up a slight gradient. This is where we
would need to make lots of empirical
measurements (perhaps), so let’s
make a guess at this and say an RPM
degradation of 50%.
Now we can say that the effective
RPM of our motor will be 80% of
8,300 RPM and then 50% of that. This
will be our max usable RPM and
calculates out at 3,320 or 55 per second, and dividing this by our required
revolutions per second, we have a
needed gear ratio of 55/4 or 13. 8. We
round that off to 14; and this is the
ratio we will use for gearing the motor.
We might also suggest a final
torque value at the wheel by multiplying the motor shaft torque by 14, this
gives 9* 14 oz-ins or 126 oz-ins. At the
circumference of the six-inch wheel in
pounds, this would be 126/3/16
which is 2. 65 pounds (the 3 in the
formula is the radius). If you were to
use two motors (differential steering),
you might have around five pounds of
force; not bad for a small bot. I hope
this makes sense.
You can go to the website and read
the specs on this motor. See Photo 11
for this setup.
The testing of this motor went
very well and to highlight some of the
• Measured max RPM was about 20
to 25 RPM.
• Amps went from about .1 to . 2 at
max speed (unloaded).
With a little effort and some off-the-shelf products, the hobbyist
should be able to build a Motor Test
Lab/Dynamometer which rivals
multi-thousand dollar versions. In any
case, you have to do something to
understand the motors you are using if
you get them at surplus suppliers,
perhaps just wet your finger and stick
it in the air. NV
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TESTING A SECOND
I’m not going to bore you with all
the details of testing a second motor,
so I will go through some of the highlights. The second motor I tested was
a geared motor I got from Zagros
Robotics, called the Max 96/99/97.
■ Medusa Research Power Analyzer
■ Matrix Orbital LCD —
■ Zagros Robotics gear motor —
■ Electronics Exchange surplus
motor — www.electronics-
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■ FreeRTOS — www.freertos.org/
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February 2007 17