Page 153 - Making things move_ DIY mechanisms for inventors, hobbyists, and artists

Chapter 6 Options for Creating and Controlling Motion 133

from (usually centered, but not always), and the motor is the end where the power is
connected.

All motor data sheets look different, and the terminology can vary, but don’t let that
scare you. Let’s look down the list in Figure 6-7.

• Gear Ratio This doesn’t tell us anything yet, because even though we know
it has 298 times more torque than the tiny motor did without the gearhead,
we don’t know anything about the tiny motor.
• Unloaded RPM This is the same as the no load speed in Figure 6-6. The
speed here at 3V is only 33 rpm—much slower than the 23,000 rpm of the
DC toy motor! The next line shows the unloaded RPM at 6V. The two values
indicate that the motor will run on anything between 3V to 6V just fine, so
the specs give you the speed for each extreme.
• Unloaded Current This is the same as the no load current spec in Figure
6-6. Milliamps are used here instead of amps. A rating of 40mA is 0.040A,
which is even less than the 0.070A required by the DC toy motor.
• Stall Current This is the current the motor needs at the stall torque.

• Stall Torque At 6V, the stall torque here is 44.90 in-oz, which is about
900 times more torque than the DC toy motor! I told you DC gearheads are
stronger.

All DC motors have similar relationships among speed, power, efficiency, current, and
torque. You’ve learned that maximum efficiency happens at about one-fourth of stall
torque. As you can see in the data sheet for the DC toy motor and Figure 6-8,
maximum power happens at one-half the stall torque.

Standard Hobby Servo Motors
There are two types of hobby servo motors: standard and continuous rotation. Standard
servos are by far the more popular. They are usually found in radio-controlled models
like planes and boats.

NOTE In industry terminology, servo refers to any motor with built-in
feedback of some sort. Feedback just means there is some way to know
where the output shaft is. I’ll call the ones covered in this book hobby servos
to distinguish them from industrial servo motors.