# Inertia for Field and Tile Shooting

It seems to me that a low moment of inertia in a flywheel, single or double, is beneficial for a base shooter (using DCLs), but a high moment of inertia is beneficial in a field robot, especially close to the low goal bar. I say that a low moment of inertia is beneficial for tile shooting because a 10 second spin up time is absurdly high, and a low moment of inertia allows for a faster response to speed adjustments. Accuracy is more important than speed at long range, although you can have both. At short range, you can use the time spent traversing the field and collecting balls, so you aren’t waiting on the flywheel to spin up, and you generally don’t need to adjust speed significantly. A higher moment of inertia means the wheel doesn’t slow down as much as you shoot, so a high enough moment of inertia allows you to shoot 4 balls in quick succession without waiting for the wheel to recover.
What do you guys think?

I think it’s really a balance. I found that balance with two five inch wheels spaced apart by an inch. It’s really a combination of the inertia of the whole system, the control systems, and how well that it’s built. I’m testing something with lower inertia over the next few weeks on my flywheel to see if it can work better, and you may be right that lower is better for full court, but I still believe you need a good amount of moment. I went from a single wheel to a double wheel and my fire rate almost doubled because the ball took so little of the wheels energy for the double compared to the single.

Test different types of wheel and have a velocity graph record your results.

@lpieroni It is true that you don’t want flywheel to take 10 sec to accelerate, but you don’t want to have very light wheel for the base shooter either. Regardless of how precise you can regulate the speed of the flywheel, the only way to ensure that balls of variable density and mass all score into the high goal is to have moment of inertia large enough.

If you have a way to adjust moment of inertia, I would suggest to run this experiment:

Find two heaviest (firmest ), two lightest (softest), and two average balls.

Launch each ball 10 times, giving flywheel enough time to get to stable speed. If you see heaviest and lightest balls consistently overshooting or undershooting, than you need to increase the moment of inertia until they start to score.

On the other hand if that doesn’t help then you need to tweak your compression and go back to the previous step to find moment of inertia cutoff.

For example, our flywheel could score very consistently if we give it enough time to stabilize the speed. However, if we rush exactly the same set of balls, then it starts to miss some of them. This tells me that our moment of inertia is just large enough to counter ball variability with stable speed, but is very close to the edge. The combination of out-of-spec ball + slightly out-of-target speed causes it to miss.

Everybody wants to feed the balls faster, so if your speed control cannot be more precise, then you could either increase flywheel’s moment of inertia and add motors to keep spin-up time tolerable or simply accept the fact that some of the balls will miss.

Interesting. I suspect that the reason for the fire rate improvement is that you have widened the bounds of the min and max velocity values that would still let you score. Please, tell us do you use any speed control algorithm for that launcher?

Quite honestly I came home from our FL state championship and thought building a launcher with more mass would help. We have a double flywheel and I noticed that when adding more mass the motors couldn’t rev the wheels back up to speed quickly enough. The result was a quick three shot burst and then a two second wait to come back to speed. In the end you need a perfect balance of mass and motor power. While more mass should mean less recovery time, if your motors can’t bring the wheels back to speed you’ve kind of defeated the purpose.

Not really, motors only replace energy lost to launching the ball. If you are feeding a constant stream of balls then recovery time almost doesn’t depend on the flywheel’s moment of inertia (accuracy still does).

However, if you are doing quick bursts of several balls in the field, then yes larger moment of inertia will help you a lot.

That’s a very good point, moment of inertia (also called angular mass) is like a mass sitting on the end of long horizontal lever that your motors need to turn. If it is too heavy you will not have enough torque to accelerate it and just end up overheating PTCs.

That’s a very good point, moment of inertia (also called angular mass) is like a mass sitting on the end of long horizontal lever that your motors need to turn. If it is too heavy you will not have enough torque to accelerate it and just end up overheating PTCs.

Exactly, more mass helps with reducing the energy lost in ball launch, but without the motor power to back it up it won’t work.

I’ m using Take back half similar to jpearman’s but with added slewrate control to protect the motors.I can post photos and code for the launcher if you like. I’m also adding feedforward for major overshoot and I get to test it today.