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detecting a magnet

B

Brian

Jan 1, 1970
0
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?
 
D

Don Pearce

Jan 1, 1970
0
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?

Roughly proportional to how fast the magnet is moving - give us a
clue.

d

Pearce Consulting
http://www.pearce.uk.com
 
B

Brian

Jan 1, 1970
0
say it crosses the plane in 2 seconds.

Are there ways of detecting a stationary magnet?
 
D

Don Pearce

Jan 1, 1970
0
say it crosses the plane in 2 seconds.

Are there ways of detecting a stationary magnet?

A Hall Effect sensor is the only option for a stationary magnet. It is
also the best choice for such a slow moving magnet.

The magnitude of signal you get depends on stuff like how strong the
magnet is.

d

Pearce Consulting
http://www.pearce.uk.com
 
S

Spehro Pefhany

Jan 1, 1970
0
A Hall Effect sensor is the only option for a stationary magnet. It is
also the best choice for such a slow moving magnet.

Ah, flat statements are (almost ;-)) always wrong. What about a
fluxgate senor? Or a reed? Or magnetoresistive (eg. GMR) sensor?
There are probably others I've forgotton

But the first would require core material, and the others don't use
the coil that the OP posited.


Best regards,
Spehro Pefhany
 
B

Bob Eldred

Jan 1, 1970
0
Brian said:
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
 
G

Gregory L. Hansen

Jan 1, 1970
0
say it crosses the plane in 2 seconds.

Are there ways of detecting a stationary magnet?


A fluxgate magnetometer (Google search terms) can probably be built easily
enough by the hobbyist. You need a high permeability material like
mumetal, a few solenoids, and electronics.

E.g. http://beale.best.vwh.net/measure/fluxgate/
 
L

Leon Heller

Jan 1, 1970
0
Don Pearce said:
A Hall Effect sensor is the only option for a stationary magnet. It is
also the best choice for such a slow moving magnet.

The magnitude of signal you get depends on stuff like how strong the
magnet is.

A SQUID will work, as well. They work best when they are very cold.

Leon
 
J

Joerg

Jan 1, 1970
0
Hi Spehro,
Ah, flat statements are (almost ;-)) always wrong. What about a
fluxgate senor? Or a reed? Or magnetoresistive (eg. GMR) sensor?
There are probably others I've forgotton

Yep. Reed is really easy, just take a reed relay and use only the inner
glass body with the contacts. That's how I usually did it. It cost next
to nothing but that might have changed. Of course, it'll age after
umpteen thousand cycles and once I had a reed contact break off and
float about in the little glass tube.

Such contacts can also be purchased separately with proper mounting
holes, matching screws and all. They are used in alarm systems to detect
when an intruder starts to move a sliding window (the magnet slides
away). Possibly a store like Radio Shack has these.

Regards, Joerg
 
B

Brian

Jan 1, 1970
0
Bob Eldred said:
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 am tryning to accomplish "mental exercise". Sometimes I see things and it
just makes me ponder how to do it.

This "application" could be many, but is similar to RFID I suppose. Tagging
something, but without identification and a completely passive tag. Could be
to see if a box is on a pallet, or a dog is in his doghouse. Basically
thinking of ways to have a flat "mat" and detect the presence of something
within 2-4 feet directly above it.

In the end, I do it cuz I am a geek :)
 
J

John Larkin

Jan 1, 1970
0
Ah, flat statements are (almost ;-)) always wrong. What about a
fluxgate senor? Or a reed? Or magnetoresistive (eg. GMR) sensor?
There are probably others I've forgotton

Semi-seriously, you could use a compass and an led-phototransistor
sensor. You might resolve milligauss that way.

John
 
M

Mark

Jan 1, 1970
0
Bob said:
it

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
 
M

Mark

Jan 1, 1970
0
Bob said:
it

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
 
B

Brian

Jan 1, 1970
0
Much smaller scale, much smaller target, proportionally much longer range.
 
G

Guy Macon

Jan 1, 1970
0
Don said:
A Hall Effect sensor is the only option for a stationary magnet.

Dang! Now I have to throw away my chunk of iron glued to a switch!
 
C

Charles Edmondson

Jan 1, 1970
0
Mark said:
size of an


of the


(DC) by


is a


extremely small


an idea




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!
 
K

Ken Smith

Jan 1, 1970
0
Ah, flat statements are (almost ;-)) always wrong. What about a
fluxgate senor? Or a reed? Or magnetoresistive (eg. GMR) sensor?
There are probably others I've forgotton


A SQUID would work. PNI makes a magneto-inductive sensor that uses a
nonlinear core to detect the field at a lower cost than a fluxgate.

BTW: A good flux gate will detect a cow magnet up to about 100 feet. A
SQUID only improves on this distance by about a factor of 20.
 
K

Ken Smith

Jan 1, 1970
0
Gregory L. Hansen said:
A fluxgate magnetometer (Google search terms) can probably be built easily
enough by the hobbyist. You need a high permeability material like
mumetal, a few solenoids, and electronics.

E.g. http://beale.best.vwh.net/measure/fluxgate/

I think 2 toroids inside two solenoids will work better.


You place a toroid inside a solenoid with its axis at right angle to the
bore of the solenoid. You drive the toroid with an AC current that just
saturates it in each direction. The solenoid coil will develop a voltage
proportional to the external field at twice the drive frequency.

Such a design will have trouble with picking up stray AC fields. If you
place 2 side by side, drive the toriods at 90 degrees to each other and
wire the coils to subtract, you get more than twice the signal and a lot
less pick up.
 
B

Bob Eldred

Jan 1, 1970
0
I am tryning to accomplish "mental exercise". Sometimes I see things and it
just makes me ponder how to do it.

This "application" could be many, but is similar to RFID I suppose. Tagging
something, but without identification and a completely passive tag. Could be
to see if a box is on a pallet, or a dog is in his doghouse. Basically
thinking of ways to have a flat "mat" and detect the presence of something
within 2-4 feet directly above it.

In the end, I do it cuz I am a geek :)

Somebody below mentioned a car detector that used the metal locator
principle of changing an oscillators frequency when a metal object (car)
alters the field of a coil. This wouldn't work very well on a dog or a
nonmetalic box but changing frequency by capacitance would. There is a
musical instrument called a Theramin where the frequency of an oscillator is
varied by waving ones hand over an antenna. This is beat with another
oscillator producing an audio tone played through an amp and speaker. You
can hear these things in space movies and other places where weird etherial
music is used. In any case you could make a dog detector that worked on
this principle; i.e., varying the frequency of an RF oscillator by changing
the capacitance in a tank circuit. An oscillator running at several hundred
kilohertz having an insulated metal plate connected to its tank could easily
detect a dog or other nearby object by change in frequency. No magnets or
metalic objects required. Do a search on Theramin and see if that idea might
meet your needs.
Bob
 
G

Gregory L. Hansen

Jan 1, 1970
0
Ah, flat statements are (almost ;-)) always wrong. What about a
fluxgate senor? Or a reed? Or magnetoresistive (eg. GMR) sensor?
There are probably others I've forgotton

Put a solenoid on a crank and spin it around.
 
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