There
are a number of technologies that are used to sense magnetic fields.
I have also included some places where magnetic fields are used
like in MRI, NMR, particle accelerators that use super conducting
magnets.
1
Oersted = 79.58 Ampere-Turns/Meter
1 nano tesla = 10 micro Oe = 1 Gamma
1 Tesla = 10,000 Gauss Papers about Magnetism -
Journal of Magnetism
and Magnetic Materials - For over 100 years, Elsevier Science
has been dedicated to facilitating the exchange of information.
The Magnetic Measurements Handbook -
The
direction of the horizontal component of the Earth's magnetic
field can be determined by a compass. These range from the
"Cracker Jack" type to very high quality models.
Note
that the Earth's
magnetic field can be thought of as having three components
(Calif. values shown):
down
in the direction of gravity = 44,862 nt
to
true North (not magnetic North) = 22,460 nt
and
an East-West component at right angles to true North = 6,479
nt
The
East - West component causes what is called declination, or the
compass points to "magnetic north".
The true North component is in the horizontal plane, but
The down component is much stronger than the North component!
If the compass is not level then it will have a large error because
it is starting to read the down component of the Earth's field.
This is why all conventional compass' need to be held in a level
position.
Chris
M. Goulet's Frequently Asked Questions - Magnetic Declination
Suunto - MC-1 G Global - unique needle
balanced for worldwide use.Although there are 12 US patents
held by Suunto, I don't see the global. Vector wrist top
computer (thermometer, altimeter, barometer, compass, watch
- I have one of these) - Although I have not experienced it,
there are reports that all batteries have ferrous metal and
typically are magnetized and might cause trouble with the compass.
The instructions say to re-calibrate the compass every time
the battery is changed. The person making the complaint
may have been in a strong magnetic field that changed the batteries
magnetic field. Recta - at Perret
Optical - at
MapSport - at
TugawarPhotos
- Silva - Products - Brunton - Pocket
Transit = Army M2 comes calibrated in either degrees or
mils (1 mil is 1 meter lateral @ 1,000 meters range) Celestaire, Inc -Marine and Air Navigation
Instruments - Compasses - Stocker &
Yale, Inc. - manufacturer of Lensatic Compass
- Cammenga - Current Lensatic Compass
manufacturer Point Research Corporation
- PointMan - drift-free dead
reckoning navigation system - Engineering Evaluation Kit is
$2,495 (2/13/99) US5583776
-
Brooke,
There are 3 magnetometers and 3 accelerometers on orthogonal
axes, plus a barometric altimeter. The accelerometer triad senses
the gravity vector to tilt compensate the compass, and is also
used for stride detection. Distance is measured by counting
the user's steps (whose length is calibrated by GPS or other
method) rather than by double integrating the accelerometer
data. GPS and DR data are combined in an on board Kalman filter.
Jack Runyon
Microstrain, Inc. - Orientation sensor 3 magnetometers &
3 accelerometers
Photos: Surveyor's
- M2
- Lensatic -
German
- Army
Marching - How Stuff Works - How
a Compass Works -
The
electron beam in a Cathode Ray Tube (CRT) used in computer monitors
and TV sets is bent by the magnetic field of the Earth.
In some applications where precision beam positioning is important
the CRT has a Mu-metal shield. There are numerious permeabilities
offered some alloys are called Netic and Co-Netic. MµShield materials - AD-Vance Magnetics - Magnetic Shield Corporation
- FAQ -Netic and Co-Netic
- Helmholtz Coil Assemblies -Zero Gauss Chambers Sunbo - Korea raw materials
& shield cases for magnetic recoreing heads Riken - Magnetic
Materials Laboratory - Japan
Active
If
a magnetic sensor is used in a feedback loop to control the
current in a Helmholtz coil and bring the sensed field to Zero
those devies in the sweet spot volume of the coil will
not see any magnetic field.
Theodern
fluxgate works by using a torroid with windings both going through
the hole and others going outside the hole.
Fluxgate
sensors were called "Flux Valves" prior to WW2. At that
time the indicator was a 400 cps Syslen and the Flux Valve was
constructed on a "spider" that had 3 legs each 120 degrees apart.
The coils were wound on bobbins and slipped onto the legs.
