JP2579280B2 - Calibration method for SQUID magnetometer - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a SQUID (Supercon
The present invention relates to a calibration method for system calibration of a magnetometer such as a SQUID magnetometer or a fluxgate that measures a magnetic field using a ducting quantum interference device (superconducting quantum interference device). Where S
A QUID is a DC current as a bias current applied to a SQUID loop, which is a superconducting loop that is maintained in a low temperature state in a heat insulating container (cryostat, Dewar, or the like) by liquid helium, liquid nitrogen, or the like and includes a Josephson junction in the loop. When a magnetic flux from the outside is coupled and applied to this SQUID loop via a pickup coil, an input coil, and the like, a circulating current is induced in the SQUID loop, and a quantum current in the Josephson junction in the loop is generated. This element measures a small change in magnetic flux by utilizing the fact that it operates as a transducer that converts a small change in the applied external magnetic flux into a large change in the output voltage due to the interference effect.

[0002]

2. Description of the Related Art Conventionally, as a calibration method for calibrating a relationship between a magnetic field intensity to be measured and an output voltage value of a SQUID magnetometer, a method of calibrating a SQUID magnetometer installed near the bottom or the like in an insulated container such as a dewar is known. A circular coil or a straight electric wire, etc., and apply a current to this circular coil or a straight electric wire to generate a magnetic field. From this current value and the distance to the SQUID magnetometer,
Calculate the theoretical value of the magnetic field strength at the ID magnetometer position,
SQ from the relationship with the output voltage value output from the ID magnetometer
A method of calibrating the magnetic field sensitivity of a UID magnetometer is known.

[0003]

However, the conventional S
In the QUID magnetometer calibration method, the SQUID
Since the D magnetometer is installed in an opaque dewar or other insulated container, it is difficult to determine its exact position. Therefore, an error occurs in the theoretical value of the magnetic field strength at the SQUID magnetometer position. However, there is a problem that an error occurs in the magnetic field sensitivity. The present invention has been made in order to solve the above-mentioned problems, and has been developed in such a manner that an SQUID in an insulated container or the like is provided.
It is an object of the present invention to provide a calibration method capable of improving the position estimation accuracy of a D magnetometer and accurately calibrating the magnetic field sensitivity of a SQUID magnetometer.

[0004]

In order to solve the above-mentioned problems, a method for calibrating a SQUID magnetometer according to the first invention of the present application is a method for calibrating an SQUID magnetometer having N known positions near a SQUID magnetometer to be calibrated. Magnetic field generating coils C _{1 to} C _N
Was placed, the magnetic field generating coil C _j among the N field generating coil C ₁ -C _N when the current value is sequentially applies a known known current to each magnetic field generating coil C ₁ -C _N is the SQUID magnetometers of the position vector r _p B the theoretical vector value of the magnetic field made by: a (r _p C _j), a unit vector indicating the direction of the detection field of the SQUID magnetometer and n _p, the magnetic field sensitivity scalar value g _p and the output voltage measurement value of the SQUID magnetometer by the magnetic field generating coil C _j to which the known current is applied is V _pj ,

(Equation 3) There configured to estimate the parameters of the SQUID magnetometer position vector r _p and the detected magnetic field direction vector n _p and the magnetic field sensitivity scalar value g _p so as to minimize. Further, the method for calibrating a SQUID magnetometer according to the second invention of the present application includes the N magnetic field generating coils C _{1 to} C _N whose installation positions are known near the SQUID magnetometer to be calibrated.
Was placed, the said current value to each magnetic field generating coil C ₁ -C _N and frequency f ₁ ~f _N is the said N magnetic field generating coil C ₁ -C _N in the case of applying a known known current time S
The theoretical vector value of the magnetic field created by the position vector r _p of the QUID magnetometer is B (r _p : C _j ), the unit vector indicating the direction of the magnetic field detected by the SQUID magnetometer is n _p , and the magnetic field sensitivity scalar value is g _The SQUID by _N magnetic field generating coils C _{1 to} C _N to which each of the known currents is applied.
When the output voltage intensity for each frequency component f _j obtained by Fourier transforming the output voltage measurement value of the D magnetometer is V _pj ,

