M-BOT
chassis includes ample breadboard
space to accommodate any of these
processors, so your bot can easily grow
in complexity and sophistication.
M-bot’s Design
significantly less expensive if purchased
online. Also, I listed 6-32 threaded rod
as an alternative to the 1-1/2 inch
standoffs I used to connect M-bot’s
two bases because it’s readily available
at hardware or hobby stores, and
considerably less expensive.
In addition to demonstrating the
feasibility of constructing an intelligent,
autonomous robot with the 08M as its
only processing power, M-bot’s chassis
is intended to serve as the foundation
for a variety of modifications and
improvements you may wish to add.
For this reason, the chassis includes two
DC motors that will enable you to add
additional circuitry (such as PWM speed
control) later on. In order to provide for
these improvements, M-bot’s design
includes an SN754410 motor controller
chip. Although we will not implement it
in this article, the SN754410 is capable
of controlling two DC motors with full
PWM speed control.
Since the 08M only has five I/O
lines, we will also use a separate logic
gate chip (the 74HC00 quad NAND
gate) to assist with the I/O tasks. If you
are unfamiliar with logic gates, take a
look at Ray Marston’s series on the subject beginning in the April ‘07 issue of
Nuts & Volts ( www.nutsvolts.com).
His article focusing on NAND and OR
gates (N&V, June ‘07) is especially
relevant to M-bot’s circuitry.
Figure 2 presents M-bot’s complete parts list with sources. Many of
the parts can easily be purchased from
other suppliers. For convenience, I have
included RadioShack part numbers for
many parts you may already have on
hand. Most of these parts would be
Assembling the
Double Gearbox
M-bot uses the Tamiya double
gearbox DC motor set. This unit
provides three major advantages for
M-bot: it’s easy to put together and
mount to the lower base, it provides a
good amount of speed and torque
with a six-volt power supply (M-bot
uses four AA batteries for this
purpose); and it can be assembled at
any one of four different gear ratios.
A small robot like M-bot could
have just as easily been driven by two
small servo motors, but DC motors
were chosen because they can provide
greater speed and power than servo
motors, and they are much more
scalable — that is, the same principles
will apply to your next creation, even if
it’s a four foot tall, 200 pound bot!
Be careful when you purchase the
Tamiya double gearbox — Tamiya also
makes a “twin-motor gearbox” which
looks almost identical to the double
gearbox. The only obvious difference is
that the gears of the twin-motor gearbox are all yellow, while those of the
double gearbox are yellow and blue. Be
sure to obtain the latter, which fits
M-bot’s base better, and provides a
reasonable level of speed, as well. Be
sure to assemble the double gearbox
according to Tamiya’s directions for
“Type C,” which provides a 114.7:1
gear ratio. When I first assembled my
gearbox, I chose “Type-B” ( 38.2:1 gear
ratio), and M-bot darted off so quickly
I couldn’t even catch up before he
smashed into a wall!
Once you have the gearbox assembled, it’s a simple matter to briefly
touch the leads of the six-volt battery
pack to each motor’s terminals to
identify which terminal on each motor
should be + 6 volts and which should be
ground to produce forward motion.
You may also want to use a magic
marker to label the motor terminals
accordingly. At this point, you can also
solder four eight-inch pieces of #22
solid wire to the motor terminals — you
may want to color-code these wires to
match your labeling on the motors.
Building M-bot’s
Chassis
Now that you have assembled and
tested the gearbox, it’s time to assemble
M-bot’s complete chassis, which consists
of two six-inch diameter circular bases.
The upper base (as you can see in Figure
3) is actually a somewhat “truncated”
circle. If you would like to construct
your own bases, you can make them
from 1/8-inch plywood, hardboard
(masonite), or plastic by downloading
the two full-sized bitmap files
( basetop.bmp and basebottom.bmp)
that are available on the SERVO website
( www.servomagazine.com).
Print out each one and use it as
a template for constructing M-bot’s
chassis. Before you begin cutting, make
sure each template is exactly six
FIGURE 3. M-bot’s Chassis. inches in diameter. If not, adjust the
magnification of your printout until
each base is correctly sized.
If you would rather avoid all the
drilling, sanding, etc., pre-cut chassis
sets (one top and one bottom base)
are available on my website (www.
JRHackett.net). These sets are professionally laser-cut (see www.Pololu.
com) from 1/8-inch smoke gray
translucent acrylic. The bases that I
designed are only presented as one
possibility; feel free to design your own
M-bot chassis, or use one you already
52 SERVO 09.2007