An outer ring was then attached to the central core and this ring
had three gaps. The whole assembly was mounted on a pendulum
and part of the calibration was to be sure that the center of
mass of the Flux Valve was centered so that it would hang plumb.
See my Fluxgate Patents
page for more on them. The MC-1 uses this "Y"
type core.
A-Plus Photo
They make fluxgate magnetometer unique in that they use a sandwich of
printed circuit boards instead of winding wire around a core.
There are 3 seperate coils, one with turns through the center
of the toriod, one ourside for X and another outside for Y.
The turns are done with traces and plated through holes.
They may be a big player in the automotive compass market. Their
fax in Japan is 03-5281-2064 & tel 03-5281-2067 Paper on
very similar idea - Applied Physics Systems
- Fluxgate Magnetometers, DC SQUID Magnetometers ASC Scientific - Magnetometers -
Barington @ ASC
Scientific - Billingsley Magnetics
- Swales Aerospace
- Atitude
Control Magnetometers - Don Lancaster's article muse104
- another muse
103 -
EMDS - (301) 840-5710 Fat Quarters Software
- They sell units made in England by Speake & Co Limited.
They have an internal oscillator and an output whose frequency
varies with magnetic field. Also carried are some ASIC (microcontrollers)
to do common functions. A number of app notes are also available,
now as HTML documents. KVH Industries - C100
- They have a commercial monocular that has an optical readout
of the magnetic bearing of the item sighted. Typically used
by mariners. USD0325041:
Multifunction monocular US5020902:
Rangefinder with heads-up display Magnetic Research, Inc. - book
with detailed circuit diagrams and fluxgate sensors MC-1 Aircraft fluxgate
compass field calibration kit - Watson Industries
- combinations of heading sensors, accelerometers, angular rate
gyros, and angle references as well as a line of magnatometers.
Magneto-Inductive
(MI)
Precision
Navigation - Precision Navigation holds a couple of patents
on the combination of the MI coil and a specified circuit:
PNI Photo
- I used a Computer Boards
model CIO-DAS08 (image)
card in my computer to bang bits to control the V2X card.
There was a strange stratification
in the data that can be seen in my LabVIEW program and PNI
could not explain. There is a posibility that there was a timing
problem with WIN3.11. It may be that averaging will give
the correct answer, but I prefer to get the correct reading in
the first place. Jameco no longer carries the 126691 Development
kit for the PNI vector modules. There may be a compatability
problem with the printer port on modern computers. Jameco
still has a couple of pages on their ftp site for the 126691.
4,851,775
Digital Compass and Magnetometer Having a Sensor Coil Wound
on a High Permeability Isotropic Core. The
idea is to use a simple IC to form an oscillator whose frequency
varies with he magnetic field. This patent (IMHO) protects
the combination of the MI sensor and a simple oscillator circuit.
5,239,264
Zero-offset magnetometer having coil and core sensor controlling
period of an oscillator circuit. The idea is the same
as in the first patent except that both polarities of DC are
used through the sensor to cancel temperature effects and is
implemented in the IC they sell. This patent (IMHO) protects
the combination of the MI sensor and a circuit that reverses
the drive polarity to improve the zero offset.
This
is a technology that makes use of semiconductor type processes.
The resistance of an element changes due to magnetic field strength.
It is now being used in the read heads of computer disk drives.
Note in this application the output signal does NOT depend on
the speed of the disk like it does with inductive heads.
Giant MR disk heads are starting to be used for disk drives, but
GMR technology is not as good as MR for Earth field sensing.
The science of magnetic materials is a practical application of
quantum mechanics. This makes the study of these materials
a Phd project. The magnetic properties relate to both the
electron orbiting and the spin of the electron it's self.
Electrons can spin in the "up" direction or the "down" direction.
The
offerings form Honneywell and Philips Semiconductor appear to
be about the same in capability. Honeywell-
Has extensive app notes on many aspects of these including the
"setting" and "re-setting" circuits. Honeywell Philips
Semiconductor - Has very little information on their magnetic
field sensors & KMZ51 units.
DigiKey
p/n 9082CA-ND
(9000 series Fig 2) is an 8-pin SOIC to 8-pin SIP surfboard
for the KMZ51 package. Scanner Image - 9082 w/KMZ51
- Leica has a
3 axis unit that is very accurate because it includes an inclinometer.
