Re: detecting a magnet
From: Charles Edmondson (edmondson_at_ieee.org)
Date: 02/04/05
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Date: Thu, 03 Feb 2005 16:59:37 -0800
Mark wrote:
> Bob Eldred wrote:
>
>>"Brian" <brian@w3gate.com> wrote in message
>>news:VJadnZKl8qofnp_fRVn-tA@centurytel.net...
>>
>>>If I had a flat, 2 foot by two foot coil lying flat, how far above
>
> it
>
>>could
>>
>>>I reasonably expect the detect a rare earth magnet, perhaps the
>
> size of an
>
>>>M&M?
>>
>>An ordinary inductive coil requires the magnet to be in motion and
>
> the
>
>>voltage induced is a function of the number of turns and the rate of
>
> change
>
>>of the flux, dphi)/dt. Therefore the speed of a moving magnet is very
>>important. Practically you might be able to sense the magnet moving a
>
> foot
>
>>away or so depending on the number of turns in the coil and the speed
>
> of the
>
>>magnet. It is a signal to noise issue and power line interference
>
> (hum)
>
>>would limit what you can detect.
>>
>> However it is possible to detect small stationary magnetic fields
>
> (DC) by
>
>>arranging and energizing coils in a device called a flux gate. These
>
> things
>
>>can be designed to measure fields as low as 1/1000 of the earths
>
> magnetic
>
>>field or even lower, less than a milligauss and could easily measure
>
> your
>
>>magnet many feet from the fluxgate. Being DC, it's easy to get rid of
>
> the
>
>>power line interference. There is also hall effect devices which has
>
> been
>
>>mentioned but they are not particularly sensitive. Beyond that, there
>
> is a
>
>>quantum mechanical device called a "squid" that can measure
>
> extremely small
>
>>fields like the field from the iron in a single blood cell. These are
>>probably way beyond what you need or could pay for but it gives you
>
> an idea
>
>>of the technology available. What are you trying to do?
>>Bob
>
>
>
> I'm guessing, maybe wrongly that the OP is interested in those vehicle
> detection loops burried in the ground at red lights. These sometimes
> fail to respond to motorcycles and some people sell magnets that you
> can attach to your motorcycle that alegedly allow the coil to see your
> motorcycle.
>
> I believe the principal of these coils is a tuned resonant tank that is
> detuned by a large hunk of metal. I don't believe that the magnet
> helps in this case but there are people out there that will swear that
> they do work.
>
> Mark
>
System used by 3M was to basically use it to tune an oscillator. When
metal entered the loop, the frequency changes. Traditionally, you had a
filter for the frequency, and detected when it changed enough to no
longer pass signal. 3M just used a digital counter to measure the
frequency. When count changed by about 5, a car was there!
-- Charlie -- Edmondson Engineering Unique Solutions to Unusual Problems
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