Fluxgate (& other) Patents

Background

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)

Brown

Flux Gate Core Material

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 -

•  319 Magnetic Compass
• 345 Magnetic
• 355R Magnetic responsive
• 360 Utilizing cathode-ray tube or photoelectric cell
• 361 Electro-magnet or inductor (e.g., flux valve, etc.)
•  708 Magnetic distance measuring

324 Electricity: Measuring and Testing

• 76.24 Measuring, Testing, or Sensing Electricity, per se
• 
  
• 200  Magnetic
• 225 With compensation for test variable
• 244 - 263 Various Magnetometers
• 244 - Magnetometers
• 244.1 - Optical
• 245 - Plural sensor axis misalignment correction
• 246 - With means to align field sensor with magnetic field sensed
• 247 - Nonparallel plural magnetic sensors
• 248 - Superconductive magnetometers
• 249 - Thin film magnetometers
• 250 - Electronic tube or microwave magnetometers
• 251 - Hall plate magnetometers
• 252 Semiconductor type solid-state or magnetoresistive magnetometers
• 253 - Saturable core magnetometers
• 254 - Second harmonic type
• 255 - Peak voltage type
• 256 Energized movable sensing coil magnetometers
• 257 - Moving coil magnetometer
• 258 - Fixed coil magnetometer
• 259 - Movable magnet or magnetic member interacts with magnetic field
• 260 -Magnetic field detection devices
• 261 - With support for article

• 300 -  Particle Precision Reasonance
• 301 -Using a magnetometer
• 302 To determine direction
• 304 -Using optical pumping or sensing device
• 323 of Geophysical Surface or Subsurface in Situ
• 326 For small object detection or location
• 327 Using oscillator coupled search head
• 328 Of the beat frequency type
• 323 By aerial survey
• 331 For magnetic field detection
• 332 With radiant energy or nonconductive-type transmitter
• 335 Employing multiple frequencies
• 345 By magnetic means

327 Miscellaneous Active Electrical Nonlinear Devices, Circuits, and Systems

• 190 - With transformer or saturable core device
  
• 191 - Blocking oscillator
  

331 Oscillators

• 3 - Molecular resonance stabilization
• 112 - Blocking oscillator type
  

340 Communications: Electrical

• 500 - Condition Responsive Indicating System
• 551 - Disturbance of magnetic field
• 572.6 - This subclass is indented under subclass 572.1.  Subject matter where the device or tag includes an element having magnetic properties of high permeability and low coercivity.

428 Stock Material or Miscellaneous Articles

• 900 - Magnetic Feature

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
References:

3614729		Fujimoto	October 1971
3626413		Zachmann	December 1971
3641569		Bushnell et al.	February 1972
3778761		Cribbins	December 1973
3877666		Itakura et al.	April 1975
3983531		Corrigan	September 1976
4234923		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
4356489		Hirota et al.	October 1982
4368428		Dijkman	January 1983
4767988		Wilson	August 1988
4920340		Mizuno	April 1990
4943805		Dennison	July 1990
4968979		Mizuno et al.	November 1990
5008666		Gebert et al.	April 1991
5091697		Roth et al.	February 1992
5113136		Hayashi et al.	May 1992
5206642		Gregoire et al.	April 1993
5331276		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

Patents

Filed 1968, issued 1972. Uses "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.

Filed 1940, Issued Sep 1944.  This appears to be the Start of what we now call the fluxgate compass.
Uses a frequency doubler to drive the indicator although that's not a part of the claims.

Filed 1939, issued 1942.  Three wires connect two devices.  The signals are seperated by 120 degrees.
There is a drive coil so each device has 5 terminals.

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.

Filed 1933, Issued 1936.  This Flux Valve 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.

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

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

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

Links

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