JP2869781B2 - Variable SQUID magnetometer - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-sensitivity SQUID magnetometer utilizing the Josephson effect, and more particularly, to a flexible SQUID magnetometer by variably calculating electric signals obtained from a plurality of SQUIDs. The present invention relates to a SQUID magnetometer constituting a simple measurement system.

[0002]

2. Description of the Related Art Conventionally, a known SQUID magnetometer comprises a dewar (or cryostat) for storing liquid helium, an SQUID probe operating in liquid helium, an amplifier and a controller operating at room temperature,
The SQUID probe in liquid helium and the room temperature amplifier are connected by a coaxial cable.

Normally, a feedback for a linear operation called FLL (flux lock loop) is formed through the room temperature amplifier to control the zero-order method. When simultaneously performing magnetic measurement at many points for measurement for biomagnetism or non-destructive inspection, several tens to several hundreds of SQUAs are stored in the dewar.
IDs are arranged in an array to obtain a spatial distribution of the magnetic field.

In practice, since a weak magnetic signal is measured, the measurement is usually performed in a magnetically shielded room.
At this time, the purpose of the magnetic shield is to shield magnetic interference noise generated from a place distant from the object to be measured.
When the pickup coil for picking up the magnetic flux to be input to the ID has a differential configuration, or when the pickup coil and the SQUID are integrated, the output of the SQUID is electrically subtracted so that the distance between the pickup coil and the SQUID can be increased. Cancellation of the disturbing magnetic field in position has been implemented.

[0005] Such a means is called a gradiometer or the like using a differential coil (also called a differential coil). The method of taking the difference with an electronic circuit after measurement with the latter magnetometer is also called a gradiometer.

[0006] Differential (differential) coil construction methods include a method in which a differential coil such as a first derivative and a second derivative is used in a single step and a method in which a differential coil is used in two steps to obtain a second derivative of the spatial distribution of a magnetic field. Thus, a configuration method for canceling a magnetic field having a first-order gradient is typical. In the case of a differential configuration using a coil, the differential configuration is created by winding a coil around a bobbin or the like, so that the differential configuration is a magnetometer (0th order), a first-order differential gradiometer, or a second-order differential gradiometer according to the measurement target. It cannot be changed to.

[0007] For example, in the case of biomagnetic measurement and magnetoencephalography measurement, when a magnetic field generated from a shallow place such as the cerebral cortex is targeted, the primary or secondary differential coil is advantageous in terms of S / N. For a magnetic field generated from a deep part of the brain, for example, from the brain stem, a magnetometer or a primary differential coil having a long base line length is advantageous.

[0008] As described above, when the pickup coil is determined, the object to be measured is limited, so that the device cannot be effectively used. The same applies to the case where the shape of the pickup coil must be determined in advance in the nondestructive inspection depending on the depth of the flaw to be searched.

On the other hand, in the case of an electrically differentiating configuration, the magnetometer is previously arranged at a position separated by the differential length, and the output linearized by the FLL circuit is electrically differentiated. There is flexibility. However, the number of elements and FLL circuits is increased by the number of magnetometers for taking a differential configuration, and the room-temperature electronic circuit (FL
L circuit) and the SQUID are connected more lines,
Heat intrusion into the dewar becomes large, and the consumption of the refrigerant may become severe, so that there is a problem that it is economically disadvantageous.

[0010]

Since the dynamic range of the magnetometer is determined by the frequency characteristics of the FLL circuit, a high-frequency and large-amplitude disturbing magnetic field is applied to a highly sensitive magnetometer. In such a case, the magnetic flux lock cannot be maintained, so-called lock jump occurs, and it becomes difficult to continue the measurement. This means
An essential drawback of using a FLL circuit is to use a magnetometer having an inherently high magnetic field sensitivity, which has been exacerbated by this drawback.

[0011]

SUMMARY OF THE INVENTION The present invention has been made in view of such conventional problems, variable according to claim 1
The differential type SQUID magnetometer is a magnetometer using a plurality of dc-SQUIDs (hereinafter, referred to as SQUIDs) having the same magnetic field sensitivity as sensors for detecting a magnetic field. Switch means for turning on / off the bias current of one SQUID, a plurality of resistors for taking out the respective voltage outputs of the plurality of first SQUIDs as currents, and electrically connected in series with the plurality of resistors. and coil has a second SQUID to or SQUID array coupled to said coil and magnetically, the summing amplifier consisting of said plurality of first SQUID
Or or is the ash is disposed such that the code of sensitivity surface adjacent arranged pickup coil for supplying magnetic flux to said plurality of first SQUID becomes opposite to each other.

