Page 87 - Build Your Own Combat Robot

Build Your Own Combat Robot
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enough to hold the output shaft still.) Apply a very low DC voltage to the motor—a
much lower voltage than what the motor will be run at. If you do not have a variable
regulated DC power supply, one or two D-cell alkaline batteries should work.
Now measure both the voltage and current going through the motor at the
same time. The best accuracy occurs when you are measuring several hundred
milliamps to several amps. The internal resistance, R, can be calculated by divid-
ing the measured voltage, V , by the measured current, I :
in in
4.14

It is best to take a few measurements and average the results.
To determine the no-load current, run the motor at its nominal operating volt-
age (remember to release the output shaft from the clamps, and have nothing else
attached to the shaft). Then measure the current going to the motor. This is the
no-load current. The ideal way to do this is to use a variable DC power supply. In-
crease the voltage until the current remains relatively constant. At this point, you
have the no-load current value. The no-load current value you use should be the
actual value for the motor running at the voltage you intend to use in your robot.
After conducting these experiments, you will now have all of the motor con-
stant parameters to calculate how the motor will perform in your robot.

Power and Heat
When selecting a motor, you should first have a good idea of how much power
that your robot will require. A motor’s power is rated in either watts or horse-
power (746 watts equal 1 horsepower). Small fractional horsepower motors of
the type that are usually found in many toys are fine for a line-following or a
cat-annoying robot. But, if your plan is to dominate the heavyweight class at
BattleBots, you will require heavyweight motors. This larger class of motors can
be as much as 1,000 times more powerful than the smaller motors.
A small toy motor might operate at 3 volts and draw at most 2 amps, for an input
requirement of 6 watts (volts × amps = watts). If the motor is 50-percent efficient,
it will produce 3 watts of power. At the other end of the spectrum are the robot
combat class motors. One of these might operate at 24 or 48 volts and draw hundreds
of amps, for a peak power output of perhaps 5 horsepower (3,700 watts) or more.
Two of these motors can accelerate a 200-pound robot warrior to 15-plus mph in
just a few feet, with tires screaming. One 1997 heavyweight (Kill-O-Amp) had
motors that could extract 1,000 amps from its high-output batteries! The power
that your robot will require is probably somewhere between these two extremes.
Your bot’s power requirements are affected by factors like operating surface. For
example, much more power is required to roll on sand than on a hard surface.
Likewise, going uphill will increase your machine’s power needs. Soft tires that
you might use for greater friction have more rolling resistance than hard tires,