Fluxgate (& other) Patents

Brooke Clarke, N6GCE

14 Feb 2005 - Changed all patent links to a more stable URL.
    Two Terminal Coil 
Core Material 
Other Catagory (in addition to Fluxgates)
Patent Class
Magnetic Anomaly Detection
Honeywell MR Sensors
Philips MR Sensors
Active Impulse Magnetometer (AIM)


Up to some point in time all the Flux gate/valve systems used a 'Y" core with the legs spaced 120 degrees to be compatible with Selsyn type indicators.  This type of indicator provides a mechanical pointer that can rotate a full 360 degrees and was the most desirable type of compass indicator for many decades and is still used in many systems.  From very early on (1940s) the second harmonic output was understood and the need to minimize the fundamental feed through and other harmonics that would distort the second harmonic.  This was done using band Pass Filters (BPF).

Variation is the difference between true North and magnetic North and is a function of Longitude, Latitude, Elevation and date.  If you know these inputs the World Magnetic Model will calculate the variation.  Also if you know you are driving on a road that goes North then you could press a button to tell the electronic compass to apply a variation correction to bring the display to 0 degrees.

Compensation refers to correcting for error due to surrounding ferrous metal and to surrounding magnets.  This is also referred to as soft and hard iron compensation.  Also called remanence.  If the fluxgate is rotated in a circle while being kept level and the X and Y sense circuit outputs are plotted the resulting figure should be a circle with it's center at 0,0.  If the center is not at 0,0 then there's a local magnetic object present.  If a number of points are measured then gain and offset corrections can be applied to the X and Y sensor circuit outputs to make the plot much closer to a circle.

There are a number of applications for the Fluxgate.
Some are used to sense the Earth's field in a non directional way like are used in sea mines.  For this application low power consumption is very important.  This is achieved by turning off the drive coil current as soon as the core saturates.
In compass applications the core saturation needs to continue until the sense coil pulse has subsided.

Two Terminal Coil (TTC)

There are a number of patents covering what I'm calling a Two Terminal Coil (TTC).  This is the simplest coil to wind.  Many other patents require a seperate drive and sense coil and a few a third feedback coil.   These other coils (complexity) may be justified by better specs for some performance parameter, but at what price?  So you can search (Find) TTC on this page to locate them.


Brown invented a number of sea mine sensors for the U.S. Navy.  These were deisgned to both work and to have very low DC power drain.  They are typically very simple transistor oscillator circuits. search (find) in this page for "Brown"

Flux Gate Core Material

There are two states of the "Gate". 
"Open" = With no bias the core has a very high permeability and a lot of the magnetic field is concentrated in the core. 
"Closed" = When the drive coil saturates the core and the mu goes to 1 the same as air.
The transition between Open and Closed causes a lot of flux lines to cross the sense coil.

Properties of a good core are that it has very high permeability.  This means that the core will concentrate a lot of the earth's field.  Air has a mu of 1 and mu-metal (aka Conectic AA) may be on the order of 350,000.   Also it's good if the material is very "soft", i.e. has a low remnance.  That's to say that it does not take on a magnetic set or become a magnet.  You want it to become non magnetic of it's not in a magnetic field.  Note that "soft" and "hard" also go along with the physical properties of iron.  Magnets are made of "Hard" iron.  The iron used in flower ties is "soft" and easy to bend.

Early cores were made of iron sheets like are used in transformers.  These typically were in a "Y" configuration with a coil on each leg and were used in 3 phase synchro type systems.

Later cores were wound using very thin strips of mu-metal inside ceramic toroid cores.  By using ceramic the finished core could be heat treated (in a reducing atmosphere so that the metal would not oxidize) at very high temperatures.  The heat treating relieved the stresses in the metal caused by the winding and about doubled it's mu.  These are very expensive to make.  Mu-metal only comes in thin sheets or foils.

Instead of using sheet metal to form the toroid core very fine wire can be used.  I have a 100' spool of extremely high mu wire that's very small in diameter and if wound on a 1 or 2 inch diameter form would not be stressed very much.  It could also be used as a straight core in a cylindrical coil wound on something like a small coffee stirrer or drinking straw.

The latest materials are the Metglas 2714 type that can have a mu of 1,000,000.  This material is used in some types of anti shop lifting strip type tags (about 45x11x1.5 mm).  It only comes as sheet material and typically is between 0.1 to 2 mils thick.

Other Catagory

In July 2004 I have expanded this page to include other magnetic sensors.  My interest now is in what I call magnetic signature as applied to a fixed magnetometer and a car driving by.  This is a much simpler case than that of an aircraft flying a Magnetic Anomaly Detector (MAD), a sub or a buoy looking for a sub.  These moving platform systems typically use a number of frequency bands and then process the amplitude signals in each band to detect subs (see the movie "Hunt for Red October").

The patents in boxes have some signal processing ideas that could be applied to my interest.

The system I would like would be placed in the driveway and announce that a UPS, Fedex, propane truck or a small, medium or large car had entered the driveway.  If it's sensitive enough it should also announce the one or some number of people were walking up the driveway.

Magnetic anomaly and MAD are key words for this type of sensor when in a moving platform.  So far I have not found the key words for a fixed sensor looking for a moving object.

Threshold levels and time measurements may be all that is needed for a micro controller to process in order to determine what is causing the filed change.

Note that a total field magnetometer can not detect a change in the direction of the field only it's magnitude and so is not as useful as a vector field sensor for signature analysis.  If a single axis vector type magnetometer was aligned to the Earth's field and there was a small perturbation in the field, it would be difficult to detect (a small change on top of a large reading).  But if the magnetometer was installed so it's sensitive axis was orthogonal to the Earth's field (say level and on the E-W magnetic axis) then the sensor output would be zero on average and it's gain could be increased to allow picking up small changes in the field.  A 2-axis sensor could be arranged so that both sensors were orthogonal to the Earth's field and would be even more sensitive to changes since it would pickup changes in a direction where the orthogonal direction would have a small change.

By using two of these sensors in a differential mode only gradients in the field would be sensed and changes in the Earth's field would be canceled.  Note that magnetic gradiometers only sense changes happening close to the two sensors.  Changes far away effect both sensors the same way and cancel out.

If you want to detect changes in the far magnetic field all that's needed is a coil with an iron core.  The core will concentrate the earth's magnetic field and when the field changes the coil will output a voltage.  It works in a fashion similar to the playback head on a tape recorder.  A coil sensor like this can be used to detect moving cars (but not stationary cars).

Patents that contain the phrase "magnetic signature" usually are refferring to an equivalent of a bar magnet aligned along the long axis of a ship.  The difference between these relates to the mass of the ship and it's speed and direction.

Patents that contain the phrase "magnetic dipole" may be talking about a way to differentiate various classes of objects even to the level of serial number.

Some of the Honeywell application notes show the X, Y and Z magnetic field of a roadside sensor as varoious vehicles drive by.  The engine and axles clearly show up making it possible to classify vehicles.

