JP4427122B2 - SQUID magnetometer - Google Patents

Description

となる。従つて、ゲインβ′_１とβ_１、β′_２とβ_２、β′_３とβ_３がそれぞれ一致していれば、第４のマグネットメータ１４は、第１および第２のマグネットメータ１１および１２で検出される磁束Φ_１およびΦ_２の差と、第３および第４のマグネットメータ１３および１４で検出される磁束Φ_３およびΦ_４の差とを更に差分した値、すなわち磁束の２階差分値に比例する電圧Ｖ_４を出力する。
【００３８】
このように、第１〜第４までマグネットメータ１１〜１４を４個使用し、第１のマグネットメータ１１の出力値Ｖ_１をゲイン倍して第２のマグネットメータ１２のフィードバック信号に加算するとともに、第３のマグネットメータ１３の出力値Ｖ_３をゲイン倍して第４のマグネットメータ１４のフィードバック信号に加算する。さらに、第２のマグネットメータ１２の１階差分出力Ｖ_２をゲイン倍して第４のマグネットメータ１４のフィードバック信号に更に加算する。これにより、第４のマグネットメータ１４から２階差分された電圧Ｖ_４が得られる。
【００３９】
本実施形態の２階微分グラジオメータは、立体的なピックアップコイルを必要としないので高温超伝導ＳＱＵＩＤにも適用可能であり、しかも参照用のマグネットメータを別途備えなけばならないという構成を採る必要がないため、製造コストの低減化も可能になる。
【００４０】
（第２の実施形態）
ところで、上述した第１の実施形態において、２階微分グラジオメータ１により検出された出力に含まれる環境磁場成分は、各々のマグネットメータ１１〜１４で検出された磁場の一様成分の約「１−（２）^１／２β′／β」倍、または、環境磁場の１次勾配成分の「１−β′_２／β_２」倍の内、大きい方の値程度まで低下する。ここで、「β′／β」は、「β′_１／β_１」および「β′_３／β_３」の典型的な値である。例えば、磁場の一様成分の大きさを約１００ｐＴ／√Ηｚ、必要な磁場分解能を１０ｆＴ／√Ηｚとすると、１−２β′／βは、１０^−４程度以下の大きさにする必要がある。つまり、β_１およびβ_３の各ゲインのぺアは１０^−４の半分程度の誤差で一致させる必要がある。また、環境磁場の１次勾配成分の大きさを約１ｐＴ／√Ｈｚとすると、１−β′_２／β_２は１０^−２程度以下の大きさにする必要があり、β_２のゲインのぺアは１０^−２程度の精度で一致させる必要がある。
【００４１】
これを実現するための具体的な実施形態を、本第２の実施形態として図２に基づき説明する。なお、これ以降の実施形態において、それまでに説明した実施形態のＳＱＵＩＤ磁束計と同一または同等の構成要素には同一符号を付して、その説明を省略または簡略化する。
【００４２】
図２に示すＳＱＵＩＤ磁束計は、第１の実施形態のものと同様に、４個のマグネットメータ１１〜１４を電気的に接続することで、ＳＱＵＩＤ磁束計としての２次微分グラジオメータ１を構成したものである。
【００４３】
同図に示す如く、マグネットメータ１１〜１４のフィードバックアンプとして抵抗素子Ｒ_１〜Ｒ_４を夫々用いている。さらに、第２および第４のマグネットメータ１２および１４のＳＱＵＩＤ１２ａおよび１４ａには、夫々、２個のフィードバックコイル１２ｂ′及び１２ｂ"、および、１４ｂ′及び１４ｂ"が備えられている。
【００４４】
第１のマグネットメータ１１のフィードバックループの抵抗素子Ｒ_１はその自身のフィードバックコイル１１ｂに接続された後、このコイル１１ｂは第２のマグネットメータ１２の２つのフィードバックコイルの内の一方のコイル１２ｂ'に直列に接続されている。第２のマグネットメータ１２のフィードバックループの抵抗素子Ｒ_２は、もう一方のフィードバックコイル１２ｂ"に接続されている。同様にして、第３のマグネットメータ１１のフィードバックループの抵抗素子Ｒ_３はその自身のフィードバックコイル１３ｂに接続された後、このコイル１３ｂは第４のマグネットメータ１４の２つのフィードバックコイルの内の一方のコイル１４ｂ'に直列に接続されている。第４のマグネットメータ１４のフィードバックループの抵抗素子Ｒ_４は、もう一方のフィードバックコイル１４ｂ"に接続されている。
【００４５】
この２次微分型グラジオメータ１は、第２及び第４のマグネットメータ１２および１４に２つのフィードバックコイルを備えることで、フィードバックゲインβ_１とβ′_１、および、β_３とβ′_３の一致度を夫々上げることができる。以下、本実施形態のフィードバックゲインの一致度および出力信号の精度に関して説明する。
【００４６】
フィードバックゲインβ_１、β′_１は、夫々、
【数９】
β_１＝Μ_１／Ｒ_１， β′_１＝Μ_２／Ｒ_１
である。Μ_１は第１のマグネットメータメータ１１におけるフィードバックコイル１１ｂとＳＱＵＩＤ１１ａとの間の相互インダクタンスであり、Μ_２およびＭ′_２は第２のマグネットメータ１２におけるフィードバックコイル１２ｂ′および１２ｂ"とＳＱＵＩＤ１２ａとの間の夫々の相互インダクタンスであり、ＳＱＵＩＤとコイルを薄膜で形成する場合、１０^−４より高い精度で形成可能である。
【００４７】
第３および第４のマグネットメータ１３および１４に関しても同様である。Μ_３は第３のマグネットメータメータ１３におけるフィードバックコイル１３ｂとＳＱＵＩＤ１３ａとの間の相互インダクタンスであり、Μ_４およびＭ′_４は第４のマグネットメータ１４におけるフィードバックコイル１４ｂ′および１４ｂ"とＳＱＵＩＤ１４ａとの間の夫々の相互インダクタンスである。従って、「１−２^１／２β′／β」を１０^４程度以下の大きさに設定できる。
【００４８】
また、フィードバックゲインβ_２およびβ′_２は、夫々、
【数１０】

であり、Μ′_２とΜ′_４は１０^−２よりも高い精度で容易に形成でき、Ｒ_２とＲ′_２も１０^−２より高い精度で一致させることができるため、「１−β′_２／β_２」に関しても１０^−２程度以下の大きさにする設定することができる。
【００４９】
２次微分型グラジオメータ１としての差分値検出機能は第１の実施形態のものと同様である。すなわち、第４のマグネットメータ１４から出力される２次微分の出力電圧Ｖ_４は、
【数１１】

である。
【００５０】
本実施形態では従来例のような差分演算器を必要とせずに、２階微分グラジオメータが構成される。従って、従来例のように２つのマグネットメータ間のゲインの違いや差分演算器の同相信号除去比によって、２階微分出力の精度が制限されるということがなく、環境磁場を高い精度で除去することができる。また、補償電流を調整する調整抵抗無しに必要な高精度な環境磁場の除去能を得ることができるため、補償電流の調整を頻繁に行なう必要も無い。
【００５１】
（第３の実施形態）
第３の実施形態を図３および４に示す。
【００５２】
この実施形態のＳＱＵＩＤ磁束計も、４個のマグネットメータ１１〜１４を用いて電気的２次微分型グラジオメータ１として構成されている。
【００５３】
マグネットメータ１１〜１４の各フィードバックループには、フィードバック電流を供給する回路として定電流回路１１ｅ〜１４ｅ（ゲインα_１〜α_４）をそれぞれ採用している。図中、Ｍ１〜Ｍ４は、それぞれ、各ＳＱＵＩＤとフィードバックコイルとの間の相互インダクタンスである。
【００５４】
また、第１のマグネットメータ１１のフィードバックコイル１１ｂと第２のマグネットメータ１２のフィードバックコイル１２ｂとを直列に接続し、一方、第３のマグネットメータ１３のフィードバックコイル１３ｂと第４のマグネットメータ１４のフィードバックコィル１４ｂとを直列に接続し、これにより、第２の実施形態で説明したと同様に、フィードバックゲインβ_１とβ′_１、および、β_３とβ′_３の一致度を向上させている。
【００５５】
さらに、第２のマグネットメータ１２の出力電圧Ｖ_２を、別の定電流回路２１（ゲインα′_２）を介して第４のマグネットメータ１４のフィードバックコイル１４ｂに加えている。
【００５６】
本実施形態で使用している定電流回路１１ｅ〜１４ｅの一例を図４に示す。この定電流回路は電圧−電流変換回路として構成されている。この回路において、図示した各抵抗素子は以下の関係を保つ値のものを用いている。
【００５７】
【数１２】
Ｒ_ｆ／Ｒ_ｓ＝（Ｒ_２＋Ｒ_３）／Ｒ_１
これにより、この定電流回路の出力電流Ｉ_Ｌは、下記式で表されるように、入力電圧Ｖ_ｓに比例した値となる。
【００５８】
【数１３】
Ｉ_Ｌ＝（Ｒ_ｆ／（Ｒ_ｓＲ_３））Ｖ_ｓ
本実施形態における２次微分型グラジオメータ１としての差分値検出機能は第１の実施形態のものと同様である。すなわち、第４のマグネットメータ１４から出力される２次微分の出力電圧Ｖ_４は、
【数１４】

