Sensors

Magnetic

Earth's Magnetic Field
Compass
Helmholtz Coil(s)
Magnetic Shielding
Magnetic Stars In Outer Space
Superconducting Magnets
Sensor Types
Fluxgate
Magneto-Inductive
Magnetoresistive (MR)
Giant Magnetoresistive (GMR) & Spin Valve
Coil Antenna
Hall Effect
Magnetic Precission
SQUID
Silicon Integrated Circuit
Medical Applications
Nuclear Magnetic Resonance (NMR)
Magnetic Reasonance Imaging (MRI)
Magnetoencephalograph (MEG) & MCG
Applications
Paleomagnetics
Recording
UXO UneXploded Ordnance Detection
MICR (Check) Processing
Magnetic Ink in U.S. Paper Money
Credit and other cards with a Magnetic Stripe
Anti-Shoplifting Garmet Tags
MRAM

Micromachined
Geophone Based Intrusion - Seperate Web Page
Thermal
Earth's Gravity
Seismometer
Vibration
Pedometer

Light      Photometers
IR        Detectors
Laser      Distance Measuring
Cloud     Turbidity

Temperature

Humidity

Tilt

Building Wall Stud

Ultrasonic

Gyroscopic   North Finding Module

Human movement

Explosive Detection

Gas

Speech

Smell

Rotary Angle

Hobo Data Logger

Fingerprint

Web Links

Magnetic

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 -

Earth's Magnetic Field

The National Geophysical Data Center lists a number of models and is the key US web site for the Earth's magnetic field.  The World Magnetic Model   is updated every five years by the DoD.  This model will give you all three components of the Earth's magnetic filed if you know the date,  Longitude, Latitude and Elevation.  They also have a good links page.  The WMM95 model is built into the current versions of the Motorola VP Oncore GPS receivers.  A magnetic compass and this GPS receiver could be used to give true bearings, not just magnetic bearings.  Now add a long range laser range finder and you can locate anything within range in absolute coordinates  . . .  just what the military needs for targeting.
Nominal value of Earth's field = 0.5 Oe = 0.5 Gauss = 39.79 A-T/m = 5E-5 t = 50 ut = 50,000 nt.
Experiment Relating to the Vertical Component of the Earth's Field -

The International Association of Geomagnetism and Aeronomy -
USGS - geomagtism page - Magnetic Observatories - National Geomagnetic Information Center  - models - links -
WWV - Official Announcements - includes geomagnetic field activity activity
Hermanus Magnetic Observatory - CSIR - Geomagnetic Field Models and Maps -
International Geomagnetic Reference Field (1995) -
British Geological Survey - Geomagnetism - Instrumentation -
Bartington Instruments Ltd - Magnetic Instrumentation Products - Mag-01H Declinometer/Inclinometer - comprises a single axis fluxgate magnetometer, with the magnetometer sensor mounted on a non-magnetic optical theodolite
Australian INTERMAGNET Observatory Information -
Schonstedt Instrument Company - Lab Magnetometers - Surveying, Unexploded Ordnance Detection, Construction, Police, Utility (Water and Electrical), and Undersea pipe and cable location.
Dunham & Morrow - surveying marker locator designed by the same people that designed the Schonstedt GA-32/52 series.
Institute for Defense Analyses - Sensors, Surveillance & Target Acquisition - Magnetic Sensors (MEMS) -  DARPA MEMS -
Leica DMC - Digital Magnetic Compass - combined compass and 2-axis tilt sensor
MicroStrain - 3DM  - Orthogonal arrays of magnetometers and accelerometers are used to compute the pitch, roll and yaw angles over a wide angular range. Dev Kit $695
Geomagnetic Field Synthesis Program (Version 3.0) Input Form - on line determination of all 3 components of field
Swedish Institute of Space Physics (IRF) - Real Time plot - Today's and Earlier Space Weather -
Home Built Magnetometer for Solar Studies - magnet suspended on "E" guitar string - AAVSO Solar Division -
The Great Magnet, the Earth by  Daniel Stern of the NASA-  Commemorating the 400th anniversary of "De Magnete" by William Gilbert

Compass

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 -

Helmholtz Coil(s)

A Holholtz coil consists of two coils aligned on a common axis.  Their fields add in such a way that there is a region at the geometric center where the magnetic field has constant magnitude and angle.  The magnitude of the magnetic field is linearly proportional to the applied current through the two coils.

This is a good way to test magnetic sensors.

A single Helmholtz coil could be positioned so that it's axis was in line with the Earth's local magnetic field and then the DC drive current adjusted to cancel the Earth's field.  An alternate procedure is to use 3 Helmholtz coils in an X-Y Z configuration.

Wood, plastic, fiberglass are suitable for either DC ar AC driven Helmholtz coils.  For Earth's field work where only DC will be used aluminum, brass, and copper can also be used because there will be no AC eddy current problems.
UC Press - Hermann von Helmholtz & the Foundations of Nineteenth-Century Science -
Magnet-Physik - Moment measuring coils - for measuring permanent magnetics
Biot-Savart law - web plotting
Walker Scientific - Helmholtz coil - Gaussmeters - Fluxmeters - magnetometers -
Rapid Magnet Evaluation -
Magnetic Applied Devices -
3m diameter Helmholtz Coil at the National Physical Laboratory -
Billingsley Magnetics - 1 meter, 2.2 meter - coils
US5126666: Method and apparatus for substantially eliminating magnetic field interference to a magnetometer caused by DC current carrying conductors
Lake Shore Cryotronics - and magnetic field instruments

Magnetic Shielding

Passive

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.

Paleomagnetics

National Geophysical Data Center - related servers -
Scripps Paleomagnetic Laboratory - analyze the magnetic properties of rocks - Equipment & Facilities - links -
Geomagnetism and Paleomagnetism Section - of the American Geophysical Union -
Gisco -geophysical instrument source - Paleomagnetics index - Magnetometers -
Coast Ranges of Oregon and Washington - Magnetics in the exploration of the ground -

Fluxgate

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.

Pacific Rim (Japan) A-Plus

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.

If someone used circuitry other than the two patented ones above they could use MI sensors without infringing the PNI patents again IMHO.

Since this technology has a very low power requirement I think it is used for digital compass applications like in a wrist watch.

The MI coil has a core made from Allied Signal METGLAS® Magnetic Alloy 2705M (Cobalt-based).  These use much less power than the classical flux gate units.

American Microsystems, Inc -  magnetic sensing ASIC implements the 5239264 patent and is available for licensing from Precision Navigation Inc.  See the March 2000 issue of Sensors Magazine for an article.

Robot Navigation Schemes - CORDIC (COordinate Rotation DIgital Computer) - sinplified coordinate rotation
Interfacing with A Precision Navigation Vector 2X Compass Module -
PNI SEN-L sensor drawing (need free Volo  view to see drawing), X-ray -

Magnetoresistive (MR)

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

Coil Antenna

The May '96 issue of Scientific American Amateur Scientist article "Detecting low-frequency electromagnetic waves" on page 98. This is a follow up to the Fraser-Smith (Stanford) detection of low frequency waves prior to the Loma Prieta Quake.
Stanford University -Space, Telecommunications, and Radioscience Laboratory -