Further, the variable differential SQUI according to claim 2 is provided.
2. The plurality of first sensors SQUI of claim 1, wherein:
ID or Oite the second SQUID to or SQUID array of claim 1, and a magnetic flux-voltage conversion ratio dV / d?
And a resistor Rn and a coupling coil Mn, a negative feedback circuit is provided so that a feedback ratio β = (dV / dΦ) × Mn / Rn ≧ 1, and a Φ−V curve in which an operating point is linearized. the flux bias circuit provided at the ash to be positioned substantially mid-point of the slope.

[0013]

Embodiments of the present invention will be described below with reference to the drawings. First, the principle of the present invention will be described. That is, a plurality of first SQUIDs (sensors S
QUID), a second SQUID or SQUID array for adding or subtracting these outputs, and an analog switch for switching the bias current of the sensor SQUID.
By turning on / off the bias current flowing through D,
Select the SQUID in the ON state.

The SQUID for detecting a signal as a sensor SQUID and the SQUID for detecting a reference signal having the same sensitivity and opposite sign are arranged in advance, so that a primary differential configuration (gradiometer) and a magnetometer can be obtained. Is switched by a switch. The output of the selected SQUID is derived as a current through a resistor, which is connected to a coil for addition or subtraction, and further amplified by a second SQUID or SQUID array magnetically connected to this coil to obtain a room temperature. It is connected to an amplifier.

By adopting such a configuration, the differential configuration can be arbitrarily reconfigured by switching the bias current flowing through the sensor SQUID and the reference SQUID. The configuration of the magnetometer can be switched by switch means. In addition, since addition and amplification are performed by the second SQUID, the S / N ratio is extremely high, and the output linearized by the FLL circuit is not calculated. It has good characteristics.

FIG. 1 shows a variable differential type SQUID magnetometer according to an embodiment of the present invention, and a circuit configuration for arbitrarily selecting a magnetometer and a second derivative gradiometer by switching a sensor SQUID. Is shown. The spatial arrangement of each SQUID will be described with reference to FIG.

In FIG. 1, reference numerals 1, 2, and 3 denote a sensor SQUID for detecting a magnetic field, in which a pickup coil and a shunt resistor are omitted. Although the negative feedback circuit described with reference to FIG. 3 is also omitted, the negative feedback circuit does not necessarily have to be included.

Reference numerals 4, 5, and 6 denote coupling resistors for transmitting signals to the next-stage SQUID.
It has a resistance value ratio of 1: 2.

Reference numeral 7 denotes a bias current for the sensors SQUID1 to SQUID1.
3 is a current limiting resistor for supplying only an appropriate value to the sensor 3 and the sensor SQUID1 turned on by the analog switch 8.
To 3, the output voltage corresponding to the input magnetic field is transmitted to the second-stage SQUID, and any of the OFF-state sensors SQUID1 to 3 is not supplied with the bias current, and the output voltage is reduced. Does not occur.

The analog switch 8 may be replaced by a mechanical switch. Reference numeral 9 denotes a bias current supply voltage source. Regarding each of the analog switch 8 and the bias current supply voltage source 9, each sensor SQUID1 to
The number of DA converters that can control the output current so that the bias current can be supplied independently of the number of the SQUIDs may be connected to the number of sensors SQUID.

Reference numeral 10 denotes ON / OFF of the analog switch 8
It is a control device for performing. 11 is each sensor SQ
This is a coupling inductance for adding output currents of the UIDs 1 to 3 and transmitting the added magnetic flux to the SQUID array 12 at the next stage. The SQUID array 12 is a single SQUID
However, addition can be performed.

Reference numerals 13 and 14 denote a current limiting resistor for supplying a bias current to the SQUID array 12 and a bias current supply voltage source. Reference numeral 15 denotes a negative feedback resistor for applying negative feedback. Reference numerals 16 and 17 denote room temperature amplifiers for amplifying the output of the SQUID array 12 to a suitable value. Reference numerals 18 and 19 are magnetic flux input coils for setting the operating point of each SQUID. Current limiting resistor 20, voltage setting power source 21 and current limiting resistor 22, voltage setting power source 2
Reference numeral 3 denotes a current supply source for applying a bias magnetic flux to the magnetic flux input coils 18 and 19, respectively.

In FIG. 1, the structure of the second derivative is basically used. However, the structure of the first derivative is as follows if the number of sensors SQUID is two and the ratio of the coupling resistors such as 4, 5 is 1: 1. Good.

