1 Oersted = 79.58 Ampere-Turns/Meter
1 nano tesla = 10 micro Oe = 1 Gamma
1 Tesla = 10,000 Gauss
Papers about
Magnetism - http://www.wsrcc.com/alison/magnetism.html
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 - http://www.webspan.net/%7Emagres/
Earth's Magnetic Field
Magnetic Field Calculator - need Lat, Lon, elevation, can choose WWM2005 or IRGF 10 models. Outputs 7 components:
Lat: 39° 11' 24'' Lon: - 123° 9' 50'' Elev: 280.00 m Declination + East - West
Inclination + Down - Up
Horizontal Intensity
North Component + North - South
East Component + East - West
Vertical Component + Down - Up
Total Field
5/26/2007 15° 6' 62° 20' 23,183.6 nT 22,382.9 nT 6040.4 nT 44,224.8 nT 49,933.1 nT Change per year - 6' per year - 1' per year -24.7 nT/year -12.7 nT/year -48.0 nT/year -69.6 nT/year -73.2 nT/year
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.
Compute Field Values for givenlocation and date -
Experiment Relating to the Vertical Component of the Earth's Field -
National Geophysical Magnetic Data Center - On line mag field calculator
The International Association of Geomagnetism and Aeronomy -
USGS - Real-time monitoring of the Earth's magnetic field - geomagtism page -
WWV - Official Announcements - includes geomagnetic field activity activity
Hermanus Magnetic Observatory - CSIR -
International Geomagnetic Reference Field - World Magnetic Model
British Geological Survey - Geomagnetism -
Bartington Instruments Ltd - Magnetic Instrumentation Products -combined with theodolite
Australian INTERMAGNET Observatory Information -
Schonstedt Instrument Company - 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 - DARPA MEMS -
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
NGDC - Utility Programs for Geomagnetic Fields
Swedish Institute of Space Physics (IRF) - Real Time plot - Today's and Earlier Space Weather -
AAVSO Solar Division - McWilliams magnetometer - bar magnet suspended on "E" guitar string photo sensors detect rotation
The Great Magnet, the Earth by Daniel Stern of the NASA- Commemorating the 400th anniversary of "De Magnete" by William Gilbert3 Axis Magnetic Field Sensor - looking for information on this
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):
The East - West component causes what is called declination, or the compass points to "magnetic north".
- 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 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- World Map, North America - Field Compass - MC-1 G Global - unique needle balanced for worldwide use. No longer on their web pages?
Suunto has many patents.
3813792 Device for Declination Correction of a Magnetic Compass, Suunto Oy, Jun 1974, 33/356 - just declination dial on bottom to manually set
7129835 Wrist-top computer, Suunto Oy, Oct 31, 2006, 340/539.11 ; 340/539.12; 455/100 - belt transmits to wrist displayVector 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 TugawarPhotos -
Silva -makes a number of quality compasses and other outdoor items like Head Lamps
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) 5583776 -
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 -
Static vs. Dynamic Compass
Note that to determine the compass bearing when static requires both sensing the Earth's magnetic field and sensing it's gravitational field. A simple way to do this to level the compass using a bubble vial. That's what's done on the Suunto wrist compass. For a conventional magnetic needle compass the needle is weighted to counteract the dip caused by the Earth's magnetic field for a limited range of latitudes. In order to get a reading the compass needs to be held level or the needle will not be free to turn.
For an electronic compass an accelerometer can be combined with a 3-axis magnetic field sensor, called an orientation sensor, to determine both North and down.
But if an orientation sensor is used in a dynamic environment, like a boat, car or plane, it will not work because the "down" direction will not be valid when the vehicle is not going in a precise straight line. To fix this a gyroscope can be added to sense turning. This is why "Gyro compass" is a term herd on vehicles.
Dip Needle
The compass works in the horizontal plane and is compensated for it's lattitude of operation to balance out the effect of the vertical component. The dip needle works in the vertical plane, which is often the stronger magnetic field.
Notice that the scale is only on half the circle and that a chain with a hook is provided to hang it so the needle axis is horizontal. Not sure why two covers rather than the single cover used on a compass, but maybe so you can see whichever side is nearest you.
This one needs cleaning up and the needle is too weak to work properly so needs to be magnatized.
Helmholtz Coil(s)
A Holholtz coil consists of two coils aligned on a common axis, wound in the same sense and connected in series. The radius of the circular coils is R and the seperation between them is also R. 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.
The magnetic field is given by B = (4/5)^(3/2) * (Uo * N * I)/R
where the R units are:
This is a good way to test magnetic sensors.
meters
cm
inch
Uo =
1.26E-6 Tm/A
1.26E-4 Tcm/A
4.95E-5 Tin/A
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 -
3m diameter Helmholtz Coil at the National Physical Laboratory -
Billingsley Magnetics - 1 meter, 2.2 meter - coils
5126666: 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
Maxwell Coils
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 numerous permeabilities offered some alloys are called Netic (soft iron) and Co-Netic (mu-metal). Metglas is a high initial permmability material.
