Re: [mowbot] A heavy mower

Davidson (Alex.Davidson nospam at
Mon, 14 Jul 1997 09:07:54 +0000

> Date: Sat, 12 Jul 1997 22:48:13 +1000
> To: mowbot nospam at
> From: Dave Everett <deverett nospam at>
> Subject: Re: [mowbot] A heavy mower
> Reply-to: mowbot nospam at

> At 04:23 PM 7/9/97 +0200, you wrote:
> >Computer: PC-XT 1 MHz
> >Command sender: 27 MHz 1/4 watt driven by serial computer output.
> >Command receiver: serial decoder chip provides 8 discrete commands,
> > five now in use.
> >Location: 17 KHz pulse received by two mikes, delay measured
> > by computer (using parallel port) allows
> > triangulation.
> >
> >Problems:
> >Sonar location repeatability ca. +/- one meter.
> >Determination of actual direction with this poor accuracy is
> >quite uncertain.
> Have you considered using 4 mikes, like a doppler direction finding
> array? The mikes would be spaced in a square.
> >Motors interfering with radio reception is maddening, fixed by
> >extensive shielding, bypassing, etc. 27 MHz is a rotten frequency,
> >need much higher frequency for dependability and short antenna.
> What are the chances of you mounting the XT on the robot to avoid the
> comms transmission problems?
> >
> Dave Everett.
Hi Dave, thanks for your interest. This weekend I replaced the
sloppy steering connection with a firmer assembly, and the thing
holds direction better; it used to wander about unpredictably.

Mounting a computer on-board is, at the moment, beyond my budget.
Good idea though.

Could you please explain how I could determine direction with four
mikes in a square? I doubt I could measure any Doppler effects at
the 14 cm/sec current crawl speed.

A recent software improvement was to take a row of pings, and select
the return with the shortest range. As the pinger warms up, the
rise time (or frequency) of the output changes, giving erratic
results. A more powerful pinger would be nice. Right now it is a
piezo tweeter. A transformer steps up the signal from 12 V
peak to peak to 70 V p-p.

Best regards,
Alex Davidson
Tel: (02203) 601 3423
Fax: (02203) 601 2875
E-Mail: alex.davidson nospam at