JPH0447283A - Fluxmeter using superconducting quantum interference device - Google Patents

Abstract

PURPOSE:To always provide the max. dynamic range and to carry out stable measurement by connecting a trap compensating circuit detecting the offset component generated by detected trap magnetic flux to a feedback circuit. CONSTITUTION:A trap compensating circuit register 6 detecting the offset compo nent generated by the trap magnetic flux phit detected by supplying a feedback current at least larger than a steady-state current is connected to a feedback circuit and a feedback coil 5 is operated while an offset current is supplied to said coil 5. Feedback magnetic flux phif sufficient to negate the magnetic flux phit can be supplied to a superconductive quantum interference element by feedback operation using a large feedback current and an operation impos sible state is avoided to make it possible to certainly detect the magnetic flux phit. External magnetism is measured while the offset component due to the magnetic flux phit thus detected is added to the coil 5 to certainly compensate the magnetic flux phit and a compensation method separately supplying an offset current can prevent the reduction of a dynamic range.

Description

[Detailed description of the invention] 【overview】

Regarding magnetometers using superconducting quantum interference devices, especially those using superconducting quantum interference devices that measure external magnetic flux by digitally processing the output of superconducting quantum interference devices, we always have the largest dynamic range. The object of the present invention is to provide a magnetic flux meter using a superconducting quantum interference device that operates stably, comprising: a magnetic flux sensor having a superconducting quantum interference device; an alternating current source that applies an AC bias to the superconducting quantum interference device; A digital SQUID magnetometer that measures external magnetic flux and has a feedback circuit that feeds an amount of magnetic flux proportional to the number of output pulses from a superconducting quantum interference device to the superconducting quantum interference device, the feedback circuit comprising: A trap compensation circuit for detecting an offset generated by the detected trap magnetic flux by supplying at least a larger feedback current than a steady state is connected, and the feedback coil is configured to operate while supplying the offset current.

[Industrial application field]

The present invention relates to a magnetometer using a superconducting quantum interference device, and more particularly to a magnetometer using a superconducting quantum interference device that measures external magnetic flux by digitally processing the output of the superconducting quantum interference device. In recent years, highly sensitive magnetometers using superconducting quantum interference devices (hereinafter referred to as rsQUI DJ) have been used to measure minute magnetic fields generated from living organisms and the like. In particular, by measuring the magnetic field distribution in the brain and heart, it is possible to estimate the current source that generates the magnetic field. It has been pointed out that it is useful for elucidation. When measuring such minute magnetic flux, it is necessary to always operate the SQUID sensor stably. However, due to the nature of the SQUID as a superconducting device, it is difficult to Even in this state, a so-called magnetic flux trap in which magnetic flux is captured by the 5QtJID itself due to excessive external noise often occurs, making the operation of the SQUID unstable. In particular, multi-channel SQUID that uses multiple SQUID sensors in order to measure magnetic field distribution at once.
[In Tokei, it is very important that the operation of each SQUID is stable, and a SQUID [flux meter] that can compensate for the effects of magnetic flux traps is desired.

