There are several ways to decrease “speed-up” time on flywheel launchers.
Add weights. Inertia is the tendency to do nothing or to remain unchanged, and one way to increase moment of inertia is to increase the mass of the object. Look at the equation for the moment of inertia of a cylinder: I=(1/2)MR^2. Moment of inertia increases at a rate directly proportional to the mass of an object.
Use a larger wheel. Using the equation I=(1/2)MR^2, we can see that increasing the radius of a cylinder increases the moment of inertia exponentially.
You can use these two methods in combination to reduce the amount of weight you add to your flywheel. Because moment of inertia increases exponentially with radius it is much more weight-efficient to have a large-diameter weight. In short, make a big, heavy flywheel.
Use more motors. More motors means more torque, which means faster acceleration of the flywheel between launches.
Programming. Things like PID, which adaptively adjust motor power, allow for quick and accurate adjustments. For example, say your motors are running at a power level of 80 and at 80 rpm. When a ball is launched, the power level stays the same, but because of the increase in load, the motor slows down to, say, 40 rpms. What PID would do is momentarily increase the power level from 80 so that the time for the motors to bring the rpm back up to 80 is reduced. Once the motors have reached that speed of 80 rpm again the power level will drop back down to 80. For PID, or any other type of velocity control, the only sensor you will need is some kind of encoder–a way to count rotations. There are numerous threads and posts on how to write velocity control code.
Ikenite’s suggestions are very good ones. You can think of the spinning mass of your flywheels as an energy storage device. Once you spin up the flywheels, they contain a certain amount of energy, and each ball that you shoot will steal away a portion of that energy. In that way, a spinning mass is analogous to an electronic capacitor. The bigger the capacitor, the more energy you can store. The more energy you can store, the less effect each launched ball will have on the speed of your spinning flywheels.
But there is a downside to increasing the mass (moment of inertia): it takes longer to “charge up” the system when you begin from a standstill. If you add too much moment of inertia, your start-up time can take longer than you might want. So when it comes to adding mass, you will want to aim for a “goldilocks point.”
Perhaps another thing to consider is the spacing between your flywheels. If the spacing is too narrow, then you will be compressing the balls more than is necessary. And that excessive compression can steal away energy that gets converted into heat. The excessive compression will not be translated into ball velocity, so you might want to check that you aren’t over-compressing the balls. You want just enough compression to adequately grip the balls and send them on their way.
One thing you might want to look into, as mentioned in another post, is the spacing between the hood and wheel. You might also consider reducing the compression time–make the distance the ball travels while being compressed shorter. It really only takes an inch or two to impart the necessary energy to the ball. Increasing the weight of the wheel should only decrease how much the ball slows it down because of the greater moment of inertia.
Do you think our gear ratio is the problem too?
Your gear ratio is faster than ours and a bit stronger. Ours is a 25:1 with a single shooter but with combined two wheels. Would making the weight of the wheel heavier make the recovery time longer?
Making the weight of the wheel heavier would mean less recovery time due to the mass of the wheel. Making a wheel heavier means a longer speed up time, but the time between shots is less because of the reduced recovery time.
I agree that the initial speed up will take longer. However, the acceleration will be less too (f=ma, or something of that sort). The speed up time between shots for all masses will be the same, though the initial acceleration will decrease. I believe that the positive of a heavier wheel is the possibility of it decreasing just a bit, which allows for multiple shots before it is at an unusable speed.
As mentioned before the contact time is another factor in speed up time. Looking for the sweet spot will take hours upon hours of testing; it is not an easy thing to do.