Make Electronics

Experiment 32: A Little Robot Cart

280 Chapter 5

FundAmentAls

All about motors

Brushed DC motor

This is the oldest, simplest design for an electric motor,

shown in very simplified form in Figure 5-101. Coils are

attached to a shaft where they can interact with sta-

tionary magnets around them. The magnetic attraction

turns the shaft a little, at which point the next coil on

the shaft is energized to turn the shaft a little more, and

then the next coil—and so on. To make this happen,

electricity has to be fed into the coils by “brushes,” often

consisting of soft carbon pads that conduct power to

a hub, known as a commutator, divided into sections,

each of which is connected to a separate coil.

This basic design has several advantages if we want to

build a small motorized gadget, such as a miniature

robot or even a model airplane:

• Widely available


• Low cost


• Simple


• Reliable


• Will run in reverse when voltage reverses
  In addition, brushed motors are often sold with reduc-
  tion gearing built in. Such units are known as gearhead
  motors or gear motors. They free you from the need to
  use your own gears or belts to adjust the output speed
  yourself. You simply choose the motor that fits your
  specification.
  DC stepper motor
  This requires a controller, consisting of some electron-
  ics to tell the motor to rotate its shaft in small, discrete
  steps. The advantages of a stepper motor are:


• Precise positioning of the shaft


• Precise speed adjustment
  Stepper motors are ideal for devices such as computer
  printers, where the paper has to roll up by a precise
  distance and the print head has to move laterally by
  an equally precise distance, but they are also useful in
  robots. If the motor is small enough to draw less than
  200mA and will run on 12 volts or less, you can control
  it with pulses from a 555 timer. I’ll describe stepper mo-
  tors in more detail in Experiment 33.

Servo motor

This is generally used in conjunction with a program-

mable microcontroller, which sends instructors to

rotate the motor shaft to a specific position and then

hold it there. I’ll mention servo motors when I intro-

duce you to microcontrollers, but we won’t be dealing

with them in detail.

Other types of motors exist, including brushless DC motors

(which require a different type of controller and are found

in computer disk drives and CD players), and AC motors

(including synchronous motors, which synchronize their

rotation with the frequency of AC voltage, and were used

extensively in clocks, before clocks mostly became digital).

In this book, I’ll be talking mostly about brushed DC motors

and DC stepper motors.

DC

Power

Magnet

Commutator

Brushes

Coil

Figure 5-101. The basic principle of a simple DC motor. The

commutator passes electricity through a coil, creating a mag-

netic field that interacts with a magnet around the motor. The

coil turns, and the commutator turns with it, until the electric

field through the coil is reversed. This causes the process to re-

peat. In reality, a motor is likely to have a commutator formed

from multiple segments, connected with multiple coils. The

principle, however, remains the same.