[mowbot] Basic platform

Terry (cefpearson nospam at accs.net)
Thu, 14 May 1998 10:23:07 -0500

I put together a mobot last year when this group was active last time,
this is what I came up with for a basic platform at a price of around
$50.00 U.S. I concentrated on useing parts that are avalible everywhere
and are cheap.
For the chassi I used a 12" X 16" sheet of PVC. The PVC sheet I made
from 4" schedule 40 PVC pipe. I formed the sheet by putting a 14" section
of the pipe in an oven at 250 degrees F for 15 minutes. I take the hot
section out, slice down one side with a knife to open the diameter, and
press the hot section between two suitably sized boards. It takes about 5
minuts for the hot PVC to form a sheet. Nice thing about using PVC as the
chasse is that with a pilot hole, wood screws can be used to bolt small
items to it.
For the drive motors I used two automobile windshield wiper motors. They
cost $10.00 apiece used. I connnected the wheels directly to the output
shaft of the wiper motors. The wheels are 4 inch sections of 6" PVC pipe
with strips of old tire tread screwed to it. the center hub is made from
PVC pipe that has been heated and formed into a sheet as described above.
Wiper motors have a threaded output shaft and they are two speed, but I use
only slow speed in my design.
The cutting motor is a heater blower motor from a car ($5.00 used). To
the output shaft I bolted on a 5" diameter disk of PVC sheet and to it I
attached two 1" sections of hacksaw blade. The blades come with a hole at
either end so I used the 1 inch section from both ends because its almost
impossible to drill through the hardened blade. The blade has to be broken
off at 1", not cut.
The electronics is simple. Some 555 timer chips turn the mower at a
random amount when it bumps somthing. I used relays to do the driving. An
electronic H bridge just has too much of a voltage drop across it to be
much good.
Altogether the thing cuts a nice path through grass 4 or 5 inches high.
The drawback is that it pulls 14 amps at 12 volts when it cuts heavy
grass. 7 amps when is just cutting over an already cut area. It can climb
a 15 degree steep hill and mow sidways on the hill without tracking down
the slope. Castor wheels at both ends keep the thing up right. The 20 amp
hour batery last about 15 miutes between charges before the voltage effect
the timer chip's duty cycle to the point that it just turns in erratic
circles. The high power requirment is a must to climb the hills I have here
and to cut grass at the same time. I tried smaller drive motors and found
them to be too small to get the job done.

Cef Pearson