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136 Making Things Move

• Operating Speed This is equivalent to the no load speed on the DC motors,
but worded a little differently. Because servos don’t rotate all the way around,
you won’t see rpm. These specs tell us that when given 6V, the motor will
move 60° (or one-sixth of a full rotation) in 0.15 second with no load on the
shaft.
• Current Drain This is similar to no load current on the DC motors. For this
servo, we see two numbers. At 6V, the servo will draw 7.7mA of current just
doing nothing, and 180mA when moving with no load on the shaft.
• Stall Torque This is the same as on the DC toy and gearhead motors. The
stall torque is the highest torque the motor can give, and happens when you
stop it or stall it when it’s trying to move. This motor shows 49 oz-in of torque
at 6V when stalled. This is equivalent to about 3 in-lbs.

• Operating Angle and Direction These tell us the servo can move 45° in
either direction, for a total range of 90°. On this particular motor model, you
can pay $10 more to get a 180° range.

• Gear Type This is the most relevant line for us in the rest of the lines in the
specification, which describe the motor parts. Lower torque servos will have
plastic gears usually made out of nylon. Stronger servos with higher torque
use metal gears.

Continuous Rotation Hobby Servos
A continuous rotation servo is a modification of the standard servo motor. Instead of
determining position, the pulses tell the motor how fast to go. You give up knowing
the position of the servo arm here, but you gain speed control and 360° movement. A
continuous rotation servo is a great option if you have something that needs to spin
continuously but you want an easy way to control the speed, such as for an electronic
toy mouse to chase your cat around.

You can either buy servos that are already modified for continuous rotation, like the
Hitec HSR-1425CR, or get a standard hobby servo and perform some surgery to
modify it yourself. If you’re wondering which servos can be modified for continuous
rotation, check ServoCity’s Rotation Modification Difficulty List (www.servocity.com/
html/rotation_modification_difficul.html).