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234 USING DC MOTORS

V+ V+

M M

Figure 22- 3 How current flows through
Switch/relay Switch/relay the double- pole, double- throw switch. As
in position in position shown here, the switch is wired in a way
A B that it literally reverses the polarity of the
V- V- battery connection to the motor.

Remember to use DPDT switches with a center- off position. When they are in the center
position, the motors receive no power, so the robot does not move.

G Try to get momentary- contact switches so that when you release them they spring back to the
center. This makes it a lot easier to control your robot by flipping the switch. Momentary- contact
DPDT switches cost a little more, but they’re much more convenient.

Figure 22- 3 shows how the switches (the technique also applies to relays, discussed next)
are wired in order to achieve motor reversal. It may look a little a little weird at first, but it’s
actually quite logical in how it works. The switches are wired so that in one position— say,
Position A— current from the battery flows through the motor in one direction. Flipping to
Position B changes the direction of battery flow to the other way around. This naturally makes
the motor go the other way.
Some things to try:

• Steer by turning on one motor only (leave the other one off). Note the speed of the turn.
• Now steer by making one motor go forward and the other motor go backward. Note the
speed of the turn . . . it’s faster. The robot actually spins, turning in place. This is how
military tanks turn. It’s referred to as tank steering, or, more commonly in robotics, dif-
ferential steering.

Operating your robot with switches is great for learning how things work, but you’ll soon
want to graduate to hands- off methods, where your mechanical creations will steer them-
selves. The remainder of the DC motor control methods concentrate on techniques that allow
for fully autonomous robots.

Motor Control by Relay

Before getting to the all- electronic methods of motor control, I want to take a moment to talk
about another, somewhat more old- fashioned approach: using relays. Yes, I know this is the
twenty- first century, and what’s all this about using something like a relay that was invented
almost 200 years ago. Daft indeed!
But there are many good reasons to look at relays, if only because they’re a natural
stepping- stone from switch control to fully electronic control. But also, small relays for small
robots are very cheap and very easy to use.

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