good speed, and a big hitting drum.
So, how do you beat what is
perhaps the ultimate drum bot? Well,
since combat robotics is a game of
rock, paper, scissors, and Cataclysm is
the ultimate scissors, then what is
needed is a better rock. I faced the
same problem a few years back when
Grande Tambor was dominating the
Beetleweight class and the answer
then was my wedge/brick Trilobite
(Figure 2).
Trilobite can be fitted with a
variety of wedges in order to match
up to whatever is its opponent's weak
point. It worked, with Trilobite
winning its very first competition and
defeating Grande Tambor a couple of
times in the process.
So, the question is can I build a
12 lb version of my 30 lber? The first
problem is that there is no equivalent
motor for 12 lbers to the Kitbots’
1,000 RPM motors I use in Trilobite.
The most obvious one to use is
BaneBots P60 with RS-550, but if you
use one per wheel the bot is forced to
become excessively wide which just
adds volume and weight. So, that
leads to using only two motors and
using a belt drive to drive the other
wheels. I have used this configuration
a couple of times before. My
Hobbyweight wedge CheepShot 3.0
and my 30 lb Featherweight
Xhilarating impaX used it with
considerable success — including a
National Championship for both in
2006.
The arrangement can be seen in
Figure 3. A motor at the front drives
the front wheel on that side and the
rear wheel on the same side through
a 5 mm HTD belt drive. Similarly, a
motor at the back drives the wheels
on the other side.
CheepShot 3.0 used cordless drill
motors; performance was adequate
but not scintillating. Its big brother
Xhilarating impaX used a pair of Mini-EV motors (RS-800s?) with planetary
gearboxes (Figure 4) that are no
longer available. Performance with
these motors was very good — even in
a 30 lber.
What I would like to do is use
these more powerful motors in a
12 lb design. They proved very reliable
in the Featherweight, but I only
have a pair of the gearboxes
and only one spare motor.
BaneBots does a P60 gearbox
with a RS-775 (Figure 5); if the
design could take those as well,
that would be a useful backup if
the old motors ever fail.
The design all starts in the
CAD model. I used Solid Works,
but any good 3D CAD software
would do. Trilobite uses 1/2"
thick UHMW main chassis
members, so I went with 1" on
the new bot design.
It should be noted that
scaling up or down a design
from one weight class to
another is not as easy as it
would seem because strength
goes up by a square rule, while
weight goes up by a cube rule.
So, in very general terms, a material
twice as thick is four times as "strong,"
but if all the dimensions are doubled
it’s eight times as heavy!
I want the chassis rails to be four
times as strong because it's in a
weight class that's four times as
heavy. However, I certainly can't afford
for them to be eight times as heavy,
so it's important to keep the bot as
small as possible to keep the weight
down.
The rails have two machined
recesses (Figure 6) to allow the width
of the bot to be kept to a minimum.
The alternate motors fit the same
recesses, as well (Figure 7); it can be
SERVO 01.2014 31
FIGURE 2.
FIGURE 3.
FIGURE 4.
FIGURE 5.
