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Originally Posted by James Critchley
This is all "correct," but clearly it is not "right." There should be some kind of free speed loss, so it's not a very good model.
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Agreed. That's why I found StevenB's suggestion alluring. It derates the free speed AND the stall torque, and creates a speed vs torque line for which the output power at every operating point is derated by the efficiency. That seemed like a reasonable thing to do. The open question is: Does it do a useful job of modeling the actual behavior of the motor+gearbox combo? At this time, I do not have access to robot parts or lab equipment. If I did I would be running some tests right now.
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Your initial post asked for "wisdom" and John has delivered that.
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John's answer was a good one, and it answered a question I was wondering about... but not the one I asked in this thread, to wit:
"What is the accepted wisdom here on CD of the "correct" way to model a motor plus gearbox combination". His model is for the whole vehicle; not the motor+gearbox separately.
Another thought occurred to me as I was back in the woods this afternoon pondering robots while mowing the walking trails. Given: Four wheel 120-pound vehicle with no drivetrain attached (just free-spinning wheels) on a flat, level, carpeted surface. Roughly how much force should it take to keep the vehicle moving at a constant speed of 13 feet per second? i.e. how much force to overcome rolling friction and wheel bearing friction? I'm thinking about 10 pounds. Does that sound about right? More? Less?
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I'm interested to hear how the banebots data works out.
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Indeed.
It would also be enlightening if someone would measure the free speed (or free current) of a stand-alone CIM and a CIM mounted in a Toughbox.