FIG. 2 is a conceptual diagram showing an example of the arrangement of a SQUID or a pickup coil for supplying a magnetic flux to the SQUID. It is assumed that a surface for detecting a magnetic field is in a direction passing through a square.

FIG. 2A shows a first-order differential configuration in which the sensor SQ is used.
The sensitivity surfaces of the UIDs (or pickup coils) are oriented in opposite directions, and are arranged on the same plane.

At this time, when a parallel magnetic field is applied, two SQUIDs are generated as magnetic fluxes having opposite signs and equal signals.
Since detection is performed at D, a differential operation is performed on the output of the adder.

On the other hand, FIG. 2B corresponds to the circuit configuration of FIG. 1, in which the sensitivity planes A, B, and C, which are opposite to each other, are arranged at equal intervals and on the same plane. It is.
And each SQUID is the sensor SQUID1 of FIG.
In the case where the number corresponds to 2, 3, a secondary differential operation is performed.

Here, the magnetic field sensitivity of the sensors SQUIDs 1, 2, and 3 is η, the magnetic flux voltage conversion rate is dV / dΦ1, the magnetic flux voltage conversion rate of the SQUID array 12 is dV / dΦ2, and the coupling resistances 4, 5, and 6 Assuming that the resistance value is R, 2R, R, and the value of the coupling inductance is M, the output of the SQUID array 12 with respect to the parallel magnetic field B is given by the following equation 1, and no voltage is output.

[0029]

(Equation 1)

When the signal source is close to the coil A, for example, a voltage is output because it is not a parallel magnetic field, as in the case of forming an ordinary secondary differential coil.

FIG. 2 (c) schematically shows the configuration of the first derivative when the sensor surface is not on the same plane, and the sensitivity surface moves in parallel even if it is not on the same plane. It indicates that it is enough to be in the arrangement.

FIGS. 3, 4 and 5 explain the above configuration in more detail. As shown by the thin line in FIG. 5, the output voltage of the SQUID changes in a sinusoidal manner with respect to the input magnetic flux. Therefore, a magnetic flux lock circuit usually called a FLL circuit is used to perform feedback correction such that a zero-order method is established so that an output voltage becomes linear with respect to a change in input magnetic flux.

However, such correction has a problem of lock jump as described in the prior art. Therefore,
The output voltage is linearized by providing a negative feedback circuit including a negative feedback resistor 25 and a coil 26 as shown in FIG. At this time, the resistance value of the resistor 25 is Rn, and the coil 26 and S
The mutual coupling inductance of QUID27 is Mn, SQUI
Assuming that the magnetic flux voltage conversion rate of D26 is dV / dΦ, the feedback rate β is given by the following equation (2).

[0034]

(Equation 2)

Here, if β ≧ 0, the Φ-V curve is deformed so that the slope of the slope becomes gentle, as indicated by the thick line in FIG. 5 (as β increases). The magnetic flux voltage conversion rate after the feedback at this time is given by the following Equation 3.

[0036]

(Equation 3)

When β is in the range of 1 to 3,
The sinusoidal Φ-V curve has a sawtooth shape, and the slope of the bold line shown in FIG. 5 has increased linearity. Further, when there is no feedback, the linear range is Φ1 at Φo / 2 or less, whereas the linear part is Φo according to the value of β (when β is around 2) as shown by Φ2.
It is enlarged to the vicinity. At this time, by applying a magnetic flux bias to the center of the linear portion (shown in FIG. 5), the externally applied magnetic field fluctuates around this point.

Normally, the magnitude of the magnetic field to be measured is about several tens pT in the case of biomagnetic measurement, but the magnetic field sensitivity η is several nT including the transmission efficiency from the pickup coil to the SQUID.
/ Φo, the dynamic range of the signal is Φo /
10 or less. On the other hand, the DC magnetic field fluctuation is about 50 nT / Φo due to the diurnal variation of the terrestrial magnetism, and is about 1/100 under the environment in the magnetically shielded room. Therefore, the fluctuation of the bias point is smaller than Φo.

Accordingly, it can be seen that there is no problem in measurement even in open loop as long as linearity can be obtained. By performing such linearization by negative feedback on the sensors SQUID1 to SQUID1 in FIG. 1, magnetic field-voltage conversion and voltage transfer can be performed while maintaining linearity.

