Re: [mowbot] motors, requirements

robin nospam at
Fri, 27 Sep 1996 00:27:04 +0100

Dave Everett <Deverett nospam at> wrote:
> George Phillip Atkins wrote:
> > Checkout WEB site:
> > They have 12V, gearhead motors rated at 1.5amps (175rpm) and 12.6V rated
> > at 600ma (40 rpm) for US $17.50. Look like motors from electric auto
> > window units to me.
> They both look like excellent possibilities to me, especially at the
> price. The 175rpm version would be good as it could be PWMed to lower
> speeds.
We also need to know some other details like the torque (start, running
and stall) and if there is a duty cycle. Some of the small, powerful
motors I've seen looked promising but aren't designed for continuous
running (typically being designed for special purposes, like window
winders:-). Either they get hot quickly, or their bearings aren't up
to carrying off-axis loads.

>From experience of parts I've seen and used, I'd say we really need to
buy motors with integral gearboxes. It is such a hassle having to mate
and mount gears and motors yourself. Getting everything just right,
and making sure it stays that way is hard. This is particularly true
given Mowbot's likely environment: dirty and wet with lots of vibration.
A sealed unit that takes current in one end and has a (say) 60rpm shaft
at the other is very attractive.

Referring to my earlier notes about rotation speed and wheel size;
assuming we don't add any extra gears, then practical considerations
that constrain wheel size necessarily constrain motor speed too.
If we expect wheels to have a radius r somewhere between 0.03m and 0.2m
(the extremes I can imagine), and the linear velocity v to be between
0.1m/s and 0.3m/s, then since the rotation speed S is given by
S = v/(2*pi*r)
the limits on S are 0.08 and 1.6 revolutions/second (5 to 96 rpm).
For rough terrain we need bigger wheels, but that probably means we
need to use a motor with a lower gear ratio and hence more torque.
We can always slow a motor down by modulating its supply, so getting
one that goes faster than needed is possible (but perhaps wasteful).

> Can you find out if these are just surplus stocks? Or are they stocked
> on a continuous basis?
I know I recently suggested that off-the-shelf parts be a requirement, but
bearing in mind our geographic dispersion, that is actually a very hard
requirement to fulfil, raising as it does the question of ``whose shelf?''
We'll be OK for semiconducters, but availability of mechanical stuff
seems to suffer a lot more variation. So perhaps we should also consider
some parts that could be got through co-operative purchase arrangements.
That way, we wouldn't automatically rule out George's motors just because
I can't get them in the UK. I am reluctant to go this route if there
is an alternative, but I thought it should be mentioned.


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