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I've built a simple robot that can move. I'm using brushless motors. I've realized that at slow speeds the motors pull a lot of current. I didn't understand the power / current / efficiency charts when I started. I see it in the charts, now, that the efficiency in electric motors is at high RPM and the low speed torque is awesome but comes at a price of high current.

For mobile robots, do people generally just provide high-current batteries for slow speeds, ignore slow speeds, or do they build robots with variable transmissions instead of fixed gearboxes? When I look around I just see designs with fixed gearboxes but I feel like I'm missing something.

If I look beyond hobby type projects I see talk about CVT and the Inception drive but I'm wondering anyone at the home / hobbiest level do anything like that.

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For a simple answer: no, most mobile robots do not use variable transmissions. I believe it is far easier to just design a motor and gearbox combination that works well in your narrow range of desired speeds, than deal the extra mechanical complexity and cost of a variable transmission.

The long answer is that it depends on what you consider a mobile robot. Let's take a quick survey of the range of "mobile robots". This can be a huge list, but I will try to span the space from very small to very large.

Plum Geek Ringo Robot. Brushed motors, no transmission at all!

Parallax BOT Bot. Brushed motors, no variable transmission.

iRobot Roomba/Create. Brushed motors, no variable transmission.

FLIR Packbot. Brushless motors, no variable transmission.

DARPA LAGR Program Robot. Electric wheelchair motors, no variable transmission.

Stanley. Desiel engine with variable transmission.

Tesla Model 3, brushless electric motor with variable transmission.

So it looks like variable transmissions are really only needed when you have gas engines. Or very high performance electric motors.

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    $\begingroup$ It’s worth emphasizing the point at the end of your first paragraph — if the OP is finding that their robot spends too much time in the low speed/high current range, it’s probably worth giving up some high-end speed by putting in a gearbox with more reduction to keep the robot in its sweet spot for a greater period of time. $\endgroup$
    – RLH
    May 23, 2020 at 16:20
  • $\begingroup$ Wow! Thank you both. Those are excellent, informative responses. $\endgroup$
    – mrtidy
    May 24, 2020 at 16:25
  • $\begingroup$ "it’s probably worth giving up some high-end speed by putting in a gearbox with more reduction to keep the robot in its sweet spot for a greater period of time" That is helpful too. I had been only thinking about the top end for the gear ratio but I see now that I need to rethink that! Thanks again. $\endgroup$
    – mrtidy
    May 24, 2020 at 16:28

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