Re: [mowbot] test

Byron A Jeff (byron nospam at
Mon, 3 May 1999 15:04:41 -0400 (EDT)

> Byron,
> On Fri, 30 Apr 1999 19:43:53 -0400 (EDT) byron nospam at (Byron A
> Jeff) writes:
> > > Drivetrain: Two car power window motors with worm gear.
> >
> > Same. Center mounted so that the circular body can spin on its axis.
> > I did most of my drivetrain work last spring. Learned quite a few
> > things:
> >
> > BTW I'm using a couple of Hyundai WW motors....
> What kind of RPM do these generate? The ones I saw at
> operated at 100RPM. Figuring for 6 inch wheels (approximately 18 inches
> in diameter) that would give a speed of 2.5 feet per second
> (approximately). This seems way too fast to me, so I am thinking that I
> need to look for a motor putting out 10-20RPM instead. Seem reasonable?

Yes. Never really checked the speed. Like all wiper motors, it has two speeds.
The slow speed pushed 8 inch wheels maybe about a foot or so a second. Not
very fast. I could easily walk with it.

> >
> > > Cutting blade: Nylon blade adapter intended for use on string
> > trimmers.
> >
> > Tried it. Not much success. My current blade is a 7 in circular saw
> > blade.
> Using a metal blade isn't feasible for me. I have small kids. What were
> the principal problems with the nylon blades?

Tangle. It's the same problem I have with the blades when used in their
proper place on a weed eater. The cut material gathers on the blade and
slows it down until it jams requiring removal from the cutting area to
have the motor regain its original speed.

I too have small kids and so far I'm perceiving this as a sensor issue.
Ideally I want the mower to come to a dead stop, both movement and cutting
blade, whenever encountering an obstacle in a 5-10 foot radius. I'm planning
on doubling this with a perimeter sensor that will shut down the bot whenever
anyone enters the area to be cut. In short it should turn off if it even gets
a whiff of an obstacle.

In the end I plan to fence in my back yard, the target cut area. I think that
I'll be able to ensure safety around the bot.

But the metal cutting blade enclosed so that neither the blade or the
shaft can attract cut or laying grass has been the only effective method
I've found so far.

I am open to suggestions....

> >
> > I used a radiator fan motor. It has the heft to carry the saw blade,
> > which is
> > smaller and flat. So the saw blade actually spins faster than the
> > original
> > fan blade.
> Any idea of the current draw for the radiator fan motor?

Nope. For prototyping I threw the power budget out the window. Also I blew up
my 10A current meter. Ooops.

> > > Grass cutting pattern: Random
> >
> > This is actually a real problem. By necessity the cut path is much
> > smaller than
> > Again I haven't gotten this far, but I'm pretty sure I'm going to
> > implement
> > a real grass height detector and cut uncut grass.
> >
> Did you see my earlier post on using an IR LED emitter/detector pair for
> uncut grass detection? I realize there are issues with the
> emitter/detector getting blocked by dirt or cut grass, but does this idea
> seem feasible?

Yes. In fact if you go back to a really old copy of Radio Electronics, you'll
find an article for a robotic mower called the Lawn Ranger. It used that
exact same technique to detect grass height.

> > a completely bounded area and bump sensors. Somthing like the very
> > popular
> > railroad ties. Anything I wish to protect will be bounded also.
> >
> Eminently reasonable. I also have been thinking along the lines of
> keeping it simple for the initial implementation just so I can get
> SOMETHING working, rather than never finishing anything due to an overy
> complex design.

Exactly. That's my plan. Having a moving cutting platform that isn't too smart
is better than having no platform at all.

> > Also I'm an advocate of dropping core knowledge here. Many of us a
> > software
> > or firmware folks. However trying to figure out how to attach an
> > axel to
> > the driveshaft of a motor is a completly different skillset. I'd
> > like to
> Speaking of which, how did you attach axles to the power window motors?
> The specs on the ones I have been looking at say that the output shaft is
> only 5/8 of an inch long.

But it is has a screw at the end of the shaft. I haven't perfected the
technique but here's the basic gist:

1) Go to Home Depot and get the following items:
- metal dowel
- titanium drill bit. Will talk about the size in a minute
- Screw tap. They're in the tool corral near the drill bits.
- Cutting oil
- Gloves and safety glasses
- lock washer

The size of the screw tap is the same size/grain of a nut that fits on the
shaft. For example the Hyundai motor has a 8mm coarse nut. So I got a 8mm
coarse screw tap. On the package it'll tell you shat size hole to drill. Get
the titanium drill bit of that size.

1) Get and cut metal rod to length. sand off rough edges.
2) As best you can center the drill bit and slowly drill a hole in the center
of the dowel along its long axis. Be sure to go slowly, and use the safety
equipment as you'll have hot flying metal shaving all over. They sting.
Trust me!
3) After drilling to the desired depth, remove the drill bit and blow/rinse
out the excess metal from the hole.
4) Fill the hole with the cutting oil and slowly screw the Screw tap into the
hole. The tap will cut the threads into the hole. Be sure to clean the area
carefully after the tap bottoms out in the hole.
5) Voila! You now can screw the rod onto the motor shaft! You'll definitely
want a lock washer. And the harder part is drilling and setting a set screw
perpendicular to the axel so that when you spin the motor in reverse, the
axel doesn't unscrew off the shaft. I used a cotter pin though the axel and
shaft but it weakened the shaft enough that the torque broke the shaft.
Next time I plan to simply abut the screw against the shaft instead of
drilling through it.

BTW there are also outside taps to that you can put threads on the motor
shaft if it doesn't have any. The tough job it getting the shaft so that it
doesn't spin while threading it.

> Thanks for the informative post,

You're welcome. Anything that gets folks out in the shops trying to piece
a mowbot together is a good thing.