Gear Motors vs. Servos

I’m a software guy that is learning about designing/building robots. My son asked me to help him with an upcoming robot competition. I’m having a hard time in determining whether to use motors or servos to drive the 2-wheel robot.

The robot will be about 4 pounds.
The wheel diameter is 3 inches.
I’m using Pololu’s dual serial motor controller along with Parallax’s Javelin stamp.
I have Tamiya gearboxes (plastic gears) that have 3V toy motors.
The competition will be conducted on a flat wooden surface (no incline).
It would be pretty sweet if we could move the 4-pound robot at least 6 inches per second.

My struggle:
Do I upgrade to 6V motors?
Do I need to upgrade from Pololu dual serial motor controller to provide more current?
Is there a tutorial that shows how to use math to help determine proper torque/motor selection without having to obtain a degree in mechanical engineering?

Any help would be greatly appreciated!!!


I think RC hobby servos generally aren’t a very good option. They are not made for continuous rotation, so you will have to either modify them yourself or buy them modified. You’re then paying for a feature you’re not using (feedback), and you don’t have as good control as you would have with something like one of our motor controllers. I think four pounds is a lot for cheap servos, and with better servos, you lose the price advantage.

Given what you already have, I think the way to go is to get the Solarbotics RM3. This is not necessarily an upgrade, but it will allow you to use your existing motor controller with the Tamiya gearboxes, which are nice for playing around and changing gear ratios.

I don’t know of the kind of tutorial you’re looking for. However, I doubt you can do much of what you want with just math. There are many factors to making a controllable chassis that you won’t have measurements for (e.g. all kinds of friction, how much acceleration you want). In general, I recommend playing with Lego or Vex sets to get some idea of what kind of performance you can expect, plus you have many options for reconfiguring your robot.

- Jan