Motor & Wheel with Encoder Selection

I am working on a small robot (must fit within a cubic foot). The pertinent requirements are that we’re looking for a velocity of 2 ft/sec (60.96 cm/sec), with acceleration of 1 ft/sec (30.48 cm/sec).

I also need to have an encoder on the wheels, so I know for certain that the wheels are all turning the same speed like they’re supposed to.

I also have the requirement that the whole of the wheel and motor have to fit inside a 2 inch radius circle.
The power supply will be about 6V, maybe 7 if needed.
Low cost is preferred…

The current combo that I’m looking at is this one:

Solarbotics RM2 (high-power motor for GM2/3/8/9)
Solarbotics RM3 (Replacement Motor for GM2/3/8/9, Regular Motor 3)

put into this:
Solarbotics GM9 143:1 Gear Motor 90deg. Output

Wheel Watcher Rotation Encoder for GM2/3/8/9

GM Series Plastic Wheels

The chart on the RM2 comparing it with the GM2/3’s motor implies that it won’t be fast enough at 5V, and I don’t think I can get the power up to 9V. Even then I don’t think it’ll be fast enough. There also aren’t torque measurements.
However, I cannot find specifications for the RM3. The page says nothing about its speed or torque.
I need an rpm of about 165 or more with those wheels to get 2 ft/sec.
As long as torque is over 15 oz-ins it should be fine.

Does anyone know the specs of the RM3?
Is it the same motor as is in the GM8/9?

Hello, maxdave.

The RM3 is indeed the standard motor used in the GM2/3/8/9. Since the RM2 typically runs about 3 times faster than the RM3 in the GM2/3 (according to the table), I think you could expect a similar increase in speed if you replace the motor in a GM9 with an RM2 (although we have not tried it ourselves). The GM9 runs at about 78 RPM with an RM3 at 6 volts, so even a conservative 2x speed increase with an RM2 should get you 156 RPM, close to what you need.

- Kevin