Re: [mowbot] The grass keeps getting longer

Byron A Jeff (byron nospam at
Sat, 30 Jan 1999 08:52:00 -0500 (EST)

> Hey, glad to hear this is not all dead. I'm still collecting parts for my
> planned mowbot but expect it to take a couple of years.
> Anyway, its good to know you're back.
> G.H. Latta

That's interesting. Last spring I put together a halfway decent prototype
in just about a month. I learned a lot:

1) Windshield wiper motors have enough torque to drive a unit. They work
fairly well because 1) They are already geared down 2) They have a screw
shaft which is easy to attach to. 3) They are relatively cheap to pick up
in an auto junkyard.

I prototyped with a couple of motors pulled from a Hyundai. The only thing
I really would like to find is a car that actually uses 2 matched motors
for the wipers. This model used 1 motor and a linkage system to drive the
passenger side wipers. So since the motors were not perfectly matched
(and the prototype wasn't perfectly aligned) the final unit drifted to the
left during testing.

2) An all metal drivetrain is a must. I prototyped with 1/2 inch wood dowels.
They broke. Metal rods could handle both the weight and the motor torque.

3) I now understand the purpose of cotter pins. Since the motors were mounted
in opposite directions, one was rotating in the direction that would unscrew
the drivetrain from the screwshaft.

4) That metalworking is possible for the hobbiest. I never realized that
I could drill, cut, and tap metal. Just some decent attachments from the Home
Depot, good eyewear, and some patience is required. I ended up with the
all metal drivetrain where the metal rod screwed onto the shaft and was
held in place by a screw used as a cotter pin.

Anyway my prototype used a 18 inch round tabletop as a base with the motors
and drivetrain center mounted and attached to 8 in lawnmower wheels which
were positively attached to the drive train using metal L shelf brackets.
The motors were driven by a 12V 33 AmpHour battery. The battery sat at the
rear of the unit which was held up by a 4 inch castor wheel.

All in all it was a good first attempt. The unit climbed 15 degree slopes and
wandered through tall grass OK. As I said earlier it drifted to the left so
it actually wandered the yard in a great big counterclockwise circle.

The unit failed due to metal fatigue in one of the motor screw shafts, the one
that required the cotter pin.

I plan to resurrect the project this year with lessons learned and new ideas.
I feel confident now in both having a base platform, and foresee no real
problems with a relatively stupid bumper based control system to keep it within
the area and out of the neighbor's yard... Also I'll couple a remote control
system stripped from some RC toy for some human control.

What I'm extremely interested in this year is cutting technology. I've had
some success with spinning a metal cutting disk from a circular saw with a
electric drill. I need to figure out how to translate that into a platform
that I can mount on the platform. So any resources about how cutting gets
done would be greatly appreciated.

This is my fourth year on this list. It always seems to start up just like
spring training with hope renewed. But this year I plan to get a prototype
that actually cuts grass. The rest is what I consider control issues which
I'm much more comfortable with.

But after last years experience I urge all of the hobbiest on the list (Dave
doesn't count: he's clearly looking at a bigger picture) to consider how to
design less and start tinkering. In less than a month and about $200 I had
a platform that could support a mowbot. Trying to pull it all together seems
overwhelming. But by splitting the project up into three or four clearly
definied areas (mobility,cutting,sensing,control/reactivity) and focusing
on one area at a time, suddenly it feels more doable.

So think about prototyping this year. You'd be surprise how much progress you
can make. I certainly was.

Good luck to everyone this year.