Re: [mowbot] cutting motor details

david steinke (dsteink nospam at
Mon, 03 May 1999 13:28:16 -0700

Lawrence Lile wrote:
> Interesting details on the cutting motor.
> Hey, why such a complex driver for the cutting motor? Is a 3phase bridge
> driver overkill? I'm planning on just using a single logic level mosfet
> IRF540. The cutting motor never has to reverse, just start and stop.
> Perhaps I am not understanding your description.

It is a brushless motor. Commutation is done by switching voltages on
the 3 winding leads, hence 6 output transistors and fancy driver
circuit. I use them because they last up to 100 times longer and are
more efficient.

> I am also curious about your use of airflow in the cutting motor. I
> understand that moving air in mowers is a big power user. Is it possible
> just to cut the grass off and let it lay, and use less power?

The problem is that grass doesn't stand up straight. When dogs and kids
get run all over the grass it really does stand up straight. Eventually
the grass will rebound and straighten out again, but the mower may have
already cut the grass. By using suction to straighten the grass blades
out all the blades of grass get cut the same length. Also a great deal
of the air moving in the power mowers is for throughing the grass into
the bag. I am shooting for the mulching blade cycle.

> I am also planning on solar power, and am trying to trim the power budget to
> the minimum.

I thought of the nibbler/nipper, where I can keep my current draw down
below my solar panel output (12 watts), and only cut in the sun with a
random pattern. I have made provisions for it (extra side bumper
sensors, etc), but am worried about the constant cutting of the grass.
Here in Arizona they say it is better to cut 1/4" off the grass 4 times
a week than 1" off once a week. I have a hard time accepting that since
you are oozing out moisture every time you cut. I have to water the lawn
within 24 hours of cutting to prevent it from browning. Anyboday a grass

David Steinke