>-----Original Message-----
>From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] 
>Sent: 15 August 2005 19:33
>To: Microcontroller discussion list - Public.
>Subject: RE: [OT] [EE] Lawn mower rock sensor
>
>
>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.

Which is why I suggested the filtering.  You are interested only in the rate of change of RPM rather than absolute RPM.  Hitting a big rock will without doubt slow the blade down very quickly, though possibly not by a very large amount and not for very long as the speed difference has to be absorbed by the transmission (which is why you would need the sensors on the blade hubs rather than the PTO shaft).  It might only slow the blade down for a part of one rotation, but with enough teeth to sense RPM this should be detectable.  

Hitting a patch of heavy grass will probably slow the blades down more as it's not such a transient load, but the rate at which the blades slow would be much lower than hitting a large rock.

A mechanical system would probably be the best soltuion in terms of robsutness, such as a clutch that releases above a preset torque and has to be manualy reset, but of course the cost of such solution would likely be prohibitive.  Another possibility is to have some kind of cush drive between the blades and the transmission which can aborb shock loads more effectively, this should lower the peak loading on the blades somewhat at least.

Regards

Mike

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