Patent Class

33 Geometrical Instruments -
324 Electricity: Measuring and Testing
327 Miscellaneous Active Electrical Nonlinear Devices, Circuits, and Systems 331 Oscillators
340 Communications: Electrical 428 Stock Material or Miscellaneous Articles

Magnetic Anomaly Detection

This is used by Navy P-3 aircraft to detect submerged subs.  It's also used by remote controlled mini subs to detect mines and other ferrous objects on the ocean floor.  I think the concepts would be applied to a fixed sensor to allow magnetic signatures to be detected.  These patents have the words "magnetic anomaly" in the abstract. (this was too restrictive, next serach just for "magnetic" and "anomaly".
6362625 Active magnetic anomaly sensing system having synchronized transceiver and discriminator
6268725 Flux-gate magnetometer with drive signal for reducing effects of Electromagnetic Interference, Scott D. Vernon, Larry E. Tyler (Medtronic), Jul 31, 2001
uses a drive waveform that vairis over time so that a fixed frequency interference will not cause interference
6121772 Magnetic sensor
6080352  Method of magnetizing a ring-shaped magnet
6043647  Magnetic anomaly detector for detecting the movement of ferrous metals
6031377  Magnetic anomaly detection buoy for detecting submarines
2404806 Submrine Detector July 30, 1946 340/850; 102/402; 324/247; 324/331 (airborne, picks upmotor or battery charger fields)
2632884 Orienting Device for Magnetic Dector Devices March 24, 1953 324/253; 318/647; 324/246; 324/331 (airborne towed MAD)
2834000 Sound Detecting DeviceMay 6, 1958 367/155; 367/173 (barrium Titnate hydrophone)
3471777 Differential Magnetometer Having Parallel Rotating Fields & Associated Sensing Circuitry Oct 7, 1669 324/247; 102/402 (used on a buoy to detect subs)
3737842 Feature Recognition Techniques June 5, 1973 340/850; 324/76.31; 324/331; 324/345
3757203 Magnetometer for Detecting & Identificatin of Immersed Bodies Sep 4, 1973 324/331; 324/326
2426622 Magnetic Field Detector Sep 2, 1947 324/254; 324/253; 324/345; 341/15
2900595 Magnetic Field DetectorApr 18, 1959 324/335; 324/250; 342/394; 342/458; 455/137; 455/141
3257608 Optical Magnetometers June 21, 1966 324/304; 324/244; 324/244.1; 324/301; 331/3; 359/281 (similar to a Rb freq std)
3398360 Magnetic Body Detector sensitive only to Magnetic Field Variations within a Predetermined Range  324/226; 324/247; 324/254; 324/345 (airborne gradient system)

3808519 Method & Device for Determining the Depth of a Magnetic Anomaly Apr 30, 1974 324/345; 324/326; 324/331
3263161 Method of determining depth and falloff rate of subterranean magnetic disturbances utilizing a plurality of magnetometers  324/331; 324/345 - geophysical application
3441841 Methods and devices for compensation of parasitic magnetic fields, in particular in an aircraft carrying a magnetometer  324/244; 324/301
3490032 Method and Apparatus utilizing a pair of spaced magnetometers for making magnetic surveys   324/345; 324/331 aircraft or sub based
3639828 Compensation of parasitic or stray magnetic fields abroad an aircraft 324/301; 324/244 - uses differential phase and frequency from two sensors
5189368 Magnetometer Feb 23, 1993 324/304; 102/417; 324/301 (Larmor precession frequency, low power)
3129410 Electron spin echo memory system 365/152; 324/304; 359/281; 365/121
3500176 Method and Apparatus for controlling a Magnetic Field employing optically pumped nuclear resonance  324/304 - spacecraft
3526002 Magnebuoy 340/852 March 31, 1960 - atomic precession sub detection
3750008 Optical Pumping Magentometer 324/301; 324/304
6208268 Vehicle presence, speed and length detecting system and roadway installed Detector therefore,
John F Scarzello, Daniel S. Lenko, Adam C. Feaga (Navy), Mar 27, 2001, 340/941 ; 324/174; 324/179; 324/207.26; 324/247; 340/933; 340/940; 701/118
Self Powered Vehicle Detection (SPVD) probably based on sonobuoy
Fujimoto October 1971
Zachmann December 1971
Bushnell et al. February 1972
Cribbins December 1973
Itakura et al. April 1975
Corrigan September 1976
Eshraghian et al. November 1980
4302746 Self-powered vehicle detection system
An improved self-powered vehicle detector (SPVD) uses a two-axis magnetomr to sense a vehicle's magnetic signature and then telemeter vehicle presence information to a roadside receiver.

Fig 3 has the look and feel of a sonobuoy & uses magnetic signatures.  The cylindrical RF antenna may be the same as used in sonobuoys.
Uses standard ICs.
John F. Scarzello, Daniel S. Lenko, Albert D. Krall, Wayne R. Grine, Robert E. Brown, George W. Usher, Milton K. Mills, Albert M. Syeles (Navy)
November 1981 340/38L; 324/247; 340/38R; 343/700MS; 343/895
Hirota et al. October 1982
Dijkman January 1983
Wilson August 1988
Mizuno April 1990
Dennison July 1990
Mizuno et al. November 1990
Gebert et al. April 1991
Roth et al. February 1992
Hayashi et al. May 1992
Gregoire et al. April 1993
Polvani et al. July 1994

5949738  Sonar target simulation
5614825  Magnetic flux leakage inspection apparatus with surface-responsive sensor mounting.
5598152  Mine sweeping system for magnetic and non-magnetic mines
5115196  Girth weld detection system for pipeline survey pig
4613816  Cryogenic magnetic probe having new substrate
4437064  Apparatus for detecting a magnetic anomaly contiguous to remote location by squid gradiometer and magnetometer systems
4422075  Proximity monitor

Vehicle Classification

6911829 Inductive signature measurement circuit June 28, 2005 324/654; 324/76.75; 324/546; 329/361
6867709 Vehicle detector and classifier March 15, 2005 340/941; 324/654; 340/933; 340/935; 343/728; 343/748
6670891 Magnetic detector of vehicle wheels Dec 30, 2003 340/933; 340/941
6590400 Inductive signature measurement circuit July 8, 2003 324/654; 324/118; 324/207.15; 331/65; 340/941
5751225 Vehicle detector system with presence mode counting May 12, 1998 340/941; 324/654; 331/65; 340/933; 340/938
5523753 Vehicle detector system with periodic source filtering June 4, 1996 340/933; 324/654; 340/939; 340/941; 377/28
5408179 Method and apparatus for analying traffic and a sensor therefor  April 18, 1995 324/253; 340/941 Sampey Scientific - Outerlab.net - nu-metrics.com - Single coil magnatometer


