Re: [mowbot] A Basic Platform

Doug Smith (dsmith nospam at
Wed, 13 May 1998 11:22:11 -0500

>This message will detail the choices I made for my Mowbot platform. I hope
>it will clarify many of the issues that come up during robot construction.

Thanks for the great info. I'm glad to see some desire to focus on the
platform because that's the stage I'm at. Here's a little of what I'm

I'm trying not to think like I'm building a standard lawn mower. (I
already have a self propelled gas mower rigged up with a cable winding on a
spool to mow large circles.) Since I'm not pushing it, it can take longer
to mow. In fact, I don't care if it mows continuously as long as it's not
annoying and it can keep up with the lawn.

So I'm going for small and light to keep the power requirements down which
keeps the battery size and weight down, which keeps the power requirements
down, and so on. And hopefully that will translate to reasonable run time
between charges or low enough power drain to be supplemented by solar
charging. (Yes, that's part of platform design considerations because that
wouldn't even be a reasonable cost option for a larger unit.)

Like many of you, I'm more of a computer and electronics person than a
mechanical one so I need some trial and error to figure out what I'm doing.
I'm just getting started with that stage.

I've been watching the surplus catalogs for powerful low current motors for
a long time. I finally picked up some very nice small gear head motors
from MECI <> for $9.95 each (No. 420-0515). They're
only about 2 1/2" long and 5/8" in diameter and appear to be very well
built. No load ratings are about 15mA / 85RPM at 3V and 26mA / 325RPM at
12V. They're extremely powerful for their size, current draw, and weight.
So now I'm building a basic experimenting platform (not for mowing) with
those motors and 5" plastic stroller wheels. My goal is to run this while
varying voltage, weight, incline, and maybe wheel size while measuring
speed and current draw. This should give me enough data to size batteries
and set a power budget for the rest of the bot. It could also tell me that
I need to scrap the whole thing and try something else. We'll see shortly.
I'll post the results here.

After that, I'll need to do the same thing with the cutting motor which I
haven't selected yet. Has anyone tried to determine what speed a cutter
needs to run in order to cut grass effectively? Blade tip speed, which
changes with the diameter of the blade, would be more important than RPM
and it would depend on the type of blade. According to one of my lawn
mower manuals, there is an ANSI safety standard maximum blade tip speed of
19,000ft/minute. It probably doesn't need to be that fast to cut well, but
based on that maximum, we can calculate a maximum RPM for various blade
diameters, which would be...

blade diameter max RPM
-------------- -------
21" 6,911
16" 9,071
10" 14,514

That's enough for now since I haven't even reached that stage yet. I hope
I haven't rambled too long, but I wanted to try to add something to the
discussion rather than just lurk. Hopefully, others will take the time to
do the same.

Doug Smith <dsmith nospam at>