Build Your Own Combat Robot

Chapter 14: Real-Life Robots: Lessons from Veteran Builders 319
should be sufficient for our robot’s motors. The internal resistance of the motors
was measured and the calculated stall current draw would be about 110 amps. I
estimated that the normal running current would be about half of the stall current
(just a guess); so the Victor 883 should work, as long as I didn’t push the stall current
rating. I ordered three of the Victor 883’s from IFI Robotics. (I needed only
two of them, but I ordered a third for a spare in case I burned one out.)
Instead of having one set of batteries power both motors, we decided to have a
set of batteries to power each motor. We used three 6-volt 7.2Ahr Panasonic
sealed lead acid batteries to power each motor. We chose these batteries because
they fit inside a 4-inch cavity requirement of our robot. They were not selected
based on their capacity. Because these batteries would be used up in each match,
and they were not the fast-charging type, we also purchased three battery chargers—and
a total of 24 batteries for the contest. We planned on swapping out six
batteries at a time between matches and recharging the batteries later. (Special
note here: what ever you do, don’t let your spouse find out that you spent $98 for
priority shipping, and you ended up not needing the batteries the next day.)
For the radio, I went against what all the experts say. I used a regular FM radio
control system. I was able to get a ground legal 75-MHz, four-channel radio from
Tower Hobbies (www.towerhobbies.com—a great place to get R/C equipment) for
$140. I didn’t want to spend a lot of money for a 72-MHz PCM radio, since that
was outside our budget. For servo mixing, I built a custom microcontroller-based
mixing system that had a built-in failsafe feature. I didn’t think I would see too
much radio interference, and the mixing circuit would protect the robot with its internal
failsafe feature. I also ordered two additional sets of frequency crystals in case
of a radio-frequency conflict at the event.
Step 5: Layout and Modeling
The rules from the contest said that the robot must fit inside a 48-by-48–inch box.
This placed a maximum geometry constraint for the robot. We decided that we
wanted the robot to fit inside a 36-by-36–inch box. We laid out how the motors,
gears, and wheels would look on a piece of wood (see Figure 14-9). Since the length
of the motors and gearboxes was 11 inches, we couldn’t directly attach them to the
wheel axles. We decided to use a two-motor approach to drive all four wheels.
Because one of the goals was to make the robot a rapid maintenance design, I
designed the robot to be symmetrical about the center of the robot. This way, one
part could be used in four different locations in the robot. After the plywood
board layout was completed, the first set of aluminum structural parts were cut out
with an abrasive waterjet. A set of 1-inch-thick aluminum standoffs were cut for the
pillow blocks so that the center line of the wheel axles would be at the same height
of the motor mount axles. The base plate was made out of a 1/4-inch-thick piece of
1100 series aluminum. (Whatever you do, don’t use 1100 series aluminum in your
robots. This is one of the softest forms of aluminum you can get. I used it because I
already had a big sheet of it, and I didn’t want to spend any more money on the robot.)
Figure 14-10 shows the next step of the fabrication process.