6917198 Hybrid hall vector magnetometer July 12, 2005 324/251; 435/4; 324/243
6867587 Excitation circuit for a fluxgate sensor  March 15, 2005 324/253
6714008 Gradiometric measurement methodology for determining magnetic fields of large objects March 30, 2004 324/261; 324/247; 324/260 (Magnetic Signature)
6546344 Magnetic anomaly sensor system April 8, 2003 702/65; 324/331 (computer state diagram and process)
6512370 Low power, frequency-mode magnetometer, John James (Elf Engineering, Ltd), Jan 28, 2003, 324/253; 33361; 331/65 - Two Terminal Coil (TTC)
Check Citations
6476610 Magnetic anomaly sensing system and methods for maneuverable sensing platforms Nov 5, 2002 324/345; 324/225; 324/247; 324/326
6407547 Biaxial flux gate type magnetic sensor Class 33/361 - wafer level 2-axis fluxgate sensor
6380735 Orthogonal flux-gate type magnetic sensor -Cylindrical core with primary rod along center axis, many claims
6301794 Vehicle compass system with continuous automatic calibration - uC corrects for local mag field uses triangle to compute center of circle  Class 33/356
6282803 Self calibration circuit for determining an accurate zero compensation for a fluxgate compass - uC zero during off time
6218831 Low power fluxgate circuit with current balance, J. Thomas Fowler (Arthur D. Little, Inc.), 324/253; 324/228; 324/258; 33361; 34087033; 361/156 -
Two Terminal Coil (TTC)
Check Citations
6208884 Noninvasive room temperature instrument to measure magnetic susceptibility variations in body tissue
6173501 Compensation system for electronic compass - MR set/reset
6100686 Magnetic field sensor with double wheatstone bridge having magneto-resistive elements August 8, 2000 324/252; 324/207.12; 338/32R
       uses 2 bridge outputs to cancel offset so no flipping is needed.
6075361 Device for detecting a magnetic field June 13, 2000 324/252; 324/225
6031377 Magnetic anomaly detection buoy for detecting submarines, James A. Watkins, Feb 29, 2000, 324/345; 324/247; 324/226
advantages over sonobuoys and airborne magnetic anomoly detectors.
2632884 ORIENTING MECHANISM For MAGNETIC (Magnetic ananomaly)
Mar 1953
3263161 Method of determining depth and falloff rate of subterranean magnetic disturbances utilizing a plurality of magnetometers  324/331; 324/345 - geophysical application Jul 1966
3737842 FILTER CHANNELS Jun 1973
3757203 Magnetometer for Detecting & Identificatin of Immersed Bodies Sep 4, 1973 324/331; 324/326 Sep 1973
3808519 Method & Device for Determining the Depth of a Magnetic Anomaly Apr 30, 1974 324/345; 324/326; 324/331 Apr 1974
4208738 Deployable sonar array with interconnected transducers operated in the bending mode Jun 17, 1980
4938136 Resonant acousticmagnetic minisweeper Jul 3, 1990
5189368 Magnetometer Feb 23, 1993

5959452 Lorentz force magnetometer having a resonator
5939881 High dynamic range digital fluxgate magnetometer
5924212 Electronic compass, Stanley J. Domanski (Donnelly Corp), Jul 20, 1999, 33/355R ; 324/247; 324/260
uses a number of PNI type two terminal coils (TTC)
5878370 Vehicle compass system with variable resolution - uses 2 diameters of circle to correct forlocak mag field
5831873 Magnetic dipole target classifier and method, Kirk K. Kohnen, Harold C. Gilbert, Wilbur W. Eaton, Nov 3, 1998, 364/559; 364/449.1
"Magnetic Dipole" may be key words for an advanced "magnetic signature" that allows differentiating classes or serial numbers of vehicles?
It changes depending on the direction of movement of the vehcle.
5744956 Low power magnetometer circuits having zero offset, Timothy J. Hawks, Apr 28, 1998, 324/253 - Two Terminal Coil (TTC)
5742092 Semiconductor structures, methods for controlling their conductivity and sensing elements based on these semiconductor structure 257/610; 257/611; 257/E29.109; 257/E29.324
5737226  Vehicle compass system with automatic calibration - uses an ellipse not circle for correction
5689445 Electronic compass and attitude sensing system
5652512 Advanced Digital Flux Gate Magnetometer - hi resolution
5642046 Low power magnetometer circuits requiring a single switching, Timothy J. Hawks, Jun 24, 1997, 324/253 -
Two Terminal Coil (TTC)
Check Citations
5621319 Chopped hall sensor with synchronously chopped sample-and-hold circuit April 15, 1997 324/251; 324/225; 327/384; 327/511; 327/551 Allegro Microsystems - Hall plate alternately driven + then - and the output synchronously detected, but not done in rotation for vector detection.
5600611 Wrist mountable compass Feb 4, 1997 368/10; 368/14; 368/281; D10/31
5495929 Apparatus and method for validation of bank notes and other valuable documents, Valeri V. Batalianets, Georgi O. Antonov, Mar 5, 1996, 194/207; 382/137
used optical and magnetic signature of inks in currency validator
5351005 Resetting closed-loop magnetoresistive magnetic sensor Sep 27, 1994 324/252; 33/355R; 324/207.21; 324/225
5270648 Single core triaxial flux-gate magnetometer Dec 14, 1993 324/253; 324/247; 336/213
5254987 Output signal processing unit of a fluxgate sensor - LPF cutoff variable depending on rate of car turning
5239474 Dipole Moment Detection and Localization - Schonstedt - improved s/n way of making core
5239290 Magnetic cores for saturable core measuring devices and methods of manufacturing such cores
5239264  Zero-offset magnetometer having coil and core sensor controlling period of an oscillator circuit, 324/253 ; 331/65
5199178 Thin film compass and method for manufacturing the same - accurate construction
5170566 Means for reducing interference among magnetometer array elements
5168223 High sensitivity saturable core magnetic field sensor with symmetrical structure - 1nT combined analog & digital
5151607 Currency verification device including ferrous oxide detection, Timothy T. Crane, Richard A. Menelly, Robert J. Danek, Sep 29, 1992, 250/556; 356/71
measures the "magnetic signature" of paper money, mainly the position and strength of the magnetic thread
5124648 Single winding saturable core magnetometer with field nulling - single coil for both drive and sense, Spencer L. Webb, A. Paul Brokaw (Analog Devices),
Jun 23, 1992, 324/253 ; 324/225; 33/361; 331/65 (TTC) Metglas

2991414 TILLMAN Jul 1961
3040247 VAN ALLEN Jun 1962
3768011 SHEET 2 CF Oct 1973
4059796 Second harmonic magnetic field detection circuit with means to rectify the sensed signal Nov 22, 1977
4060761 Tubular magnetic field sensor with means to rectify the sensed signal Nov 29, 1977
4277751 Low-power magnetometer circuit with constant current drive Jul 7, 1981
4305034 Magnetic field intensity measuring device with frequency change indication Dec 8, 1981
4305035 Magnetic field amplitude detection sensor apparatus Dec 8, 1981
4314200 Method and apparatus for detection of magnetization Feb 2, 1982
4321536 Magnetic field detection by differential phase lag Mar 23, 1982
4402142 Electronic compass for automotive vehicles Sep 6, 1983
4447776 Pulse driver for flux gate magnetometer May 8, 1984
4539706 Mobile vehicular repeater system which provides up-link acknowledgement signal to portable transceiver at end of transceiver transmission Sep 3, 1985
4626782 Magnetometer operated at a self-resonant frequency by a sense winding Dec 2, 1986
4677381 Flux-gate sensor electrical drive method and circuit Jun 30, 1987
4728888 Magnetometer with time coded output of measured magnetic fields Mar 1, 1988
4733181 Single-winding magnetometer with Schmitt trigger output circuit Mar 22, 1988

5091697 Low power, high accuracy magnetometer and magnetic field strength measurement method -
5039945  Single winding magnetometer with automatic distortion compensation 324/253; 33/361 Metglas
5015953 Magnetometer for detecting DC magnetic field variations, Lucian G. Ferguson, Lincoln H. Charlot, Jr. (Security Tag Systems, Inc), 324/244; 324/209 -
 Two Terminal Coil (TTC) Metglas
Check Citations

