Page 275 - Robot Builder's Bonanza

244 USING DC MOTORS

or tantalum electrolytic capactor (47 F to 330 F) across the motor voltage and ground con-
nections, as close to the transistors as possible.

Motor Control by Bridge Module

Circuits for controlling motors are big business, and it shouldn’t come as a surprise that doz-
ens of companies offer all- in- one solutions for running motors through fully electronic means.
These products range from inexpensive $2 integrated circuits to sophisticated modules cost-
ing tens of thousands of dollars. Of course, we’ll confine our discussion to the low end of this
scale!
Motor control modules incorporate the H- bridge design you learned about in the previous
section. The module may consist of just an IC, or it may be a premade circuit board with the
H- bridge electronics on it. Either way, motor control bridges have two or more pins on them
for connection to control electronics and, of course, connections to power and to the motors.
Typical functions for the pins are:
• Motor power. Connect these to the battery or other source powering the motors. I like to
use a completely separate battery pack for the motors and the rest of the robot’s electron-
ics. Using the motor power pins on the motor bridge, this is very easy to do.
• Motor enable. When enabled, the motor turns on. When disabled, the motor turns off.
Some bridges let the motor “float” when disabled; that is, the motor coasts to a stop. On
other bridges, disabling the motor causes a full or partial short across the motor terminals,
which acts as a brake to stop the motor very quickly.
• Direction. Setting the direction pin changes the direction of the motor.
• PWM. Most H- bridge motor control ICs are used to control not only the direction and
power of the motor but its speed as well. The typical means used to vary the speed of a
motor is with pulse width modulation, or PWM. This topic is described more fully under
“Controlling the Speed of a DC Motor.” On many H- bridge ICs, the motor enable and
PWM input are the same
• Brake. On bridges that allow the motor to float when the enable pin is disengaged, a
separate brake input is often used to specifically control the braking action of the motor.
• Motor out. These are outputs for connecting to the motor.

The better motor control bridges incorporate overcurrent protection circuitry, which pre-
vents them from being damaged if the motor pulls too much current and overheats the chip.
Some even provide for current sense, useful when you want to determine if the robot has
become stuck.
Recall from earlier in this chapter that DC motors will draw the most current when they are
stalled. If the robot gets caught on something and can’t budge, the motors will stop, and the
current draw will spike.

USING THE L293D AND 754410 MOTOR DRIVER ICS
Among the most common— and least expensive— motor bridge ICs are the L293D and its
close cousin, the 754410. Both come in small 16- pin IC packages, and their hookup is iden-
tical. The big difference between the two is the maximum amount of current that the chip can
handle.

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