Catapult-bot on wheels

In my middle school Intro to Robotics class, a group of three 6th grade girls built and programmed a catapult-bot for a game they designed. They used a Touch LED sensor to activate a motor that rotated the catapult arm. The catapult arm design was inspired by the Clawbot arm. They also installed a cross-bar to stop the arm. If they had more time, they would have liked to install motors to control the wheels and incorporated the controller to make the game experience more interactive. At full speed, the catapult was able to hurl a ping pong across a small table. If anybody has ideas for how to hurl the payload a farther distance, I would love to hear your ideas.

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Good job building the catapult. I like how they used the rubber band to put some asymmetry to the pull vs. push action of the motor.
If you want a real fast release, driven freely by the rubber band with the motor only used in one direction to actually accumulate the energy, you can use the mechanism like this









Hello nenik,
Thanks for the sharing your idea for an alternative design. I noticed you used similar gears for both the drive and driven gear. I’m thinking if you had the drive gear as the larger gear and the driven as a much smaller gear, we could get even more speed.

That is a correct observation. One of the first things I do with my robotics teams, as this gets useful in many designs down the line. The very next thing I want my students notice is that while they gain speed, they loose torque. Force. Strength. (Whatever word would your target audience understand, but never allow the word “power”, power stays constant, sans losses).

Now, you don’t need to get this mechanism any faster. What provides speed here is the abrupt release of the energy (accumulated by the rubber band) between steps 4-5. I would actually use the opposite gear, slowing down, but providing more force, so you could use stronger rubber band. Energy equals power * time, so with the same available power, you can store more energy if it takes longer.

Hi nenik, this a problem my 5th graders are trying to solve. I understand your undersatnd you suggestion, conceptually, but I’m not sure I follow with respect to how to guide my team. If we’re using the rubber band to charge the catapult in one direction…how does the suggested approach get incorporated? Is the rubber band connected to the small shaft connected to the arms connected to the upper gear? Thanks for any advice. Paul

I hope it would be obvious enough from this photo.

I’ve been asked in a private message, so posting the detailed explanation here too:
(But, well, just let them build it ;-))

The key detail is how the axle is protruding just a single plate thickness. As the gear turns, the first arm (1x3 beam in my photo) first gets pulled backward (second render), but instead of being released, it gets stuck on the rubber collar (3rd render) and keeps pulling the second half of the arm (the 1x6 beam) up to the point (shortly after 4th render), where the join between the two beams gets over the level of beam-gear join. At that point, the force vector on the join would no longer push the 1x3 beam against the collar but to the other side, so the 1x3 beam would be free to rotate around the beam-gear connection, letting the beam-beam join quickly and freely swing forward (to the position in the 5th render). Please note that between 4th (fully engaged) and 5h (fully released), there is almost no rotation of the gear.

Nenik,

first time poster. I’m working with my 4th and 5th graders to build the catapult you have shown in the posts dated 11-20-2015 and 1-21-2016. We’ve built them, but not sure how to incorporate the rubber band. Can you post some more photos similar to those from the 1-21-2016 post to show how to incorporate the design with a firing mechanism.

Many thanks.

Osodad.

If you mean a full catapult, try this (cad file at Google drive). This was never built, so may not work or may need a lot of tuning.
For using the release mechanism alone as a shooter, the picture from 1/21/2016 does have the rubber band in working position, but your students would likely need to add a plate or something protruding more out of the mechanism.

OK…got it!!