(Equation 4) There configured to estimate the parameters of the SQUID magnetometer position vector r _p and the detected magnetic field direction vector n _p and the magnetic field sensitivity scalar value g _p so as to minimize. In the above, the installation position vector r, the direction vector n, and the value S = (r, n, i) of the current value i are known near the M SQUID magnetometers calibrated by the calibration method of the SQUID magnetometer. Install an evaluation magnetic field source, measure the output voltage value V ^* of the M SQUID magnetometers by the evaluation magnetic field source, based on the output voltage value V ^* ,
The position vector r ^* and the direction vector n of the evaluation magnetic field source
^*, Current value i ^* values S ^* = (r ^*, n ^*, i ^*) to estimate, the estimated value S ^* and the formula E is the difference between the true value S (r _e, n _{e, ie} ) = S (r, n, i) -S ^* (r
^* , N ^* , i ^* ), the estimation error by the calibration method may be evaluated.

[0005]

The SQ according to the first invention of the present application having the above-mentioned structure is provided.
According to the calibration method of the UID magnetometer installation position in the vicinity of the SQUID fluxmeter to be calibrated is placed N known magnetic field generating coil C ₁ -C _N, the respective magnetic field generating coil C ₁ -C _N the N theoretical vectors of the magnetic field the magnetic field generating coil C _j is made of the position vector r _p of the SQUID magnetometer of the magnetic field generating coil C ₁ -C _N when the current value is sequentially applies a known known current Let the value be B (r _p :
C _j ), the unit vector indicating the direction of the magnetic field detected by the SQUID magnetometer is n _p , and the magnetic field sensitivity scalar value is g
_{Let p} be the magnetic field generating coil C _j to which the known current has been applied.
When the V _pj output voltage measurements of the SQUID magnetometer according to the following formula

(Equation 5) There can be estimated the parameters of the SQUID magnetometer position vector r _p and the detected magnetic field direction vector n _p and the magnetic field sensitivity scalar value g _p so as to minimize. Accordingly, this allows the magnetic field sensitivity g _p of the SQUID magnetometer to be accurately calibrated, and at the same time, the SQUID in an insulated container or the like.
The exact position of the ID magnetometer can be known. According to the SQUID magnetometer calibration method according to the second aspect of the present invention having the above configuration, the SQ to be calibrated is
N magnetic field generating coils C _{1 to} C _N whose installation positions are known are installed near the UID magnetometer, and the respective magnetic field generating coils C _1
N of the magnetic field generating coils C when current values and frequencies f _{1 to} f _N with known currents are simultaneously applied to.
₁ -C _N is the SQUID magnetometer of the position vector r _p B the theoretical vector value of the magnetic field made by: a (r _p C _j), a unit vector indicating the direction of the detection field of the SQUID magnetometer and n _p, the field sensitivity scalar value and g _p, the frequency components obtained by Fourier-transforming an output voltage measurements of the SQUID magnetometer according to the N pieces of the known current is applied to the magnetic field generating coil C ₁ -C _N f _j When the output voltage intensity for each is V _pj ,

(Equation 6) There can be estimated the parameters of the SQUID magnetometer position vector r _p and the detected magnetic field direction vector n _p and the magnetic field sensitivity scalar value g _p so as to minimize. Accordingly, this allows the magnetic field sensitivity g _p of the SQUID magnetometer to be accurately calibrated, and at the same time, the SQUID in an insulated container or the like.
The exact position of the ID magnetometer can be known. Further, an evaluation magnetic field source whose installation position vector r, direction vector n, and current value i = (r, n, i) are installed near the M SQUID magnetometers calibrated by the above method. ,
The output voltage value V ^* of the M SQUID magnetometers by the evaluation magnetic field source is measured, and based on the output voltage value V ^* , the position vector r ^* , direction vector n ^* , and current value of the evaluation magnetic field source are measured. i ^* values ^{S * = (r *, n} *, *) to estimate, the estimated value S ^* and the difference between the true value _{S E (r e, n e} , i e) = S (r, n, i) -S ^* (r
^* , N ^* , i ^* ), whereby the estimation error by the above calibration method can be evaluated, and the reliability of the calibration value by the above method can be evaluated.

[0006]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a conceptual diagram for explaining a SQUID magnetometer calibration method according to a first embodiment of the present invention. As shown in FIG.
A UID magnetometer 1 is provided. The output of the SQUID magnetometer 1 is sent from the SQUID output circuit 3 to the computer 4.
And the output voltage value is monitored and recorded.

The above-mentioned SQUID magnetometer 1 is a SQUID magnetometer.
The magnetic flux input to the loop is converted to the magnetic flux voltage conversion coefficient dV / dΦ
(V: voltage, Φ: magnetic flux) is converted into an electric signal and output as a voltage. This magnetic flux voltage conversion coefficient dV / dΦ is equal to SQUA
The magnetic field sensitivity of the ID magnetometer 1 is shown.

In the vicinity of the SQUID magnetometer 1, a plurality of magnetic field generating coils C _{1 to} C _{N as} magnetic field generating sources are arranged. It is assumed that the individual coil positions of each of the magnetic field generating coils C _{1 to} C _N (for example, the vector amount from a predetermined origin) are known. Further, each of the magnetic field generating coils C _{1 to} C _N
Are connected to the current source 5 via the changeover switch 6, and the switching operation of the changeover switch 6 causes the alternating current having a constant amplitude value to be sequentially applied to the individual magnetic field generating coils as shown in FIG. Is configured. It is assumed that this applied current value (for example, the waveform of the magnitude and frequency of the current) is also known.

[0009] In the present embodiment, the SKU is
The computer estimates and converges parameters of the magnetometer position of the ID magnetometer 1 (for example, a vector amount from a predetermined origin), a detected magnetic field direction (for example, a unit vector in a predetermined direction), and a magnetic field sensitivity (scalar value). 4 First, among the _N magnetic field generating coils C _{1 to} C _N , the magnetic field generating coil C _j is located at the position r of the SQUID magnetometer 1.
Let B (r _p : C _j ) be the theoretical value (vector quantity) of the magnetic field created by p (for example, the vector quantity from a predetermined origin).
This B (r _p : C _j ) is expressed by the following equation from Biot-Savart's law.

(Equation 7) Is represented by In the above equation (1), μo represents a scalar value representing the magnetic permeability of vacuum, J represents a vector value representing a current density, and r ′ represents a vector value representing a current position.

A unit vector indicating the direction of the magnetic field detected by the SQUID magnetometer 1 is represented by n _p , and a magnetic field sensitivity scalar value g _p (unit: T / V, where T represents Tesla and V represents volt) The theoretical value of the output voltage of the SQUID magnetometer 1 is given by

(Equation 8) Is represented by In the above equation (2), B (r _p : C _j ) ·
n _p represents the inner product of both vectors. Also, 1 / g
There is a relation of _p = dV / dΦ.

Next, the theoretical value V expressed by the above equation (2)
If calibration is performed by the computer 4 so that the _theory and the output voltage measurement value V _pj monitored by the computer 4 by the SQUID magnetometer 1 actually output by the magnetic field generating coil C _j to which the current is applied become equal, the actual Magnetic field strength and SQ
The output voltage of the UID magnetometer 1 will correspond exactly. However, in practice, the measured values vary, so using the least squares method,

(Equation 9) Is an objective function f (x), and each parameter is searched using a numerical solution so as to minimize the objective function f (x).

In this embodiment, the so-called "iterative method"
The iterative method is a method of searching for a numerical solution by the following procedure. That is, first, “initial setting” is performed as a first procedure. The initial setting refers to, for example, giving the value of the initial point x _o at k = 0 among the variables x _k . Next, as a second procedure, when a predetermined stop condition is satisfied is considered a solution that x _k.
If there is no x _k that satisfies the predetermined stop condition, the process proceeds to the next third procedure. In the third procedure, the search direction d _k is determined. Next, in a fourth procedure, the “step width α _k ” in the d _k direction is _obtained by a “straight line search method” or the like. Next, in the fifth procedure, x _{k + 1} = x _k + α _k · d _k (4) Then, as a sixth procedure, k = k + 1, and the above-described second and subsequent procedures are executed again.

