Re: [mowbot] Motors arrive

Dave Everett (deverett nospam at
Wed, 05 Mar 1997 17:03:38 +1000

Byron A Jeff wrote:
> I was going to come in a post this morning too. It has been raining like
> crazy here in Atlanta and there have been record highs (80 degrees!). Little
> tuffs of grass are starting to pop up! Sigh!

Hehe I know what you mean. We had a few days of rain and now the grass
is about 1/3 up my shin.
> Anyway just to re-interate my agenda in priority order:
> 1) Mobile platform than can move in a straigt line at up to 25o incline.
> 2) Cutting tool that reasonably cuts grass.
> 3) Everything else (edge detection, low power go home and feed circuit, auto
> grass cutting)
> A couple of weeks ago I was intently examining a childs 16 inch real bike
> wheel. It seemd to be real easy to mount, uses a standard bike chain
> (obviously) and seemed really sturdy. Does anyone have any comments on the
> possibility of driving a pair of wheels with gearmotors such as above. Of
> course the gearing for bike wheel are such that they only drive in one
> direction, the other causes braking. However it seems to me that steering
> can be accomplished by stopping one wheel and driving the other, which would
> effectively spin the bot around the braked wheel.

Yep, but how would you backup if you need to? I can imaging the mower
going right up to a wall to mow and then not being able to turn because
the front is right against the wall. 16inches sounds pretty big, I've
been trying to keep the robot as small as possible to make it
maneuverable as possible. The wheels I'm using are Rover lawnmower
wheels, sealed plastic (so they don't fill with water), a good tread and
about 175mm in diameter (7 inches). At 30rpm I get around 1km/hr while
keeping the required torque down. 16 inch wheels will move faster with a
slower motor, but they will increase the required torque.
> Here's a couple of really crufty ASCII pictures to illustrate:
> You get the basic idea. I figure if the body is wide enough that it'll be
> difficult to tip it. I have it low to the ground to keep the Center of
> Gravity Low.

This is fine on a flat surface, but what will happen when you encounter
rough ground? It looks like the driven wheels could get lifted off the
ground if the robot travsersed a small dip. I'll be using 2 driven
wheels near the front with a single idler at the rear, this way all 3
wheels should always be in contact with the ground.
> This isn't real well thought out. I'm not a mechanical type guy. I mostly
> do computers and electronics. But if I can put together a platform that can
> wander around in my yard then I'm moving a lot closer to a MowBot.

I'm no mechanical guy either <g>, like you I just need a platform, I'm
quite comfortable with the other issues.
> Any updates on cutting technology? Anyone consider simply using a real
> lawn mower attached to a base such as above. Real lawn mowers definitely
> have the correct cutting technology, they just don't have the correct
> platform for autonomous (sp) motion.

I've looked at using a standard mounting wheel from a mower with the
swingout metal blades, but it just looked too heavy. The electric motor
to drive it would be quite a drain on the power supply. I did find a
light small plastic blade for cutting grass, when the parts come back
from the engineer I'll be able to run some experiments with it. I'm
getting him to put together a leadscrew lift system for the blade motor
so I can vary the cut height.
As an aside I was considering the problem of speed regulation on sloping
ground. The gearmotors I got are worm drive so they are not
backdriveable. This is important. If you imagine the robot going down a
slope under PWM control, it will pickup speed and be uncontrollable.
Worm drive fixes this as there is no freewheeling point because of the
worm mechanism. If someone is considering using a backdriveable motor I
would recommend switching to a PWMed brake control during movement down
a slope. the LM18200 motor driver IC has a brake input so you could
switch to that mode under those circumstances.

Dave Everett.