I tried common metal objects like food cans, but the soft iron just translates and concentrates the Earth's magnetic field. Using Co-Netic material (Netic has a much higher saturation level and is for strong fields) in a can shape both inside and outside a 10" length of 3" diameter ABS plastic pipe results in a reasonable shield. Using the same length, but with 2.5" diameter PVC would be better (longer L/D), but the 2.5" PVC is not available at my local hardware stores.
The material in the Magnetic Shield Corp. LK-110 is in 10" x 15" sheets.
When working with magnetic sensors it's difficult to tell what the zero field output is. There is a continuous bipolar change from the max North indication and the max South indication. Because of offsets half way between these two is not the zero field output. But with a zero field can you can tell what's going on. A Helmholtz coil aligned with the Earth's magnetic field vector or a 3-axis coil system could also produce a zero field, but with much more complication and also with some uncertainty.
The 10 & 4 mil sheets are easy to cut using ordinary paper scissors. Although not as good as spot welding an overlapped joint can be spot soldered.
When a FGM-3 sensor is pointed alone the Earth's field vector it reads 37 kHz or 135 kHz depending on polarity, but when pointed in the same directions inside the can it reads 58 or 66 kHz.
If the Earth's field is + or -50 uT and that corresponds to a span of 98 kHz then the sensitivity of the FGM-3 is about 1.02 uT per Hz. The field remaining in the can is about 8 * 1.02 or 8.1 uT. It seems that the can should be doing more, but maybe it's too short or needs spot welds, or thicker material. (it was too large in diameter)
17 Nov 2005 - I now have a third can that fits inside the above two cans and can be seperated from the inner can above by using bubble wrap or cardboard.
The attenuation for a cylinder is proportional to (material thickness) / (diameter) for a single can.
When two cans are nested and magnetically isolated from each other the final attenuation is the product of the two attenuations. So if you double the thickness on a single can you get 2 times the attenuation, but if you use the same material to make two cans you get about 4 times the attenuation. So you get more attenuation for the dollar by using a number of thin wall cans than a single thick wall can.
There appear to be different effects at work:
(1) as a probe is moved along the centerline of a can starting from the open face there is a 1/(e^(a*x)) type of decrease in field strength. where x is expressed in terms of L/D with L being the distance down into the can and D the ID of the can. This suggests that the inner can should be shorter than the outer can.
(2) after some distance the field strength flattens out and this must be related to the material thickness and permeability as well as the can I.D.
(3) It may be that the overlap joint in the can has some leakage that prevents the field from becoming smaller and smaller at the deeper depths.
(4) There is no material saturation problem with these materials and only the Earth's magnetic field at work here.
A plot of field strength for the 2.4" dia x 9.75" high can shows that the field increases slightly from the center value to the cap indicating that the cap has some leakage because of the folding done to make it. From 2 to 6 inches up from the cap the field is constant at about 2.8 uT compared to a theoretical value of 44.3 uT for the vertical field.
A much better shield could have been made by making the I.D. much smaller.
Note that an open cylinder that was twice as long as a closed can should give the same results without the need for making a cap, but that takes twice as much expensive material.
Some of the material supplied in the Magnetic Shield Kit is soft iron. It's good for knocking down a very high field since it's saturation level is higher than mu-metal. But the initial permeability of soft iron is many many orders of magnitude lower than mu-metal and so even a very thick shield does little to attenuate the earth's field.
AD-Vance Magnetics -
Magnetic Shield Corporation - FAQ -Netic and Co-Netic - Helmholtz Coil Assemblies -Zero Gauss Chambers - Lab Kit 110 with 10" x 15" sheets of material.
Magnetic Shields Limited - UK mu metal supplier that has formed shapes, not just sheets or rolls.
National Electronic Alloys - mu metal and other shielding alloys in sheet or roll (they call it coil) forms.
Sunbo - Korea raw materials & shield cases for magnetic recording heads
Riken - Magnetic Materials Laboratory - Japan
MuMetal - Specifications - MµShield materials - Gauss meter rental - Equations
U. of Texas - Magnetic shielding - note equation (611) at the bottom of the page.
Reference Data for Radio Engineers, Handbook of Chemistry and Physics, and other books have tables of relative permeability for various materials. What Magnetic Shield corp. calls Netic (plain cold rolled iron) mu is about 200, cast iron is 90, machine steel is 450. Mu-metal is 20,000 and Supermalloy is 100,000. So you can see why using iron pipe, soup cans or cookie boxes will not give you much shielding.
where B is the field inside a sphere and Bo is the field outside the sphere
a is the radius of the sphere
d is the thickness of the material
mu zero is the permeability of free space 1.257E-6 Henry/meter
mu is the relative permeability of the shield material.
So to improve shielding you can make the sphere smaller, use thicker material, use material with a higher mu.
The high mu materials also are designed to have very low coercivity (they do not become magnets).