である。
【００５９】
このように、第１の実施形態と同様に差分演算器を用いることなく、２次微分グラジオメータが構成される。従来例のように、差分演算器の同相信号除去比によって２階微分出力の精度が制限されることなく、環境磁場を高い精度で除去できる２次微分グラジオメータを提供できる。さらに、補償電流を調整する調整抵抗無しに必要な精度を得ることができるため、補償電流の調整を頻繁に行なう必要のない２次微分グラジオメータを提供できる。さらにまた、第１の実施形態で示したような、第２、第４のマグネットメータそれぞれにフィードバックコイルを２個設ける必要がないため、構成が簡単化されると共に、それらのコイル間のクロストークによる歪みの発生も解消される。
【００６０】
（第４の実施形態）
第４の実施形態を図５に基づき説明する。この実施形態のＳＱＵＩＤ磁束計も、４個のマグネットメータ１１〜１４を用いて電気的２次微分型グラジオメータ１として構成されている。
【００６１】
この２次微分型グラジオメータ１は、第３の実施形態と同様に、フィードバック電流を供給する回路として定電流回路１１ｅ〜１４ｅを各マグネットメータに採用している。その一方で、第１のマグネットメータ１１のフィードバックコイル１１ｂと第２のマグネットメータ１２のフィードバックコイル１２ｂとを直列に接続し、同様に、第３のマグネットメータ１３のフィードバックコイル１３ｂと第４のマグネットメータ１４のフィードバックコイル１４ｂとを直列に接続し、これにより、フィードバックゲインβ_１とβ′_１、及び、β_３とβ′_３の一致度を向上させている。
【００６２】
とくに、第３の実施形態のグラジオメータと異なるのは、第１および第３のマグネットメータ１１および１３の出力電圧に比例する電流を、新たに付加した定電流回路により第２および第４のマグネットメータ１２および１４に夫々供給する回路が加えられた点である。すなわち、第１のマグネットメータ１１の出力電圧Ｖ_１は定電流回路２２（ゲインγ_１）を介して第２のマグネットメータ１２のフィードコイル１２ｂに加えられ、また、第３のマグネットメータ１３の出力電圧Ｖ_３は定電流回路２３（ゲインγ_３）を介して第４のマグネットメータ１４のフィードコイル１４ｂに加えられている。
【００６３】
本実施形態における２次微分型グラジオメータ１としての差分値検出機能は第１の実施形態のものと同様である。すなわち、第４のマグネットメータ１４から出力される２次微分の出力電圧Ｖ_４は、
【数１５】

である。
【００６４】
上述した新たに加えられた定電流回路２２，２３は、第３の実施形態において相互インダクタンスΜ_１，Μ_２間およびＭ_３，Ｍ_４間の値が僅かに異なっていた場合、β_１とβ′_１、及び、β_３とβ′_３に要求される高い一致度を確保できなくなる点を改善するためのものである。具体的には、第１と第２（及び第３と第４）のマグネットメータ１１，１２（及び１３、１４）のフィードバックコイル１１ｂ，１２ｂ（及び１３ｂ，１４ｂ）に共通に供給される電流Ι_１＝α_１Ｖ_１（及びΙ_３＝α_３Ｖ_３）に比べて、小さい電流Ｉ′_２＝γ_１Ｖ_１（及びΙ′_４＝γ_３Ｖ_３）を第２のマグネットメータ１２のフィードバックコイル１２ｂ（及び第４のマグネットメータ１４のフィードバックコイル１４ｂ）に供給することにより、相互インダクタンスＭ_１，Μ_２（及びＭ_３，Ｍ_４）の値の僅かな違いを補正するものである。
【００６５】
この新たに加えられた定電流回路２２，２３はそのゲインが可変になっている。このため、各マグネットメータに一様な磁場を与えたとき、第２及び第４のマグネットメータ１２、１４の出力が最も小さくなるように調整される。この定電流回路２２，２３のゲイン調整は、例えば、前述した図４の抵抗Ｒ_３の値を調整することで達成される。理想的に調整されたときのゲインは、
【数１６】
γ_１＝α_１（Ｍ_１／Ｍ_２−１）
及び
【数１７】
γ_３＝α_３（Ｍ_３／Ｍ_４−１）
という値になる。これらのゲインは、相互インダクタンスＭ_１，Ｍ_２が互いに高精度に一致していればα_１、α_３よりも相当小さな値となるため、これらの値が出力値の精度に与える影響は小さく、従来の方法に比べて調整が容易である。
【００６６】
また、相互インダクタンスＭ_１，Μ_２，Μ_３，Μ_４のばらつきは主に製造時に発生するものであって、時問的な変化は極めて小さいため、一度調整を行なった後は、長期間にわたって調整する必要は無い。
【００６７】
本実施形態によれば、第２の実施形態と同様の効果に加え、ＳＱＵＩＤのフィードバックコイルと超伝導リングとの間の相互インダクタンスの僅かなばらつきに因る２階微分グラジオメータの出力精度の劣化を、ゲイン調整可能な定電流回路によって補償することができる。このため、２階微分の出力精度をさらに上げることができる。また、そのための調整作業も頻繁に行なう必要がないという利点がある。
【００６８】
（第５の実施形態）
第５の実施形態を図６及び７に基づき説明する。この実施形態のＳＱＵＩＤ磁束計も、４個のマグネットメータ１１〜１４を用いて電気的２次微分型グラジオメータ１として構成されている。
【００６９】
この２次微分型グラジオメータ１は、第３の実施形態と同様に、フィードバック電流を供給する回路として定電流回路１１ｅ〜１４ｅを採用している。第１のマグネットメータ１１のフィードバックコイル１１ｂと第２のマグネットメータ１２のフィードバックコイル１２ｂとを直列に接続するとともに、第３のマグネットメータ１３のフィードバックコイル１３ｂと第４のマグネットメータ１４のフィードバックコイル１４ｂとを直列に接続することにより、フィードバックゲインβ_１とβ′_１の一致度、及び、β_３とβ′_３の一致度を上げている。
【００７０】
また、第３の実施形態の構成に加えて、第２のマグネットメータ１２の出力電圧Ｖ_２を増幅回路３２（ゲインγ_２）で増幅して加算回路３３を介して第４のマグネットメータ３４の出力に加えるようになっている。
【００７１】
さらに、第１のマグネットメータ１１の出力電圧Ｖ_１に比例する電圧を、新たに加えた増幅回路３０（ゲインγ_１）により生成し、この電圧を加算回路３１により第２のマグネットメータ１２の出力電圧に加算する構成が新たに設けられている。第３のマグネットメータ１３の出力電圧Ｖ_３に比例する電圧を、新たに加えた増幅回路３４（ゲインγ_３）により生成し、この電圧を加算回路３５により第４のマグネットメータ１４の出力電圧に加算する構成が新たに設けられている。
【００７２】
本実施形態における２次微分型グラジオメータ１としての差分値検出機能は上述した実施形態と同様である。すなわち、第４のマグネットメータ１４から出力される２次微分の出力電圧Ｖ_４は、
【数１８】

である。
【００７３】
この新たに加えられた増幅回路３０、３４および加算回路３１、３５は、第３の実施形態において相互インダクタンスＭ_１，Ｍ_２、Μ_３，Ｍ_４およびα_２とα′_２が僅かに異なっていた場合、２次微分成分のみを出力すべき出力値に一様成分や１次勾配成分が混入する問題を改善することができる。
【００７４】
増幅回路３０、３２、３４はそのゲインを変更可能に構成されており、各マグネットメータに一様な磁場を加えたときに第２、第４のマグネットメータの微分出力が０となるようにゲインγ_１、γ_３を調整し、各マグネットメータに一様な勾配磁場を加えたときに第４のマグネットメータの出力が０になるようにγ_２を調整する。
【００７５】
図７にゲインを調整するための具体的な回路構成を示す。コンピュータ４０内のゲイン制御部４１からゲイン値がデジタル信号として出力されると、このゲイン値はＤ／Α変換回路４４によって対応するアナログ電圧に変換される。各マグネットメータのから出力された出力電圧（入力信号１）とアナログ電圧に変換されたゲイン信号とが乗算回路４５に入力され、両者の積信号が演算される。この積信号は、他のマグネットメータのＦＬＬ回路からの出力電圧（入力信号２）に加算回路４６で加算される。この加算信号は、信号処理回路４７により、ゲイン調整、フィルタ処理などの信号処理が施された後、コンピュータ４０の信号収集部４３に送られる。この信号収集部４３によってデジタル信号に変換され、ゲイン制御部４１でのゲイン制御に供せられる。また、コンピュータ４０は磁場発生コイル制御部４２を備えており、この制御部がＳＱＵＩＤセンサ付近に設置した参照用の磁場発生コイル４８からの磁場発生を制御できるようになっている。本構成によれば、コンピュータ４０からの指令に応じて増幅回路３０、３２、３４のゲインを任意に変更することができる。
【００７６】
なお、このコンピュータ４０は、この２次微分型グラジオメータ１の検出出力（少なくとも第４のマグネットメータの出力電圧Ｖ_４）を受けて磁場源などを解析する装置と兼用することができる。
【００７７】
次にゲイン調整の手順を説明する。ゲイン制御部４１は全ての増幅回路３０、３２、３４のゲインをまず一度、零に設定する。次いで、磁場発生コイル制御部４２は、ＳＱＵＩＤセンサ付近に設置された複数の磁場発生コイル４８に予め決められたパターンの磁場を発生させる。信号収集部４３ではその時の各磁束計の出力を収集し、コンピュータ内のメモリに記憶する。次いで、図示されてない演算部により、例えば特開平９−２０６６２６号に記載されている方法を用いて各増幅回路のゲインを決定し、ゲイン制御部４１はこの決定したゲイン値を元に各増幅回路のゲインを制御する。
【００７８】
本実施形態によれば、Μ_１，Μ_２，Μ_３，Ｍ_４のばらつきによる出力精度の劣化を防止することができる。相互インダクタンスΜ_１，Ｍ_２，Ｍ_３，Ｍ_４のばらつきは主に製造時に発生するものであり、時問的な変化は極めて小さいため、一度調整を行なうと、その後、長期間にわたり再調整することは殆ど必要無くなる。
【００７９】
本実施形態によれば、第３の実施形態と同様の効果に加え、ＳＱＵＩＤのフィードバックコイルと超伝導リングとの間の相互インダクタンスのわずかなばらつきに因って２階微分グラジオメータの出力精度が劣化するような場合であっても、ゲイン調整可能な増幅回路および加算回路により、これを補償することが可能になる。したがって、２階微分の出力精度をさらに高くすることができ、そのための調整作業も頻繁には行なう必要が無いという特徴がある。
【００８０】
なお、本実施形態では、新たに追加した増幅回路および加算回路が電気的に構成されている場合を例示したが、必ずしもこれに限定されるものではない。他の構成例として、ソフトウェア処理によるものであつてもよい。ソフトウェア処理により増幅及び加算を行なうと、増幅のためのゲイン調整がソフトウェア処理により容易に行なえるため、図７に示したような特別なゲイン調整手段が不要になり、回路構成が大幅に簡単化できるという効果がある。
【００８１】
（第６の実施形態）
第６の実施形態を図８に基づき説明する。この実施形態のＳＱＵＩＤ磁束計は、３個のマグネットメータを用いて電気的２次微分型グラジオメータ１として構成されている。このグラジオメータは、本発明に係る２次微分グラジオメータのほかの基本的な構成を成す。
【００８２】
２次微分グラジオメータ１は３個のマグネットメータ１１〜１３から成り、各マグネットメータは３つのＳＱＵＩＤ１１ａ（〜１３ｃ）と駆動回路としてのＦＬＬ回路１１ｃ（〜１３ｃ）から成っている。また、各マグネットメータにおいて、ゲインβ_１（〜β_３）のフィードバックアンプ１１ｃ（〜１３ｃ）により、出力電圧に比例した電流が夫々のフィードバックコイル１１ｂ（〜１３ｂ）に戻されるようになっている。
【００８３】
これに加えて、第１のマグネットメータ１１の出力電圧Ｖ_１がゲインβ′_１のフィードバックアンプ５１により増幅されて、第２のマグネットメータ１２のフィードバックループに挿入されている加算器５２に印加される。また、第３のマグネットメータ１３の出力電圧Ｖ_３がゲインβ′_３のフィードバックアンプ５３により増幅されて加算回路５２に印加される。加算器５２で加算された第１〜第３のマグネットメータの出力電圧は第２のマグネットメータ１２のフィードバックコイル１２ｂに与えられる。
【００８４】
この結果、第１と第３のマグネットメータ１１、１３は通常のマグネットメータとして動作し、その一方で、それらの出力電圧Ｖ_１、Ｖ_３を夫々β_１、β_３倍した大きさの磁束が第２のマグネットメータ１２のＳＱＵＩＤ１２ａに、第２のマグネットメータ自体の出力に比例した磁束と共に印加される。このため、第２のマグネットメータ１２の出力電圧Ｖ_２は
【数１９】

