Build Your Own Combat Robot

Chapter 13: Robot Sumo 281
were initially chosen so that bots could easily detect the color change and thus recognize
the edge of the ring.
For most competitions, this type of ring is sufficient. The official international
rules specify that the sumo rings be made from an aluminum cylinder with a height
of 5cm and a diameter of 154cm. The top of the sumo ring will be covered with a
hard black rubber surface. The official specification for the surface material is to use
a long-type vinyl sheet NC, No. R289 made by Toyo Linoleum, Inc., in Japan. Unfortunately,
this material is not available outside of Japan, and most vinyl sheet
manufacturers in the United States do not make solid black vinyl sheets over 3 feet
wide. Lonseal out of Carson, California, sells a solid black vinyl sheet that measures
6 feet wide. This material is called Lonstage, and is a flooring material. There are
two different black color numbers to choose from: number 102 is for glossy black,
and 101 is for flat black. Either one will work for the sumo ring surface. Lonseal recommends
their adhesive number 555 to bond the vinyl to a plywood surface.
This material is generally not stocked in other flooring material warehouses,
and you’ll have to custom-order it. This material is fairly expensive, so only use it
on official competition sumo rings. Regular painted plywood sumo rings will
work for all other uses, including testing your sumo bot.
Mini Sumo
Mini sumo robots are becoming the most popular of the sumo classes because
they’re small, easy to build, and inexpensive, and you can easily carry their smaller
sumo ring with one hand. This section will explain how to build a simple mini sumo
bot that will be ready to compete in a contest or just show off to your friends.
Modifying an R/C Servo for Continuous Rotation
The first step in building a mini sumo bot is to modify two standard R/C servos so
that they can rotate continuously around instead of having the normal 180 degrees
of motion. This is a fairly simple modification to make. Use the Hitec
HS-300, Futaba FP-S148, Tower Hobbies TS-53, or Airtronics 94102. If you use
larger servos, then the completed bot will be wider than the 10cm specifications.
To modify the servos, remove the four screws from the bottom of the servo. Remove
the servo horn so that only the small output shaft’s spline is showing. With
your thumb on the spline and your two forefingers under the front and real
mounting tabs, push down on the spline. This will cause the top part of the case to
come off. Figure 13-2 shows a servo with the top of the case removed. You’ll then
see a set of four gears on the top of the servo. Carefully lift the top middle gear off
the center spindle shaft, and set down inside the top of the case. Then pull the output
gear/shaft from the servo.