Quote:
Originally Posted by trycatch
What are you using to add mass to the wheel? We're using a pneumatic wheel... I guess we could add weight to the hub somehow..
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A good way is just to add something with more mass to the drive shaft. I have seen some robots that just bore out a piece of solid stock and put that on the driveshaft. Another option might be to use something like an AM performance wheel (made of aluminum, mass is further away from the center).
In a physical sense, you are trying to increase the mass moment of inertia of the shaft driving your shooter (or another shaft in the system), such that the entire system has a greater angular momentum while at the desired operating speed. Assuming that in all cases the fuel is being launched in an identical way, the impulse that the shooter will impart on the ball (impulse is a force over a given time, or alternatively an acceleration over a given time), each fuel fired should "take away" a constant amount of angular momentum away from the system.
Now, here's the fun part. As you increase the mass moment of inertia of your system, you increase its angular momentum at the operating speed. So, when you fire a fuel out of your shooter, that loss in momentum (the impulse applied to the fuel) is a smaller fraction of the total momentum. So, the angular speed (the rpm's) of your system will decrease by a lesser amount. This means that the system is closer to the operating speed after firing a fuel.
To get back to operating speed, the motor has to apply the same energy in all cases, because the fuel still left with the same energy it did is the previous case (neglecting friction because hopefully you used decent bearings). HOWEVER, since your speed loss was less because the fraction of angular momentum lost was less compared to the overall angular momentum of the system, the chances are that your system is closer to or still within the tolerance range for acceptable shooter speeds to reach your target (ie. you can still make your shots because your rpm's only changed a little). So, by increasing the rotational inertia of your system, the speed will decrease less per shot, meaning you can make more shots more quickly while holding your speed within the appropriate ranges.
Now, you still will lose some speed after every shot. This is why you need to design your system such that, after a shot, the controller (probably a PID loop on a Talon) can add more power to the system. By gearing so that the right speed comes at ~50% (plus or minus a bunch depending on your taste) the maximum speed, when fuel is fired, there is more overhead for available power from the motor to be delivered to the system, so you can recover more quickly. Additionally, if you design to run at 100% speed, you run into issues where battery voltage and draw from other robot systems might prevent you from ever reaching your target speed.
So, back to your question of how to add mass (increase inertia):
Add something with mass to a spinning part of the system (think flywheel). As you add mass, the system will take longer to reach speed ("spin up"), but once there, it will remain at speed for a longer time (all the discussion above about momentum). If you are driving the system with something like a VP, the only real place to add mass is on the main output shaft. However, if you are doing a custom system, or have a stage after the planetary like a chain reduction, you could also add more mass before the final stage. The faster it spins, the more inertia it has, so by adding mass to an earlier stage in the system, you can theoretically get more bang for your buck by increasing momentum more while allocating less of your robot's weight to the flywheel. Also, something like a performance wheel is great because the mass is farther from the rotational axis, so its inertia is greater than just a simple hub, even though they have the same mass.
Another thing: be safe. Think about where the mass is located. Can it grab someone's finger? Is the shaft it is spinning on well supported so that the spinning mass doesn't fly apart? Is this much angular momentum safe (see all the spinning Battlebots that are basically gyroscopes that fly all over the place when disturbed)? Is the torque required on startup to great for the motors being used (it is probably possible to have too much mass)?
OK that's a lot of words.
tl:dr;
-spin up as much mass, located as far from the rotational axis as possible, as fast as possible (without creating a spinning death machine).
-gear smart so you have more power available to recover speed lost to firing fuel