On 12/4/2012 8:11 AM, Joseph Gwinn wrote:
....

The problem with SawStop has been that each false alarm requires
replacing various mechanical components, which takes time and money. It
doesn't make too many false alarms to make guards more economical.

My recollection is that the sensor measures resistance (not capacitance)
between saw blade and ground, so it won't work with wet wood or metal
stock. The patents should tell the tale.

AFAIK there's no data available from SawStop on the number of false
positives (and, of course there's no data on how many users turn the
bypass control on to be sure to avoid any chance of same when working
with wet wood such as PT).

The best one can tell from the patent is

"In many of the exemplary embodiments and implementations described above
and in the references incorporated herein, detection subsystem 22 relies
on the inherent electrical capacitance and/or resistance of the user's
body to modify the effective capacitance and/or resistance of the
cutting tool or operative structure."

The patent is (as are all well-constructed ones) written to cover not
only the actual "embodiment" but every possible variation of it that the
applicant can manage to stretch the colorful language to cover. In
early interviews it was pretty clear that it is capacitive coupling that
was used at least in the initial product and I presume it is likely
still the same fundamental technology used in the current saws w/ some
improvements owing to the experience factor. It's the same idea as the
"touch switches" on table lamps, etc. -- the body has much higher
capacitance owing to the water content than does dry lumber and if the
blade monitor sees a drop it presumes that's not good and fires.

It's a clever technology and worthwhile--my complaint w/ Gass is his
attempt to now force it on the market to make a bundle by
legislative/mandatory means rather than by winning the competitive
battle in the marketplace.

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