or bearings can handle. On some gearheads, you will see a
rating for overhung load. This is the maximum weight the
manufacturer recommends you support from the output
shaft. Another consideration is the torque limit of the
gearhead. This can be less than the motor can actually
supply. If you exceed this value for any length of time, you
could destroy your gearhead. Better motors use better
gearheads. One way to tell the quality of motor you are
using is by the noise it makes when running. The louder the
motors, the less efficient the gearhead motor is. High
quality gearheads are very quiet, but they are also
Battery selection is another key aspect of getting the
most out of your drive motors. The right battery will need
to supply current for the desired robot run-time. Batteries
are specified with two key values: voltage and amp-hours.
An amp-hour is defined as the amount of time — in hours —
the battery will supply one amp of current. To get an
estimate of how long your robot will run on a given
battery, just take the battery rating and divide it by the
average current draw of your robot. Do not forget this
should be for the entire robot, not just the drive motors.
For example, my robot has a 12 amp-hr battery. My robot
normally draws less than two amps, so I expect to get five
to six hours of run-time with a fully charged battery. To get
a ball park of the power your batteries will need to supply
to your motors, you can use the relationship between
power, voltage, and current.
We’ve completed an exercise in basic motor sizing for a
mobile robot. In most cases, you should shoot for
exceeding your calculated requirements by two or three
times. If you purchase undersized motors, it usually costs
more than purchasing oversized motors because you will
end up buying a second set of motors that are large
enough to supply the power you need.
The free body diagram and basic physics is a great
place to start when sizing a motor in any situation. Our
example was very simple; you can always add more detail
to the free body diagram. This can help you answer other
questions. For example, how much of an incline can your
robot handle before it tips over?
If you want more information, a great source of
information is the book Mobile Robots – Inspiration to
Implementation by Joseph L. Jones and Anita M. Flynn. The
Zagros Robotics lab notes section also includes information
on many topics, including DC motor selection.
( www.zagrosrobotics.com). SV
Tips For Selecting DC Motors
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