のように表される。
【００８５】
そこで、β′_１／β_１、β′_３／β_３を夫々１／２にすることにより、２次微分に相当する出力を得ることができる。この回路構成の場合、１次微分の出力を得るには、第１と第３のマグネットメータの出力電圧の差分演算を行なう必要はあるが、その一方で、従来５組必要であったＳＱＵＩＤ及び駆動回路の組が３組で済むため、構成が非常に簡素化されるという容易になるという利点がある。
【００８６】
上述した図８の原理的な２次微分グラジオメータ１を更に具体的に構成した例を図９に示す。この構成は、上述した各実施形態と同様の手法を用いることができ、１次微分出力を直接得ることができない点を除いて、上述のものと同等のの効果を得ることができる。
【００８７】
具体的には、マグネットメータ１１〜１３のフィードバックループには定電流回路１１ｅ〜１３ｅ（ゲインα_１〜α_３）が夫々挿入されている。また、マグネットメータ１１〜１３の夫々において、ＳＱＵＩＤ１１ａ（〜１３ａ）に対するフィードバックコイルは２個ずつ備えられている。
【００８８】
第１のマグネットメータ１１のフィードバックコイル１１ｂ'および１１"（相互インダクタンスはＭ_１、Ｍ'_１）は互いに直列に接続されて、自分の出力のフィードバック電流を受けるとともに、その直列接続のコイルは更に第２のマグネットメータ１２の２個のフィードバックコイル１２ｂ'および１２"（相互インダクタンスはＭ_２、Ｍ'_２）の内の一方１２ｂ'に接続されている。同様に、第３のマグネットメータ１３のフィードバックコイル１３ｂ'および１３"（相互インダクタンスはＭ_３、Ｍ'_３）は互いに直列に接続されて、自分の出力のフィードバック電流を受けるとともに、その直列接続のコイルは更に第２のマグネットメータ１２の一方のフィードバックコイル１２ｂ'に接続されている。第２のマグネットメータ１２では、残りの一方のフィードバックコイル１２ｂ"に自分の検出出力に比例した電流がフィードバックされている。
【００８９】
このグラジオメータ１の出力は第２のマグネットメータ１２から与えられ、その値は、
【数２０】

で表される。１個のＳＱＵＩＤに対して２個備えられているフィードバックコイルの超伝導リングヘの相互インダクタンスＭが全て同一になるように構成すると、２次微分に相当する出力を得ることができる。
【００９０】
第２の実施形態と同様に、２次微分出力を得るために減算回路などを使用しないので、高精度に磁場の２次勾配成分を計測することが可能になる。
【００９１】
（第７の実施形態）
第７の実施形態を図１０に基づき説明する。この実施形態のＳＱＵＩＤ磁束計は、３次以上のグラジオメータとして、電気的３次微分型グラジオメータとし構成したものである。
【００９２】
図１０には、３次微分型グラジオメータ１の１つの基本的な構成を示す。このグラジオメータ１は、８個のマグネットメータ１１〜１４、６１〜６４を備える。この内、図面左側に表した４個のマグネットメータ１１〜１４は前述した第１の実施形態のものと同一に構成されている。図面右側に記載した残り４個のマグネットメータ６１〜６４も左側の４個のマグネットメータ１１〜１４と同様に構成されている。さらに、第４のマグネットメータ１４の検出出力Ｖ_４がフィードバックアンプ７０を介して第８のマグネットメータ６４のフィードバックループに供給されている。
【００９３】
したがって、第１、第３、第５及び第７のマグネットメータ１１，１３，６１及び６３にマグネットメータ出力Ｖ_１，Ｖ_３，Ｖ_５及びＶ_７、第２及び第６のマグネットメータ１２及び６２に１次微分出力Ｖ_２及びＶ_６、第４のマグネットメータ１４に２次微分出力Ｖ_４、並びに第８のマグネットメータ６４に３次微分出力Ｖ_８を得ることができる。
【００９４】
（第８の実施形態）
第８の実施形態を図１１に基づき説明する。この実施形態のＳＱＵＩＤ磁束計も、３次以上のグラジオメータとして、電気的３次微分型グラジオメータとし構成したものである。
【００９５】
図１１には、３次微分型グラジオメータ１の別の基本的な構成を示す。このグラジオメータ１は、４個のマグネットメータ１１〜１４を備える。このマグネットメータ１１〜１４のＳＱＵＩＤ、フィードバックコイル、ＦＬＬ回路及びフィードバックアンプによる基本回路は今まで説明したものと同様である。
【００９６】
これに加えて、第３のマグネットメータ１３のフィードバックループには、加算回路７１が挿入されており、この加算回路７１には自前の検出出力Ｖ_３に加えて、第２のマグネットメータ１２の検出出力Ｖ_２をフィードバックアンプ７２（ゲインβ′_２）を介して、また第４のマグネットメータ１４の検出出力Ｖ_４をフィードバックアンプ７３（ゲインβ′_４）を介して加えるようになっている。さらに、第４のマグネットメータ１４のフィードバックループには別の加算回路７５が挿入されて、自前の検出出力Ｖ_４をゲイン倍した電流と第１のマグネットメータ１１の検出出力Ｖ_１をフィードバックアンプ（ゲインβ′_１）でゲイン倍した電流とを加算してフィードバックコイル１４ｂに与えるようになっている。
【００９７】
したがって、ゲインに関して、β′_１／β_１とβ′_２／β_２を１、β′_４／β_４を１／２や１／３などのある定数に設定することで、第３のマグネットメータ１３から３次微分出力Ｖ_３が
【数２１】

