Team 539's Robot Shooting!

[Nuttyman54]

Originally Posted by Alex.q

One last question: In our prototype, we have a massive amount of vibration when running the motors that fast. Did you guys encounter this, or does anyone have any sugestions for this?

Most likely this is because the shaft/wheel assembly is not balanced. "Balanced" in this case means the center of gravity is slightly different than the true center of rotation. At low speeds or with light weight things, this isn't a problem, but when you start spinning larger objects like shooter wheels and flywheels at 3000+ rpm, you will start to notice it. At certain frequencies, the unbalanced vibration will match one of the vibration modes of the assembly, and you'll hit resonance (which is really bad).

This can be dealt with in a few ways:

1) Add some kind of damping to your system, like making it much stiffer. Stiff systems (short, stiff shafts, good bearings, etc) will have a very high first mode, making it hard to trigger a resonance. Even if you're not at the resonant frequency, getting close to it can start to cause your assembly to vibrate pretty wildly. You can see from this simple graph the effect of damping on the amplitude of the vibrations, as the driving frequency (the speed the wheel is spinning), gets closer to the resonant frequency (where it peaks).


2) Make sure you have good tolerances on your parts. This will help ensure that your system's CG is as close to the axis of rotation as you can get it. You can also have whatever your spinning be dynamically balanced by a shop that specializes in that. There are specifications depending on how close you need the CG of the wheel assembly to be to the true center of rotation. They will drill holes or add small weights at specific places around the wheel to move the CG around very slightly. Cox & Sprague's website has a good explanation of how this is done.

If you're truly interested in the physics and math behind this kind of analysis, take a look in a book about rotor dynamics, it's got all kinds of cool information and ways to calculate the critical frequencies and see if your mechanism is likely to explode.