SawStop science

Hi all ;

Can anyone provide a scientific .. yet explanation - of how the saw-stop _detection_ works ? I saw a demonstration - and cannot figure out why : : it works on a finger : it works on a hot dog - being held by a finger .. but not on a hot dog that is not held by a finger !

I'm really missing something - and it's driving me crazy !

John T.

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Reply to
hubops
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SawStop's saws apply a small amount of electric voltage to the blade of the saw. The current through the blade is continuously monitored. If the saw detects a change in this current (as would occur if a hand or other body part came into contact with the blade) an automatic braking system is activated, forcing an aluminum brake block into the blade. The saw stops within five milliseconds, and angular momentum lowers the blade into the table. The operator suffers a small nick instead of an amputation or other more serious injury.[2] The design takes advantage of the difference in "electrical conductivity" (similar to a GFI circuit) between wood and flesh.[3]

Elijah

------ the dastardly movie villians need to use older saws now

Reply to
Eli the Bearded

Capacitance.

Better explanation than I can type:

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Reply to
Swingman

snipped-for-privacy@ccanoemail.com wrote in news: snipped-for-privacy@4ax.com:

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Reply to
Doug Miller

On 08/19/2014 5:13 PM, snipped-for-privacy@ccanoemail.com wrote: ...

...

The demo's w/ the hotdog I've seen had it on a stick swinging into the blade...altho thinking about it, perhaps it was a metal rod so there was still a sizable object of capacitance in effect in contact...

As the mechanical design article says, it's on the capacitance-matching; similar in principle to the capacitance-sensing lamp switching.

Reply to
dpb

Not sure what you mean by not on a hot dog that is not held by a finger. Have you really looked into this at all? Have you asked any questions of the manufacturer?

Reply to
Mike Marlow

I guess that is what I need to have explained ... Thanks for the web links everyone - but none of them explain - - to me - how a finger stops the saw .. but a hotdog that is not held by a finger does not stop the saw ? Anyone have any idea ? John T.

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Reply to
hubops

It was explained in the seminar < but not demonstrated > that if you fed a hotdog into the blade - but not holding it in your hand - - ie just having the hotdog held between 2 pieces of wood - the sawstop would not function to stop the blade ... it would cut the hotdog ! That's what I couldn't understand. That's what I need to have explained. Regards ; John T.

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Reply to
hubops

Not exactly electrical conductivity. Rather, as Swingman posted, it detects the capacitance of the blade. If it touches your hand, the capacitance will go up.

Reply to
krw

Question the manufacturer - not yet - the seminar was just today. Valuable info and answers from a usenet group ... you bet ! John T.

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Reply to
hubops

Capacitance is the ability to store an electrical charge (your body has capacitance).

There must also be a conductor to allow that electrical charge to flow (or show a difference in potential) along a certain path (to the mechanism of the sawstop).

Wood, is not a good conductor (unless wet) of electricity, nor does it have sufficient capacitance to store enough of charge on it own.

Therefore the hot dog, when affixed between two pieces of wood (having no, or a very week, electrical charge; and not being a good conductor) will likely not create the difference/electrical flow necessary to trip the sawstop circuit.

That's a pretty simplistic explanation, and it has been a long time ...

Reply to
Swingman

snipped-for-privacy@ccanoemail.com wrote in news:gpo7v9peibpve3l21bidlos50v750iga8r@

4ax.com:

As has already been pointed out, SawStop uses an electronic circuit to determine the capacitance -- the ability to store electric charge -- of the material being cut.

Surely it is obvious that an 80kg human body has a far greater ability to store electric charge than a 50g hotdog. Their compositions are similiar, after all, and one would expect that 1600x the mass would probably have a subtantial effect on capacitance.

Hence the circuit might be expected to react similarly to 80kg of human, and 80.05kg of human + hotdog -- but quite differently to 50g of hotdog alone.

Of course, you could always email the manufacturer and get the information direct from the authoritative source.

Reply to
Doug Miller

Maybe you should describe your knowledge about electricity and electronics first. Understanding this is not as simple as eating a hotdog. You may need to brush up before you properly approach the question.

Reply to
Bill

A capacitor "absorbs" and stores electricity up to its capacity, then releases any addition electricity sent its way. A hotdog, although having similar capacitance characteristics, is only a small mass, and capacitance of human flesh (and hotdogs) functions as a relationship of mass to capacitance content, or think of it as volume. If a human is holding the hotdog, then the hotdog and the human essentially become one and there is enough mass and capacitance "volume" that the saw can sense the change and trigger the Sawstop. The hotdog held by sticks just does not have enough mass and capacitance "volume" for the Sawstop to recognize the change.

Does that get you to the answer?

Reply to
Morgans

And to just throw a wrench in the equation, the saw stop has an override switch for cutting wet lumber and or ferrous materials.

My experience with cutting damp PT lumber is that the brake does not trip but does shut down the saw. So this probably backs up what Doug was indicating. The wet wood does not have enough capacity to store an electrical charge to trip the brake but apparently enough to shut down the saw.

Reply to
Leon

------------------------------------------------------------- "Bill" wrote:

-------------------------------------------------------------------------- Capacitance technology is older than dirt.

Most capacitance probes are found in level measurements in process vessels.

Over the years I nave probably sold 1,000+ probes that have involved either continuous level measurements in vessels that contained conductive or non-conductive contents.

The same is true of point level devices (Saw-Stop is a point level device).

Again the probe can be engineered to monitor either conductive or non-conductive materials.

The vessel can be either conductive or non-conductive.

I've sold probes to operate at temperatures of 1,000F.

Although the science of capacitance probes is rather straight forward and well documented, the real work is the application engineering required to properly define the design of the probe such that the probe solves the problem at hand.

I was fortunate to have a great application engineer, but father time punched his ticket a couple of years ago, far ahead of his time, but that is life.

As Bill has suggested, getting involved in an in depth discussion of Saw-Stop technology is a little more complex than one might think.

Lew

Reply to
Lew Hodgett

Doug's correct in the human body being the source of capacitance sufficient for the mechanism's sensors to detect the necessary difference to operate.

The moisture in the wood is what is allowing the detection of a difference in the capacitance from your body to the mechanism's sensors, not that it is storing an electrical charge. IOW, the moisture in the wood is providing the path. (Perhaps enhanced by the chemicals used to treat it.)

For the OP's benefit, here is simple way to illustrate what I was trying (poorly) to describe in response to his request for a _simple_ explanation of the basic principle.

Take an iPad or any device that uses a capacitance touch screen.

Tightly roll/fold a _dry_ paper towel to thickness of a pencil. Which would be the wood, holding the hot dog, in the OP's question.

Holding it in your bare fingers, try to use it as a stylus on your touch screen. No dice.

Wet the paper towel "stylus", squeeze the water so that it is not saturated, but moist, grasp it between your bare fingers, and stylus away.

The water has provided a path from your fingers, through the wet "wood", to allow the capacitance screen's sensors to detect the difference required to operate.

If you now wrap that wet stylus in sufficient dry paper that your fingers are no longer be in contact with moisture (the hot dog held by wood, instead of fingers in the OP's demo), the stylus will no longer work

Not exactly the same as the Sawstop, but a simple way to observe part of the underlying principle the OP requested.

(Theoretically you can do the same thing by grounding both the device and the stylus to the same plane)

Reply to
Swingman

Probably fewer than 1 in 10 have a clue, though I think the MF part would catch on fast.

Reply to
Bill

Please excuse my poor attempt at humor. Ya'll provided very good examples.

Reply to
Bill

Nice edit on the MF. ;)

Reply to
Swingman

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