が得られる。このようにして、４組のマグネットメータだけで３次微分出力を得ることができる。
【００９８】
同様にして、本発明に係る４次以上の微分型グラジオメータも構成することができる。
【００９９】
（第９の実施形態）
第９の実施形態を図１２に基づき説明する。この実施形態のＳＱＵＩＤ磁束計は、フィードバック回路の改善に関する。
【０１００】
高温超伝導ＳＱＵＩＤを用いたＳＱＵＩＤ磁束計では、低周波雑音を低減することを意図して、交流バィアス回路を採用し、フィードバック電流に加えた変調信号をフィードバックコイルに入力させる構成を採ることが多い。このような回路の構成例は特公平６−３８１０３に詳細に記述されている。
【０１０１】
図１２は、かかる回路構成を用いたＳＱＵＩＤ磁束計の好ましい実施形態を示す。ＳＱＵＩＤ磁束計１は、複数のマグネットメータ９１、９２、…を有し、各マグネットメータは高温超伝導リング９１ａ（９２ａ）、フィードバックコイル９１ｂ（９２ｂ）、駆動回路９１ｃ（９２ｃ）、及びフィードバック系に挿入された電圧・電流変換用の定電流回路９１ｄ（９２ｄ）を備える。駆動回路９１ｃ及び９２ｃは夫々、増幅器Ｄａ、復調器Ｄｂ、積分器Ｄｃを入力側からこの順に備える。超伝導リング９１ａ，９２ａのフィードバックコイル９１ｂ，９２ｂは、自己のフィードバックループに夫々、直列に接続されながらも、相互に直列に接続されている。
【０１０２】
複数のマグネットメータ９１、９２、…には共通の交流バイアス回路９３が装備されている。この交流バイアス回路９３は、発信器９４、分周器９５、排他的論理和回路９６、および定電流回路９７を備える。
【０１０３】
この交流バイアス回路９３は、フィードバックコイル９１ｂ、９２ｂ、…が直列に接続された複数のマグネットメータ９１、９２、…に対して、変調信号を発生する発振器９４を１台だけ設け、その変調信号を定電流回路９７で電圧・電流変換し、変換した電流を直列接続のフィードバックコイル９１ｂ、９２ｂ、…に共通に供給するという特徴を有する。また、駆動回路９１ｃ，９２ｃの夫々は、ＳＱＵＩＤ９１ａ，９２ａ，…の夫々の出力電圧を増幅器Ｄａで増幅した後、共通の発振器９４を起源とする参照信号を用いて復調器Ｄｂで復調することを特徴とする。この回路構成は、前述した第１〜第８の実施形態の各々に対して適用され、フィードバック電流は第１〜第８の実施形態の各々に記されている方法で生成される。
【０１０４】
本実施形態によれば、高温超伝導ＳＱＵＩＤに顕著な低周波雑音を効果的に抑制することができるため、高温超伝導ＳＱＵＩＤに対して好適に適用できるようになる。さらに、駆動回路夫々に共通の発振器により生成された変調信号が供給され、共通の発振器に基づく参照信号により復調されるので、チャンネル間の意図しない相互作用に因って、出力電圧に歪みが発生しても、その歪みはチャンネル間に常に一定に作用するので、この相互作用は各チャンネルのばらつきと同様の歪みとして現れる。このため、第５の実施形態で説明した手法によって測定及び、補正することが可能である。したがって、第５の実施形態と組み合わせて実施することによってチャンネル問の相互作用の影響を排除して、磁場勾配をさらに高精度に計測できるという効果がある。
【０１０５】
上述した実施形態では、電気的な回路によるグラジオメータを例に説明したが、本発明の主要な構成要素はソフトウェア的にも実行可能なものであり、その場合にも電気的に構成されたのと同様の効果が得られる。
【０１０６】
【発明の効果】
以上説明したように、本発明のＳＱＵＩＤ磁束計によれば、参照用の余分なマグネットメータを設ける必要がなく、２次以上のグラジオメータを電気的に又はソフトウェア的に構成できる。このため、空間的に立体的なピックアップコイルが構成困難な高温超伝導ＳＱＵＩＤを用いて、補償電流の調整を頻繁に行なう必要が無く、且つ環境磁場の高い除去率を実現する高次微分グラジオメータを提供することができる。したがって、安価で運用の容易な生体磁気計測装置を提供することができる。
【図面の簡単な説明】
【図１】図１は、本発明の第１の実施形態に係るＳＱＵＩＤ磁束計としての２次微分型グラジオメータの概略を示す構成図。
【図２】図２は、本発明の第２の実施形態に係るＳＱＵＩＤ磁束計としての２次微分型グラジオメータの概略を示す構成図。
【図３】図３は、本発明の第３の実施形態に係るＳＱＵＩＤ磁束計としての２次微分型グラジオメータの概略を示す構成図。
【図４】図４は、フィードバック回路に用いる定電流回路の回路図。
【図５】図５は、本発明の第４の実施形態に係るＳＱＵＩＤ磁束計としての２次微分型グラジオメータの概略を示す構成図。
【図６】図６は、本発明の第５の実施形態に係るＳＱＵＩＤ磁束計としての２次微分型グラジオメータの概略を示す構成図。
【図７】ゲイン調整法を説明する回路図。
【図８】図８は、本発明の第６の実施形態に係るＳＱＵＩＤ磁束計としての２次微分型グラジオメータの別の基本形を示す構成図。
【図９】図８の２次微分型グラジオメータを具体例を示す構成図。
【図１０】図１０は、本発明の第７の実施形態に係るＳＱＵＩＤ磁束計としての３次微分型グラジオメータの一例を示す構成図。
【図１１】図１１は、本発明の第８の実施形態に係るＳＱＵＩＤ磁束計としての３次微分型グラジオメータの別の例を示す構成図。
【図１２】図１２は、本発明の第９の実施形態に係るＳＱＵＩＤ磁束計の概略構成を示すブロック図。
【図１３】従来技術を説明するために用いピックアップコイルおよびグラジオメータの構成図。
【符号の説明】
１ ２次微分型グラジオメータ（ＳＱＵＩＤ磁束計）
１１〜１４ マグネットメータ
１１ａ〜１４ａ ＳＱＵＩＤ（超伝導リング）
１１ｂ〜１４ｂ フィードバックコイル
１１ｃ〜１４ｃ ＦＬＬ回路
１１ｄ〜１４ｄ フィードバックアンプ
１１ｅ〜１４ｅ 定電流回路
１１′ｂ，１１"ｂ，１２′ｂ，１２"ｂ，１４′ｂ，１４"ｂ フィードバックコイル
１７〜１９ フィードバックアンプ
２１ 定電流回路
２２，２３ 定電流回路
３０、３２、３４ 増幅回路
３１、３３、３５ 加算回路
４０ コンピュータ
４４ Ｄ／Ａ変換回路
４５ 乗算回路
４６ 加算回路
４７ 信号処理回路
４８ 磁場発生コイル
５１，５３ フィードバックアンプ
５２ 加算回路
６１〜６４ マグネットメータ
７１，７５ 加算回路
７２，７３，７４ フィードバックアンプ
９１，９２ マグネットメータ
９１ａ，９２ｂ 高温超伝導リング
９１ａ，９２ｂ フィードバック回路
９３ 交流バイアス回路[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a SQUID magnetometer using a high-sensitivity magnetic sensor called SQUID (Superconducting Quantum Interference Device), and in particular, using high-temperature superconducting SQUID as SQUID, removing extraneous magnetic noise. The present invention relates to a SQUID magnetometer that employs a configuration of a gradiometer that is effective. This SQUID magnetometer is mainly suitable for a biomagnetic measuring device.
[0002]
[Prior art]
Currently, multi-channel SQUID magnetometers using many SQUIDs have been actively developed, and biomagnetic measurements such as magnetoencephalograms and magnetocardiograms using this magnetometer have been studied. This biomagnetic measurement has already been found to be effective for the study of cerebral physiology and the diagnosis of ischemic heart disease and arrhythmia.
[0003]
Since the SQUID operating with extremely low sound requires a large cooling mechanism, there is a problem that the entire configuration of the biomagnetic measuring device is enlarged and the operation cost is high.
[0004]
Under such circumstances, in recent years, SQUID using a high temperature superconducting material is approaching practical use. The high temperature superconducting SQUID can be cooled by liquid nitrogen which is easy to handle. For this reason, by incorporating the high-temperature superconducting SQUID into the biomagnetic measuring device, it is expected that a large-scale cooling mechanism will be unnecessary, the overall size of the device can be avoided, and the operating cost can be reduced. Yes.
[0005]
By the way, in order to measure biomagnetism using a SQUID magnetometer, since the magnetic field generated from the living body is very weak, the magnetic field generated from other than the living body (hereinafter referred to as “environmental magnetic field”). Unless it is removed), effective measurement cannot be expected. For this reason, the technology for removing the environmental magnetic field forms one of the main fields of biomagnetic measurement.
[0006]
  FIG. 13 shows some of the conventional methods for removing the environmental magnetic field. FIG. 5A is called a first-order differential pickup (detection) coil for detecting magnetic flux used in a SQUID magnetometer. This pickup coil is a coil employed in a SQUID magnetometer that mainly operates at liquid helium temperature, and two loops are wound in opposite directions by superconducting wires. This primary differential pickup coil shows a sensitive detection characteristic for a magnetic field generated from the vicinity of this coil, such as the heart and brain, while it has two coils for an environmental magnetic field generated far away.AlmostSince magnetic fluxes of the same magnitude are interlinked, they are canceled.
[0007]
FIG. 2B shows a pickup coil having a configuration in which two primary differential pickup coils described above are used and connected in a state of being stacked in the opposite direction in the central axis direction. This pickup coil is called a secondary differential pickup coil, and can exhibit a higher ability to remove an environmental magnetic field than the primary differential type. A magnetometer that includes these differential pickup coils and measures a gradient component of a magnetic field is called a “gradiometer”.
[0008]
Techniques for realizing this gradiometer electrically or in software are proposed by, for example, US Pat. No. 5,122,744, JP-A-4-264281, JP-A-5-232202, and JP-A-6-138197. Known as. These electrical or software methods can be applied to a cryogenic SQUID magnetometer, but it is difficult to form a big-up coil as shown in FIGS. 13 (a) and 13 (b). It can be suitably applied to a SQUID magnetometer.
[0009]
Of these, in particular, Japanese Patent Application Laid-Open No. 5-232202 discloses an example in which a second-order differential gradiometer is configured in terms of software, and this is schematically shown in FIG. According to the figure, four magnetometers are used, the difference between the outputs of each of the two magnetometers is calculated to obtain the first derivative output, and the difference between the difference outputs is again calculated. A second derivative output is obtained by calculation. In addition, a separate magnetometer for reference is prepared, and a current proportional to the measured value of the magnetometer for reference is supplied as a compensation current to the other four magnetometers, so that they are commonly input to the four magnetometers. The spatially uniform environmental magnetic field having a large value is subtracted from each detected value in advance.
[0010]
By configuring the second-order differential type gradiometer in this way, it is possible to increase the ability to remove the environmental magnetic field as compared with the first-order differential gradiometer. Naturally, the higher the differential order of the third order or higher, the more the environmental magnetic field can be removed.
[0011]
[Problems to be solved by the invention]
Since the second-order differential gradiometer described in Japanese Patent Laid-Open No. 5-232202 described above calculates the difference after removing the uniform component of the environmental magnetic field having a large value in advance, the dynamic range of each magnetometer is Even if it is small, the first-order differential output value that is the output of the first-stage difference calculation can provide a highly accurate measurement value.
[0012]
However, the second-order differential gradiometer described in the same publication has a problem that its ability to remove a first-order gradient component of a magnetic field having a large value that is also included in the environmental magnetic field is extremely low.
[0013]
That is, most of the primary gradient component is mixed in two primary differential outputs. Therefore, a small secondary gradient component included in the primary gradient component of the large environmental magnetic field must be extracted by the second-stage difference calculation. Therefore, if a secondary differential output value is to be obtained with high accuracy, a large dynamic range and a strict gain setting are required for the primary differential output value. In order to ensure a large dynamic range and a strict gain setting for the primary differential output value, it is necessary to increase the dynamic range of each magnet meter. Therefore, strict gain setting is required for the SQUID driving circuit and each amplifier.
[0014]
  For example, the peak value of the uniform component of the environmental magnetic field is about 10 nT, the primary gradient componentpeakThe value is about 100pΤpeakThe value is 1pΤ, and the environmental magnetic field per unit frequency is 100pT / √Hz for the uniform component, 1pT / √Hz for the primary gradient component, and 10fT / √Hz for the secondary gradient component, and is measured with the minimum resolution of 10fT / √Hz. I want to do it.
[0015]
In this case, a 100 pT signal is output at the peak of the output of each magnetometer and the primary differential output, so that the minimum resolution of the primary differential output is at least 10 fT / √Hz or less and a peak value of 100 pT. It must be designed so that it cannot be shaken out. Further, the signal value of the first derivative output is 1 pT / √Hz per unit frequency, and it is necessary to take out a signal of 10 fT / √Hz from this, so that the accuracy of setting the gain of each magnet meter, amplifier, and difference calculator In addition, the in-phase signal rejection ratio of the difference calculation at the first stage and the difference calculation at the second stage needs to be at least 40 dΒ. Furthermore, since it is necessary to take out the first derivative output with the minimum resolution of 10 fT / √Hz from among the 100 pT / √Hz uniform component of the environmental magnetic field, the uniform component of the magnetic field is obtained by a reference magnetometer. The amount of compensation given to each magnet meter to eliminate must be matched with an accuracy of 80 dB.
[0016]
In the case of the conventional example described in this publication, a compensation current adjustment circuit (variable resistor) is provided to correct the amount of compensation given to each magnet meter with an accuracy of 80 dB, and corrects the variation of each magnet meter. It has a configuration.
[0017]
However, it is troublesome to manually adjust the variation of all magnet meters. In addition, even after the adjustment is completed, the resistance value temporally fluctuates due to changes in the temperature of the adjustment resistor and wiring resistance. It was.
[0018]
In addition to the four magnet meters, there was a problem that a magnet meter had to be added separately for reference.
[0019]
The present invention has been made in consideration of the conventional problems found in the above-mentioned secondary differential gradiometer described in JP-A-5-232202, etc., and it is difficult to form a spatially three-dimensional big-up coil. High-order differential gradiometer that can be applied to superconducting SQUID, does not require a reference magnetometer, does not require frequent adjustment of compensation current, and can realize a high removal rate of environmental magnetic field It is an object of the present invention to provide a SQUID magnetometer.
[0020]
[Means for Solving the Problems]
In order to achieve the above object, according to the SQUID magnetometer according to the present invention, three or more SQUIDs, a drive circuit that independently drives each of the three or more SQUIDs, and each of the three or more SQUIDs. And a feedback coil that feeds back a magnetic flux having a magnitude proportional to the output of each of the drive circuits. And means for applying a feedback magnetic flux having a magnitude proportional to the output of the drive circuit of the other SQUID to at least one SQUID among the three or more SQUIDs via the feedback coil of the SQUID. And means for additively applying a feedback magnetic flux having a magnitude proportional to the output of the drive circuit of the other SQUID other than these SQUIDs to the at least one SQUID via the feedback coil of the SQUID. Features. Thereby, a secondary or higher-order gradiometer is configured.
[0021]
Further, by providing a fine adjustment circuit of the feedback gain in each drive circuit of the gradiometer configured as described above, it is possible to prevent the output accuracy from being deteriorated due to subtle variations in each SQUID.
[0022]
According to another aspect of the present invention, a SQUID comprising a plurality of SQUIDs, a drive circuit that independently drives each of the plurality of SQUIDs, and a feedback coil attached to each of the plurality of SQUIDs. In the magnetometer, means for applying a feedback magnetic flux having a magnitude proportional to the output of the drive circuit of the SQUID to each of the plurality of SQUIDs via a feedback coil of the SQUID, and a plurality of SQUIDs of the plurality of SQUIDs Means for additively applying a feedback current supplied to another SQUID to each feedback coil, and at least one SQUID of a plurality of SQUIDs to which the feedback current is additively applied The feedback coil has another SQ of the plurality of SQUIDs. Wherein means for further additive applied to the magnitude of the feedback current proportional to the output voltage of the driving circuit of the ID, and that a possible change means a conversion factor to the current from the output voltage.
[0023]
Thus, the present invention constitutes a SQUID magnetometer having a second or higher order electrical difference gradiometer. To summarize one aspect of this configuration, each of the two first-order differential gradiometers is configured with a magnet meter, and the feedback current of one first-order differential gradiometer is added to the other first-order differential gradiometer. It is the structure to apply. With this configuration, it is not necessary to provide an extra magnetometer for uniform magnetic field removal, and a high-order differential gradiometer having a wide dynamic range and good characteristics can be easily configured.
[0026]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.
[0027]
(First embodiment)
The SQUID magnetometer according to the first embodiment will be described with reference to FIG. This embodiment shows a basic configuration for carrying out the present invention.
[0028]
This SQUID magnetometer is configured as an electrical second-order differential gradiometer using a magnet meter. The SQUID incorporated in each magnet meter is formed of a high temperature superconducting material.
[0029]
As shown in FIG. 2, the second-order differential gradiometer 1 includes first to fourth four magnetometers 11 to 14. Each magnet meter 11 (˜14) includes a SQUID (superconducting ring) 11a (˜14a) made of a high-temperature superconducting material, a FLL (flux locked loop) circuit 11b (˜14b) that drives this SQUID, and a SQUID. A feedback coil 11c (˜14c) for feeding back magnetic flux and a feedback amplifier 11d (˜14d) inserted in a feedback loop connected to the feedback coil are provided. The gains of the feedback amplifiers 11d to 14d are β₁~ Β₄It is.
[0030]
Of the four magnet meters, the second and fourth magnet meters 12 and 14 are provided with addition circuits 15 and 16, respectively. Each of the adder circuits 15 and 16 is inserted between the feedback coil 12b (14b) and the amplifier 12d (14d) in the feedback loop. In addition to the original feedback loop as one input, the adder circuits 15 and 16 receive inputs from other magnetometers and feed back their combined inputs to the SQUID.
[0031]
A branch path branched from the output end of the FLL circuit 11 c of the first magnetometer 11 is connected to the adding circuit 15 of the second magnetometer 12. This branch has a gain β ′₁Amplifier 17 is inserted.
[0032]
The adder circuit 16 of the fourth magnetometer 14 is connected to a branch path branched from the output ends of the FLL circuits 12c and 13c of the second and third magnetometers 11. These branches have a gain β ′₂And β ′₃Amplifiers 18 and 19 are respectively inserted.
[0033]
A bias current is supplied to the SQUIDs 11a to 14a of the first to fourth magnetometers 11 to 14 by a bias current supply circuit (not shown). Further, a modulation magnetic flux is applied to the SQUIDs 11 to 14 from a modulation circuit (not shown) in the FLL circuits 11c to 14c. The outputs of the SQUIDs 11a to 14a are amplified by the amplifiers in the FLL circuits 11c to 14c, detected and demodulated by the detection circuit, integrated by the integrator, further amplified again as necessary, and supplied as the output voltage V1. (Amplifier, detector circuit, integrator, etc. are not shown).
[0034]
In the first SQUID 11, the feedback circuit (loop) is “β proportional to the output voltage V1.₁・ V₁Is applied to the SQUID 11a via the feedback coil 11b. The FLL circuit 11c is externally applied to the SQUID 11a, that is, the magnetic flux Φ detected from the living body.₁And feedback magnetic flux β₁・ V₁Behaves to be equal,
[Expression 1]
Φ₁= Β₁・ V_I
The output voltage V_IIs
[Expression 2]
V₁= Φ₁/ Β₁
It becomes. That is, the first magnetometer 11 receives the input magnetic flux Φ₁Output voltage V proportional to_IIs output.
[0035]
The SQUID 12a of the second magnet meter 12 has its output voltage V₂Proportional to₂・ V₂”And the output voltage V of the first magnetometer 11.₁Proportional to₁・ V₁A magnetic flux summed with a feedback magnetic flux of a magnitude of “is applied. That is,
[Equation 3]
Φ₂= Β '₁V₁+ Β₂V₂
Output voltage V₂Is
[Expression 4]
V₂= (Φ₂− (Β ′₁/ Β₁) Φ₁) / Β₂
It becomes. Therefore, gain β ′₁And β₁Are equal to each other, the second magnet meter 12 detects the magnetic flux Φ detected by the first magnet mechanism 11.₁And magnetic flux Φ detected by the second magnetometer 12₂, The voltage V proportional to the first-order difference value of magnetic flux₂(= (Φ₂−Φ₁) / Β₂) Is output.
[0036]
The SQUID 13a of the third magnet meter 13 has its output voltage V₃Proportional to₃・ V₃”Is applied. That is,
[Equation 5]
Φ₃= Β₃V₃
And its output voltage V₃Is
[Formula 6]
V₃= Φ₃/ Β₃
It becomes. Therefore, the third magnetometer 13 detects the magnetic flux Φ detected.₃Voltage V proportional to₃Is output.
[0037]
The SQUID 14 a of the fourth magnet meter 14 has an output voltage V of the second magnet meter 12.₂Β ′ proportional to₂V₂Feedback magnetic flux of the magnitude and the output voltage V of the third magnetometer 13₃Β ′ proportional to₃V₃Feedback magnetic flux of the magnitude and the output voltage V of the fourth magnetometer 14₄Β proportional to₄V₄A magnetic flux summed with a feedback magnetic flux of a certain magnitude is applied. That is,
[Expression 7]
Φ₄= Β '₂V₂+ Β '₃V₃+ Β₄V₄
And its output voltage V₄Is
[Equation 8]

