Robot Building for Beginners, Third Edition

CHAPTER 17 ■ DC MOTORS

Summarizing the Characteristics of DC Motors

When examining an advertisement or datasheet for a motor, here are the things to think about.

Speed – Does the RPM at the given voltage (or scaled proportionally) meet the

robot’s needs?

To r q u e – It is unlikely that you’ll be sure of your robot’s exact torque needs. Yet,

torque is still useful in choosing between otherwise equal motors in a potential

purchase. One way to get a feel for the torque of a motor you are considering is to

compare it with a known motor that has either been adequate or inadequate for

you in the past.

Voltage – Is the motor’s specified voltage within 50% to 125% of the voltage the

robot’s batteries are prepared to provide? Consider the voltage of the batteries

when fresh and when declared exhausted.

Current – Can the robot’s electronics handle the maximum (start up/stall)

current? How long will the battery last based on the minimum current (no-load)

of the motors?

Efficiency – If the speed and torque of the motor is a good match for the robot,

then the highest efficiency motor has, by definition, the lowest current for a

given voltage. High efficiency suggests high quality, which may be true for other

characteristics of that same motor.

Audible Noise – Although every motor can be expected to generate some audible

noise, excessive noise can indicate a problem or, at the very least, be an irritant.

Electrical Noise – Every brush motor generates some electrical noise, but it

shouldn’t be so bad as to interfere with the electronics. Motors that are high

efficiency or have multiple windings/commutator segments are less likely

to disturb circuitry. Liberal use of capacitors in the robot’s circuitry helps

immensely.

Mass – Pick a motor whose weight can be supported by the body materials you’re

comfortable using. Where mass-limit rules apply, watch out for running out of

weight due to motors, thus being forced to skimp elsewhere in the robot.

Dimensions – The motors must fit into the selected robot body, or else select

a body for the chosen motors. Unless gearing or a fancy drive train is included,

motors are usually positioned end to end, thus dictating the minimum width of

the robot.

DC Gearhead Motors

Gearhead motors are sometimes called gearmotors or geared motors. They usually consist of a DC brush
motor (either iron core or coreless) with a gearbox attached to the shaft. You can immediately identify a
motor as a gearhead by two distinct segments connected together (see Figure 17-23).