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Build Your Own Combat Robot
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Driving with an H-Bridge
Relay control gives you only two speeds—full speed or stopped. Some weapon
systems require that you reverse the direction of the motor, and the motors of your
robot’s drive train will also need to be reversible. Running a motor in both direc-
tions will necessitate that you switch both sides of the motor between the plus and
minus sides of the battery. The circuit for doing this is called an H-bridge.An
H-bridge gives you the ability to reverse direction, but you’ll still be going full
speed in whichever direction you choose. When can you get away with this?
Most weapons don’t need more than simple on/off control. A saw or spinner
weapon usually needs a single relay to switch it on or off. Large high-inertia spin-
ners may need a second relay for braking purposes. Hammer and lifting arm
weapons will need an H-bridge arrangement for reversing direction, but they usu-
ally do not need to run at variable speeds. An H-bridge using solenoids for motor
control is shown in Figure 7-6.
An H-bridge uses four relays, one from each motor terminal to each battery ter-
minal. In Figure 7-6, relays A and B connect one motor terminal to the positive
and negative sides of the battery, respectively, and relays C and D connect the
other side of the motor to the positive and negative sides of the battery. When you
look at Figure 7-6, imagine a vertical line passing between relays A and B, and a
vertical line passing between relays C and D. Then imagine a horizontal line passing
through the center of the motor, connecting to the two vertical lines. These lines
now form the letter H; hence the term, H-bridge.

FIGURE 7-6
Typical H-bridge
configuration using
motor starter
solenoid relays.