FIGURE 2. The MadeUSA base is constructed from 5/8"
HDPE. A separate battery compartment deck is laser-cut
from sturdy cast acrylic.
base that give the MadeUSA its Medusa look. I elected
to not use all 10 for Big Brute, saving a few for other
(If you don’t want or need all 10 Pings))), you can
purchase the MadeUSA component parts separately which
shaves several hundred dollars off the total price. Check
out the MadeUSA page on the Parallax.com site for a
listing of the separate components. Mix and match what
Air-filled pneumatic tires are provided with the kit,
along with custom machined aluminum wheel hubs. To
prevent excess stress on the motor shaft, the MadeUSA
uses a heavy-duty pillow block bearing for each wheel.
(The block is sturdy enough to support a horse, let alone
an 18” robot.) All the weight of the robot bears down on
this bearing, literally. That helps to prolong the life of the
motors, and keeps the motor shafts parallel with the base.
Getting From Here to There
Power for the motors comes from a 12 volt seven
amp-hour battery that you provide. The MadeUSA kit
comes with a clever battery holder that adjusts for
variations in size and shape of the battery, holding it snugly
between a set of aluminum standoffs. (See Figure 3 for
details.) The battery holder is placed on the underside of
the robot which keeps its center of gravity low. That makes
it highly unlikely the MadeUSA will tip over, even on a
Complementing the heavy-duty motors are a pair of
25 amp motor drivers, Parallax model HB- 25. By
themselves, these drivers are operated as if they are radio
control (R/C) servo motors. Provide a train of pulses of
1,500 microseconds and the motor stops. Shorter pulses
make the motor turn one direction; longer pulses make it
turn in the other direction. Speed control is affected by
altering the pulse width.
Should you be interested in such things, the fan cooled
HB- 25 uses mini-blade style fuses. The fuse will blow should
there be a short or other serious condition in the motor or
its wiring. Each HB- 25 comes with 25 amp fuses preinstalled.
The HB- 25 drivers are augmented by a set of precision
wheel encoders. The motor and position encoder kit is
shown in Figure 4. These encoders are commanded
through standard serial signals. You can opt not to use the
encoders — you can always control the HB-25s directly.
Since the encoders provide a means to accurately maneuver
the robot, there’s little reason to disregard them. In use,
your microcontroller is coupled to the wheel encoders, and
the wheel encoders are connected to the HB- 25 drivers. The
HB- 25 drivers, in turn, are attached to the motors.
FIGURE 3. The 12 volt lead
acid battery is held captive in
its compartment holder using
screw-on standoffs. You can
adjust the spacing of the
standoffs for variations in
FIGURE 4. MadeUSA uses two automotive power window motors
for locomotion. Precise control is provided by an optical encoder.
SERVO 01.2012 39