Fast Line-Follower Motor Selection? What to Look For

I want to build a fast line-follower. From details I seen through Google, it looks like 1 meter per second is atainable with the right hardware and software. My questions…

How do I equate 1 meter per second with a specific motor? I know the math is easy - wheel circumference times RPM can give me that - right? Do I plan for a speed of 1-1/2 meters per second so the motor still has some acceleration for the straight-away?

Now, if I want the least mass possible (more mass = harder to get through a turn fast), then do I want the lightest motor that will give me the RPM I need?

And what about torque? How does that figure in? For a given size/motor weight, does double the torque mean I can increase speed faster? Or does more torque mean more weight? …Or just that from a standstill, higher torque will get me started faster, or what? What would be the trade-off questions I’d ask to select from several torque/speed possibilities?

I plan to use PWM and have a PIC18F control left and right motor speed. I’d start slow and work up speed and add PID as I learn how to apply it.


I recommend that you begin with a slower platform and move up to the 1 m/s target for your second robot. If you use the Tamiya double gearbox, you can start with a higher gear ratio for a slower robot and then re-use the same parts for a faster robot. That gearbox, with either the sports tires or narrow tires, can definitely get you past 1 m/s.

I think the mass doesn’t matter that much, at least on courses like the ones we use for the Las Vegas robot club, which use only 6" radius turns. The biggest difficulty is navigating those turns at high speed, and wheels slipping start becoming a problem. With more weight, you get more friction, so that kind of cancels out. I think the bigger concern is to keep the weight low so that the robot doesn’t flip over when it hits the turns.

I think the torque of the gearbox output doesn’t matter that much, either. Obviously, it matters if you don’t have it, but most gearmotors with a few hundred RPM output will have enough, so your concern will be with controlling the motor. Having more torque should simplify things, especially if you have no feedback about the wheel speed, since your PWM setting will correlate better to the speed you get. You don’t want to get into a situation where 23% PWM will not get the robot moving but 24% will get it accelerating enough that it’s too fast by the time it gets to the end of a long straightaway.

To sum things up, I would look for a small (maybe around a few watts going in) DC gearmotor with a 20:1 to 30:1 gear ratio with wheels around 1-2 inches in diameter, shoot for making the whole thing weigh around half a pound, and just start building it. If I had tons of money to spend, I might consider something like, but I usually prefer to get everything built for under $100 rather than spending three times that just for motors.

- Jan

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