CHAPTER 18 ■ ADDING GEARHEAD MOTORS
Since speed decreases linearly with voltage, the 9 V RPM can be estimated as:(RPM nominal / V nominal) × V desired = RPM at desired voltage
(225 RPM / 12 V) × 9 V = 168 RPM
That’s a little faster than sought after, but it’s also the no-load speed, which is unrealistically fast. The
torque required to move the load of the robot decreases the RPM. Still, a slightly slower motor would be better.
The motors are available from Jameco, part #162191, manufacturer’s part number #HNGH12-1324Y-R,
for $17.95 each. These were the least expensive gearhead motors that met the criteria. Even so, at almost $36
a pair, the motors are more expensive than all the other line-following parts combined. You’ll face this reality
again and again. Motors generally make up the largest portion of a robot’s cost.
If you decide to purchase different motors, or use some you already own, make sure the motors meet
the first four criteria listed at the beginning of this chapter.
Solarbotics provides a wide variety of lower-cost motors. They selected their GM2 motor for their
Sandwich kit. The motors are slower (which improves line following), half the weight, and cost less than^1 / 3
of the price (only $6.90 each when purchased in pairs).
Furthermore, Solarbotics sells matching-colored wheels (see Figure 18-2), part #GMPW, for as little as
$2.68 each when purchased with the motor. This allows you to tear out Chapters 19 (selecting wheels) and
20 (attaching wheels) of this book.
Figure 18-1. A pair of Hsiang Neng gearhead motors
Figure 18-2. Solarbotics GM2 motor with matching wheel