So it turns out there is a bewildering array of DC gear head motors out there that one might employ in a robotic adventure. I finally decided on these guys from Electronic Goldmine (they are a surplus dealer so no guarantee of how long the link is good)
http://www.goldmine-elec-products.com/prodinfo.asp?number=G18120
They mount with 3mm metric screws and at 7.5 volts they draw about 0.5 amps and seem to draw less than an amp with a pretty good load on them. We shall see how they fare in actual use. My guess is total stall current is around 2-3 amps. With four of them that makes total stall around 8-12 amps, which is really high but that is a concern with any differential drive or multimotor robot.
So just for those who dont know there are a couple of different motor types and things to consider. Cheap DC motors are designed to run efficiently around 1800+ RPM. The speed of a motor varies directly with the voltage applied (more voltage more speed - until you burn it out). The current a motor draws is a different story. At "free running speed" meaning optimum voltage and no load, is where a motor draws the LEAST amount of current. As the loading increases the current draw increases until the motor hits stall (which is what it sounds like). This is where the most current load occurs. This is true for both brushed and brushless designs.
To slow these little guys down you use a motor controller and pulse-width modulation but even then they tend to run a little fast so you employ a gear head to slow them down. This also has the benefit of increasing the torque and the slower you run the more shaft torque you will have. For really accurate positioning you need to employ some sort of wheel encoder to track the number of wheel revolutions. I dont have time to talk about stepper motors but I am curious to try one for informations sake.
More later about the electronics to run these guys.
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