[Prior Art] A conventional 5QTUD [flux meter] is shown in FIG. 2, and is configured to transmit magnetic flux picked up by a pickup coil 8 to a SQUIDI via a magnetic flux transmission transformer 9. As a digital SQUID that can obtain pulse output,
5QLIID (Japanese Patent Laid-Open No. 63-299
(Refer to Publication No. 79) and analog-operated dc SQU.
A device that adds the voltage output of the ID to a superconducting comparator or a 1-bit A/D converter to obtain a pulse output (D, D
Rung, Cryogenics, vol. 26.
pp623-627.1986) is known. SQUIDI connects two or more Josephson junctions 10.1
0 and a superconducting loop 11, and is magnetically coupled to the magnetic flux transmission transformer 9. As shown in FIG. 3(a), this SQU I D 1 has the amount of combined magnetic flux and
Adjustment is made so that the transition occurs between the no-voltage state in the hunting region and the voltage state outside it by changing the bias current value rc, and the transition occurs at the same positive and negative bias current values when the amount of magnetic flux Φ to be coupled is 00. has been done. The SQUIDI configured as described above is supplied with a bias current 1c (1) whose maximum value is equal to the critical current from the AC current source 3, and in this state, a positive external magnetic flux Φ8+ is applied to the pink-up coil 8. Then, since the positive critical current transmitted to the SQUIDI via the magnetic flux transfer transformer 9 becomes smaller, the SQUID 1 transitions to a voltage state in the positive half cycle of the bias IC (t) and outputs a positive pulse. Similarly, when negative magnetic flux is applied, a negative pulse is output from SQUIDI. The output pulses are amplified by the preamplifier 12, counted by the up/down counter 4, converted into analog by the D/A converter 13, and negatively fed back to the SQUIDI via the feedback coil 5. Pulse counting by the up/down counter 4 is performed until the magnetic flux applied to the 5QUTDI is always constant or becomes 0 and pulses from the SQU I D 1 are no longer output, and the pickup coil 8 is linked based on the counted number. Magnetic flux Φ. is required. On the other hand, when a magnetic flux trap occurs, trap magnetic flux Φt is given to SQUID 1 in the initial state, so Φ-■. As shown in FIG. 3(b), the characteristics are shifted laterally by the amount of the trap magnetic flux Φt. When feedback is applied in this state, magnetic flux Φ is input to the pickup coil 8. If not, feedback is applied to compensate for the magnetic flux trap, so the Φ-■ characteristic becomes as shown in Figure 3 (C), but the compensated Φt is generated in the up/down counter 4, so D /The full range RF of the A converter 13 shifts and the external magnetic flux Φ to be detected. The dynamic range for this will be reduced. For this reason, the feedback circuit includes an offset voltage control circuit 14 that detects the count value of the up/down counter 4 for the trap, and an offset voltage generator that applies an offset voltage to the feed bank circuit based on a signal from the offset voltage control circuit 14. The offset due to the magnetic flux trap is always compensated for by the offset voltage control circuit 14 to prevent a reduction in the dynamic range. [Problems to be Solved by the Invention] However, in the conventional example described above, when the amount of feedback magnetic flux per pulse is reduced in order to reduce the influence of quantization noise and increase the sensitivity of the magnetometer, the fourth As shown in the figure, the feedback current is D/
Even if the A converter 13 supplies the trap magnetic flux Φ1 to the feedback circuit, it is no longer possible to compensate for the trapped magnetic flux Φ1, and the feedback operation cannot be performed after the power is turned on, resulting in the disadvantage that it does not operate as a magnetometer. The present invention has been made to eliminate the above-mentioned drawbacks, and an object of the present invention is to provide a magnetometer using a superconducting quantum interference device that always has the maximum dynamic range and operates stably.

[Means to solve the problem]

According to the present invention, the above object, as shown in FIG. 1 corresponding to an embodiment, includes a magnetic flux sensor 2 having a superconducting quantum interference element 1, and an alternating current 8 that provides an AC bias to the superconducting quantum interference element 1.
3, and a feed bank circuit that feeds back an amount of magnetic flux proportional to the number of output pulses from the superconducting quantum interference device 1 to the superconducting quantum interference device 1, and measuring external magnetic flux. , The feedback circuit includes a trap compensation circuit 6 that detects an offset generated by the detected trap magnetic flux Φt by supplying a small (or larger than steady) feedback current.
This is achieved by providing a magnetometer using a superconducting quantum interference device, which is characterized in that it is operated while supplying the offset current to the feedback coil 5. Further, the feedback circuit is provided with a plurality of feedback resistors Rfl, Rf2... having different resistance values,
The feedback resistors Rfl, Rrz... may be selected by a switch 7.7'' to set the feedback current at the time of detecting and measuring the trap magnetic flux Φt; When detecting the trap magnetic flux Φ1 by changing the resistance value of the variable resistor with a switch 7,
It is also possible to set the feedback current at the time of detection and measurement of the trap magnetic flux Φt by making the full range of the feedback circuit variable and changing the full range of the feedback circuit. Further, at least one or more feedback coils are connected to a trap compensation circuit 6 that detects an offset generated by the detected trap magnetic flux Φt by supplying a feedback current larger than a steady state, and a feedback coil 5 is connected to the feedback circuit. It is also possible to provide each independently.