It becomes. Therefore, gain β ′₁And β₁, Β ′₂And β₂, Β ′₃And β₃Are equal to each other, the fourth magnet meter 14 detects the magnetic flux Φ detected by the first and second magnet meters 11 and 12.₁And Φ₂And the magnetic flux Φ detected by the third and fourth magnetometers 13 and 14₃And Φ₄The voltage V proportional to the second-order difference value of the magnetic flux₄Is output.
[0038]
Thus, four magnetometers 11 to 14 are used from the first to the fourth, and the output value V of the first magnetometer 11 is used.₁Is multiplied by a gain and added to the feedback signal of the second magnet meter 12, and the output value V of the third magnet meter 13 is added.₃Is multiplied by a gain and added to the feedback signal of the fourth magnet meter 14. Further, the first floor differential output V of the second magnetometer 12₂Is multiplied by a gain and further added to the feedback signal of the fourth magnet meter 14. As a result, the voltage V obtained by subtracting the second floor from the fourth magnet meter 14 is obtained.₄Is obtained.
[0039]
Since the second-order differential gradiometer of this embodiment does not require a three-dimensional pickup coil, it can be applied to a high-temperature superconducting SQUID, and it is necessary to adopt a configuration in which a reference magnetometer must be provided separately. Therefore, the manufacturing cost can be reduced.
[0040]
(Second Embodiment)
By the way, in the first embodiment described above, the environmental magnetic field component included in the output detected by the second-order differential gradiometer 1 is about “1” of the uniform component of the magnetic field detected by each of the magnetometers 11 to 14. -(2)^1/2β ′ / β ”times or“ 1-β ′ of the primary gradient component of the environmental magnetic field₂/ Β₂The value will drop to the larger value. Where “β ′ / β” is “β ′₁/ Β₁”And“ β ′₃/ Β₃"Is a typical value. For example, if the magnitude of the uniform component of the magnetic field is about 100 pT / √Ηz and the required magnetic field resolution is 10 fT / √Ηz, 1-2β ′ / β is 10^-4The size must be less than or equal to. That is, β₁And β₃Each gain pair is 10^-4It is necessary to match with an error of about half of that. If the magnitude of the primary gradient component of the environmental magnetic field is about 1 pT / √Hz, 1−β ′₂/ Β₂10^-2Must be less than or equal to₂The gain pair is 10^-2It is necessary to match with a degree of accuracy.
[0041]
A specific embodiment for realizing this will be described as a second embodiment with reference to FIG. In the following embodiments, the same or equivalent components as those in the SQUID magnetometer of the embodiments described so far are denoted by the same reference numerals, and the description thereof is omitted or simplified.
[0042]
The SQUID magnetometer shown in FIG. 2 constitutes the second-order differential gradiometer 1 as a SQUID magnetometer by electrically connecting four magnetometers 11 to 14 as in the first embodiment. It is a thing.
[0043]
As shown in the figure, a resistance element R is used as a feedback amplifier for the magnetometers 11-14.₁~ R₄Are used respectively. Further, the SQUIDs 12a and 14a of the second and fourth magnetometers 12 and 14 are provided with two feedback coils 12b 'and 12b "and 14b' and 14b", respectively.
[0044]
Resistance element R of the feedback loop of the first magnetometer 11₁After being connected to its own feedback coil 11 b, this coil 11 b is connected in series to one of the two feedback coils 12 b ′ of the second magnetometer 12. Resistance element R of the feedback loop of the second magnetometer 12₂Is connected to the other feedback coil 12b ". Similarly, the resistance element R of the feedback loop of the third magnetometer 11 is connected.₃After being connected to its own feedback coil 13 b, this coil 13 b is connected in series to one of the two feedback coils 14 b ′ of the fourth magnetometer 14. Resistance element R of the feedback loop of the fourth magnetometer 14₄Is connected to the other feedback coil 14b ".
[0045]
The second-order differential type gradiometer 1 includes two feedback coils in the second and fourth magnetometers 12 and 14 so that a feedback gain β₁And β ′₁And β₃And β ′₃The degree of coincidence can be increased. Hereinafter, the degree of matching of the feedback gain and the accuracy of the output signal according to the present embodiment will be described.
[0046]
Feedback gain β₁, Β ′₁Respectively
[Equation 9]
β₁= Μ₁/ R₁, Β ′₁= Μ₂/ R₁
It is. Μ₁Is a mutual inductance between the feedback coil 11b and the SQUID 11a in the first magnetometer meter 11,₂And M ′₂Is the mutual inductance between the feedback coils 12b 'and 12b "and the SQUID 12a in the second magnetometer 12, and when the SQUID and the coil are formed of a thin film,^-4It can be formed with higher accuracy.
[0047]
The same applies to the third and fourth magnetometers 13 and 14. Μ₃Is the mutual inductance between the feedback coil 13b and the SQUID 13a in the third magnetometer meter 13,₄And M ′₄Is the mutual inductance between the feedback coils 14b 'and 14b "and the SQUID 14a in the fourth magnetometer 14. Therefore," 1-2^1/2β ′ / β ”is 10⁴It can be set to a size less than about.
[0048]
Also, feedback gain β₂And β ′₂Respectively
[Expression 10]

