by Fred Eady
There are times when a standard hobby servo just won’t do the job.
The same goes for small DC motors. Sometimes parts of aluminum
humans require a bit more torque than servos and low-load DC
motors can provide. In these cases, a hefty DC motor fills the bill.
However, a meaty DC motor needs a beefy DC motor driver.
Fortunately, there is inexpensive and easy
to use DC motor driver technology that is
available to us that will drive heavy iron.
This month, we’ll pull together what it takes to design,
build, and code a heavy duty DC motor driver module.
First, we’ll look at the electrical theory behind the DC motor
driver electronics. Then, we’ll build up the DC motor driver
module’s “intelligence” and meld it with the DC motor
driver’s “brawn.” If all of that passes the smoke test, we’ll
code a simple RS-232 interface, which will allow you to
control the big DC motor with simple serial commands.
The DC motor driver IC of choice for this project is the
Allegro MicroSystems A3959. The A3959 is a DMOS Full-Bridge PWM Motor Driver IC. Before we begin our DC motor
driver IC walk-thru, let’s take a look at the problem at hand.
Big Mama Gearmotor
The behemoth you see in Photo 1 is an Anaheim
3000-R326 DC brush planetary
gearmotor. This motor is really
not a “problem.” It’s actually our
project design point. The BDPG-
60-110 weighs in at four pounds,
10 ounces. From shaft to tail, the
measures in at 8.08 inches. The
planetary gear section is 2.65
inches in length while the shaft
adds 1.10 inches to the overall
length. The actual motor cylinder
comes in at 4. 33 inches in length.
44 SERVO 07.2008
The BDPG-60-110’s maximum diameter at the motor cylinder
is 2.36 inches.
The BDPG-60-110’s name tells much of its story. All of
the planetary gear motors in the BDPG-60-110-24V-3000-
R326’s class share the same part number including the “R.”
The numbers that follow the R provide us with more
information about the planetary gearmotor. The R326 is
the most powerful gearmotor in the bunch as it can provide
4,166 ounce-inches of continuous torque. The BDPG-60-110
can peak at twice the amount of continuous torque. The
326 also tells us what the gear ratio is. In our case, the
BDPG-60-110-24V-3000-R326 has a gear ratio of 326:1. At
326:1, the BDPG-60-110 rotates its shaft at a brisk 9.2 RPM
unloaded. If you work the gearmotor, the shaft rotation will
fall to a loaded value of 7. 7 RPM. Those RPM figures may
seem too low to perform any real work. However, when
you add external gearing to the
BDPG-60-110, I can assure you
that heavy loads will move about
relatively quickly. The 3000
number in the BDPG-60-110’s
part number is the speed of the
motor before the gearbox.
I initially gave you the
dimensions in terms of inches.
However, in reality the BDPG-60-
110 likes to be described
metrically. The body diameter is
called out in the part number as
PHOTO 1. This is one heavy
duty motor all packed up in
a relatively tiny package.