They do not say which technology is used in the top web page,
maybe further reading will reveal the technology. Pegasus - InSb Magnetoresistors
- San Diego Magnetics
- MR, GMR sensors for tape and a new type of magnetic card
US5260653
Thin film very high sensitivity magnetoresistive magnetometer
having temperature compensation and simple domain stability
US5119025
High-sensitivity magnetorresistive magnetometer having laminated
flux collectors defining an open-loop flux-conducting path
Searching for ( (magnetoresistive) <in> AB) and ( (Eastman
Kodak) <in> PA) at http://www.patents.ibm.com/boolquery
- yeilds 24 patents that may now be assigned to SDM
Giant
Magnetoresistive (GMR) & Spin Valve
These
are technologies being looked into for the next generation computer
hard disk drives. GMR Physics - NonVolatatile Electronics Inc.
- makes a number of magnetic sensor based products
These
are not sensitive enough for Earth's field applications.
There is a cleaver product (model 1490) that uses
Hall Effect sensors combined with a rotating magnet (compass)
for coarse direction indication made by Dinsmore Instruments. 1814-T
Remell St. Flint, MI 48503 4482 USA Tel: 810-744-1330 Fax: 810-744-1790
There are a couple of new sensors Model 1525 and 1655 (uses Allegro 3507) that
may use a moving magnet (like in a compass) and a couple of analog
Hall sensors to allow getting a bearing reading. This will require
reading 2 analog signals and computing a vector. I
have one on order and will have more information when I have had
a chance to test it.
US5157841:
Portable electronic compass US4402142:
Electronic compass for automotive vehicles 1983 compass needle
& 4 Hall sensors US4250626:
Electronic compass - Lazar; Harold - Feb. 17, 1981 / Aug. 20,
1979
Not
by Dinsmore an interesting combined sphercial LED tilt and
compass sensor based on 3 flux gates. US5526022:
Sourceless orientation sensor - 1993
Integrated
Resonant Magnetic Field Sensor - a current carrying coil formed
in Silicon mechanically vibrates and it's motion is effected by
the Earth's magnetic field. The effect is sensed by means
of capactive sensors. US5731703:
Micromechanical d'arsonval magnetometer
Magnetic Stars
In Outer Space
On
27 Aug. '98 high energy gamma rays from outer space passed through
our solar system. These came from a Magnetar. This had effects of HF radio propagation.
Press
release. This is very interesting since it came from
outside our Solar system.
Magnetic
Ink in U.S. Paper Money - NOT - well OK
there is something here
The
magnetic stripe idea is a myth, in the new paper money.
I received an email for the Treasury
and the stripe is a plastic, it glows in UV light and it's position
in the bill indicates the denomination. Coin Currency & Document Systems
Inc. - Currency
Processing Equipment - MD-3
Magnetic Ink Detector - I don't know what this is
4/25/99 - I have been told that the ink used to print US currency
has magnetic properties and a magnetic sensor is used in some
bill changers. The Nov 2001 issue of Sensors has an article about
reading the magnetic signature of a new $20 bill.
Anti-Shoplifting Garmet
Tags - Electronic Alarm Systems (EAS)
Although
some of these systems depend on magnetic materials others use
RF loops and/or loops with diodes. Electronic Shoplifting Deterrent
Systems - Magnetic tag notes - Knogo - US3747086: Deactivatable
Ferromagnetic Marker for Detection of Objects Having Marker Secured
Thereto and Method and System of Using Same - this patent is called
out by 47 other
patents so it is a good place to start US5288980: Library
check out/check in system - references 40 patents going
back to 1939 US3990065: Theft
detection system - magnetic method US4642640:
Signal receptor-reradiator and surveillance tag using the same
- mixer type US4139844:
Surveillance method and system with electromagnetic carrier and
plural range limiting signals - mixer type US4429302: Electronic
security system with noise rejection - sweeping through a reasonance
and counting the pulses out put frequency - Checkpoint? US5349332:
EAS system with frequency hopping - calls out 10 prior art patents
including the two above US5821859:
Concealed magnetic ID code and antitheft tag
MRAM
Magnetic Random Access Memory
Conventional
DRAM chips use the electric field on an electron to store a binary
state. These need to be refreshed frequently or they loose
their data. By using the spin on the electron to store data
magnetically a nonvolatile memory can be made that promises to
be faster than DRAMS. I think the first patent was by Honeywell
US4455626: Thin
film memory with magnetoresistive read-out
Filed March 21, 1983 for a military version. Then Nonvolatile Electronics, Inc started
development of commercial versions. They have a good online
white paper. HP, IBM, Motorola
and others have dozens of patents in the area. This technology
uses standard CMOS chips with the magnetic element sitting over
a single CMOS FET. It has the potential to replace FLASH
chips and hard disk drives.
