Page 101 - Build Your Own Combat Robot

Build Your Own Combat Robot
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the wire so that it is easy to handle. Temperature causes the resistance to change,
so use the wire at room temperature and don’t use it so long that it heats up.
1. Place the resistor in series with your robot’s battery.
2. Measure the voltage across the resistor (a 6.2-foot-long coil of #12 wire,
or the high-wattage resistor) with the robot running in normal battle-like
conditions. When measuring this voltage, the value will likely be variable
and may appear unstable. Take the maximum reading, and then take
a reading that appears to be the nominal or average value. The robot’s
motors must be loaded to simulate those of a real battle, or else you will
measure a value that is much too low—up to five to ten times too low
than battle-use values.
3. When you have gathered these voltage values, calculate the current by
placing the voltage readings into the formula current = voltage / 0.01 ohms.
The 0.01 ohms is the resistance of the 6.2-foot-long wire. If you are using
a high-wattage resistor, then substitute the 0.01 ohms for the resistance
of your resistor. For example, suppose that when running the experiment,
you noted a maximum voltage of 1.2 volts and an average of 0.5 volts.
Plugging these values into the formula yields a maximum current value
of 120 amps (120 amps = 1.2 volts / 0.01 ohms) and a typical current of
50 amps (50 amps = 0.5 volts / 0.01 ohms). After you have found the
maximum current value and the typical current value, you have the
information that you need to choose the correct battery for your robot.

Blowing Fuses on Purpose?

An alternative method for measuring current draw is one of the easiest methods
and is fairly accurate. You can use the fuse holder that is in-line with your robot’s
battery to measure draw. Fuses are commonly used for testing, but few people use
fuses during an actual competition. It is usually better to risk an electrical fire than
to blow a fuse and be a sitting duck for your opponent to destroy your bot with
impunity. A blown fuse in battle also means an automatic loss!
To use this method, you’ll need a handful of fuses of various amperages. Start
with a fast blow fuse, and select values that you think it will survive. Install this
fuse and test run your robot in battle-like conditions.

note It is important that you test your robot in battle-like conditions, or else the
measurement will yield a current draw that is lower than what will occur in the robot arena.

Keep changing the fuse values until you find the fuse value that will survive and
the highest fuse value that fails. Between these two values is your robot’s maximum