4953305 Vehicle compass with automatic continuous calibration - removes jumps from display. uC flow diagram
4940966 Article detection and/or recognition using magnetic devices July 10, 1990 340/551; 340/572.6; 428/900 - anti shoplifting tags
4939459 High sensitivity magnetic sensor July 3, 1990 324/247; 324/249; 324/257; 324/258 (car compass)
4929899 Fluxgate magnetometer apparatus and adjustment method to maintain accuracy over a wide temperature range - construction 7 overdriving primary
4914381 Direct-coupled fluxgate current sensor - core construction method
4894615 Stress-relieved amorphous metal ribbon magnetometer having viscous fluid interface - piezo vibration = linear response
4887033 Magnetometer having coil formed by two spiral shaped conductors - GM core winding mathod
4859944 Single-winding magnetometer with oscillator duty cycle measurement, Spencer L. Web (Analog Devices), Aug 22, 1989  324/253 (TTC) Metglas
One way around the PNI patent
4851775 Digital Compass and Magnetometer Having a Sensor Coil Wound on a High Permeability Isotropic Core.  Metglas
This is the first PNI patent for an MI type sensor, but note that Ripka says it's really a fluxgate. Two Terminal Coil (TTC) Calls:
       3416072 Thin Film Magnatometer  Employing Phase Shift Discrimination 324/249 -  Two Terminal Coil (TTC) Metglas
       3936949  Static non-swinging course determining device on a vehicle 324/247 - uses op amp circuit &  Two Terminal Coil (TTC) 0.1 x 50 mm mumetal wire
       4182987  Magnetometer employing hard axis thin film inductor 324/249 -  Two Terminal Coil (TTC) recording head Laminated core
       4241317  Frequency generator suitable for use as position-frequency transducer 331/65 -  Two Terminal Coil (TTC) & op amp make oscillator
       4305034  Magnetic field intensity measuring device with frequency change indication 324/253 -  Two Terminal Coil (TTC) simple oscillator Permalloy wire
4825166 Bobbin for a magnetic sensor, Douglas C. MacGugan (Sundstrand Data Control), Apr 25, 1989, 324/346 ; 29/606; 324/253; 336/213; 336/221
copper bobbin matches thermal coeficient of expansion of wire preventing strain over temperature
4769599 Magnetometer with magnetostrictive member of stress variable magnetic permeability
4763072 Magnetic azimuth detector - improved drive winding and mechanical construction
4739263 Magnetic Sensor Using The Earth's Magnetism.- TDK - core wires threaded through coils
4733181 Single-winding magnetometer with Schmitt trigger output circuit, Harald Bauer (Robert Bosch), Mar 22, 1988, 324/253; 33361 - Two Terminal Coil (TTC)
4677381 Flux-gate sensor electrical drive method and circuit -
4646015 Flux gate sensor with improved sense winding gating - Etak - core in saturation < 1% of the cycle
4641094 Magnetometer circuit for measuring the period of beat frequency maxima Feb 3, 1987 324/302; 324/260; 324/328
4640016 Means for indicating direction and a method of determining a direction Feb 3, 1987 33/356; 33/363Q; 324/252 similar to Dinsmore
4626782 Magnetometer operated at a self-resonant frequency by a sense winding, Richard W. Lewis (Boeing), Dec 2, 1986, 324/253 ; 33/361
By driving the F.G. at the self reasonant frequency of the drive coil noise in the second harmonic signal is reduced.  By using feedback instead of adjusting each serial number device this is accomblished automatically.
2480265 August 1949 Rubenstein
2565799 August 1951 Brattain
2861242 November 1958 Leavitt
3001129 September 1961 Knowles
3040247 June 1962 Van Allen
3286169 November 1966 Slonczewski
3427534 February 1969 Maxwell
3626280 December 1971 Van Englehoven et al.
4447776 May 1984 Brown