FIG. 4 shows a case where the same negative feedback is provided in the SQUID array to linearize the output voltage. That is, a negative feedback circuit including the negative feedback resistor 28 and the coil 29 is provided. At this time, the resistance 2
8 is Rn, the mutual coupling inductance between the coil 29 and the SQUID array 30 is Mn, and the magnetic flux voltage conversion rate of the SQUID array 30 is dV / dΦ, the feedback rate β is given by the above equation (2).

As described above, the embodiment of the present invention has been described. However, the present invention is not limited to the embodiment shown here, and unless the structure described in the claims is changed, It can be carried out as appropriate.

[0042]

As described above, in the variable differential SQUID magnetometer of the present invention, the sensor SQUID is selected by the switch means, and the summing amplifier (a resistor for taking out current,
A coil connected to the resistor and magnetically coupled to the coil
A second SQUID or SQUID array
) , So that the sensor surface is arranged so that it can be configured differentially ( sensors adjacent to each other).
Differential output can be obtained by arranging such that the signs of the degrees are opposite to each other) .

Then, the sensor SQUID and the reference SQUI
Since the switching of the bias current flowing through the ID is performed by the switch means, the differential configuration can be arbitrarily reconfigured, and the same configuration can serve as the magnetometer and the first and second derivative gradiometers, Therefore, the number of wirings can be reduced as compared with the case of performing electrical differentiation.

Further, the sensor SQUID and the reference SQUID
Since the output of D is added and amplified by the second SQUID,
Since the S / N ratio is extremely high, the addition and subtraction are directly performed by the second SQUID without using the configuration of the FLL circuit, the frequency characteristics are extremely good.

On the other hand, if the switch means is installed in the cryostat in the same manner as the SQUID, the number of wirings can be further reduced, thereby reducing the heat penetration of the cryostat, thereby reducing the helium running cost and the circuit scale. Is small.

Further, since the measurement is performed in an open loop, even if the measurement is temporarily hindered by an excessive input or power supply noise, it is possible to obtain a great effect that the measurement can be continued continuously.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating a variable differential SQUI according to an embodiment of the present invention.
It is a principal part circuit diagram of an ID magnetometer.

FIGS. 2A, 2B, and 2C each show a sensor S of a variable differential SQUID magnetometer according to an embodiment of the present invention.
It is a conceptual diagram for showing the example of arrangement of a QUID or a pickup coil.

FIG. 3 is a circuit diagram showing a configuration of a negative feedback circuit of SQUID.

FIG. 4 is a circuit diagram showing a configuration of a negative feedback circuit of the SQUID array.

FIG. 5 is a characteristic diagram showing input / output characteristics of a SQUID.

[Explanation of symbols]

 1, 2, 3, 27 Sensor SQUID 4, 5, 6 Coupling resistor 7, 13, 20, 22 Current limiting resistor 8 Analog switch 9, 14 Bias current supply voltage source 10 Controller 11, 26, 29 Coupling inductance 12 , 30 SQUID array 15, 25, 28 Negative feedback resistor 16, 17 Voltage amplifier 18, 19 Magnetic flux input coil 21, 23 Power supply for voltage setting 24 Power supply for bias

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int.Cl. ⁶ , DB name) G01R 33/035 ZAA H01L 39/22

Claims (2)

(57) [Claims] 1. A plurality of dc-SQUs having equal magnetic field sensitivities.
In a magnetometer using an ID (hereinafter, referred to as SQUID) as a sensor for detecting a magnetic field, a sensor SQUID comprising a first plurality of sensors, a switch means for turning ON / OFF a bias current of the plurality of first SQUIDs, A plurality of resistors for taking respective voltage outputs of the plurality of first SQUIDs as a current, a coil electrically connected to the plurality of resistors in series, and a second SQUID magnetically coupled to the coil. Or SQ
Has a UID array, a summing amplifier comprising a said plurality of first SQUID to or above a plurality of first SQU
Arranged adjacent to the pickup coil that supplies magnetic flux to the ID
A variable differential type SQUID magnetometer, wherein the signs of the provided sensitivity surfaces are opposite to each other . 2. The plurality of first sensors SQUID or
Is in the second SQUID or SQUID array
Here, it is assumed that the magnetic flux voltage conversion rate is dV / dΦ and that the resistor Rn and the coupling coil Mn are provided, and the feedback rate β = (dV / dΦ) ×
A negative feedback circuit is provided so that Mn / Rn ≧ 1, and a magnetic flux bias circuit is provided for positioning an operating point at a substantially middle point on a slope of a linearized Φ-V curve. 2. The variable differential SQUID magnetometer according to 1.