Re: [mowbot] Questions?

david steinke (dsteink nospam at
Mon, 03 May 1999 09:33:49 -0700

Byron A Jeff wrote:
> > My mechanics are close to being done. The robot can rotate within its
> > own area. It has a ground speed of 2 inches per second and it can go
> > foward, reverse, and rotate in either direction. It can also count wheel
> > rotations/angle change from an optical encoder on the drive motors.
> I think I saw your web page URL from the PICLIST mailing list. Can you repost
> it? Also tell us more about the drivetrain. What kinds of motors? Axels?
> How is the encoder implemented? This is an area of extreme interest for me
> because I'm trying to finalize my platform/drivetrain design.

Here is how I am set up. My platform is a 15" diameter circle. I have
two 6" drive wheels mounted within the inside edge on a main diameter.
The drive motors are Pittman brushless with HP optical encoders. The
controller is a combination of a PIC 16C84 and an Allegro UDN2936
brushless controller/driver. Each motor drives an 100:1 worm gear box.
The optical encoder feeds directly into the PIC. I can program speed and
run length. No load current is 90 ma, and running current is about 150
ma. I program the motor controller through an opto isolator using RS-232
type format. The platform uses a 3rd freely pivoting wheel mounted on
the same radius as the drive motors, with the battery weight mounted
over it to maintain balance.

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 oriented.
> > 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 I'm
> thinking of have solar powered self contained units that will act as a
> responder when the mowbot asks for a position, Thus not requiring stringing
> 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.

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