Build Your Own Combat Robot

International Robot Sumo Class
Motors
The general functionality of the international robot sumo class is basically the same as
a mini sumo, except that they are heavier, faster, and smarter. The size of the international
robot sumo class is 20cm square. This is a 4x increase in the area over mini
sumos. The maximum weight increases by a factor of six, to 3kg. This allows for
more powerful propulsion systems, more sensors, and improved microcontrollers,
which increases the flexibility in the designs of the 3kg sumo robots.
Most 3kg sumo bots don’t use modified R/C servos. This is because most
R/C-style servos are not strong enough for the increased weight or fast enough to
rapidly move the robot across the larger sumo ring. Typical motors include
high-powered 12-volt and 24-volt gearhead motors from Pittman Motors, Barber-Coleman;
planetary gearheads from cordless screwdrivers; and the electric
motors from high-performance R/C racing cars. Most gearhead motors are purchased
from surplus stores, because they usually cost 1/10th the cost of a new one
purchased directly from the manufacturer.
When using stand-alone electric motors, you must build a custom gearbox. The
advantage to this is that the gear reduction ratios can be set up to maximize the desired
speed and torque range of the motors. Otherwise, you will have to use what
is available in the regular gearhead motors. The drawback to this approach is that
it requires custom machining of the gearboxes, which can be expensive. Because
of this, most people use off-the-shelf gearhead motors and vary the motor voltage
to get the performance they want.
Motor Controllers
Chapter 13: Robot Sumo 299
Using high-powered motors requires high-powered motor controllers. The motor
controllers are commonly called electronic speed controllers (ESCs). In mini
sumos, the peak motor current requirements are usually around 1 A. For the international
robotic sumo class, peak motor current demands can exceed 100 A.
This really depends on the type of motors selected for the sumo bot. Usually,
higher-voltage motors require less current. The most cost-effective ESCs are the
ones made for the R/C racing car industry. These controllers are designed to handle
large amounts of current for short periods of time. They are also easy to integrate
into a sumo bot.
When looking at electronic speed controllers, make sure that yours has a reversible
speed controller. More than half of the electronic speed controllers made
today are for forward use only. A sumo bot will be spending about half its time going
backward as compared to going forward. The other factor to consider is the
current handling capacity when operating in reverse. Of the ESCs that are reversible,