Also note that the same materials that work well for shielding would also work well as the core of a fluxgate sensor and for the same reason, they concentrate the filed lines.
Lakeshore 4060 Zero Gauss Chamber
The cylindrical space inside is 0.51" dia x 2.04" inside the plastic sleeve. When the sleeve is removed the space is 0.605 dia x 2.114".
The field at the bottom of the hole is essentially zero. Comes with the cloth bag.
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 - Magnetometers -
Coast Ranges of Oregon and Washington - Magnetics in the exploration of the ground -
Fluxgate
The idea of a flux gate is that when the core is saturated it essentially disappears and when it's out of saturation it bends the earth's magnetic field lines which cross the coil. The switching of the core in and out of saturation is gating the earth's magnetic filed through the coil thus generating a voltage according to Faraday's law.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.
See: O. V. Nielsen, B. Herhando, J. R. Tetersen, and R. Primdahl,
"Miniaturization of low cost metallic glass fluxgate sensors", J. Mag.
Mag. Matr., 83, 405, 1990, for a general background on fluxgate
magnetic sensors.
There are a number of different ways to process the signal from a
fluxgate sensor. See the Fluxgate
Patents web.
The most common ones are:
- looking at the second harmonic of the drive frequency
- looking at the difference between the positive and negative peaks of the waveform
- looking at the frequency of an oscillator
- looking at the period of an oscillator
Many fluxgate circuits make use of two coils, one for driving the core and another one or two coils for sensing. But there are many fluxgate circuits where there's only a single coil that's used for both drive and sensing. In all cases there needs to be a way for the sensing circuitry to ignore the drive signal and maximize sensitivity to the output signal. I have tagged patents that have a Two Terminal Coil (TTC) on the Fluxgate Patents web since these would be less expensive both in terms of winding the coil and simpler circuitry.
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 -
Bartington Instruments - Instruments, not sensors
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.
SAM (Simple Auroa Monitor) - PIC uC kit for using the FGM-3
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. D0325041: Multifunction monocular 5020902: Rangefinder with heads-up display
Magnetic Research, Inc. - book with detailed circuit diagrams and fluxgate sensors - he died no further info
MC-1 Aircraft fluxgate compass field calibration kit -
Multilayer Air Core Inductor Calculator - good for fluxgate coil design with the core removed. (IE only, not Netscape)
Simple Fluxgate Sensor - FGM overview & App notes Single cal coil details start at pg 9
Speake & Co - make FGM- series of sensors carried by Fat Quarters
Stefan Mayer Instruments -
Watson Industries - combinations of heading sensors, accelerometers, angular rate gyros, and angle references as well as a line of magnatometers.
These are some fluxgate type magnetometers that use the Metglas material, yet are different from the PNI use of the material, yet also still have low power consumption.
5124648 Single winding saturable core magnetometer with field nulling 324/253; 33/361; 324/225; 331/65
5039945 Single winding magnetometer with automatic distortion compensation 324/253; 33/361
4859944 Single-winding magnetometer with oscillator duty cycle measurement 324/253; 33/361; 331/65
A History of Vector Magnetometry in Space -
Cores
The early cores were "Y" laminations like are used in transformers. Later Tape Would Cores (torrids) were used. Note that in both cases the core material comes in sheets. The current highest initial permeability material is Metglas.
Magnetics - Strip Wound Cores -
Arnold Magnetics -
Magcore -
Vacuumschmelze GmbH - ?Tape Wound Cores?
MK Magnetics - only does large cores
Applied Physics Systems - makes magnatometers and cores
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.
- 4851775 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.
- 5239264 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.
These use much less power than the classical flux gate units.
-
If someone used circuitry other than the two patented ones above they
could use MI sensors without infringing the PNI patents again IMHO.
- 2 digital drive pins with the sensor and a resistor in series between the pins. The resistor sets the current. One pin is high, say +5 or +3.3 Volts and the other pin is low say 0 Volts.
- 1 pin is an analog input, either an A/D or comparator with a threshold near 1/2 way between the rails. It is connected to the junction of the sensor and the resistor.
Idea in Public Domain (28 Dec 2002)
Use 3 pins on a microcontroller as follows:This approach, in my opinion, is outside of the two patents and has the advantage over the patents of providing a faster response time. Both of the PNI patents use counting the frequency of an oscillation which is good for averaging the reading, but at the expense of fewer readings per second.
Have not tested this idea.
Maybe PNI patent 6084406 "Half-cycle saturable-core magnetometer circuit " covers this idea.
Since this technology has a very low power requirement I think it is
used for digital compass applications like in a wrist watch.
6512370
Low power, frequency-mode magnetometer James; John Elf Engineering 324/253;
33/361; 331/65 - another circuit that gets around the PNI patent.
The MI coil has a core made from Allied Signal METGLAS®
Magnetic Alloy 2705M (Cobalt-based). A small sliver of the
Metglas material can be inserted into the center of a coil or molded
into the plastic rod that is the former a coil is wound over.
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 -
