Re: [mowbot] RFD: Define reference 'grassbox' for mowbot

Dave Everett (deverett nospam at
Wed, 13 May 1998 11:42:33 +1000

At , you wrote:
>> >However after all this time I still feel like I'm lacking to knowhow,
>> >and equipment to develop a working mowbot.

I will start a new thread after this message called 'A basic platform' to
add this issue, it will nto deviate into sensor design, intelligence or how
much cheaper it would be to hire 5 gardeners for fifty years to mow your
lawn for you <G>

>I just think if we can define something, anything that folks can get
>their hands dirty with, it'll jumpstart the discussion.
>> The biggest problem with motors is that the most suitable one for your
>> project is always very expensive. Mostly I've restricted my projects to
>> what is cheaply available. This is not always a good idea, it's pointless
>> designing a great robot if it has trouble pushing itself along.
>OK help me here. Let's presume the playfield is low voltage DC motors
>(12-24 volts). I know of a two types:
>1) Typical small hobby motors.
>2) Large motors like winch drivers and car starters.

I chose motors used for car electric windows, they came from
and cost $14.97 each and outputs 40RPM, the part number is DCM-64. All
Electronics have other motors that may be suitable, and specifically
another electic window motor with an output speed of 120RPM.

Worm drive means, the motor spindle is connected to a gear that looks like
a threaded rod, another ordinary gear meshes with the worm gear at 90deg,
these motors are not backdriveable, that is, you cannot turn the output
shaft by hand because the output gear locks in the worm gear threads.
>BTW what is expensive. All the toy motors are cheap. The others?

Toy motors generally require extensive gearing to do the job and don't have
the power required.
>What about salvage. Do motors from equipment like cordless screwdrivers
>or cordless drills fit the bill? They are commodity enough to purchase
>and strip.

Would probably be ok, but they often use the drill case as the motor
housing, holding the bearings and brushes.
>OK. Define wormdrive please. What's the RPM, torque, amps, voltage, price,
>and distributor of suck motors? What's the drivetrain.
Well they don't exactly suck, but here are the details ;-)
The RPM is 40, torque I haven't calculated, amps are between 0.6-4, 12V,
>Am I asking stupid questions because this all on the website? I haven't
>looked in over a year...

A little of it is on the website

>> One problem with direct drive motors to wheels is that going down an
>> incline, you can exceed your safe speed (for cutting and stopping). If you
>> were normally PWMing the motors to control speed, you now need to PWM the
>> dynamic braking input or imitate dynamic braking if your H-bridge doesn't
>> have one built in.
>I think the bridge that Paul suggested has what's called regenerative
>braking. For less than $30 US I think that's a component I'm willing to
>buy instead of build.

Regenerative braing means that when not accelerating, the motor can become
a generator restoring a small amount of the battery power consumed, I don't
know how successful this would be on a mowbot travelling at slow speeds, I
think you need around 1500rpm at the motor to get regeneration. I'd also
check on the current required to run the boards without motors, it mightn't
be too much but you need to know.

I just checked the website, no mention of supply current, however I note
that braking requires a separate magnetic brake, you get some slowing from
regen, but obviously to stop fully you need the separate electromagnetic

>> On your cutter motor problem. I will be using 2 cutters so I can monitor
>> average voltage to determine cut and uncut areas roughly. I have chosen a
>> Mabuchi motor that is readily available at hobby stores RS-380PH. The motor
>> is small, can run up to 14000rpm and has fair torque (I used the motor in a
>> lead screw portion of a walking robot some years ago). If you need more
>> torque, you could try the motors used for electric model aircraft.
>Aha! That helps. So I need to visit my local hobby shop. WIll do.
I have a ROBBE Power 700/13 Turbo motor that I used in an electric edger
project. It directly drove a plastic blade that is used for grass cutting,
it cost me about 40 bucks though. If you need more power, then gearing is
the way to go. You need speed to cut the grass cleanly, and torque to keep

I'll address the other stuff in my next message

Dave Everett

Mowbot Project Website