Catapult help!!!

Our team is trying to build a catapult with motors to launch little stress balls out at football games and outreach events. We are trying to stay away from pneumatics and go with springs so there is little maintenance needed. Has anyone ever done this before or do you have any ideas for this?

The big solutions here are a cam and a choo choo mechanism. Choo choo’s were very popular on 2014 robots, and you an see a great example of how they work here:

and on a robot (sorta) here:

My team (1836) used a cam on our 2016 catapult, and a decent example of how they work can be found here: *

To see a cam catapult in action, check out our not-so-helpful practice match video here:

https://www.youtube.com/watch?v=dWfzvqZG2ao (thanks for letting us use your practice field Code Orange <3)

Both solutions can work well, all depends on your execution, which mechanism makes the most sense to your designers, and how you want to implement it.

*If someone has a better resource on how a cam works in a catapultey context please post it here cause my lego videos aren’t always that helpful…

You can also do something similar to what my team did in 2014 with a winch and latch system. Basically, you use a winch to pull back a spring powered arm. When the arm gets all the way down, you close a latch to grab the arm in the down position. You then run the winch in reverse to give the string slack (you may need something to keep the slack string out of the way). When you’re ready to fire, open the latch and the springs pull the arm quickly back to the start position. Lather, rinse, repeat. IMO it’s easier than a choo-choo or cam mechanism both to build and to control.

Sound like a cool solution!! Could you provide some pictures/video/cad/documentation to help me and OP (as well as probably others) understand better?

I just uploaded some pictures where you can see the mechanism here.

We actually used pneumatics to pull back the string and trigger the latch, but it wouldn’t be hard to switch them with motors if you wanted to. The large pneumatic cylinder pulled on the rope, which went through two pulleys to pull down the catapult arm. You can see the bar that catches in the latch at the top of the first picture, and the latch in the last picture. The latch was originally just a regular gate latch, but we replaced the latch piece to adapt it to the cylinder. All in all, it was an easy mechanism to put together and just as easy to program.

Looks cool!

I guess in theory, one could achieve variable shot angles with this type of setup too? If it were a motor pulling back the winch, could you only let up half the slack? Might be interesting for a 2016 catapult, as our cam catapult was limited to one shot angle. If I understand you right, could you have coded 3 separate shot angles for outer works, just off the batter, and a batter shot with this mechanism?

With catapults you generally want to keep your retracted position the same regardless of shot type, because there is usually some assortment of mechanisms actively holding onto the ball in the retracted position. So you need to change spring power or release angle to change the shot, which this mechanism alone wouldn’t do.

Theoretically you could vary the shot angle that way while still keeping the resting position the same by only letting or a certain amount of slack, then letting out the rest after firing. The catapult arm would always rest in the same place, but the firing angle would change. However, I wouldn’t do that with a motor winch because there would be no hard stop, only the rope pulling on the motors. Instead of immediately stopping, the motors would slow the arm to a stop and not launch as far. You could do it with a 3-position pneumatic cylinder or some other method of ensuring that when the string runs out of slack the arm immediately stops instead of slowing to a stop.

Another wrinkle you could do with the motor and latch to allow different tensions (and therefore launch speeds) would be to attach the motor/gearbox to the core of a wound spring, and the arm to the outer edge. Then, you could latch the arm, and wind the spring loosely or tightly before releasing the latch. You could probably manage launch speed/range with just an encoder on the spring side and a limit switch on the arm side.

(Caveat: Just an idea; I’ve never done this.)

You might find this thread helpful: https://www.chiefdelphi.com/forums/showthread.php?t=152739

In 2014, we used a pretty easy spring loaded catapult: We pulled back the catapult arm with a webbing strap, wrapped around a drum made of two hex bore hubs. The hub was attached to a two-speed shifting gearbox, with one speed missing a gear so that when you shift to it, the gearbox freewheels and the catapult launches. In its place we put a ratcheting wrench so that we could hold the catapult back without having to stall the motors constantly. It was super easy to make, and worked like a champ (without any finicky latches). The downside was the ball shifter internals had to be replaced frequently, as the hard shifts damaged them (vex since improved the design).

Does anyone have any advise on adjustable hard stops for catapults?

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In 2014 we used motors to move the Catapult arm.
It was loosely based on a quick robot build.

We used 2 CIMs and 2 mini CIMs for power.
This setup was able to launch a yoga ball about 8 feet.
No mechanical energy storage.

It was very simple to build. Just the motors, transmission, chain/sprockets and the arm.

I understand your desire to go motors over pneumatics, but mechanically the cylinder that pushes up the arm seems simpler and less likely to break than the spring system. That said, 343 has done a winch retracted catapult both in 2014 and 2016 and we were pleased with the results both times, but they have been fairly high maintenance machines especially now that they are abused demonstration robots. We have a ratchet on the motor and a dog shifter on the spool so we can engage the spool to retract and disengage the spool to release. in 2014 we did this with a custom transmission and ratcheting wrench, but in '16 we used an AM sonic shifter with high gear removed and a 3D printed insert with a piece of spring steel where the 2nd motor would go for the ratchet. Our 2014 pin in the dog is a frequent maintenance item (we do it regularly like an oil change so we have less instances of it being dead during a demo) and the spring steel needs readjustment frequently in our 2016 robot. Most of the issues that come with catapults (really no matter the design) is the violence that they require. We try to avoid having hard stops on our catapult arms (we instead make sure the ball is completely released before the arm has completed its spring-allowed travel) to help reduce the jarring, but it is still an issue.

I will add that pneumatics are a great tool for catapults, and a off-season robot is the perfect time to practice using pneumatics. Whether you go with a straight pneumatic catapult or a pneumatic retracted spring catapult, catapults only need to maintain 2 positions and they actuate with a fair amount of force. Those are the strong suits of pneumatics. Sure you could use motors, but why make life harder for yourself?

Team 2052 in 2016 is a good example off the top of my head.

3098 in 2014 had what I feel to be, and even simpler solution than the choochoo mechanism. I have seen something like this in industry, but the simplicity is really neat.

It uses a chain attached trolley that runs in a track that retracts catapult. There is some good on robot video of it here.

The chain has two trolleys attached to it, so as one is exiting the track for a shot, one is entering the track at the pivot end. The entrance of the track is near the pivot end, so it is always picked up.

Check out the Ri3D teams from 2014 since some of them had some pretty clever designs. Additionally, check out 148s 2014 robot since I believe that it used springs to power their catapult.

My team used a variation of the winch design on a spring loaded catapult in 2016. We had a second arm attached to the catapult pivot axle that was connected to a motor and gearbox. The arm could push the catapult arm down, where it engaged a latch, then move back up and out of the way before the latch released. This eliminates the annoying cable/ webbing management of a winch design. We designed a clutch to make it possible to use the secondary arm as a hard stop without destroying the gearbox, but we never finished implementing it.