[Effect]

Based on the above configuration, in the present invention, the trap magnetic flux Φt can be used to direct a large feedback current to the feedback coil 5.
Detected by supplying Feedback operation using a large feedback flow creates a tiger-knob magnetic flux Φ in the SQUIDI. It is possible to supply enough feedback flux Φt to cancel out the trapping flux Φt, avoiding the inoperable state.
This makes it possible to reliably detect The trap magnetic flux Φ1 is reliably compensated for by measuring the external magnetism while adding the offset due to the trap magnetic flux Φt detected as described above to the feedback coil 5, and the compensation method for separately supplying the offset current is as follows: In the invention according to claim 2 characterized by a reduction in the dynamic range, the selection of the offset current comprises:
This is done by selecting feedback resistors Rfl, R4, etc. In the invention according to claim 3, the feedback resistor Rfl is made variable, and in the invention according to claim 4, the full range of the feedback circuit is changed. This compensates for the offset current. Furthermore, in the invention set forth in claim 5, the feedback magnetic flux Φt to the SQUID 1 can be changed by selecting a plurality of feedback coils 5.

【Example】

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. FIG. 1 shows an embodiment of the present invention. In the figure, 1 is a SQUID having two Josephson junctions 10, 10 in series, and the magnetic flux Φ8 captured by the pickup coil 8.
is supplied via the magnetic flux transfer transformer 9. Further, the SQUID 2 is supplied with an AC bias current 1c(t) from an AC current source 3, and the Φ shown in FIG.
- Outputs a pulse voltage based on the 1c characteristic. After the output pulse voltage is amplified by the preamplifier 12,
It is counted by the up/down counter 4. D/A converter 13, the up/down counter 4
The D/A converter 13 converts the output from
The output from is supplied to the feedback coil 5 from a feedback circuit in which a feedback resistor Rfl is inserted. The feedback current supplied to the feedback coil 5 is given in the direction of canceling the input magnetic flux Φt from the pickup coil 8, until the input magnetic flux Φt and the feedback magnetic flux Φt match and the output from SQ[ID1 becomes 0. The external magnetic flux is measured from the count number of the up/down counter 4, that is, the amount of feedback. In this invention, the compensation for the trap is first
This is performed by detecting the offset amount by the trap magnetic flux Φt captured by QtJID1 by an operation with a large amount of feedback, and setting it in the trap compensation circuit 6. In the embodiment shown in FIG. 1, a feedback system including a resistor 16, a D/A converter 13'', and a feedback resistor RfZ is provided in order to supply a larger feedback current than the steady state to the feedback coil 5, and the compensation control Either or both of them are selected by switching the switch 7 according to a command from the circuit 17.The two feedback systems are used when a steady feedback system is used to detect the trap magnetic flux Φt and compensate for it. In this case, an operation occurs due to a decrease in the dynamic range with respect to the external magnetic flux Φ, or when the trapping magnetic flux Φt is large, it is not possible to supply feedback current sufficient to cancel the trapping magnetic flux Φt to the feedback coil 5. In the embodiment shown in Fig. 1, a D/A converter 13'' is provided to avoid a power failure state and has the same dynamic range as the D/A converter 13 used in a steady feed bank system. is the feedback resistor R7
By setting resistance value 2 to be smaller than Rfl, the amount of feedback per pulse is increased. Therefore, according to this embodiment, in order to detect the trap magnetic flux Φt interlinked with the SQUID 1, the switch 7 is opened, the switch 7° is closed, and the feedback resistor R
f! This causes a feedback operation to be performed. At this time,
Since the resistance value of the feedback resistor Rf2 is smaller than Rfl, the amount of feedback per pulse becomes large. Since most of the magnetic flux detected by this operation is due to the trap magnetic flux Φt, the count value by the up/down counter 4 is taken into the register 16 as the trap magnetic flux Φ. Next, when the switch 7 is turned on, trap compensation is performed by the D/A converter 13' and the feedback resistor Rfz, and the operation with a small amount of feedback, that is, the measurement of the external magnetic flux Φt, is performed as follows.
This is done simultaneously by the D/A converter 13 and the feedback resistor Rfl. In addition, in FIG. 1, two feedback resistors R
Although the case where the amount of feedback is controlled by selecting fl and Rfl has been shown, more feedback resistors may be provided, the feedback resistor Rfl may be made variable, and the full range of the feedback circuit may be changed. It can also be configured as Furthermore, in order to compensate for the trap magnetic flux Φ1, the feedback current supplied to the feedback coil 5 is not limited to only control, but many types of feedback coils 5 are provided, and the feedback flux supplied to the SQUIDI is adjusted with the same feedback current. It is also possible. FIG. 2 shows an embodiment in which the present invention is applied to a one-chip SQUID.
It is an AND gate consisting of a Josephson circuit that ANDs the output pulse from 1 and the bias pulse,
If switch 7 is open, write gate 22
The output pulse of SQUIDI is not input to the input terminal. The write gate 22 writes a positive flux quantum into the feedback coil (superconducting storage loop) 5' if the output pulse of SQUIDI is positive, and writes a negative flux quantum if the output pulse is negative. In addition, the output of SQUIDI is supplied to an up/down counter 23 to determine the input magnetic flux of the pickup coil at room temperature. In the magnetic flux needle according to this embodiment configured as described above, first, the switch 7 is opened to disable the feedback loop consisting of the write gate 22 and the feedback coil 5''. At this time, the switch 7'' is disabled.
is closed, and a feedback loop consisting of an up/down counter 4 with a large amount of feedback per pulse and a D/A converter 13 is activated to detect and feed back the trapped magnetic flux. Next, by opening switch 7'' and closing switch 7, and operating a feedback loop consisting of write gate 22 and feedback coil 5'' with a small amount of feedback per pulse, the effects of traps can be eliminated and the dynamic Operation using the range effectively is possible.In this case, as shown in FIG. 22' and a loop consisting of the feedback coil 5.

