I have already selected and purchased the gear motors I plan to use for the robot in the mechatronics course. They come with shaft encoders, metal mounting brackets, and wheels, and cost me $25.51 for two motors with shipping from AliExpress. The specs sounded very promising:

Wheel diameter: approx. 65mm

Shaft diameter: approx. 4mm

Voltage: DC 6V

Speed: 210 rpm

Encoder motor end: 11 signals

Rated voltage: DC 6V

No-load speed: 210RPM 0.13A

Max efficiency: 2.0kg.cm/170rpm/2.0W/0.60A

Max power: 5.2kg.cm/110rpm/3.1W/1.10A

Stall torque: 10kg.cm 3.2A

Retarder reduction ratio: 1 : 34

Hall resolution: hall x ratio 34.02 = 341.2PPR

I did some testing on the motors earlier and found that they do not meet the specs given on the web site at all.

The gear ratio is 2 (11/5)^3 = 21.296, determined both by comparing turns of the shaft to encoder counts and by counting teeth on the gears, not 34. I was a bit leery about disassembling the gearbox, but I managed to take it apart and put it back together without mishap.

The encoders do give 11 pulses per motor shaft revolution, but that translates to 234.256 pulses per output shaft rotation, not 341.2. With four edges in the quadrature encoding, I could, theoretically, measure rotation with a resolution of 0.384°.

At 5.99V, the no-load current is 64mA and the shaft speed is 189.4 rpm (determined from the gear ratio and the frequency of the encoder ticks).

I did not measure stall torque, but stall current was 1.3A at 5.72V, for a resistance of 4.4Ω, which would be about 1.36A at 6V, not 3.2A. I should be able to estimate the torque, if I assume a gearhead efficiency of about 50%, which seems to be typical for cheap gearhead motors. Using natural units (Nm/A or V s/radian) for the motor coefficients, I have K_{e}≈0.3 Vs/radian, so K_{t}≈0.15 Nm/A, which would provide a maximum torque of about 0.2Nm (28 oz in or 2 kg cm) at 1.36A. I refined this estimate a little using gnuplot (still with the 50% efficiency assumption).

Since I plan to run my motors at 6V, not 12V, this motor is near the middle of the current range that Gabriel Elkaim suggested in class, though I’m near the high end for rpm, so torque may be less than desirable for moving the robot.

I have thought about two ways of installing the motors:

I believe that I will install the motors above the MDF, since that would put optical sensors for tape very close to the floor (improving signal-to-noise ratio) and allow me to mount battery and other power electronics on the same level as the motors, keeping the center of gravity low.

If the motors are placed between the wheels as close together as possible while staying coaxial, the space between the tires is about 13.5cm and the outside edges of the tires are about 19cm apart. If the wheels are not outboard, then 3.5cm by 6.5cm wheel wells will need to be cut in the MDF, with 13cm between the inner edges of the wells.