Not sure the blade speed monitor will work, I'm talking about large 
rocks that are partially buried. When the blades hit a rock, they 
don't really slow down much.....mostly because the housing tends to 
vibrate and flex to allow the blades to continue turning.

Also, the blades slow down alot during the course of regular 
mowing...... grass tends to be a variable load, not constant at all. 
I'm afraid anything that worked by detecting speed reduction of the 
blades would detect normal mowing and not hits to partially buried rocks.

We do have a guy with a bulldozer coming soon to dig some of the 
rocks up.....a process that is repeated often here. But, we always 
miss a few and/or want to reclaim another brush overgrown area to 
mowable lawn' status.

When a rock is hit, it makes a very loud noise which doesn't have 
alot of high frequency components..so a sensor that is more sensitive 
to higher frequency sound or vibration probably won't do much good.

I'm really open to suggestions though, blade sharpening and 
replacement ain't cheap. Also, a slow responding 
electronic/mechanical sensor is still 100 or more times faster than 
my reaction time and will really reduce the damage caused by hitting rocks.

Also, sorry for posting without a title to the message, didn't notice 
it until I saw the message on my email list. Thanks for adding the 
title in for me.

Art



At 11:46 AM 8/15/2005, you wrote:


> >-----Original Message-----
> >From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu]
> >Sent: 15 August 2005 14:49
> >To: Microcontroller discussion list - Public.
> >Subject: [OT] [EE]
> >
> >
> >Good day all,
> >
> >I have to mow lawn that has rocks and similar lawnmower blade
> >damaging hazards. I use a John Deere riding lawnmower that has a
> >solenoid to engage and disengage the drive to the blades.
> >
> >It's hard on the blades when they hit rocks and other obstructions
> >because I can't react fast enough to disengage the solenoid to stop
> >the blades from turning.
> >
> >Needless to say, the blades get dull fast and rarely last more then 2
> >years and have to be re-sharpened every month or so.
> >
> >I'd like to put some sort of sensor in that would disengage the
> >blades automatically when the blades hit something solid and I would
> >like some suggestions from the group.
>
>
>A possible solution would be to monitor the RPM of the blade hubs 
>using an inductive sensor and teeth on the outside of the hub (i.e. 
>just like a crank position sensor on a fuel injected engine).  As 
>long as you have enough teeth, you can detect changes in RPM very 
>quickly, and inductive sensors are robust, water tight and readily available.
>
>To prevent the unit tripping too frequently you would probably need 
>to implement some basic filtering to weed out small but fast changes 
>caused by small stones or sticks, or larger but slow changes caused 
>by e.g. a patch of thick grass
>
>Regards
>
>Mike
>
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