Electricity: measuring and testing – Magnetic – Magnetometers
Reexamination Certificate
2001-10-12
2004-02-10
Strecker, Gerard R. (Department: 2862)
Electricity: measuring and testing
Magnetic
Magnetometers
C324S260000, C336S200000, C336S221000
Reexamination Certificate
active
06690164
ABSTRACT:
TECHNICAL DOMAIN
The present invention relates to a fluxgate magnetometer with perpendicular detection and its process of realisation. This invention applies mainly to the domain of magnetic field measurement and more precisely for:
Localisation and steering of medical instruments
Non-destructive control
Monitoring of aiming system attitude
Navigational aids
Traffic and access control
Magnetic cartography
PRIOR ART
The magnetometers said of fluxgate type (or fluxvalve type) involve the physical properties of saturable ferromagnetic materials. Such a magnetometer usually uses a ferromagnetic core, an excitation coil and a pick-up coil. An alternating current (sine- or triangle-shaped) circulates in the excitation coil and creates a variable magnetic field, that is a variable induction in the core. As soon as the saturation of the material is reached, its permittivity decreases strongly. A supplementary field which should be applied would not lead to any significant increase of induction, as if the “gate” offered to the magnetic flux was closed. Conversely, when the current in the excitation core crosses zero, the permittivity of the material is high and the gate is open to the flux, hence the “fluxgate” expression.
The variable induction in the core then creates a variable magnetic flux in the detection coil, which provokes an electric signal on the terminals of the said coil. The existence of an external magnetic field creates a distortion of this signal from which the external field value can be derived. The development of the micro-technology in the magnetic measurement domain led to the realisation of micromagnetometers, said of “microfluxgate” type.
In particular, through the use of electrolytic deposits, it is possible to obtain magnetic layers and coils with layered conductors whose dimensions are very small. One finds a description of these fluxgate micro-magnetometers in the following articles:
Microfluxgate magnetic sensing elements using closely-coupled excitation and pick-up coils” from Shoji KAWAHITO et al. in “Transducers 95-Eurosensors”IX 290-A12;
A miniaturised magnetic field sensor system consisting of a planar fluxgate sensor and a CMOS readout circuitry from R. GOTTFRIED-GOTTFRIED et al. in “Sensors and Actuators” A 54 (1996), pp. 443-447;
“High-resolution microfluxgate sensing elements using closely coupled coil structure” from Shoji KAWAHITO et al. in “Sensors and Actuators” A 54 (1996) pp. 612-617:
A microfluxgate magnetic sensor using micromachined 3-Dimensional planar coils” from Trifon M. Liakopoulos et al. in “Solid State Sensor and Actuator Workshop”, June 98.
The
FIGS. 1A and 1B
show a top view and a cross section of a fluxgate magnetometer of current type. It is composed of two ferromagnetic bars
10
a
and
10
b,
a double excitation coil
12
and a pick-up coil
14
. The whole is realised on a non-magnetic substrate
16
, for example a silicon substrate. The cross-section—FIG.
1
B—is drawn along a plane containing one of the strips of the coil
12
. Insulating material
18
is provided as shown in FIG.
1
B.
In the case of this arrangement, the pick-up coil
14
sees no global flux, as the induced fields in the bars
10
a
and
10
b
are of opposite signs. When submitted to an external field, the equilibrium is broken and the signal picked-up at coil
14
terminals reveals this external field. More precisely, this signal reveals the component of the field which is parallel to the substrate
16
, and, moreover, parallel to the longitudinal axis of the bars
10
a
and
10
b.
For some applications, it is necessary to know the amplitude and direction of the measured field, which requires to measure three components of the field along three orthogonal directions. The magnetometers of current type are sensitive to the component of the field which is parallel to the substrate and consequently permit the measurement along two orthogonal directions X and Y, provided that two identical micro-magnetometers are available on the same substrate, whose orientations are orthogonal (respectively X and Y). With two identical micro-magnetometers orientated along Y and Z, the field components along Y and Z will be obtained. Thus, with three micro-magnetometer orientated along X, Y and Z, the three components of the field can be measured. Usually, the two magnetometers are located on two adjacent faces of a cube. The precision of the measurement is altered due to defects on the cube machining (faces are not perfectly perpendicular) or a bad position of the magnetometers resulting from poor bonding process etc.
The purpose of the present invention is to overcome these drawbacks.
DESCRIPTION OF THE INVENTION
To that effect, the invention proposes a micro-magnetometer sensitive to the field component which is perpendicular to the substrate (and not to the one which is parallel to the said substrate), i.e. orientated along the Z axis if the X and Y axes are parallel to the substrate. Hence, the measurement of the three components of a given field can be derived by combining the use of the micro-magnetometer object of the invention, which delivers the component along Z, with the use of two classical micro-magnetometers located on the same substrate and delivering the components along X and Y.
Precisely, the object of the present invention is a fluxgate magnetometer featuring a substrate which includes at least one ferromagnetic core fitted with at least one excitation coil and one pick-up coil, this core and these coils having the same axis, this magnetometer being characterised by the fact that the said axis is perpendicular to the substrate.
The object of the present invention is also to disclose the realisation method of the said micro-magnetometer, characterised by the following steps:
one uses a non magnetic substrate whose one part at least is an insulating material;
one creates in the insulating material two conducting coils surrounding a central area;
one etches this central area to create a an imprint whose axis is perpendicular to the substrate;
one fills up this imprint with a ferromagnetic material.
REFERENCES:
patent: 2321355 (1943-06-01), Berman
patent: 2971151 (1961-02-01), Mierendorf et al.
patent: 4623842 (1986-11-01), Bell et al.
patent: 5199178 (1993-04-01), Tong et al.
patent: 5479099 (1995-12-01), Jiles et al.
patent: 6429651 (2002-08-01), Choi et al.
patent: 721731 (1955-01-01), None
Cuchet Robert
Duret Denis
Fedeli Jean-Marc
Gaud Pierre
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