Make Electronics

What Next? 281

Experiment 32: A Little Robot Cart

Take-home messages from this experiment include the following:

• You can buy simple DC motors with reduction gearing built in, providing
  your choice of RPM. Literally hundreds of websites will sell you small mo-
  tors for robotics projects.


• When you reverse the voltage to a DC motor, the motor runs in reverse.


• A DPDT relay can be wired so that when it closes its contacts, it reverses a
  power supply to a motor.


• You can use two limit switches and a pair of diodes to stop a motor at two
  positions. In each of its stopping positions, the motor consumes no power
  and you won’t have the risk of it burning out.

What other projects can you imagine using this simple set of techniques?

Mechanical Power

In the United States, the turning force, or torque, of a motor is usually mea-
sured in pound-feet or ounce-inches. In Europe, the metric system is used to
measure torque in dynes.

A pound-foot is easy to understand. Imagine a lever pivoted at one end, as
shown in Figure 5-102. If the lever is one foot long, and you hang a one-pound
weight at the end of it, the turning force is one pound-foot.

One foot

Rotational

force

(torque)

of one

pound-foot

1 lb. weight

Figure 5-102. The rotational force created by a motor is known as “torque,” and in the
United States it is measured in pound-feet (or ounce-inches, for small motors). In the
metric system, torque is measured in dynes. Note that the torque created by a motor will
vary according to the speed at which the motor is running.