【Effect of the invention】

As is clear from the above explanation, according to the magnetometer according to the present invention, the trapped magnetic flux can be reliably compensated for without reducing the dynamic range, so magnetic flux can always be measured in the maximum dynamic range. It is possible to improve the reliability of the operation.

[Brief explanation of drawings]

Fig. 1 shows an embodiment of the present invention, Fig. 2 shows a second embodiment of the invention, Fig. 3 shows a third embodiment of the invention, and Fig. 4 shows a conventional example. FIG. 5 is a diagram showing the Φ-IC characteristic, and FIG. 6 is a diagram showing the offset state due to the trapped magnetic flux. In the figure, 1 is a superconducting quantum interference device (SQUID), 2 is a magnetic flux sensor, 3 is an alternating current source, 4 is an amplifier down counter, 5 is a feed hack coil, 6 is a trap compensation circuit, 7.7' is a switch , Rfls Rft is the feed hack resistance, Φt is the trapping flux, Φ. is the external magnetic flux.

Claims (1)

[Scope of Claims] [1] A magnetic flux sensor (
2), an alternating current source (3) that applies an AC bias to the superconducting quantum interference device (1), and an amount of magnetic flux proportional to the number of output pulses from the superconducting quantum interference device (1) to the superconducting quantum interference device (1). A digital S that has a feedback circuit that feeds back to the quantum interference element (1) and measures external magnetic flux.
In the QUID magnetometer, the feedback circuit is supplied with at least a feedback current larger than a steady state to generate a detected trap magnetic flux (Φ_t
) is connected to the trap compensation circuit (6) that detects the offset generated by the feedback coil (5).
A magnetometer using a superconducting quantum interference device, characterized in that it is operated while supplying the offset current to the magnetic flux meter. [2] The feedback circuit includes a plurality of feedback resistors (R_f_1, R_f_2, R_f_2,
・ ・ ) is provided, and the feedback resistor (R_f_1
, R_f_2, . Total. [3] The feedback resistor (R_f_1) in the feedback circuit is formed by a variable resistor, and the resistance value of the variable resistor is changed by a switch (7) to set the trap magnetic flux (Φ
3. A magnetometer using the superconducting quantum interference device according to claim 1 or 2, wherein a feedback current is set at the time of detection and measurement of _t). [4] The superconducting quantum according to claim 1, 2 or 3, wherein the full range of the feedback circuit is variable, and the feedback current at the time of detecting and measuring the trap magnetic flux (Φ_t) is set by changing the full range of the feedback circuit. A magnetometer using an interference element. [5] In the digital SQUID magnetometer according to claim 1, 2, 3, or 4, the detected trap magnetic flux (
Φ_t) One or more feedback coils connected to a trap compensation circuit (6) for detecting an offset generated by Φ_t) and a feedback coil (5) connected to the feedback circuit are each independently provided. A magnetometer using a conduction quantum interference element.