Page 100 - Build Your Own Combat Robot

It’s All About Power
Chapter 5:
from which to draw the current. You might ask yourself, “Do I really need to buy a 81
battery to test what size battery I need?” Yes, you do, if you want to be able to mea-
sure the current draw. The battery voltage of this test battery must not droop while
testing for the current draw. In other words, the voltage must remain constant
throughout the tests. The advantage of using a large lead acid battery for the current
draw tests is that, because it will provide a long run time, you can use this battery
during the initial testing phases of the robot. After you have selected the appropriate
batteries for your robot, you can use them for all of the final test phases.
In most cases, fighting robots will draw a lot of current—much more than the
maximum current rating of most multimeters. The best tool to use to measure the
current draw is a high-current ammeter capable of measuring more that 100 amps.

Using Ohm’s Law to Measure Current Draw

You can also measure the resistance of the motor and calculate the current draw
from this measurement using Ohm’s Law. The formula to do this is current = volt-
age/resistance.Thisformuladoesn’tnecessarilyprovideareliablemeasure,however,
because, first, the resistances are very low for competition motors and most ohm
meters are not accurate at such low resistance levels. Second, if this measurement
is made accurately, it must be made considering the resistances of the complete
wiring harness, motor drivers, and motor. Last, even if the measurement is done
accurately, the calculated current will be much higher than actual due to frictional
and heat losses.
In all fairness, if measured accurately, the peak motor currents can be deter-
mined using an ohm meter and this formula:

5.1

Here, the current, I, is in amps; the voltage, V, is in volts; and the resistance, R,
is in ohms. To use this method, place a high-power, small-resistance-value resistor
in series with your robot’s battery supply. Then, using a voltmeter, measure the
voltage across this resistor.

Suitable Resistor and Measurement Basics

If you have access to a low-value, high-wattage resistor, you should use it to per-
form your measurements—but resistance, high-wattage resistors are hard to find.
The resistance should be less than 0.01 ohms. If your motor’s expected peak cur-
rent draw is 100 amps, you will need at least a 100-watt resistor. If you don’t have
access to such a resistor, a 0.01-ohm resistor can be made with 6.2 feet of readily
available #12 copper wire. The wire needs to be slightly longer than 6.2 feet, but
you can connect the voltmeter at the place on the wire that is 6.2 feet from the bat-
tery. In addition, it is a good idea to keep the insulation on the wire and to coil up