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Re: Operational method of Sensys sensors
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Bob Shanteau
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Jul 20, 2009 22:25 PDT
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John Forester wrote:
| | This is a further item of information that, perhaps, has not been
noticed by some of you. The Sensys sensors operate, so Sensys says on
their website, by passively disturbing the earth's magnetic field, and
then broadcasting that disturbance to a nearby data-gathering item.
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Not quite. They sense changes in the earth's magnetic field made by
ferromagnetic materials. Since they don't have to generate a field like
an inductive loop does, they use a lot less energy, which in turn allows
them to last about 10 years before needing replacement. A rather elegant
way to detect ferromagnetic materials, to tell you the truth.
Sensys lent me a demo system early last year to test. Here are the
results: <http://rmshant.googlepages.com/Sensystest3-22-08.pdf>.
| | The only common materials that will disturb the earth's magnetic field
are iron and steel. No other common materials will do so. Many modern
bicycles contain very small amounts of these materials. Probably the
most are in the chain, the control cables, and, in most, but not all
cases, the spokes.
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The Sensys detector did not detect the back wheel of my road bike at
all. When I mentioned this to their CTO, he said it was because any
ferromagnetic material in my wheel was too far from the detector to be
picked up. I asked about my stainless steel spokes, and he said that
some alloys of stainless steel are non-ferromagnetic, and that I could
test them with a magnet. I did that and, sure enough, a magnet was not
attracted to the spokes of that wheel, although it was to spokes of
other wheels I own.
| | This is an entirely different situation than exists with the loop
detectors, for they detect materials that form loops of electrically
conductive material. There have been many misguided claims over the
years that loops detect magnetic materials, but that is false; the
detection of iron or steel depends, in their case, only on their
conductivity, not on their magnetic material.
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I discuss how inductive loops work and how they detect bicycles
(actually the metal rims) in this presentation that I gave to the AB
1581 Subcommittee:
<http://rmshant.googlepages.com/BicycleDetectionatTrafficActuatedSig.pdf>.
Bob Shanteau
Transportation Engineering Liaison
California Association of Bicycling Organizations
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