And Μ ′₂And Μ ′₄10^-2Can be easily formed with higher accuracy than R₂And R '₂10^-2Since it is possible to match with higher accuracy, “1-β ′₂/ Β₂10^-2It can be set to a size less than about.
[0049]
The differential value detection function as the secondary differential type gradiometer 1 is the same as that of the first embodiment. That is, the output voltage V of the second derivative output from the fourth magnetometer 14.₄Is
## EQU11 ##

It is.
[0050]
In the present embodiment, a second-order differential gradiometer is configured without requiring a difference calculator as in the conventional example. Therefore, unlike the conventional example, the accuracy of the second-order differential output is not limited by the difference in gain between the two magnetometers or the common-mode signal rejection ratio of the difference calculator, and the environmental magnetic field is removed with high accuracy. can do. In addition, since it is possible to obtain a highly accurate removal capability of the environmental magnetic field without an adjustment resistor for adjusting the compensation current, it is not necessary to frequently adjust the compensation current.
[0051]
(Third embodiment)
A third embodiment is shown in FIGS.
[0052]
The SQUID magnetometer of this embodiment is also configured as an electrical second-order differential type gradiometer 1 using four magnetometers 11-14.
[0053]
In each feedback loop of the magnetometers 11 to 14, constant current circuits 11e to 14e (gain α₁~ Α₄) Respectively. In the figure, M1 to M4 are mutual inductances between each SQUID and the feedback coil.
[0054]
The feedback coil 11b of the first magnetometer 11 and the feedback coil 12b of the second magnetometer 12 are connected in series, while the feedback coil 13b of the third magnetometer 13 and the fourth magnetometer 14 are connected. The feedback coil 14b is connected in series, so that the feedback gain β is the same as described in the second embodiment.₁And β ′₁And β₃And β ′₃The degree of agreement is improved.
[0055]
Further, the output voltage V of the second magnetometer 12₂Is connected to another constant current circuit 21 (gain α ′₂) To the feedback coil 14b of the fourth magnet meter 14.
[0056]
An example of the constant current circuits 11e to 14e used in the present embodiment is shown in FIG. This constant current circuit is configured as a voltage-current conversion circuit. In this circuit, each resistance element shown in the figure has a value that maintains the following relationship.
[0057]
[Expression 12]
R_f/ R_s= (R₂+ R₃) / R₁
As a result, the output current I of this constant current circuit_LIs the input voltage V_sThe value is proportional to.
[0058]
[Formula 13]
I_L= (R_f/ (R_sR₃)) V_s
The differential value detection function as the secondary differential type gradiometer 1 in the present embodiment is the same as that of the first embodiment. That is, the output voltage V of the second derivative output from the fourth magnetometer 14.₄Is
[Expression 14]

It is.
[0059]
As described above, the second-order differential gradiometer is configured without using the difference calculator as in the first embodiment. As in the conventional example, it is possible to provide a second-order differential gradiometer that can remove the environmental magnetic field with high accuracy without limiting the accuracy of the second-order differential output by the common-mode signal rejection ratio of the difference calculator. Furthermore, since the necessary accuracy can be obtained without the adjustment resistor for adjusting the compensation current, it is possible to provide a second-order differential gradiometer that does not require frequent adjustment of the compensation current. Furthermore, since it is not necessary to provide two feedback coils for each of the second and fourth magnet meters as shown in the first embodiment, the configuration is simplified and crosstalk between these coils is simplified. Generation of distortion due to is also eliminated.
[0060]
(Fourth embodiment)
A fourth embodiment will be described with reference to FIG. The SQUID magnetometer of this embodiment is also configured as an electrical second-order differential type gradiometer 1 using four magnetometers 11-14.
[0061]
As in the third embodiment, the secondary differential type gradiometer 1 employs constant current circuits 11e to 14e as circuits for supplying a feedback current in each magnet meter. On the other hand, the feedback coil 11b of the first magnetometer 11 and the feedback coil 12b of the second magnetometer 12 are connected in series, and similarly, the feedback coil 13b of the third magnetometer 13 and the fourth magnet. The feedback coil 14b of the meter 14 is connected in series, so that the feedback gain β₁And β ′₁And β₃And β ′₃The degree of agreement is improved.
[0062]
In particular, the gradiometer of the third embodiment differs from the gradiometer of the second and fourth magnets in that the current proportional to the output voltage of the first and third magnetometers 11 and 13 is newly added by a constant current circuit. This is the point where circuits for supplying the meters 12 and 14 are added. That is, the output voltage V of the first magnetometer 11₁Is a constant current circuit 22 (gain γ₁) To the feed coil 12b of the second magnetometer 12 and the output voltage V of the third magnetometer 13₃Is a constant current circuit 23 (gain γ₃) To the feed coil 14b of the fourth magnet meter 14.
[0063]
The differential value detection function as the secondary differential type gradiometer 1 in the present embodiment is the same as that of the first embodiment. That is, the output voltage V of the second derivative output from the fourth magnetometer 14.₄Is
[Expression 15]

