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'[OT] [EE] Lawn mower rock sensor'
2005\08\15@114743 by Michael Rigby-Jones

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{Quote hidden}

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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2005\08\15@143301 by KY1K

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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 removed}

2005\08\15@145133 by Michael Rigby-Jones

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>-----Original Message-----
>From: piclist-bouncesspamKILLspammit.edu [.....piclist-bouncesKILLspamspam.....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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2005\08\15@191308 by Dwayne Reid

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At 12:33 PM 8/15/2005, KY1K wrote:
>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.

I'm not sure if its been suggested yet, but how about installing a set of
swing blades?  These swing loosely on the hub and centrifugal force caused
by high-speed rotation causes them to open to nearly the full
diameter.  They simply fold out of the way and make loud noises when the
tip of the blade hits an immovable object.

They aren't perfect but certainly seem to minimize the kind of damage you
mentioned.

dwayne

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Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
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2005\08\15@224234 by Denny Esterline

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One other thought, you might want to look into a different type of mower
altogether. A chain flail does a fine job and barely twitches if you hit a
rock.

(A flail mower is a long slender drum run horizontally with cutters attached
along it with chain links. It rotates at high speed and when one of the
cutters strikes something hard, it just folds over. Centrifugal force pushes
it back out in time for the next blade of grass. Commonly used for the sides
of highways and other 'rough' areas)

I had one many years ago on a small farm tractor (about 12 hp) worked great.

-Denny

2005\08\16@161423 by Peter

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On Mon, 15 Aug 2005, Dwayne Reid wrote:

> At 12:33 PM 8/15/2005, KY1K wrote:
>> 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.
>
> I'm not sure if its been suggested yet, but how about installing a set of
> swing blades?  These swing loosely on the hub and centrifugal force caused by
> high-speed rotation causes them to open to nearly the full diameter.  They
> simply fold out of the way and make loud noises when the tip of the blade
> hits an immovable object.
>
> They aren't perfect but certainly seem to minimize the kind of damage you
> mentioned.

Even better, why not use the 'string' blade type ? That and a rounded
hub and who cares about stones and concrete ? No ?

Peter

2005\08\17@100757 by John Ferrell

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I have and use a "DR String trimmer" The biggest problem is the short life
of the strings, I spend almost as much time maintaining as I do cutting.

John Ferrell
http://DixieNC.US

{Original Message removed}

2005\08\17@155126 by Peter

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On Wed, 17 Aug 2005, John Ferrell wrote:

> I have and use a "DR String trimmer" The biggest problem is the short life of
> the strings, I spend almost as much time maintaining as I do cutting.

I understand that the more serious units are not nylon but thin steel
wire of square (?) profile. Also there is a little reel of wire in the
hub and when the outer part gets too short you pull out more. It has
been a few years since I saw these, and they were not tractor mounted. I
was told you can trim dry wood with the steel wire (!)

Peter

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