With brushless motors easily
delivering over 10,000 RPM, I have
seen robots with weapons spinning so
fast that they just can't get any bite on
their opponent. This makes an
otherwise great design ineffective.
Although you sacrifice some energy by
reducing weapon speed, it may
significantly increase the overall
effectiveness of your weapon. It
doesn't matter how much energy your
weapon can store if it is unable to
transfer it to the opponent.
There are a few major factors in
designing your weapon: weapon
speed, diameter, material, and mass.
So, let's spin it faster and make it
bigger. Remember to consider the
trade-offs of each design decision
(there is always a trade-off). For
example, if you make it heavier, you'll
have more energy in your spinning
weapon, but it may increase your spin-up time or you may need to sacrifice
armor to meet the weight limit.
The biggest advantage to using an
asymmetric weapon is that it allows
you to spin your weapon faster
without the trade-off of reduced bite.
In the example that we used earlier,
our weapon design achieved 0.27
inches of bite with two teeth. If we
instead use an asymmetric weapon
with one tooth, we could double the
weapon RPM in Step 2 and still get
the same 0.27 inches of bite.
The relationship between kinetic
energy and weapon speed is
exponential: kinetic energy = 0.5 *
(mass) (angular velocity) 2. This
means that doubling your weapon
speed (angular velocity) quadruples
your kinetic energy. This results in a
bigger advantage than doubling the
weight (mass) of your weapon!
Sound too good to be true? One
trade-off with choosing an asymmetric
weapon is that it is more complex to
design, but it is well worth the effort.
Before you start your design, it is
important to understand the concept
of "center of mass."
Imagine you take an object and
try to balance it on the tip of your
index finger. The point at which the
object balances on your finger without
falling is the center of mass of that
object. In other words, the center of
mass of your weapon is the point that
represents the average position of the
mass of the weapon.
In your design, it is critical for the
center of mass to be at the center of
rotation (this will be the center of the
weapon shaft). Figure 3 shows the
center of mass on an asymmetric disk
with no counterweight. This may seem
like a small deviation from the center,
but it will make your robot completely
uncontrollable once your weapon is
spinning at thousands of revolutions
per minute. Typically, most weapons
have two teeth directly opposite each
other, so one tooth offsets the weight
of the other tooth. In designing a
weapon with only one tooth, you have
to counterbalance that tooth to
balance your weapon.
Don't worry. With the help of a
CAD program, this is easier than it
sounds. In next month's issue, we'll
take a set-by-step look at the process
of designing an asymmetric disk using
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