It is.
[0064]
The above-described newly added constant current circuits 22 and 23 are different from each other in the third embodiment.₁, Μ₂M and M₃, M₄If the values between are slightly different,₁And β ′₁And β₃And β ′₃This is to improve the point that the high degree of coincidence required in the above cannot be secured. Specifically, the current supplied to the feedback coils 11b and 12b (and 13b and 14b) of the first and second (and third and fourth) magnetometers 11 and 12 (and 13, 14) in common.₁= Α₁V₁(And Ι₃= Α₃V₃) Smaller current I '₂= Γ₁V₁(And Ι ′₄= Γ₃V₃) To the feedback coil 12 b of the second magnetometer 12 (and the feedback coil 14 b of the fourth magnetometer 14), so that the mutual inductance M₁, Μ₂(And M₃, M₄) To correct slight differences in values.
[0065]
The gains of the newly added constant current circuits 22 and 23 are variable. For this reason, when a uniform magnetic field is applied to each magnet meter, the output of the second and fourth magnet meters 12 and 14 is adjusted to be the smallest. The gain adjustment of the constant current circuits 22 and 23 is performed by, for example, the resistor R in FIG.₃This is achieved by adjusting the value of. The gain when adjusted ideally is
[Expression 16]
γ₁= Α₁(M₁/ M₂-1)
as well as
[Expression 17]
γ₃= Α₃(M₃/ M₄-1)
It becomes the value. These gains are the mutual inductance M₁, M₂Are consistent with each other with high precision α₁, Α₃Therefore, the influence of these values on the accuracy of the output value is small, and adjustment is easier than in the conventional method.
[0066]
Mutual inductance M₁, Μ₂, Μ₃, Μ₄This variation mainly occurs at the time of manufacture, and the change with time is extremely small. Therefore, once the adjustment is performed, it is not necessary to adjust over a long period of time.
[0067]
According to this embodiment, in addition to the same effects as those of the second embodiment, the output accuracy of the second-order differential gradiometer is deteriorated due to a slight variation in mutual inductance between the feedback coil of the SQUID and the superconducting ring. Can be compensated for by a constant current circuit capable of gain adjustment. For this reason, the output accuracy of the second order differentiation can be further increased. In addition, there is an advantage that adjustment work for that purpose does not need to be performed frequently.
[0068]
(Fifth embodiment)
A fifth embodiment will be described with reference to FIGS. The SQUID magnetometer of this embodiment is also configured as an electrical second-order differential type gradiometer 1 using four magnetometers 11-14.
[0069]
As in the third embodiment, the secondary differential type gradiometer 1 employs constant current circuits 11e to 14e as circuits for supplying a feedback current. The feedback coil 11b of the first magnetometer 11 and the feedback coil 12b of the second magnetometer 12 are connected in series, and the feedback coil 13b of the third magnetometer 13 and the feedback coil 14b of the fourth magnetometer 14 are connected. Are connected in series, the feedback gain β₁And β ′₁Of agreement and β₃And β ′₃The degree of coincidence is raised.
[0070]
In addition to the configuration of the third embodiment, the output voltage V of the second magnetometer 12₂Amplifying circuit 32 (gain γ₂) And added to the output of the fourth magnetometer 34 via the adder circuit 33.
[0071]
Further, the output voltage V of the first magnetometer 11₁Is added to the amplifier circuit 30 (gain γ₁), And this voltage is added to the output voltage of the second magnetometer 12 by the adder circuit 31. Output voltage V of the third magnet meter 13₃Is newly added to the amplifier circuit 34 (gain γ₃), And this voltage is added to the output voltage of the fourth magnet meter 14 by the adding circuit 35.
[0072]
The differential value detection function as the secondary differential type gradiometer 1 in the present embodiment is the same as that in the above-described embodiment. That is, the output voltage V of the second derivative output from the fourth magnetometer 14.₄Is
[Expression 18]

It is.
[0073]
The newly added amplifier circuits 30 and 34 and adder circuits 31 and 35 have mutual inductance M in the third embodiment.₁, M₂, Μ₃, M₄And α₂And α ′₂Are slightly different from each other, it is possible to improve the problem that a uniform component or a first-order gradient component is mixed in an output value for outputting only the second-order differential component.
[0074]
The amplification circuits 30, 32, and 34 are configured so that their gains can be changed, and the gains are set so that the differential outputs of the second and fourth magnetometers become zero when a uniform magnetic field is applied to each magnetometer. γ₁, Γ₃Γ so that the output of the fourth magnet meter becomes zero when a uniform gradient magnetic field is applied to each magnet meter.₂Adjust.
[0075]
  FIG. 7 shows a specific circuit configuration for adjusting the gain. When the gain value is output as a digital signal from the gain control unit 41 in the computer 40, the gain value is converted into a corresponding analog voltage by the D / Α conversion circuit 44. The output voltage (input signal 1) output from each magnet meter and the gain signal converted into the analog voltage are input to the multiplication circuit 45, and the product signal of both is calculated. This product signal is added by the adder circuit 46 to the output voltage (input signal 2) from the FLL circuit of another magnetometer. The added signal is subjected to signal processing such as gain adjustment and filter processing by the signal processing circuit 47 and then sent to the signal collecting unit 43 of the computer 40. This signal collecting unit 43 converts the signal into a digital signal, which is used for gain control in the gain control unit 41. Further, the computer 40 includes a magnetic field generation coil control unit 42, and this control unit can control magnetic field generation from the reference magnetic field generation coil 48 installed in the vicinity of the SQUID sensor. According to this configuration,ComputerThe gains of the amplifier circuits 30, 32, and 34 can be arbitrarily changed according to the command from 40.
[0076]
The computer 40 detects the detection output of the second-order differential gradiometer 1 (at least the output voltage V of the fourth magnetometer).₄) And can also be used as a device for analyzing a magnetic field source and the like.
[0077]
Next, a procedure for gain adjustment will be described. The gain controller 41 first sets the gains of all the amplifier circuits 30, 32, and 34 to zero once. Next, the magnetic field generation coil control unit 42 causes a plurality of magnetic field generation coils 48 installed in the vicinity of the SQUID sensor to generate a magnetic field having a predetermined pattern. The signal collecting unit 43 collects the output of each magnetometer at that time and stores it in a memory in the computer. Next, the gain of each amplifier circuit is determined by a calculation unit (not shown) using, for example, the method described in Japanese Patent Laid-Open No. 9-206626, and the gain control unit 41 sets each amplification based on the determined gain value. Control the gain of the circuit.
[0078]
According to this embodiment,₁, Μ₂, Μ₃, M₄It is possible to prevent the output accuracy from being deteriorated due to variations in the output. Mutual inductance Μ₁, M₂, M₃, M₄Since the variation of this occurs mainly at the time of manufacture and the change with time is extremely small, once the adjustment is made, it is almost unnecessary to readjust for a long time thereafter.
[0079]
According to this embodiment, in addition to the same effects as those of the third embodiment, the output accuracy of the second-order differential gradiometer is reduced due to slight variations in mutual inductance between the feedback coil of the SQUID and the superconducting ring. Even in the case of deterioration, it is possible to compensate for this by an amplifier circuit and an adder circuit capable of gain adjustment. Therefore, the output accuracy of the second order differentiation can be further increased, and there is a feature that the adjustment work for that purpose does not need to be performed frequently.
[0080]
In the present embodiment, the case where the newly added amplifier circuit and adder circuit are electrically configured has been exemplified, but the present invention is not necessarily limited thereto. As another configuration example, software processing may be used. When amplification and addition are performed by software processing, gain adjustment for amplification can be easily performed by software processing, so that special gain adjustment means as shown in FIG. 7 is not necessary, and the circuit configuration is greatly simplified. There is an effect that can be done.
[0081]
(Sixth embodiment)
A sixth embodiment will be described with reference to FIG. The SQUID magnetometer of this embodiment is configured as an electrical second-order differential type gradiometer 1 using three magnetometers. This gradiometer constitutes another basic configuration of the second-order differential gradiometer according to the present invention.
[0082]
The secondary differential gradiometer 1 includes three magnetometers 11 to 13, and each magnetometer includes three SQUIDs 11a (to 13c) and an FLL circuit 11c (to 13c) as a drive circuit. In each magnet meter, gain β₁(~ Β₃) Feedback amplifier 11c (˜13c), a current proportional to the output voltage is returned to each feedback coil 11b (˜13b).
[0083]
In addition to this, the output voltage V of the first magnetometer 11₁Is gain β ′₁Are applied to an adder 52 inserted in the feedback loop of the second magnetometer 12. Also, the output voltage V of the third magnetometer 13₃Is gain β ′₃Are amplified by the feedback amplifier 53 and applied to the adding circuit 52. The output voltages of the first to third magnetometers added by the adder 52 are given to the feedback coil 12 b of the second magnetometer 12.
[0084]
As a result, the first and third magnetometers 11 and 13 operate as normal magnetometers, while their output voltage V₁, V₃Β₁, Β₃The doubled magnetic flux is applied to the SQUID 12a of the second magnet meter 12 together with the magnetic flux proportional to the output of the second magnet meter itself. Therefore, the output voltage V of the second magnet meter 12₂Is
[Equation 19]

It is expressed as
[0085]
Therefore, β ′₁/ Β₁, Β ′₃/ Β₃By making each ½, an output corresponding to the second derivative can be obtained. In the case of this circuit configuration, in order to obtain the output of the first derivative, it is necessary to perform the difference calculation of the output voltages of the first and third magnetometers, but on the other hand, the SQUID and the five previously required SQUID and Since only three sets of drive circuits are required, there is an advantage that the configuration can be easily simplified.
[0086]
FIG. 9 shows an example in which the above-described principle second-order differential gradiometer 1 of FIG. 8 is more specifically configured. This configuration can use the same method as in the above-described embodiments, and can obtain the same effects as those described above, except that the first-order differential output cannot be obtained directly.
[0087]
Specifically, the feedback loops of the magnetometers 11 to 13 include constant current circuits 11e to 13e (gain α₁~ Α₃) Are inserted respectively. Moreover, in each of the magnetometers 11 to 13, two feedback coils for the SQUID 11a (to 13a) are provided.
[0088]
Feedback coils 11b 'and 11 "of the first magnetometer 11 (the mutual inductance is M₁, M '₁) Are connected in series with each other to receive feedback current of their own output, and the series-connected coil further includes two feedback coils 12b ′ and 12 ″ of the second magnetometer 12 (the mutual inductance is M₂, M '₂) Is connected to one of 12b '. Similarly, feedback coils 13b ′ and 13 ″ of the third magnetometer 13 (the mutual inductance is M₃, M '₃) Are connected to each other in series and receive a feedback current of their own output, and the series-connected coil is further connected to one feedback coil 12 b ′ of the second magnetometer 12. In the second magnetometer 12, a current proportional to its own detection output is fed back to the remaining one feedback coil 12b ".
[0089]
The output of this gradiometer 1 is given from the second magnetometer 12, and its value is
[Expression 20]

