Hi all ;
Can anyone provide a <somewhat > scientific .. yet
<simple> 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 !
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. The design takes advantage of the difference in
"electrical conductivity" (similar to a GFI circuit) between wood
the dastardly movie villians need to use older saws now
On 08/19/2014 5:13 PM, firstname.lastname@example.org 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.
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.
On 8/19/2014 7:04 PM, email@example.com wrote:
Capacitance is the ability to store an electrical charge (your body has
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 ...
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
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
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)
Exactly however I assumed that any path at all was enough to trigger the
brake. I have been very careful,so far, to not cut into my aluminum
miter gauge fence, something that we all apparently do. ;~) My
assumption was that the wet wood would have the same effect as cutting
the fence while touching the fence with you hand or finger and or
cutting the hot dog while touching the dog with your hand or finger.
In the case of the wet wood there was no brake trip rather after cutting
a few inches the saw coasted to a stop repeatedly until I engaged the
over ride switch. So there must be some measure of the signal from the
blade to a conductive source that decides whether to brake or simply
shut down the saw.
Apparently the saw can detect the difference and for that matter I would
suspect that the saw top also has the ability to absorb the charge at
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 ?
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.
firstname.lastname@example.org wrote in news:gpo7v9peibpve3l21bidlos50v750iga8r@
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
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?
Capacitance technology is older than dirt.
Most capacitance probes are found in level measurements in
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
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.
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