currenty doing reasearch for school on catapults, and I have some questions for teams that experimented with catapults:
does the cradle have to resemble the game piece or can it be completely different? I saw some working prototypes with both so I want to hear many opinions about this
what effects the shooting more, the range of angles between the starting position and the final position of the catapult, or the shooting speed?
what in your experience is the best way to load and actuate the catapult? I saw many ways to do this including surgical tubing and a motor, purely a motor, purely pneumatics, etc.
what do you feel are the downside of using a catapult?
please share photos/videos/other info about your prototype/robot catapults
I was typing a response to each point, and then I realized the answers were all basically the same.
These are the types of questions you answer when prototyping, and will vary specifically based on the game object you’re launching and the results you desire. What works ideally for one game object or one goal trajectory may be wildly different from another game object or goal trajectory. Launching a large inflatable ball at any arbitrary point above a bar (like in 2008 or 2014) is very different from launching a small foam ball into a goal only about 200% the size of the ball (like in 2016) is very different from launching tiny foam sacs into a top-loaded goal (like in 2021’s VIQ challenge). The re-loading techniques will vary wildly between loading a single shot at a time vs. a mechanism that you want to be able to rapid fire.
Basically, these are (some of) the reasons in which you prototype for a specific challenge.
It can be completely different. While you may want to hear many opinions about this, there are really only two (it must resemble/it does not have to resemble). If you find examples of cradles different from the projectile that work well, you have your answer. Watch 8 seconds of this video from FRC 2014. Two robots shooting well. One (the near one) has a cradle that is not shaped like the ball. It shoots great. The other (the far one) has a cradle that is rounded like the ball surface. It shoots great also. Either can work.
Probably depends what you are shooting, as well as the trajectory and range you are trying to get.
By load, I’m assuming that you are asking about how to add potential energy to the catapult. Take a look at robots from 2014. You’ll find excellent robots that used metal springs, elastic tubing, pneumatic cylinders, and more. Multiple sources of potential energy can be utilized in effective mechanisms. Tension-based systems can be effectively “loaded” by linkages or winches. Actuating pneumatic systems is much different than actuating systems that store energy in deformed materials. For pneumatics, it’s a matter of delivering sufficient airflow. For tension systems, it’s mostly about something that will latch securely, but not require excessive force to release. A classic mechanism is the choo-choo.
Some quick thoughts on downsides include shoot-reload cycle time, and relative complexity of changing range/trajectory.
Look at FRC robots from 2014. Tons of different catapult mechanisms.
We accidentally came across this in 2014, but there is a difference between the traditional medieval catapult (ice cream scoop that hurls the projectile) and a geometry that acts more like a jai alai cesta. When we went from a scoop that held the ball all the way through the arm movement to a more curved spatula, the ball greatly increased it’s launch velocity and we could then get a flatter trajectory. There isn’t a bunch of video of it out there since the robot had a bunch of other fatal flaws.
Off subject trivia fact: The energy storage device was a garage door torsion spring. Pretty knarley.
Is the catapult going to be stationary through the loading and firing sequence or will it be moving around and possibly stopping to fire? In 2014, our catapult worked reasonably well when the robot chassis was stationary but the shot became inconsistent if we were moving, stopped and shot. We noticed others had the same issues and implemented countermeasures but due to the design of our robot, it was difficult to add the countermeasures.
3467 made a quick powerpoint for design training discussing potential energy mechanisms, such as spring or surgical tubing powered catapults. (Coming soon to the build blog but you can check it out here). Included is a discussion of different ways to store and then release potential energy.
As many posters have said, it’s going to be quite game-specific but I wanted to highlight
The catapult style vs the jai alai cesta style vs an impact (think punching a ball instead of throwing it) all behave quite differently. Three examples below, and all three robots were excellent, despite approaching that problem from very different angles.
3467 from 2014 threw the ball straight with little-to-no spin:
