Data processing: measuring – calibrating – or testing – Measurement system – Orientation or position
Reexamination Certificate
2000-06-26
2003-03-25
Hilten, John S. (Department: 2863)
Data processing: measuring, calibrating, or testing
Measurement system
Orientation or position
C702S152000
Reexamination Certificate
active
06539327
ABSTRACT:
TECHNICAL FIELD
This invention relates to a process for determining the position of a moving object using magnetic gradientmetric measurements.
STATE OF THE ART
We will start by describing existing positioning/tracking methods based firstly on the use of electromagnetic measurements.
The Various Means of Measuring an Electromagnetic Field
If the quasi-static signature of the electromagnetic source is used, the useful pass band varies from a few kilohertz to 10
−5
Hz. Document reference [1] at the end of the description contains an analysis of different means of measuring the electrical and magnetic field within this frequency band.
Vector Measurement Means
The electrical field E and the magnetic field B are intrinsically vector magnitudes. Directional systems capable of measuring each of the three components of the magnitude are frequently used in order to measure all information available in this type of signal.
For electrical measurements, a distinction is made firstly between the measurement of a potential difference between two electrodes. The electrical system must then have a large input impedance so that the measurement is not disturbed as described in document reference [2]. Another technique provides access to the electrical field in a conducting medium through a measurement of the electrical current passing between two electrodes, the contact impedances of the electrodes being perfectly adapted to the ambient medium; this is called the “current collection method” (see document reference [3]).
Vector magnetometers also measure vector components of the magnetic field in the directions of each of the magnetometer axes. For example there are “Fluxgate” magnetometers that are based on the principle of generating a compensation current to compensate the field to be measured in an iron core characterized by its hysteresis cycle, as described in document reference [4]. Another technique is based on a direct measurement of the magnetic flux in “Fluxmeter” bars, but this type of instrumentation is not well adapted to the measurement of magnetic fields at low frequencies. Finally, SQUID (“Superconducting Quantum Interference Device”) magnetometers as described in document reference [5] are some of the most efficient devices since their resolution can be as high as 10
−6
nT.Hz
−½
(the nanoTesla or nT being the most frequently used unit). Their superconducting technology requires an expensive cryogenic module that is difficult to use, and are usually used in gradientmetry due to their high precision; the difference between two nearby measurements is equivalent to a differentiation that automatically eliminates remote noise sources.
Scalar Magnetometers
One serious difficulty with magnetic field measurements is the presence of the earth's magnetic field that can be considered as constant in time for the scales considered, and that has an amplitude of 45000 nT in France. However it is not always possible to guarantee that there is no movement in the measurement direction, and it is very difficult to compensate for these displacements when they induce variations of the field with time within the useful frequency band. For example, a movement of 1 degree in a 45000 nT field generates 100 times more noise than the noise level of sensors in the zero to one Hertz band.
Scalar magnetometers overcome this difficulty by measuring the modulus of a total magnetic field, in other words the modulus of the vector sum of the earth's magnetic field (about 45000 nT in France) and the vector disturbance considered (a few nT). Nuclear magnetic resonance probes thus measure the precession frequency of protons or electrons (Larmor frequency) that is proportional to the modulus of the ambient field. The resolution can be as high as 10
−3
nT.Hz
−½
.
Given the relative disproportion between the modulus of the earth's field and the value of the disturbance due to the presence of the object, a good approximation as described in document reference [6] consists of assuming that the sensors measure the modulus of the earth's field plus the projection of the disturbance onto the earth field vector, as shown in FIG.
1
.
FIG. 1
illustrates the comparison between the vector sum of the earth's magnetic field B
t
and the measured magnetostatic disturbance B
signal
, and the algebraic sum of the earth's magnetic field and the projection of the disturbance onto the earth's magnetic field vector. Scalar magnetometers measure B
total
and not B
signal
. Assuming that a high pass filter was sued to eliminate the DC component and therefore the modulus of the earth's field, the scalar magnetic measurement is then written:
B
scal
=B
signal
−U
t
One of the advantages of these devices, apart from the precisions achieved, is that there is no measurement direction, which makes them easier to use; probes may be placed in any mobile system, and are generally easier to deploy than vector systems.
However, document reference [7] emphasizes the degenerescence of the projection of an essentially vector magnitude (the disturbance) onto a constant vector (the earth's field). Therefore, this type of magnetometer is conventionally used in detection, but this type of sensor has never yet been used for positioning of sources from a single observation site.
Gradientmetric Devices for Measuring the Magnetic Field
Vector or scalar magnetic devices are used to make two types of measurements, firstly the field which is the basic technique since sensors usually provide uniform magnitudes in the magnetic field, and secondly the spatial gradient of the field.
For SQUID magnetometers, the gradient measurement consists of taking the difference between two spatially close measurements, which is intrinsically equivalent to a differentiation.
In practice, this technique eliminates the contribution of far sources that are usually sources of noise (earth's magnetic fields, geomagnetic noise, etc.). Thus, although equations for the decay in the amplitude of signals are degraded due to the differentiation (1/r
4
instead of 1/r
3
for field measurements), the best reduction of disturbing noises can result in equivalent ranges (for example SQUIDs).
Note also that some positioning techniques are intrinsically based on gradient measurements, for example magneto-encephalography (MEG) techniques. This is also the case of this invention, and of the technique most resembling it discussed throughout the rest of this document.
One difficulty lies in adjusting the base length of the gradientmeter; this length can be fixed by carrying out a parametric study based on COR (Curves of the Operating Receptor) detection curves, that are a conventional tool according to detection theory, while respecting the assumption of a differential measurement; therefore, it is checked that the sensor spacing remains small compared with the distance from the source.
This latter constraint makes it possible to intrinsically talk about a “single observation site”, even when several sensors are used to make the gradientmeters. In this case, it is assumed that the overall gradientmetric measurement device forms a single measurement site.
Use of Magnetostatic Indiscretions in Positioning
We will now consider magnetostatic positioning/tracking techniques using a single observation site.
Magnetostatic indiscretion of a ship or a vehicle is due to the ferromagnetic properties of the materials from which it is made, including the steel structure, metal plates, engines, propeller shafts, etc. This object type is modeled with good precision by a single magnetic dipole, as soon as the distance from the measurement system is a few times more than the largest dimension of the object.
The magnetic moment M of this object is then broken down into two parts:
A permanent magnetization that reflects the magnetic history of the object; it is a constant magnetization within a coordinate system related to the object, regard
Blanpain Roland
Dassot Gilles
Flament Bruno
Jauffret Claude
Commissariat a l'Energie Atomique
Hayes & Soloway P.C.
Hilten John S.
Washburn Douglas N
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