A computer that used MRAM would be like the early computers that
had magnetic core memory. You would have protection from
power outages and have "instant on" operation. Whatever
program you were running when you powered down would come up running
when you power on. The military is
very interested in this technology and Honeywell is offering products.
In Dec
2000 NVE stopped sub licensing this technology in exchange
for a million dollars.
Jewell
- Inertial Sensors, Accelerometers , Inclinometers - Systron Donner 4310 series
- web page with patent links (maybe the same as Jewel)
Micromachined
These
have a beam with many fingers that are interleaved with other
fingers that are at ground. The capacitance between the
fingers on the beam and the grounded fingers is measured.
In this was acceleration can be measured. Analog Devices - Made for automotive
air bag, New ADXL150 & ADXL250 lower noise versions of the
popular older ones. They have a large number of app notes
on their web site covering all kinds of practical areas of concern
like temperature dependency in DC coupled systems.
ADXL202
- Low-G, 2- axis - Crossbow Evel
Kits - has PWM output for direct microcontroller interface.
Note that this is the first acceleration sensor designed for
static gravity work. The other sensore have been aimed
at the automotive air bag crash detection market.
Rocket
Altimeter Project (RAP) - PIC micontroller based with
ADXL190
Applied MEMS - 3
axis accelerometer withlow noise. double integration for
60 seconds gives less than 0.1 inch movement on a fixed sensor.
Motorola - Like
the Analog Devices model it is a micromachined Silicon sensor
aimed primarily at automotive air bag applications.
US05814727
09/29/1998 Semiconductor accelerometer having reduced sensor
plate flexure US05590241
12/31/1996 Speech processing system and method for enhancing
a speech signal in a noisy environment US05545912
08/13/1996 Electronic device enclosure including a conductive
cap and substrate US05487305
01/30/1996 Three axes accelerometer US0531383505/24/1994
Integrated monolithic gyroscopes/accelerometers with logic
circuits US05249465
10/05/1993 Accelerometer utilizing an annular mass US05044201
09/03/1991 Double-integrating silicon acceleration sensing
device US04677913
07/07/1987 Safe and arming device US04378510
03/29/1983 Miniaturized accelerometer with piezoelectric FET
US05818093
10/06/1998 Semiconductor device having a movable gate US05400644
03/28/1995 Knock detection system
Fujikura Ltd.
- FACS
Accelerometers -Single Chip 2 and 3 axis signal
conditioned, 1 and 2g Oceana Sensor Technologies
- Low cost, Piezoelectric Accelerometers - Eval Kits - Morgan Matroc - PZT piezoelectric
ceramics manufactured for specific application Mitsubishi Electric Corporation
- Semiconductor
Acceleration Sensor - Because of their ability to detect
accelerations of static characteristics, low-acceleration sensors
can be applied to Inclination meter, Vibration detection, game-playing
equipment, etc. High-acceleration sensors have a high frequency
response and can be applied to airbag systems based on analysis
of shock waveforms and other signal waveforms. Summit Instruments
- ? Technology Measurement Specialties Incorporated (MSI)
- piezo thin films, analog test printed circuit board
- with schematic EG&G
(now called PerkinElmer, Inc) -Data Sheets -3031,
3038, 3022, 3028. 3140- 2G, 3145-2G 0.5 -
5.0 V out. 3255- 50, 500 G, 3265 - 5G - 3052 &
3058 -2G have internal temp comp capability good for quality
DC apps
GS Sensors Inc. -FA1131 - Ephrata, PA
A
device to determine how far a person (horse?) has gone.
Used in modern dead reckoning systems. US5724265:
System and method for measuring movement of objects - this patent
references many others US4991126:
Electronic-automatic orientation device for walkers and the
blind - distance & direction
Sensors
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