4616424 Magnetic Direction Sensor - Hitachi - core made from ribbon wound with overlap
4590679 Compass systems - measure 20 cycles of sensor coil pulses
4546551 Electrical control system - schematic for uC system
4497034 Heading detecting apparatus - slope and offset correction of X & Y outputs
4495467 Apparatus for simultaneous measurement of magnetic field components in mutually perpendicular directions _X & Y outputs
4462165 Three axis orientation sensor for an aircraft or the like July 31, 1984 33/361; 33/355R; 33/363Q
4447776 Pulse driver for flux gate magnetometer - Navy low power sea mine "Brown" 4 terminal coil and 3 transistors, but needs more coils for the output.
4445279 Magnetic course detector - offset & slope correction on output of sensor coils
4425717 Vehicle magnetic sensor - mounting on rear view mirror to isolate from car metal
4424631 Electrical compass - uses digital BPF & synchronous phase detector instead of bandpass filters to select second harmonic
4413424 Azimuth determinating apparatus - Nippondenso - hard or soft iron correction
4399615 Direction indicator -
4384254 Oscillator/driver circuit for fluxgate magnetometer May 17, 1983 324/253; 327/190; 327/191; 331/112- "Brown"-Navy low power by letting core saturation stop drive for sea mines Very simple one transistor two winding drive circuit.  But still needs balanced sense coils. 
It may be that if a cylindrical coil is used then the sense coils are not needed and the output is just the frequency of the oscillator?
2957145 Oct., 1960 Bernalien 331/112.
3649908 Mar., 1972 Brown 324/253.
3681711 Aug., 1972 Hanby 307/275.
4308501 Digital peak sensor, Thomas E. Tuccinardi, Michael Conner, (Army), 327/58 ; 102/212; 102/417; 102/427; 324/260
looks first for above threshold then for negative slope in "magnetic signature"
4305035 Magnetic field amplitude detection sensor apparatus Dec 8, 1981 324/255 Hughes -Peak voltage detection like the FLC-100 Sensor has 2 coils over a few 0.005" dia wires from Spang Industries which is out of business but National Electronic Alloys makes similar metals
4305034 Magnetic field intensity measuring device with frequency change indication Dec 8, 1981 324/253; 102/427; 324/258; 331/65 Hughes uses CD4007 IC and a simple Two Terminal Coil (TTC) type sensor Permalloy wire
4300095 Self Excited Field Saturable Core Magnetic Field Detection Apparatus - Rockwell MR sensor
4292590 Magnatometer Apparatus with Detector Immoblized in Wax, J.H. WIlson (Westinghouse), Sep 29, 1981, 324/226 ; 174/559; 324/244; 324/260; 324/345; 33/305; 53/472
4290018 Magnetic Field Strength Measuring Apparatus With Triangular Waveform Drive Means 324/255
4286261 Apparatus for discriminating between strain and magnetic stimuli in magnetic stimuli in magnetic cored solenoid type transducer line sensors,
Gilbert F. Wagner, J. Louis Berger (Army), Aug 25, 1981,340/565 ; 324/327; 340/551; 340/566; 367/136
Prior art systems just use a cable that was sensitive to strain AND magnetic disturbances so an alarm did not tell you which triggered it
This system adds an oscillator driving one end of the cable so strain can be differentiated from magnetic signatures outdoor intrusion detector
4277751 Low-power magnetometer circuit with variable freq drive - uses DC feedback to cancel the Earth's field thereby reducing the drive power to allow battery operation, non 90 deg correction**
4262427 Flux Valve Compass System - Sperry Corp - sense windings around core not over it
4260949 Magnetic field detection apparatus April 7, 1981 324/302
4228395 Feature recognition system Oct 14, 1980 324/76.24; 324/76.12; 324/76.15; 324/76.29; 324/76.42; 324/76.58; 324/76.59; 324/331 (sub MAD)
3310784 Informatin Processing Aparatus March 21, 1967 703/13; 706/20; 706/38
3453540 Circuit that Analyzes Transient Signals in both the Time and Frequency Domains July 1, 1969 324/76.29; 324/76.44; 324/76.45; 324/76.68; 324/103R; 327/52
4241317 Frequency generator suitable for use as position-frequency transducer Dec 23, 1980 331/65; 331/151; 331/167; 331/181 - This Bosch patent seems to relate to a variable capactance, but also shows a variable inductance and so is related to the PNI patents.
4194150 Method and apparatus for reducing magnetometer errors (torpedo or mine)
4193029 Pulsed helium magnetometer 324/301; 331/3
4187779 Marine mine, Harry S. Jones, David B. Kirkpatrick, Robert G. Wilson (Navy), Feb 12, 1980 [app: Apr 19, 1945] , 102/209 ; 102/417 - 35 year delay in pub
combines hydrostatic pressure and "magnetic signature" to fire
4185552 Mine firing control system, Whitman D. Mounce, Ralph W. Mann (Navy), Jan 29, 1980 [app: May 16, 1945], 102/212 ; 102/417 - 35 year delay in pub
uses induction coil to sense "magnetic signature"
4182987 Magnetometer employing hard axis thin film inductor Jan 8, 1980 324/249; 360/111 -
This Kodak tape head patent is for a MI sensor where DC and AC are applied to the Two Terminal Coil (TTC) type sensor.
4183301 Induction ground mine and firing mechanism therefor, Richard J. Burke, George Renholt, Jr. (Navy), Jan 15, 1980 [application: Aug 7, 1944] 36 year delay in pub
uses Sensitrol relay to detect "magnetic signature" of ship.  vertical induction coil senses magnetic field.  The Sensitrol is a meter that has relay contacts.
"magnetic signature" is part of the design.
4068164 Harmoic flux gate magnetometers and gradimeters and windings, Malcolm M. Schwartz, James R. Jaquet ( Infinetics), Jan 10, 1978,
324/226 ; 102/212; 102/417; 102/427; 324/243; 324/253; 340/551; 367/93; 463/47.3
This is a Royer type oscillator in the form of a stick and two can be combined - low cost
2783384  ELECTRICAL INVERTER CIRCUITS, R.L. Bright & G.H. Royer, Feb 26 1957, 331/113A ; 290/4R; 307/401; 331/181; 363/133 - magnetic saturation of the transformer is a key parameter.
4059796 Second harmonic magnetic field detection circuit with means to rectify the sensed signal - three wndings, one for feedback, in newer patents the thrid winding was replaced with a cap and the sense winding was driven with the feedback DC signal
4030204 Remote indicating solid state magnetic sensor - Hall sensors and compensating magnets
3971981 Magnetism detecting system - external field rotated while looking for under sea cable
3959889 Method and apparatus for determining a measured direction relative to an external magnetic direction - ?
3942257 Index Error Correction for Flux Valve Heading Repeater System - Sperry Rand - "Y" core like servo
3936949 Static non-swinging course determining device on a vehicle  Feb 10, 1976 33/352; 33/354; 33/361; 324/247; 324/255
This uses a simple Two Terminal Coil (TTC) type sensor and uses the coil output voltage like in the FLC-100 sensor. 0.1 x 50 mm wire
2204292 Directional Indicating Means Jun., 1940 Barth 33/204 - TTC hi L/D permalloy wire
2464057 Earth Inductor Compass Mar., 1949 Phair 33/356 - rotating "Z" metal bar
2597125 Earth Induction System May., 1952 Noxon (Bendix) 33/361 - Flux gate 2nd harmonic metal Laminated? core dual winding
2597135 Earth Induction System May., 1952 Stuart (Bendix) 33/361 - Flux gate 2nd harmonic metal Laminated? core dual winding
2671275 Magentometer Mar., 1954 Burns (RCA) 33/361 - Flux gate 2nd harmonic ferrite core 
3133244 Magnetic Field Detector and Resolver having a two Section Housing for the Detector May., 1964 Wojtulewicz (General Precision) 33/356 - long slender rod saturable inductor
3135199 Magnatometer Jun., 1964 Brown 324/43 - reports total field using 3 orthogonal square law rod sensors
3584388 Heading Reference System Jun., 1971 Petrov et al. 33/349 -
3628254 Nonpendulous Flux Valve Compass System  Dec., 1971Burmeister(Sperry Rand) 33/225 - strapped-down syncro resolver type
3678593 Compass System and Components Therefor Having Automatic Feild Cancellation Jul., 1972 Baker (Sperry Rand) et al. - "Y" core
3696518 Vehicle Direction Sensing and Steering Systems Using Magnetic Flux Responsive Means Oct., 1972 Leat 33/357 - two fine wire cores

Older Patents have Images only

3903610 Apparatus for Measuring Magnetic Field Direction - 288 kHz divided to 400 Hz, triangle drive, the SIN adn COS coil utputs are "in resolver format", 2nd harmonic is bandpass filtered, phase detector as part of a PLL. PLL ooutput is phase compared to drive signal using digital counter with 1 deg per count.
3889181 Magnetic System for Discriminating, Charles B. Greer (Army), Jun 10 1975, 340/943 ; 340/554; 340/941
used as sensor for intellengience as in an outdoor intrusion detector
Vehicle and people channel is 0.1 to 2 Hz and Vechicle only channel is 6 to 40 Hz on the "magnetic signatore"
3343079 September 1967 Crouch
3508238 April 1970 Baker
3609679 September 1971 Updegraff et al.
3723988 March 1973 Kardashian
3727206 April 1973 Geil et al.
3745450 July 1973 Wilt
3754223 August 1973 Shtrikman
3812484 May 1974 Miller et al.

3873913 Shock Resistant Magnetic Detector, Arthur W. Obenschain (Navy), Mar 25 1975, 324/262 ; 324/247 -
Ceramic used as form for magnetic material
2219107 October 1940 Langmuir
2722734 November 1955 Grant
2981885 April 1961 Schonstedt
3076930 February 1963 Schonstedt

