Re: [mowbot] Locomotion/chassis

robin nospam at
Tue, 24 Sep 1996 17:27:50 +0100

Dave Everett <Deverett nospam at> wrote:
> robin nospam at wrote:
> > I am still inclined to
> > carry on the successful scheme I have used on the table-top: two
> > drive wheels (D) with their axes on a diameter of a circular outline, and
> > a third wheel on a swivel for support. On a smooth surface, this doesn't
> > even have to be a wheel, just a rounded foot is adequate.
> > ___
> > .- S -.
> > / c \
> > | D D |
> > \ /
> > `-___-'
> I was going to leave this till we were a bit closer to deciding a
> chassis but,,
> There is a flaw in using the diameter axled differential drive system.
> Firstly I see you opted for only one skid, what happens when the robot
> reverses?
If the centre of gravity is at the point marked ``c'' and low enough,
there isn't a problem.

> If you add another skid (or castor) this will cause problems in hills and
> valleys in the lawn, dips will cause the castors to lift the drive wheels
> off the ground and Mowbot will happily mow the air until the batteries
> run out ;-)
Yes, I mentioned this problem in the next paragraph.

> ... just move the axle forward about half way and place the castor at
> the rear. Of course doing this negates the ability to perform turns in
> the robots own diameter
It seems to me that this is quite a disadvantage, since the algorithm
for getting out of confined spaces has a lot less freedom. Not that it
is impossible of course, but it does mean you need a lot more collision
sensors and a clever backtracking routine.

> I would suggest that a slightly more rectangular or square base
> would be more suitable as it will allow us to 'thin' the robot to the
> width of the axle base.
Once we know the width of the cutter, it should be more obvious whether
the chassis can be circular. Since the cutter is going to be relatively
light, we might be able to put it in the otherwise empty space forward
of the drive wheels:
.- S -.
/ c \
| D D |
\ xxxxx /
This does have the potential problem of grounding that Dave mentions.
Though as we might well want to dynamically adjust the cutting height,
the cutter can perhaps be lifted out of harm's way in these circumstances.


R.M.O'Leary <robin nospam at> +44 973 310035  P.O. Box 20, Swansea SA2 8YB, U.K.