Re: [mowbot] cutting motor details

Lawrence Lile (lilel nospam at
Mon, 3 May 1999 13:50:14 -0500

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.

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?

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

-----Original Message-----
From: david steinke <dsteink nospam at>
To: mowbot nospam at <mowbot nospam at>
Date: Monday, May 03, 1999 12:04 PM
Subject: Re: [mowbot] Questions?

>My cutting motor is a Ametek brushless motor from a 24 vdc blower that
>they make. On 12 volts it turns 4200 rpm unloaded and draws 0.6 amps.
>The controller uses a PIC 16C84, International Rectifiers IR2130 three
>phase mosfet bridge driver, and IRIZ48G mosfets. My cutting blade is a
>4" diameter aluminum disk with two 3" wall paper scraper blades
>extending out 2" from the disk's edge, for a total of 8" cutting
>diameter. I have a Master Airscrew 8x6 prop mounted on top of the disk
>for air suction to lift the grass. I have four 1" holes cut into the
>aluminum disk for the down thrust of air. Current draw with the prop and
>cutting blades (not cutting grass) is 2.6 amps and rpm drops to 4000.
>Putting the chasis out to cut grass that is about 1/2" too high and
>traveling at a speed of 2" per second increases the current draw to 3.4
>amps. With drive motors the total current draw is 3.7 amps.
>Right now my power source is a 12 volt 7 amp/hour gel.
>> > b. Could I possibly do it with an infared beam pointed south along the
>> > eastern edge? If I use a standard 40khz infared sensor with a narrow
>> > horizontal aperture (less than 1 degree) and a simple 40khz modulated
>> > beacon, the robot could rotate on the eastern edge until it is
>> > Since the robot can also rotate precisely (more or less), the robot
>> > could just reorientate the on the eastern edge. Initial homing would be
>> > more time consuming, but still easy.
>> This is more along the lines of what I was looking to do. I plan to have
>> enough units (3 or 4) that pulse at different rates so that the bot can
>> triangulate its position from the beacons. Like I said in my last post
>> thinking of have solar powered self contained units that will act as a
>> responder when the mowbot asks for a position, Thus not requiring
>> a bunch of wire around the perimeter.
>I was hoping of using a single beacon powered by solar power (converted
>solar powered path light). Since I do have 3 straight sides of my yard,
>I only have to adjust for 1 ragged edge. I can reorientate myself once
>every back and forth loop. The only time I acutally care where I am is
>at the beginning and the end. All the rest of the time I only care about
>> >
>> > c. Is there a simpler way?
>> Not sure. I'd experiment with the compass. I kinda like that idea because
>> navigation is completely self-contained on the bot.
>> BAJ
>The more I read about the compass, the bigger the problems I find. As an
>experiment, I took a standard compass and moved it withing 1 foot of my
>chasis with everything off. The compass went haywire depending on its
>location to the chasis. I turned everything on and the compass was still
>acting up. I don't see how an electronic compass would correct for this.
>I could shield my motors and wires and try again I guess.
>David Steinke