grease. One of the wires in the servo
cable was stripped off to leave a
two-wire cable to power the
motor. The ends of the cable were
soldered to the motor terminals and
a knot was tied in the cable to act as a strain relief inside
the servo housing (the knot along with a small blob of
hot glue). I then used the small mounting screws that
came with the servo to attach the servo to the basswood
Mounted to the shaft of the servos is the largest servo
horn that came with them. I bolted the 1-3/4 inch diameter
juice bottle lids to the servo horns (with #2-56 bolts) for the
robot’s wheels. A rubber band wrapped around the rim of
the lid provides traction for the wheels. The robot’s third
wheel is a tail dragger. I covered a #6-32 bolt with a plastic
tube and capped the end with a nylon acorn nut. An
alternative I might try is using a chromed acorn nut. It will
add weight, but may have less friction with the ground.
If you’d like to keep up-to-date on my design work, testing, and
experiments, then check out my blog and/or subscribe to my tweets.
http://nearsys.blogspot.com and NearSys on Twitter.
terminals. Since there’s no significant force acting on the
switches after they’re mounted to the robot body, don’t
twist the wires around the switch terminals. Just tin the
wires and the switch terminals, solder them together, and
cover them in heat shrink. Before installing batteries and
powering up the robot controller, check for shorts between
the positive and negative terminals of the battery holders.
Then, check for a short between power and ground
on the PCB (it’s easier to do this on the bottom side of the
PCB). If the robot controller passes (and it should), then
install the PICAXE-08M and batteries. Write a program
with the single command, DEBUG. Download to ensure
the PICAXE is programmable and can return data back to
the program editor.
Building a Miniature CheapBot Robot Body
A small robot controller is asking for an equally small
robot body. I made one that looks like a miniature version
of my CheapBot robot body from two pieces of Syntra and
some bolts and plastic tubing. The bottom Syntra sheet
(bottom deck) measures 3 x 4 inches and holds the robot
controller and motors. The top deck measures 2-1/2 x 3-1/2
inches, and mounts the battery holders and toggle switches.
Four #6-32 bolts 1-1/2 inches long hold the top deck above
the bottom deck. The bolts used to attach the decks
together and attach the battery holders to the top deck are
flat heads. Plastic tubes (1/4 inch in diameter) are the spacers
between the two decks. There’s just enough room on the
top deck for the battery holders and power switches.
The robot’s motors mount to the
underside of the bottom deck by
motor rails. The rails are two strips of
basswood with 3/8 x 3/8 inch sides.
The rails bolt to the underside of the
deck with #2-56 hardware. All nuts
on the robot body are nylocks (
nylon-insert locking nuts) so everything will
hold together for years to come.
The motors are modified E-flite
S75 sub-micro servos. They were
modified for 360 degree rotation by
removing their drive electronics and
by shaving off the plastic stop inside
the case. All that is left is a motor
(it looks like a pager motor) and
gear drive train. I replenished the
grease around the gears with Team
Associated silicone grease because I
got so much of it on my hands doing
the modification, I figured the gears
might tear themselves apart if I tried
driving the motors with the remaining
Sample Drive Code
The code in Listing 1 is what I use to drive the robot.
The forward, backwards, and stop routines are pretty
standard. It’s the steering routines that are a bit different
because of the turns the motors take turning the robot.
That’s a Wrap!
That completes the CheapBot-08 controller and robot
body. In the next article, I’ll describe the sensor boards
I’ve designed and tested for this robot. Remember, they
will plug directly into the robot body and will require mild
modification if you want to add them to a different robot
controller. Until then, your assignment is to make a
CheapBot-08 robot. Class dismissed. SV
SERVO 11.2009 51