3850100 Mine Firing Control System, W. D. Mounce (Navy), Nov 26 1974 [application: Aug 28, 1943], 102/417, 31 year delay in publishing
replaces relays and motors with vacuum tubes.  The prior art devices were both sensitive to air drop shock and generated magnetic fields that caused problems.
Also the power used is reduced.  Uses "magnetic signature".
3840726 Position Locator - combines 3-axis fluxgate and gravity and steps
3835784 Mine Fuse, Miller et al. (Army), Sep 17 1974, 102/218 ; 102/209; 102/402 - anti-tank uses "magnetic signature"
Moving tanks or other armored vehicles have several phenomena, herein called signatures, associated with them -- e.g., weight, vibration, and magnetic field.
A reed senses vibration and a disk connected to a bar magnetic rotates in response to changes in mag field.
3833901 Magnetic Compass Having Remote Digital Readout - compass card that can be read digitally
3825823 Aparatus for Detecting the Direction of a Magnetic Field Relative to a Marker Direction Sequentially Scanning Plural Stationary Sensors - title says it all
3778703 Apparatus for Detecting Unlevelness of a Pendulously Supported Flux Valve by Subjecting the Valve to a Vertical Magnetic Field - used for calibrating flux valves so that their center of mass casues them to hang true.
3775674 Flux Gate Gradiometer using a Pulse generator to Drive the Energizing Windings as well as Actuate the Detector - USAF - Class 324/253 - 12 uS on 500 uS period
3722409 Mine Firing Control Appratus, Plumley, Mar 27 1973 [application: Aug 28 1943], 102/417 ; 102/209 - 30 year delay in publishing 
uses "magnetic signature"
3714523, 3714559, 3829883 Magnetic Field Sensor Jan 30, 1973 TI, 257/252; 257/426; 257/E29.323; 324/252; 327/510; 327/581 a dual insulated gate FET
3705980 Controlled Magnitude Repeater for Synchro and Resolver Signals - servo based output but sin/cos out of fluxgate
3681711 Blocking Oscillator with Extended Variable Pulse Aug 1, 1972 331/112; 327/191; 331/149
3678593 Compass System and Components Thereof Having Automatic Field Compensation - Sperry Rand - "Y" core & nulling Earth's field
Filed 1968, issued 1972. Uses "Y" core.
3649908 Magnetic Field Gradiometer Utilizing a Pair of Cores Driven by a Blocking Oscillator "Brown" - Navy - 2 cores, for sea mines
3605011 Control Apparatus - single coil with PWM output- non directional 500 uW
3601899 Single Core Solid State Compass - Navy -Goniometer as prior art
3548284 Synchro Data Transmission Aparatus ... - not Compas
3546579 Thin-Ferromagnetic-Film Magnetoresistive Magnetometer sensitive to easy axis Field Components and Biased to be insensitive to Hard axis Field Components, Dec 8, 1970 Sperry Rand, Minneapolis 324/252 - may be the Honeywell Patent
3541432 Magnatometer Capable of Operating with a Very Low Input Power - core is spiral wound tape
3460029 Magnetometer Incorporating Probe in the form of a Saturable Ferromagnetic Core Subjected to the Magnetic Field to be Measured and to an Auxiliary Exciting Alternating Field - not 2nd harmonic, but rather direct comparison of the currents for + and - saturation of the core
3416072 Thin Film Magnetometer Employing Phase Shift Discrimination 324/249 Dec 10, 1968 - RF frequency change
3344379 Magnetically Operated Electrical Switch, Warren P. Morrow (Army), Sep 26, 1967, 335/206 ; 102/427 - "magnetic signature" tank mine
3319161 Flux Gate magnetometer Utilizing a Thin Helical Strip of Magnetic Material as its Core -
3260932 Magnet-Field Mesuring Device with Galvanomagnetic Probe July 12, 1966 324/252; 338/32R - adding AC mag field to Hall device
3252081 Optical Magnetometer and Gradiometer, May 17, 1966, K.A. Ruddock  324/301; 324/304; 331/3
3233332 Earth's Magnetic Field Responsive Systems - Synchro "Y" type cores
3218547 Flux Sensing Device Using a Tubular Core with Toroidal Gating Coil and Solenoidal Output Coil Would Theron - NASA -
 for 0.1T(?) fields (like in outer space) looks like a difficult to wind cylinder
3185947 Inductive Module for Electronic Devices - printed inductor - not compass
3172032 Magneto-Resistive Device Mar 2, 1965 323/294; 324/252; 338/32H; 338/32R; 365/158; 708/841 - more on Hall effect + MR
3135199 Magnatometer T. B. "Brown" 102/417 ; 102/212; 324/247; 324/253 - three orthogonal sensors for sea mine total field (i.e. position independent) -
Fluxgate with oscillator to driving coil
3109985 Magnetoresistive Elements and Devices Nov 5, 1963 323/368; 310/27; 310/72; 324/252; 338/32R; 365/158 -
using Hall effect (when the output of a Hall sensor is short circuited, the resulting current increases the sensivity of the device).
3077538 Aparatus for Measuring Radiant Flux Feb 12, 1963 250/352; 324/224; 324/248; 374/122; 505/849
3031766 Magnetic Compass Compensating System - similar to 2,924,865/6
3202912 Method of Utilizing Tunnel Diodes to Detect Changes in Magnetic Fields Aug 24, 1965 - only usable at very high field levels, not Earth
3052827 Magnetic Discriminator, D. E. Olsen, Sep 4, 1962, 361/180 ; 324/101; 324/117R; 324/258; 324/98; 361/204
a single transistor oscillator works with feedback winding on sensing transformer to detect "magnetic signature"
3017833 Magnetic Firing Control Mechanism for a Mine, S.W> Booth, (not assigned) Jan 23, 1962 [application: Jun 8, 1943],
102/417 ; 318/444; 318/450
cam and motor based timing mechanisim can survive air drops better than stepping relay, uses "magnetic signature"
3015273 Magnetic Mine Firing Control Mechanism, R. H. Park et al (not assigned), Jan 2, 1962 {application: Apr 6, 1942], 102/417
not sensitive to magnetic storms or movement of the mine due to tides but will trigger when a large metal object goes by
2238072 Method and means for Locating Concealed Bodies, Nelson et al. (Western Union Telegraph), Apr 15 1941,
324/67 ; 102/406; 324/243; 324/326; 324/345; 405/173
a towed sled contains a ghradient detection system
2399523 Control System and Device therefore for submarine mines, Van Atta et al., Apr 30 1946, 102/417 ; 29/455.1 - resistant to sweeping
3012177 Magnetic Position Sensing Device Dec 5, 1961 
318/647; 33/361; 174/153R; 318/653; 323/362; 324/207.17; 324/207.25; 324/249; 324/253; 336/110; 336/188; 336/218; 340/870.33 - Fluxgate, freq doubler
3011440 Mine Firing Mechanism, Hugh Bradner 9not assigned), Dec 5, 1961 [application: Sep 13, 1944], 102/417 - "magnetic signature"
3010395 Mine Firing Mechanism, W. R. Maltby (not assigned), Nov 28 1961 [applifcation: Oct 19, 1943], - 28 year delay in publication -
combines magnetic and accoustic sensors "magnetic signature" uses stepping relays
2998727 Heading Reference System for Navigable Craft SEP5 1961 - parallel rod flux gates 400 Hz in 800 Hz out
2997648 Magnetic Field Detector Aug 22, 1961 324/252; 318/647; 338/32R; 340/870.43 uses square law change in resistance of Permalloy
2993440 Congtrol Device (Mine Firing)
2959896 Error Compensated Directional Systems - vry similar to 2,959,865 but specific for deviation compensation electronically not with magnets
2959865 Electrical Error Compensation Arrangements - for improving Fluxgate compass systems
2957145 Transistor Pulse Generator Oct 18, 1960 331/112; 331/109 - becomes a magnetometer when the proper type of transformer is used
2924886 Transmitting Magnetic Compass Systems - Hall effect sensor tracks the needle of a normal compass
2922227 Transmitting Magnetic Compass Systems - ring of mu metal senses needle of normal compass
2920562 Mine Firing Mechanisim - "magnetic signature"
2892404 Pressure Responsive Device - combines pressure and "magnetic signature"
2887782 Gyro Magnetic Compass System - adds synchro that is turned by a geared motor to allow rapid alignment of the directional gyro
2881702 Mine Firing Mechanism, Glennon et al. (not assigned), Apr 14 1959  [appliation: Aug 9, 1941] 102/417 ; 102/212; 102/402
mentiones "magnetic signature" and uses it in sea mine
2852859 Flux valve Compensating System - "Y" core combines directional gyro, verticle gyro and Coriolis compensation
2828396 Magnetoresistive Device Mar 25, 1958 338/32R; 324/252; 327/510; 330/62; 338/234; 338/308
2755562 Compass System - Bendix - Flux Valve senses a compass needle
2744235 Magnetic Field Extrapolating Apparatus and Method, G. Breit, May 1 1956 [app: Feb 4, 1946], 324/200 ; 335/220
the variation of the ship's magnetic field along some straight line parallel to the keel is called the ship's magnetic signature.
2671275 Magnetometer March 9, 1954 33/360; 33/361; 324/247; 331/40
Claims to be the first Magnetometer where there are no moving parts.  Simple Two Terminal Coil (TTC) type sensor.
Inductance of coil is a function of mag field and is used to control the frequency of an oscillator.  This is mixed with a frequency determined by a crystal oscillator to produce a new frequency which has larger percentage changes with field strength. The mixing scheme can be used with the FGM-3 sensors.
2652545 Deviation Compensor - uses multiple serews in a circle to compensate compass systems
2611191 Telemetric or Repeatere system - Repeats a compass needle
2599550 Fluxmeter and Probe Therefore, Jun 10, 1952 324/252; 338/10; 338/32R; 338/229 MR bridge
2555209 Method and apparatus for measuring the values of magnetic fields, Gary Muffly, Vacquier Victor V, Gulf Research Development Co, Nov 1, 1943, May 29, 1951 -
Key use detecting submarines from aircraft (maybe why the 8 year delay in publication).
1895373 Apparatus for measuring magnetic fields, Brucke (Gen Electric), Jan 24, 1933, 324/250, 33/360
                deflection of cathode ray tube mounted on transit
1919982 Compass Mayer, et al (Gen Electric), Jul 25, 1933, 33/360, 340/971, 324/250
                deflection of cathode ray tube
1988521 Gyro earth inductor compass, Sperry, Jr.(Sperry Gyroscope Co Inc), Jan 22, 1935,
                 3/317.00R, 324/246, 33/316, 324/345, 33/362, 33/321
                combined Earth Inductor and gyro
2016977 Direction responsive system, Thomas (Gen Electric), Oct 8, 1935, 33/361, 340/870.32, 324/255
                One of the early flux gate patents done to eliminate the moving parts of the induction compass (Wiki)
2027393 Cathode ray device, McCreary, Jan 14, 1936,
                33/360, 348/827, 361/191, 318/675, 324/250, 315/8, 33/363.00L, 33/355.00D, 318/293, 315/364
                CRT on motorized Az/El mount
2047609 Magnetic field direction and intensity finder, Antranikian, Jul 14, 1936, 33/301, 33/362, 324/247, 340/870.33
                Flux gate type - rotatable sensor and L - R instrument to maintain aircraft course (not spinning induction compass wear)
2124825 Process and apparatus for the indirect determination of earth and air-electrical conditions, Machts et al, Jul 26, 1938
                324/344, 324/345, 324/257, 324/334
                CRT senses both magnetic and electrical fields
2213357 Direction indicating means, Barth (Siemens ), Sep 3, 1940, 3/361, 324/253
                parallel rod flux gate
2246259 Apparatus for making geophysical or other measurements, Machts, Jun 17, 1941, 324/345, 324/250
                CRT with Left and Right targets to drive vacuum tube and meter
2325365 Magnetic compass, Britten (Monroe Calculating Machine), Jul 27, 1943,
                33/357, 324/259, 33/358, 33/360, 33/363.00K, 324/244
                photo electric reading of a needle compass
2412612 Directive radio system, Godet (Gen Electric), Oct 8, 1941, Dec 17, 1946,
                342/425, 244/3.19, 318/675, 89/41.7, 318/16, 343/765, 244/177, 327/519, 318/478, 342/78
                SCR-584 (Wiki) fire control radar?
2468968 Magnetic field strength indicator, Felch et al (Bell Telephone Labs), Apr 20, 1943, May 3, 1949
                324/246, 340/870.33, 324/253, 324/345
                3-axis flux gate

