>-----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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