The “stop condition” in the second procedure of the above-mentioned iterative method is a gradient vector magnitude _{ f (x _k )}.
And the sequence of points {x _k} of variation ‖x _k -x _k-1 ‖ has converged to a solution for determining Once decreased to a certain extent, and often considered. As an example of the above iterative method, first, the “steepest descent method (Stee
pest Descent Method) ", but since the gradient vector of the objective function indicates the direction that maximizes the function locally, if you proceed in the opposite direction,
This is based on the idea that the value of f (x) could be reduced along the maximum slope. According to the steepest descent method, the k-th search vector d _k is given by the following equation: d _k = − ▽ f (x _k ) (5) However, the above search direction is not always the best search strategy when searching for a global solution, and often stagnates on the way. The steepest descent method guarantees global convergence if the line search is performed well, but has the disadvantage that the convergence speed is very slow.

As another method, the Newton method (Newton method)
Method)). Newton's method uses the objective function x
The minimum of the model function f (x _k + d) ≒ f (x _k ) + ▽ f (x _k ) ^T d + (1/2) d ^T ▽ ² f (x _k ) d... (6) The direction toward the point is selected as the search vector.

That is, the k-th search direction is obtained by solving the simultaneous linear equation ▽ ² f (x _k ) d = − ▽ f (x _k ) (7). By using this search direction, local quadratic convergence is realized.

However, the Newton method has a drawback that a "Hessian matrix" of a function is required, and it takes a lot of work to analytically calculate the Hessian matrix. It also has the disadvantage that global convergence cannot be guaranteed. Therefore, a numerical solution that can make up for the shortcomings while utilizing the advantages of the steepest descent method and the Newton method has been sought. Widely used.

As described above, the parameters (r _p , n _p , g _p ) relating to the position of the SQUID magnetometer, the direction of the detected magnetic field, and the magnetic field sensitivity that minimize the above equation (3) are estimated (initial setting). By repeating the above procedure, each parameter value (r _p , n _p , g _p ) can be made to converge to an accurate value. In the above equation (3), N indicates the number of the magnetic field generating coils.

Next, a SQUID magnetometer calibration method according to a second embodiment of the present invention will be described with reference to FIG. In the second embodiment, the current source 5 and the changeover switch 6 in the first embodiment shown in FIG.
A dedicated current source is connected to each of all the magnetic field generating coils C _{1 to} C _N. Then, alternating currents having different frequencies and current values as shown in FIG. 3A are simultaneously applied to the respective magnetic field generating coils C _{1 to} C _N from the respective current sources. That is, the magnetic field generating current having a frequency f ₁ is the coil C _1, the current of frequency f ₂ is the magnetic field generating coil C _2, ..., a current of a frequency f _N is applied to the field generating coil C _N.
Accordingly, magnetic fluxes corresponding to the applied current are generated from the respective magnetic field generating coils C _{1 to} C _N , and these magnetic fluxes simultaneously pass through the SQUID magnetometer 1.

As a result, the change over time of the output voltage value V of the SQUID magnetometer 1 becomes a curve as shown in FIG. By subjecting the measured voltage value of the SQUID magnetometer 1 to Fourier transform by the computer 4, an output voltage value intensity V _pj for each frequency component f _j is obtained as shown in FIG. Since the intensity value V _{pj in} FIG. 3C corresponds to the measured voltage value V in the above equation (3), each of the intensity values V _pj is set so that the above equation (3) is minimized, as in the first embodiment. By estimating (initial setting) the parameters (r _p , n _p , g _p ) and repeating the above procedure, each parameter value (r _p , n
_p , g _p ) can be converged to an accurate value.

However, when the calibration value of a SQUID system such as a SQUID magnetometer or a SQUID sensor array is obtained by the above-described calibration method, the value is an estimated value, and it is considered that the value includes some error. Can be Therefore, it is necessary to evaluate the reliability of the calibration value obtained by the above-described calibration method. However, since it is difficult to know the true calibration value, the magnitude of the estimation error cannot be known. For this reason, the reliability evaluation of the above-described calibration method cannot be performed based on the true calibration value. Therefore, the reliability of the calibration values obtained in the first and second embodiments is evaluated by the SQUID magnetometer calibration method according to the third embodiment of the present invention described below.

