Build Your Own Combat Robot

Chapter 7: Controlling Your Motors 145
means that each of the poles of the motor’s armature has 16 turns of wire wrapped
around the pole. As the number of turns decreases, the diameter of the wires increases,
which results in a higher torque motor that has a higher current draw.
Current Capacity in Hobby ESCs True current capacity of a hobby ESC can be
difficult to determine; and the ratings given by the manufacturer are generally misrepresented,
since they reflect the instantaneous peak current capacity of the semiconductor
material in the FETs rather than a realistic measure of the current the
controller can handle. Real current capacity of a hobby motor controller will be
determined largely by the builder’s ability to ensure that the little heat sink on the
speed controller stays cool enough to keep the electronics inside from cooking.
Since most hobby controllers are designed for low-average currents and with a
high airflow in mind, continuous high-current operation will likely cook a hobby
controller even with cooling fans installed.
Many of the cheaper hobby controllers are non-reversible, which means that
they’re designed for running the motor in one direction only. These controllers
should not be used in a combat robot. Hobby controllers that are reversible usually
have a lower current rating in reverse than in forward—the FETs used in the
reverse-going side of the H-bridge have a lower current capacity than the forward-going
FETs. Many hobby controllers designed for R/C car or truck use have
a built-in reverse delay, so that, when the throttle goes from forward to reverse
quickly, the controller will brake the motor for a preset interval before starting to
reverse. In an R/C car, this helps controllability and lengthens the life of the motor
and geartrain; but in a combat robot, it can make smoothly controlled driving difficult—if
not impossible.
Many hobby-type controllers have what is known as a battery eliminator circuit
(BEC). The speed controller contains an internal 5-volt regulator that generates
the power for the electronics inside the speed controller. This power is then fed out
through the ESC with the intention being the ability to power the R/C receiver
from the main drive batteries. While this is a great help in an R/C car, where the
extra weight of a radio battery can make a real performance difference, the more
powerful drive motors of a competition robot create a lot more electrical noise
that can cause radio interference in the receiver. A robot builder can defeat the
BEC by popping the power pin out of the ESC’s servo connector and then use a
separate battery pack to supply power to the receiver.
Hobby ESCs in Combat Robotics Hobby ESCs have been proven to be usable in
small combat robots. These are usually seen in weight classes of 30 pounds and
under, but rarely in larger robots. Determining the appropriate hobby controller
can be a challenge. If you enter a larger hobby shop that specializes in model boat
and car racing, or check out catalogs or Web pages of some of the main suppliers,
you will find literally hundreds of models to choose from. Your first instinct may
be to talk with an employee for advice, but keep in mind this person might know a
lot about cars and/or boats but absolutely nothing about the use of ESCs in robots.