2542018 Compass Feb 20, 1951 33/360; 33/362; 33/363Q; 73/504.14 - vibrating master compass to drive slaves
2535068 Submarine Detecting Device, F.A. Johnson (not assigned), Dec 26 1950 [application Sep 6, 1941], 89/1.61 ; 102/402; 102/417; 114/18; 324/257; 434/1
uses a gradient method to cancel the Earth's mag field and mentiones "magnetic signature"
2519058 Deviation Compensator - just correction of deviation for use with gyros
2502786 Telemetering of compass Indication - Wurlitzer - 2 pots allow 360 degree operation
2491736 Magnetic Pickup Device and Method of Balancing the Device Dec 20, 1949   324/245; 324/253; 340/870.33; 340/870.43
2476723 by L. F. Beach, referenced in 3012177 but is a typo
2475593 Self Synchronous Flux Valve System - coil output frequency divided by 2 (not doubled) to drive a Selsyn and to have less distortion
2454184 Telemetric Compass - continuous pot, 4 taps, wiper at ground
2426470 Remote Indicating Magnetic Compass System - deviation compensation
2415985 Permanent Magnet Telemetric System Feb 18, 1947 318/693; 324/252; 338/32R; 340/870.34; 340/870.38 uses MR elements in a "Y" configuration as the input and what amounts to a synchro as the output, but driven by a DC battery.
2389146 Flux Valve - Sperry Gyro - 8 or 12 sided polygon with a coil on a bar as each leg
2383461 Flux Valve Compass System - Sperry Gyro -
2383460 Magnetic Field Responsive Device - "Y" core
Filed 1941, Issued Aug 1945. Adds both a second harmonic filter and feedback to null the Earth's filed in each sense coil.
2357319 Flux Valve Magnetic Compass - Sperry Gyro - "Y" core outputs look like a Selsyn, pendulum to keep horizontal
Filed 1940, Issued Sep 1944.  This appears to be the Start of what we now call the fluxgate compass (see 2047609 for prior art).
Uses a frequency doubler to drive the indicator although that's not a part of the claims.
2342637 Means for reproducing Motion - electrical circuit to remotely read a compass
2335117 Magnetic Surveying Aparatus, E.P. Harrison Nov 23, 1943 324/252; 106/DIG.7; 324/345 MR with A.C. drive & amplification
2308566 Magnetic Repeater or Compass - fluxgate senses compass needle
2269602 Means for reproducing motion - This is the heart of Synchro, Selsyn, etc.
Filed 1939, issued 1942.  Three wires connect two devices.  The signals are separated by 120 degrees.
There is a drive coil so each device has 5 terminals.
2260589 Magnetic Flux Meter, G.S. Smith Oct 28, 1941 324/252; 324/117H uses MR bridge using Bismuth wire (oldest MR sensor patent)
2158500 Magnetic Filed Direction Finder - 33/361; 324/247; 324/254
2053154 Direct Current Indicator - "Y" core driven with DC and AC that can detect magnetic flux
Filed 1935, Issued 1936.  The second harmonic has been discovered.  This patent is mainly aimed at a new way to measure DC currents, but it states that the device can also be used for measuring magnetic flux.
2047609 Magnetic Field Direction and Intensity Finder - Compass with saturating magnetic cores rotated to null
Filed 1933, Issued 1936.  This Flux Valve (the first time this term was used according to patent 2357319) is an improvement over the needle type compass and over the "inductor" type compass that uses a coil that is rotated with a motor to sense the Earth's magnetic field.
This patent uses two coils at right angles and rectifies the sensor coils output and sums them in a galvanometer so that the indicator needle shows "0" when on course.  The whole sensor is mechanically rotated to set the desired direction.
1988521Gyro Earth Induction Compass Jan 22, 1935  33/317R; 33/316; 33/321; 33/362; 324/246; 324/345
1886336 Induction Compass Nov. 1, 1932   33/362; 33/361; 324/253; 340/870.33
1863415 Magnetometer June 14, 1942 F. Rieber (GE)  324/256; 33/361; 33/362; 340/870.33 - vibrating wire in gap with mag field - to replace compass needles and rotating field induction meters.
1840911 Induction Compass
Filed 1925, issued 1932.  Class 33/301 This was part of the state of the are prior to the Flux Valve. This patent removes the effect of the vertical component of the Earth's magnetic field by using two counter rotating coils.   The coils are driven by Pelton wheels fed high pressure air so as to not have any magnetic field disturbing the sensors.  Vacuum tubes are used for signal processing.
Re Issued 19,038 - 1934
1778795 Electrical Measuring Instrument Oct 21, 1930 P. H. Craig 324/120; 310/26; 324/76.11; 324/101; 324/115; 324/251
1525844 Vacuum Tube Aparatus Feb 10, 1925  313/157; 324/250 (plate current varies with mag field)
1454592 Wireless Direction Finding Means for and Method of Piloting Aircraft May 8, 1923  342/441; 324/247; 340/979; 342/448
1448050 Compensation System for Compasses Mar 13, 1923  33/301; 33/356; 33/357; 324/244
711131 Art of Transmitting Intelligence Oct 14, 1902 329/347; 324/252 (the resistance of Bismuth may vary with magnetic field)
661520  Automatic Steering Device for Toropodos, Nov 13, 1900, 114/21.1 ; 114/20.1; 318/283; 324/243
RE23397 Magnetic Field Strength Indicator Aug 7, 1951 324/246; 318/34; 318/647; 324/253; 324/345
RE21970 Earth Inductor Compass Dec 9, 1941 33/362; 33/361; 324/247; 324/258