FIG. 4 shows a SQUID according to a third embodiment of the present invention.
It is a conceptual diagram for explaining the D magnetometer calibration method. As shown in the figure, M SQUI
A sensor array 11 composed of a D magnetometer is installed, and near the sensor array 11, a plurality of magnetic field generating coils C _{1 to} C _{N as} magnetic field generating sources are arranged. Sensor array 1
The position of each SQUID magnetometer (for example, the vector amount r _pi from a predetermined origin) is unknown. It is assumed that individual coil positions of the magnetic field generating coils C _{1 to} C _N (for example, vector amounts from a predetermined origin) are known. Each of the magnetic field generating coils C _{1 to} C _N is connected to a current source 15 via a changeover switch (not shown), for example, as in the first embodiment. It is configured such that an alternating current having a constant amplitude value is sequentially applied to the generating coil. It is assumed that this applied current value (for example, the waveform of the magnitude and frequency of the current) is also known.

In the present embodiment, first, each of the M S elements of the sensor array 11 is obtained by the method shown in the first and second embodiments.
Estimation and convergence of each parameter of the magnetometer position of the QUID magnetometer (for example, a vector amount from a predetermined origin), a detected magnetic field direction (for example, a unit vector in a predetermined direction), and a magnetic field sensitivity (scalar value) are performed.

Next, an evaluation magnetic field source 20 such as a coil is installed on the calibration apparatus shown in FIG. It is assumed that the position vector r, the direction vector n, and the value S = (r, n, i) of the current value i of the magnetic field source 20 are known. The magnetic field generated from the magnetic field source for evaluation 20 is measured by the sensor array 11 which has been calibrated by the above method, and the output voltage value V ^* of each SQUID magnetometer constituting the sensor array is obtained.

Next, based on the measured SQUID output voltage value V ^* , the position vector r ^{* of} the magnetic field source 20 for evaluation,
Direction vector n ^* , current value i ^* value S ^* = (r ^* , n
^* , I ^* ). The position is, for example, 3 of x, y, z
If the coordinates and the like are determined, the direction is uniquely determined. For example, if the two directions of the zenith angle and the plane direction angle are determined, the direction is uniquely determined. If the current value is determined as one value, Since it is uniquely determined, if the magnetic field from the evaluation magnetic field source 20 is measured by at least six SQUID magnetometers, the above-mentioned S ^* = (r ^* , n ^* ,
i ^* ) can be estimated. In addition, seven or more S
In the case of the QUID magnetometer, S ^* = (r ^* , n ^* , i ^* ) can be estimated by using the least squares method in the same manner as described above.

The difference between the estimated value S ^* obtained as described above and the true value S E (r _e , _ne , i _e ) = S (r, n, i) −S ^* (r ^* , n ^* , i ^* ) (8) is the estimation error of the magnetic field source 20 for evaluation. Since the magnitude of E is caused by an estimation error of the calibration value calibrated by the calibration methods of the first and second embodiments, the magnitude of E is
The estimation accuracy of the above-mentioned calibration method can be evaluated based on the size of the calibration method. The magnitude of this estimation error indicates the accuracy of the magnetic field source estimation by the SQUID magnetometer.

The present invention is not limited to the above embodiment. The above-described embodiment is an exemplification, and has substantially the same configuration as the technical idea described in the claims of the present invention, and any device having the same function and effect can be realized by the present invention. It is included in the technical scope of the invention.

For example, in the first embodiment, the N magnetic field generating coils C _{1 to} C _N are connected to the current source 5 via the changeover switch, and are sequentially switched so that the SQUID generated by the magnetic field from each magnetic field generating coil. was recorded by monitoring the magnetometer 1 of the output voltage value, the present invention is not limited to this example, any magnetic field generating coil C _j current value is a known make with SQUID magnetometer 1 position r _p It is sufficient that the theoretical vector value B (r _p : C _j ) of the magnetic field can be calculated, and it need not necessarily be a constant current source.

