In our case we had a rope hard stop that defined the max angle of release for the catapult (i.e. long range shot) and a cylinder to change the length of that rope (via a pulley and a second cylinder handling the lock) for the short range shot. (This was all post season work that it got resolved)

Mind you, this is all based on recollections of a robot that was nearly 3 years ago so some of it may be hazy.

I think I catch your drift. To avoid not being able to re wind because of the hard stop, you could simply wind the mechanism up the opposite direction and it would work exactly the same (I think).

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No? Our mechanism only ran one way.

sighs ok back to square one. Do you have any photos, video, or diagrams?

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Here is an example of a simple one done on the VEX BuildBlitz and what FRC3005 did in 2014.

If you search choo-choo mechanism you’ll find more good pictures, and it isn’t the easiest thing to explain when you don’t have one in your hands… but I’ll give it a try. We happened to use nylon webbing connected outside of the choochoo mechanism to provide a “stop” for the catapult, and adjusted the length manually to tune in the shot.

Lets call the short linkage connected to the large gear “1” and the longer linkage that connects the catapult to linkage 1, “2”. There is a bolt head embedded in the gear that is long enough to catch linkage 1, but not 2.

At full extension, the gear is rotated such that the bolt connecting 1 to the gear is in the position closest to the catapult. Linkage 1 and 2 could make a straight line to the catapult if the lengths were nearly that of the hard stop, or they might have a bit of a dog leg if the hard stop is holding it short.

Regardless, after the shot, the gear begins to rotate counterclockwise to rewind (in this example). By moving the connection from gear to link 1 backwards (away from the catapult, you straighten out the link between 1 and 2, and begin to “rooster? thx overly strict forum filter” the catapult. After 180deg of rotation of the gear, your bolt head in that gear now rests against linkage 1. Now as the gear continues to rotate (the picture is showing this), you start to double linkage 1 over itself until you have essentially the gear/1 connection point very close to the starting point, link 1 pointing away from the catapult, and link 2 doubled over it, and the embedded bolt head holding a tiny bit of load. This is the cocked position. If you advance the gear maybe 10 deg, the linkages break over the stable position and are allowed to snap open (roughly 2x the length of link 1), firing the catapult.

If this is still crazy unclear, I might try to make a model in CAD some time in the next day or two.

I saw this thread, and remembered coming across a video of this sort of mechanism while researching catapult ideas.

They have a slow-motion video side-by-side which demonstrates how the mechanism functions rather well.

Here is an animation of a choo choo linkage.

Serves as a great example of why you should include mechanical hardstops to avoid destroying the linkage and mounts.