Honeywell MR Sensors (CCL/324/252 AND AN/honeywell)

6850057 Barber pole structure for magnetoresistive sensors and method of forming same
6831458 Magnetic differential field sensor using hysteresis field in AMR films
6750751 Integrated magnetic signal isolator with feedback
6717403 Method and system for improving the efficiency of the set and offset straps on a magnetic sensor
6700371 Three dimensional conductive strap for a magnetorestrictive sensor
6529114 Magnetic field sensing device
6376933 Magneto-resistive signal isolator
6304082 Printed circuit boards multi-axis magnetometer
6297628 Magnetoresistive bridge array
6232776 Magnetic field sensor for isotropically sensing an incident magnetic field in a sensor plane
5952825 Magnetic field sensing device having integral coils for producing magnetic fields
5667879 TaN/NiFe/TaN anisotropic magnetic sensor element
5644230 Miniature magnetometer and flexible circuit
5596272 Magnetic sensor with a beveled permanent magnet
5572058 Hall effect device formed in an epitaxial layer of silicon for sensing magnetic fields parallel to the epitaxial layer
5500590 Apparatus for sensing magnetic fields using a coupled film magnetoresistive transducer
5477143 Sensor with magnetoresistors disposed on a plane which is parallel to and displaced from the magnetic axis of a permanent magnet
5455510 Signal comparison circuit with temperature compensation
5435070 Simplified compass with multiple segment display capability
5351005 Resetting closed-loop magnetoresistive magnetic sensor
5247278 Magnetic field sensing device
4847584 Magnetoresistive magnetic sensor
4589038 Position sensor
4533872 Magnetic field sensor element capable of measuring magnetic field components in two directions
4447781 Magnetoresistive transducer apparatus

Philips MR Sensors (CCL/324/252 AND AN/philips)

6946834 Method of orienting an axis of magnetization of a first magnetic element with respect to a second magnetic element, semimanufacture for obtaining a sensor, sensor for measuring a magnetic field
6756782 Magnetic field measuring sensor having a shunt resistor and method of regulating the sensor
6580270 Magnetoresistive sensor or memory elements with decreased magnetic switch field
6577124 Magnetic field sensor with perpendicular axis sensitivity, comprising a giant magnetoresistance material or a spin tunnel junction
6501678 Magnetic systems with irreversible characteristics and a method of manufacturing and repairing and operating such systems
6501271 Robust giant magnetoresistive effect type multilayer sensor
6501261 Evaluation circuit for a magnetoresistive sensor
6486662 Magnetic field sensor comprising a spin-tunnel junction
6384600 Magnetic field sensor comprising a spin tunneling junction element
6275033 Magnetic field sensor having nickel oxide and cobalt containing ferromagnetic layers
6225802 Apparatus for automatic measurement of resistance of integrated MR device
6215301 Magnetoresistive detector comprising a layer structure and a current directing means
6100686 Magnetic field sensor with double wheatstone bridge having magneto-resistive elements
6075361 Device for detecting a magnetic field
5936402 Magneto-resistive magnetic field sensor with a constricted region
5904996 Method of manufacturing a magnetic field sensor
5689185 Compact device for measuring magnetic fields having magnetoresistors and an auxiliary magnetic field generator
5686837 Magnetic field sensor and instrument comprising such a sensor
5602471 Angle sensor including angularly spaced sensor units
5583436 Miniature magnetic field sensor with compact magnetization coil
4694248 Magnetic field sensor compensated for offset error
4686472 Magnetic sensor having closely spaced and electrically parallel magnetoresistive layers of different widths
4617600 Magnetic head having a thin strip of magnetoresistive material as a reading element
4568906 Sensor having a magnetic field-sensitive element with accurately defined weight and thickness dimensions in the nanometer range

Active Impulse Magnetometer (AIM)

3105934 Method and apparatus for the remote detection of ore bodies utilizing pulses of short duration to induce transient polarization in the ore bodies (Selco Exploration Company Ltd)

3852659 Geophysical prospecting method and apparatus utilizing correlation of received waveforms with stored reference waveforms (Barringer Research Ltd)
Pulsed field system for detecting the presence of a target in a subsurface environment (US Navy)
Active impulse magnetometer with bipolar magnetic impulse generator and fast fourier transform receiver to detect sub-surface metallic materials (Hughes)


Magnetometer Patents - Projects using different types of sensors -
Proton Precession Magnetometers, Rev 2 by Koehler
Practical guidelines for Building Magnetometers by Hobbyists - Proton Precession Magnetometer
Metal Detector Patents for UXO, including fluxgates
3919630, 4068164, 4277751, 4293815, 4439732, 4677381, 4887033, 5168223, 5670882, 5939881

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