Hi guys, my team and I have a problem, and someone out there has a solution.
My team is using a 2-ball catapult, and we have it set up with kind of a snowman of gears.
The smallest/top gear is the one motorized by two torque gears
The medium/middle gear is two gears side by side, one is a normal gear meshed with the top one, and the other is a slip gear meshed with the bottom one.
The biggest/bottom one is the one attached to the arm.
We have a shaft encoder attached to the middle axle to help with keeping the arm in the right position while loading.
Our problem is this: we have to use a single wire encoder(we have no quadrature encoders and we don’t have time to get any), which counts a positive tick for both directions, so we can’t tell if the arm is too low or too high. This means we have no way of coding it to stay in the same position against the pull of the rubber bands.
We could just give it a constant power to the motors, but then it might be too much or too little due to battery issues.
We could build a ratchet system on the middle/medium gear so that it doesn’t even have any stress at all keeping it in place.
We could build a mobile physical stop that slides in to block the arm when it loads, but slides out when it fires.
What do you think? Let me know your opinion!
Hello!
I would suggest using a potentiometer on the arm (your largest gear, which is on the bottom). This way, you measure the exact position of the arm rather than counting ticks. This was my solution when I built a 2-ball catapult, and it works well as far as I am concerned.
I would also build a ratchet (however you want, it doesn’t really matter) to not put as much strain on the motors. This way you simply shut off the motors when the catapult reaches loading/firing position, and the ratchet holds it in the correct place.
Hope this helps (and that you have a potentiometer to do it)! 
Thanks, I think we have some potentiometers, but the problem is I think our arm rotates more than 90 degrees. I will try bot though, thanks for the help!
potentiometers rotate 265 degrees?
Oh… OK then, thanks!
When installing the potentiometer make sure not to go past the two end points - if you force it, you can break it.
yeah, the rubber bands can tend to do that…
It works!! I put a potentiometer on it, and it works perfectly! there is no slop or anything, and it is exactly 91 % accurate for full-court shots! I didn’t put on a ratchet, but we drove with it for multiple hours and it burned out exactly 0 times.
Awesome! Good luck at your competition whenever it is.
Thanks for the help!
My comp is today, i hope our bot is enough, one of the teams there is 929u, they were worlds finalists last year
Best of luck! With BO1, maybe you can come up with some crazy strategy to throw 929U off their game. Won’t be easy.
yup good luck
We won skillz!!! If you have ever heard of 929U, they are a team made mostly of seniors last year who went to the dome at worlds last year, and they have V5. My team is all freshmen, so this is our 1st time with vex edr, and we are using cortex. We tied 929U 21-21 in driver skills and beat them 8-7 in programming. Thanks for all the help, we’re going to states!
we were also finalists in teamwork, but 929U beat us in the last match