Page 251 - Robot Builder's Bonanza

220 CHOOSING THE RIGHT MOTOR

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Reverse battery
to change direction
Figure 21- 1 A menagerie of DC motors, Figure 21- 2 The direction of rotation of most DC motors
with and without gear reduction boxes. These may be altered simply by reversing the polarity of its power
motors operate at low voltages, all of them connections. With a battery or other DC power source
under 12 volts and most under 6 volts— ideal connected one way, the motor spins clockwise. Reverse the
for the typical battery- run robot. polarity, and the motor spins counterclockwise.

The best and easiest test for reversibility is to try the motor with a suitable battery, as shown
in Figure 21- 2. Apply the power leads from the motor to the terminals of the battery or sup-
ply. Note the direction of rotation of the motor shaft. Now reverse the power leads from the
motor. The motor shaft should rotate in reverse.

Continuous or Stepping Motor?

DC motors can be either continuous or stepping. Here is the difference: with a continuous
motor, the application of power causes the shaft to rotate continuously (hence the name). The
shaft stops only when the power is removed or if the motor is stalled because it can no longer
drive the load attached to it.
With stepping motors, the application of power causes the shaft to rotate a few degrees,
then stop. To keep rotating the shaft, power must be pulsed to the motor. Stepping motors
are used when you want to control how far a motor turns, in either direction. They don’t have
the mechanical torque that a continuous DC motor has, but they’re useful for certain tasks
that don’t need brute force. For example, one application is to spin a turret on top of a robot;
on the turret might be a sensor of some kind or maybe a gun that shoots foam bullets. Chap-
ter 22, “Using DC Motors,” focuses entirely on continuous motors.

Stepping motors were covered in previous editions of Robot Builder’s Bonanza. Because
ON THE
stepping motors are not as heavily used in robots as they used to be, the material has been
removed from the printed book and is available on the RBB Online Support site (see Appendix
W E B
A) as a free download.

Servo Motors

A special subset of continuous motors is the servo motor, which in typical cases combines a
continuous DC motor with electronic feedback to ensure the accurate positioning of the
motor. A common form of servo motor is the kind used in model and hobby radio control
(R/C) cars and planes. We use these a lot in robotics, so this is a vital motor to get to know.

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