Further, in the second embodiment, an example in which alternating currents having different frequencies and current values are simultaneously applied from the _N magnetic field generating coils C _{1 to} C _N has been described, but the present invention is not limited to this example. The present invention is not limited thereto, and currents having the same current value but different frequencies may be simultaneously applied.

In the first and second embodiments,
Although the example in which one SQUID sensor is used has been described, the present invention is not limited to this, and the present invention can be implemented without any problem even if a sensor array including a plurality of SQUID sensors is used. In this case, the position of the SQUID magnetometer (for example, 3 of x, y, z)
Coordinates, etc.), direction (for example, two directions of a zenith angle and a plane direction angle), and magnetic field sensitivity (one value) may be determined, and simultaneous equations are provided if at least six magnetic field generating coils are provided. Each parameter can be determined by solving. Further, in the third embodiment, the SQUA
The example in which the ID sensor has N sensor arrays has been described. However, the present invention is not limited thereto. The position (for example, three coordinates of x, y, z, etc.) and the direction (for example,
It is sufficient that the current value (one value) and the zenith angle and the plane direction angle can be determined.
If a D sensor is provided, each parameter can be determined by solving simultaneous equations.

[0030]

As described above, according to the method for calibrating a SQUID magnetometer according to the first invention of the present application having the above-described configuration, N pieces whose installation positions are known in the vicinity of the SQUID magnetometer to be calibrated are known. Magnetic field generating coils C _{1 to} C
Established the _N, the magnetic field generating coil C _j among the N field generating coil C ₁ -C _N when the current value is sequentially applies a known known current to each magnetic field generating coil C ₁ -C _N is The theoretical vector value of the magnetic field created by the position vector r _p of the SQUID magnetometer is B (r _p : C _j ), the unit vector indicating the direction of the detected magnetic field of the SQUID magnetometer is n _p ,
When the magnetic field sensitivity scalar value is g _p and the output voltage measurement value of the SQUID magnetometer by the magnetic field generating coil C _j to which the known current is applied is V _pj ,

(Equation 10) There can be estimated the parameters of the SQUID magnetometer position vector r _p and the detected magnetic field direction vector n _p and the magnetic field sensitivity scalar value g _p so as to minimize. Accordingly, this allows the magnetic field sensitivity g _p of the SQUID magnetometer to be accurately calibrated, and at the same time, the SQUID in an insulated container or the like.
The exact position of the ID magnetometer can be known. According to the SQUID magnetometer calibration method according to the second aspect of the present invention having the above configuration, the SQ to be calibrated is
N magnetic field generating coils C _{1 to} C _N whose installation positions are known are installed near the UID magnetometer, and the respective magnetic field generating coils C _1
N of the magnetic field generating coils C when current values and frequencies f _{1 to} f _N with known currents are simultaneously applied to.
₁ -C _N is the SQUID magnetometer of the position vector r _p B the theoretical vector value of the magnetic field made by: a (r _p C _j), a unit vector indicating the direction of the detection field of the SQUID magnetometer and n _p, the field sensitivity scalar value and g _p, the frequency components obtained by Fourier-transforming an output voltage measurements of the SQUID magnetometer according to the N pieces of the known current is applied to the magnetic field generating coil C ₁ -C _N f _j When the output voltage intensity for each is V _pj ,

[Equation 11] There can be estimated the parameters of the SQUID magnetometer position vector r _p and the detected magnetic field direction vector n _p and the magnetic field sensitivity scalar value g _p so as to minimize. Accordingly, this allows the magnetic field sensitivity g _p of the SQUID magnetometer to be accurately calibrated, and at the same time, the SQUID in an insulated container or the like.
The exact position of the ID magnetometer can be known. Further, an evaluation magnetic field source whose installation position vector r, direction vector n, and current value i = (r, n, i) are installed near the M SQUID magnetometers calibrated by the above method. ,
The output voltage value V ^* of the M SQUID magnetometers by the evaluation magnetic field source is measured, and based on the output voltage value V ^* , the position vector r ^* , direction vector n ^* , and current value of the evaluation magnetic field source are measured. i ^* values ^{S * = (r *, n} *, *) to estimate, the estimated value S ^* and the difference between the true value _{S E (r e, n e} , i e) = S (r, n, i) -S ^* (r
^* , N ^* , i ^* ), whereby the estimation error by the calibration method can be evaluated, and the reliability of the calibration value by the method can be evaluated.

