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Help - DC Motor Selection
QUESTION:I am in the process of designing an mobile robot. The robot will use
two motors for differential drive. I need some help to determine the
motor specifications. The robot should weigh less than 50 lb. and will
be used in an outdoor environment (My yard). The terrain is not very
steep or rough. The drive train has a 3.4:1 reduction so far (more can
be added with the motors), and the wheels are 8 in. in diameter. What
process can I follow to determine the ratings for my motors. I know how
to determine the RPM required but am having trouble with the torque
specifications.
I also need a source for inexpensive motors and reduction gears. I
would like to get the lowest cost drive system that will do the job.
ANSWER: The "easiest" way to do this is to buy an electric car designed for
children. The local toy store sells them and they are designed to
carry up to about 70 lbs over moderate terrain. (moderate = sidewalks,
compressed dirt).
You might consider looking up "Mobile Robots: Inspiration to Implementation"
by Jones and Flynn. (published by AK Peters) It has a good introduction
to the physics behind mobile robots and a discussion for calculating
motor requirements.
If the limit of 50lbs on the robot is a hard limit, I'd start by subtracting
off things that you know the weight of. Electronics, drive train, body,
batteries, etc. Of course depending on how long you want this run will
affect what your batteries weigh. Once you've subtracted off all of these
things, get the largest motors that come in under the weight limit. If
they won't power your 'bot then you'll have to figure out if weight is
more important or run time is more important.
Surplus places are most inexpensive and most unpredictable. Check the
FAQ for a variety of sources.
I have found from building a four wheel drive robot that uses skid-steer
(forward on the left two motors, backwards on the right two spins the
robot in place (to the right )) The torque required to skid steer is
at least 10 times the amount of torque needed to move.
If you use some sort of steerable wheels you can get away with much
smaller motors.
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