Catapult vs Wheeled Shooter

[happyWobot]

Originally Posted by 3072Cap

I'm curious on what went through your heads when you were deciding between a catapult and a wheeled shooter when designing your robot. Team 3072 went with the wheeled shooter so that an opposing robot couldn't hit us hard enough, then dislodge our ball out of our shooter. I've noticed about an equal number of top level robots that have both designs.

That's actually a more complex question than it might seem. Several things first:

Cons - catapult

Catapults take up a lot of space.
Energy transfer and release can be problematic without the right equipment
Dependent upon the stored energy you use, it can take time to replenish
Release angle can be limited without extra articulation or features
Need extra systems to load it

Pro-catapult
Energy transfer is more consistent because the game piece is in contact with the catapult longer.
Simple and reliable if built right

Cons -wheeled
The need to understand the relationship between wheels, game piece, compression, energy loss variability, damage to game piece, flywheel effect, flywheel size, flywheel weight, speed, and loading.
The need to understand the difference between speed, torque, motor kickback, and density of game piece
Outside the robot they are vulnerable to damage
Enormous amount of energy required to articulate if on an arm
Harder to set at precision angle
Harder to design especially if outside perimeter

Pros - wheeled
Instantly reset for use
Can perform both ingress and egress with same design
Variable control over power
Variable control over angle if on a pivot (can batter shoot or long shoot for example)



Someone mentioned Aerial Assist. Thats agreat example. That game piece was very elastic, it needed lots of time in contact with the energy being transferred so it could retain it. That's a really good example where a catapult is better

But you have to consider other things. In this game particular chassis comes first, not the shooter. We used large wheels with a high chassis floor. A narrow frame so it would not get trapped going through a defense at angle. After that it would not been difficult to make a catapult but would have been difficult to load it. Tall wheels take a lot of space. Point is the wheeled shooter was the best choice in this game for us based upon how we wanted to play the game and the kind of chassis we used. The bumper rules also drove that decision as well forcing the shooter outside the frame.

But as for how difficult it is to make a wheeled shooter I wouldn't say it's that hard. Several things though.

Small wheels are harder than large ones. Weight on outer perimeter of wheel is more important than its diameter. Rpms is not enough. You can pair a mini cim direct with a heavy 6-8 inch wheel or a high rpm 775pro at 4:1 with a light 4 inch wheel. Either will work. But a mini cim with a light 4 inch isn't going to shoot far. The heavy wheel reduces kickback. And the amount of compression on the ball and the contact material between wheel and ball matter significantly. A lot of energy loss can be taken there if it's the wrong contact material. And no matter how much math you throw at it, you can never be prepared for the variability in the balls surface, density and elasticity characteristics. You have to adjust everything to take that into account but it's more gut, intuition, and feel than science. In that regard I recommend you listen to what your bot is telling you. Literally. Listen for the kickback, the smoothness of egress, grinding, etc. Dust and marks on the ball tell you a lot too.

Finally some one mentioned 401. They were loading in at one event and their students brought the bot in to the pit. I asked one of the students about it and he looked at me with this serious look and said "this thing is scary". Was he not joking. Thats a wheeled shooter that doubles as a Howitzer. They overcome a lot of the game piece variability by shooting in a straight line but they need high precision control over angle to do that and hit the mark. Their design is really quite remarkable. Ballistic trajectory is what most people including us use, 401 is shooting laser beams.

The point? Can't shoot "laser beams" with a catapult. Only ballistic. If you store enough energy to catapult in a straight line its probably pushing the limit of legal. Besides even if you do it would be crazy difficult to control angle for threading the needle the way 401 does it.You use catapult or wheeled shooter depending on what you are trying to accomplish. Neither is better than the other until you define what it is you want to do. But that has to take into account all aspects of your design including chassis and navigation.