[Brief description of the drawings]

FIG. 1 is a conceptual diagram illustrating a first embodiment of a SQUID magnetometer calibration method according to the present invention.

FIG. 2 is a timing chart illustrating a SQUID magnetometer calibration method shown in FIG. 1;

FIG. 3 is a timing chart illustrating a second embodiment of the SQUID magnetometer calibration method according to the present invention.

FIG. 4 is a conceptual diagram illustrating a third embodiment of the SQUID magnetometer calibration method according to the present invention.

[Explanation of symbols]

1,11 SQUID magnetometer 2 Dewar 3 SQUID output circuit 4 Computer 5,15 Current source 6 Changeover switch 20 Magnetic field source for evaluation C _{1 to} C _N Magnetic field generating coil

 ──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Hisashi Kado 1-1-4 Umezono, Tsukuba, Ibaraki Pref. Naoki Nakatsuka, Examiner at the Electronic Technology Research Laboratory, National Institute of Advanced Industrial Science and Technology (56) References JP-A-1-250775 (JP) , A) JP-A-7-12906 (JP, A) JP-A-7-122789 (JP, A)

Claims (3)

(57) [Claims] 1. N magnetic field generating coils C _{1 to} C _N whose installation positions are known are installed in the vicinity of a SQUID magnetometer to be calibrated, and current values are known to the respective magnetic field generating coils C _{1 to} C _N. wherein the magnetic field generating coil C _j among the N field generating coil C ₁ -C _N in the case of successively applying a known current SQU
ID magnetometer position vector r _p at making the magnetic field of the theoretical vector value B: a (r _p C _j), a unit vector indicating the direction of the detection field of the SQUID magnetometer and n _p, the magnetic field sensitivity scalar value g and _p, when the output voltage measurements of the SQUID magnetometer according to the known current is applied magnetic field generating coil C _j set to V _pj, the following equation ## EQU1 ## The SQUID magnetometer position vector r _p , the detected magnetic field direction vector n _p, and the magnetic field sensitivity scalar value g _p are estimated so as to minimize the SQUID.
Calibration method for ID magnetometer. 2. N magnetic field generating coils C _{1 to} C _N whose installation positions are known are installed in the vicinity of a SQUID magnetometer to be calibrated, and the current value and frequency f are assigned to each of the magnetic field generating coils C _{1 to} C _{N. The N} magnetic field generating coils C _{1 to} C _N when _{1 to} f _N simultaneously apply known known currents are connected to the SQUID.
D magnetometer position vector r _p B the theoretical vector value of the magnetic field made by: a (r _p C _j), a unit vector indicating the direction of the detection field of the SQUID magnetometer and n _p, the magnetic field sensitivity scalar value g _{Let p} be an output voltage value intensity for each frequency component f _j obtained by performing a Fourier transform on the output voltage measurement value of the SQUID magnetometer by the _N magnetic field generating coils C _{1 to} C _N to which the known currents are applied. _Where V _pj is The SQUID magnetometer position vector r _p , the detected magnetic field direction vector n _p, and the magnetic field sensitivity scalar value g _p are estimated so as to minimize the SQUID.
Calibration method for ID magnetometer. 3. An installation position vector r, a direction vector n, and a current value i in the vicinity of the M SQUID magnetometers calibrated by the SQUID magnetometer calibration method.
A magnetic field source for evaluation having a known value S = (r, n, i) is installed, the output voltage value V ^* of the M SQUID magnetometers by the magnetic field source for evaluation is measured, and the output voltage value V ^* , The position vector r ^* , the direction vector n ^* , and the current value i ^* of the evaluation magnetic field source S ^* = (r ^* , n ^* , i ^* )
E (r _e , _ne , i _e ) = S (r, n, i) −S ^* (r), which is the difference between the estimated value S ^* and the true value S.
^The calibration method for a SQUID magnetometer according to claim 1 or 2, wherein an estimation error by the calibration method is evaluated based on ^{( *} , n ^* , i ^* ).