It is represented by If the mutual inductances M of the feedback coils provided for two SQUIDs to the superconducting ring are all the same, an output corresponding to the second derivative can be obtained.
[0090]
As in the second embodiment, since a subtracting circuit or the like is not used to obtain a secondary differential output, it is possible to measure the secondary gradient component of the magnetic field with high accuracy.
[0091]
(Seventh embodiment)
A seventh embodiment will be described with reference to FIG. The SQUID magnetometer of this embodiment is configured as an electrical third-order differential type gradiometer as a third-order or higher-order gradiometer.
[0092]
FIG. 10 shows one basic configuration of the third-order differential type gradiometer 1. This gradiometer 1 includes eight magnetometers 11 to 14 and 61 to 64. Among these, the four magnetometers 11 to 14 shown on the left side of the drawing are configured in the same way as in the first embodiment described above. The remaining four magnet meters 61 to 64 shown on the right side of the drawing are configured similarly to the four magnet meters 11 to 14 on the left side. Furthermore, the detection output V of the fourth magnet meter 14₄Is supplied to the feedback loop of the eighth magnet meter 64 via the feedback amplifier 70.
[0093]
Therefore, the first, third, fifth and seventh magnetometers 11, 13, 61 and 63 have a magnetometer output V.₁, V₃, V₅And V₇, First and second differential outputs V to the second and sixth magnetometers 12 and 62.₂And V₆The second derivative output V is output to the fourth magnet meter 14.₄And the third derivative output V to the eighth magnet meter 64₈Can be obtained.
[0094]
(Eighth embodiment)
An eighth embodiment will be described with reference to FIG. The SQUID magnetometer of this embodiment is also configured as an electrical third order differential gradiometer as a third or higher order gradiometer.
[0095]
FIG. 11 shows another basic configuration of the third-order differential type gradiometer 1. This gradiometer 1 includes four magnetometers 11-14. The basic circuits including the SQUID, feedback coil, FLL circuit, and feedback amplifier of the magnetometers 11 to 14 are the same as those described so far.
[0096]
In addition, an adder circuit 71 is inserted in the feedback loop of the third magnetometer 13, and the adder circuit 71 has its own detection output V.₃In addition to the detection output V of the second magnetometer 12₂The feedback amplifier 72 (gain β ′₂) And the detection output V of the fourth magnet meter 14₄Feedback amplifier 73 (gain β ′₄) To be added via. Further, another adder circuit 75 is inserted in the feedback loop of the fourth magnetometer 14 so that its own detection output V₄Current multiplied by the gain and the detection output V of the first magnetometer 11₁The feedback amplifier (gain β ′₁) Is added to the current multiplied by the gain and given to the feedback coil 14b.
[0097]
Therefore, with respect to gain, β ′₁/ Β₁And β ′₂/ Β₂1, β '₄/ Β₄Is set to a constant such as 1/2 or 1/3, and the third-order differential output V is output from the third magnetometer 13.₃But
[Expression 21]

Is obtained. In this way, a third-order differential output can be obtained with only four sets of magnetometers.
[0098]
Similarly, a fourth-order or higher-order differential gradiometer according to the present invention can be configured.
[0099]
(Ninth embodiment)
A ninth embodiment will be described with reference to FIG. The SQUID magnetometer of this embodiment relates to the improvement of the feedback circuit.
[0100]
In a SQUID magnetometer using a high-temperature superconducting SQUID, an AC bias circuit is often used to reduce low-frequency noise, and a modulation signal in addition to a feedback current is input to the feedback coil. . An example of the configuration of such a circuit is described in detail in Japanese Patent Publication No. 6-38103.
[0101]
FIG. 12 shows a preferred embodiment of a SQUID magnetometer using such a circuit configuration. The SQUID magnetometer 1 includes a plurality of magnetometers 91, 92,..., And each magnetometer includes a high-temperature superconducting ring 91a (92a), a feedback coil 91b (92b), a drive circuit 91c (92c), and a feedback system. A constant current circuit 91d (92d) for voltage / current conversion is provided. Each of the drive circuits 91c and 92c includes an amplifier Da, a demodulator Db, and an integrator Dc in this order from the input side. The feedback coils 91b and 92b of the superconducting rings 91a and 92a are connected in series to each other while being connected in series to their own feedback loops.
[0102]
A plurality of magnetometers 91, 92,... Are equipped with a common AC bias circuit 93. The AC bias circuit 93 includes an oscillator 94, a frequency divider 95, an exclusive OR circuit 96, and a constant current circuit 97.
[0103]
This AC bias circuit 93 is provided with only one oscillator 94 for generating a modulation signal for a plurality of magnetometers 91, 92,... Connected in series with feedback coils 91b, 92b,. A voltage / current conversion is performed by the constant current circuit 97, and the converted current is commonly supplied to series-connected feedback coils 91b, 92b,. In addition, each of the drive circuits 91c, 92c amplifies the output voltage of each of the SQUIDs 91a, 92a,. Features. This circuit configuration is applied to each of the first to eighth embodiments described above, and the feedback current is generated by the method described in each of the first to eighth embodiments.
[0104]
According to the present embodiment, remarkable low-frequency noise can be effectively suppressed in the high-temperature superconducting SQUID, so that it can be suitably applied to the high-temperature superconducting SQUID. In addition, each drive circuit is supplied with a modulation signal generated by a common oscillator and demodulated by a reference signal based on the common oscillator, causing distortion in the output voltage due to unintended interaction between channels. Even so, since the distortion always acts between channels, this interaction appears as distortion similar to the variation of each channel. For this reason, it is possible to measure and correct by the method described in the fifth embodiment. Therefore, by implementing in combination with the fifth embodiment, there is an effect that the magnetic field gradient can be measured with higher accuracy by eliminating the influence of the channel interaction.
[0105]
In the above-described embodiment, the gradiometer using an electric circuit has been described as an example. However, the main components of the present invention can be executed in software, and in that case, the gradiometer is also electrically configured. The same effect can be obtained.
[0106]
【The invention's effect】
As described above, according to the SQUID magnetometer of the present invention, it is not necessary to provide an extra magnetometer for reference, and a secondary or higher-order gradiometer can be configured electrically or in software. Therefore, it is not necessary to frequently adjust the compensation current using a high-temperature superconducting SQUID in which a spatially three-dimensional pickup coil is difficult to construct, and a high-order differential gradiometer that realizes a high removal rate of the environmental magnetic field Can be provided. Therefore, an inexpensive and easy-to-operate biomagnetic measuring device can be provided.
[Brief description of the drawings]
FIG. 1 is a configuration diagram showing an outline of a second-order differential type gradiometer as a SQUID magnetometer according to a first embodiment of the present invention.
FIG. 2 is a configuration diagram showing an outline of a second-order differential type gradiometer as a SQUID magnetometer according to a second embodiment of the present invention.
FIG. 3 is a block diagram showing an outline of a second-order differential type gradiometer as a SQUID magnetometer according to a third embodiment of the present invention.
FIG. 4 is a circuit diagram of a constant current circuit used for a feedback circuit.
FIG. 5 is a block diagram showing an outline of a second-order differential type gradiometer as a SQUID magnetometer according to a fourth embodiment of the present invention.
FIG. 6 is a block diagram showing an outline of a second-order differential type gradiometer as a SQUID magnetometer according to a fifth embodiment of the present invention.
FIG. 7 is a circuit diagram illustrating a gain adjustment method.
FIG. 8 is a configuration diagram showing another basic form of a second-order differential type gradiometer as a SQUID magnetometer according to a sixth embodiment of the present invention.
9 is a configuration diagram showing a specific example of the second-order differential type gradiometer of FIG. 8. FIG.
FIG. 10 is a block diagram showing an example of a third-order differential type gradiometer as a SQUID magnetometer according to a seventh embodiment of the present invention.
FIG. 11 is a block diagram showing another example of a third-order differential type gradiometer as a SQUID magnetometer according to the eighth embodiment of the present invention.
FIG. 12 is a block diagram showing a schematic configuration of a SQUID magnetometer according to a ninth embodiment of the present invention.
FIG. 13 is a configuration diagram of a pickup coil and a gradiometer used for explaining a conventional technique.
[Explanation of symbols]
1 Second derivative gradiometer (SQUID magnetometer)
11-14 Magnetometer
11a-14a SQUID (superconducting ring)
11b-14b Feedback coil
11c-14c FLL circuit
11d-14d feedback amplifier
11e-14e constant current circuit
11'b, 11 "b, 12'b, 12" b, 14'b, 14 "b Feedback coil
17-19 Feedback amplifier
21 Constant current circuit
22, 23 Constant current circuit
30, 32, 34 Amplifier circuit
31, 33, 35 Adder circuit
40 computers
44 D / A conversion circuit
45 Multiplier circuit
46 Adder circuit
47 Signal processing circuit
48 Magnetic field generating coil
51,53 Feedback amplifier
52 Adder circuit
61-64 Magnet meter
71,75 Adder circuit
72, 73, 74 Feedback amplifier
91,92 Magnet meter
91a, 92b high temperature superconducting ring
91a, 92b feedback circuit
93 AC bias circuit

Claims (9)

  Three or more SQUIDs, a drive circuit that independently drives each of the three or more SQUIDs, and a magnetic flux that is attached to each of the three or more SQUIDs and is proportional to the output of each of the drive circuits In a SQUID magnetometer having a feedback coil for feeding back a feedback magnetic flux having a magnitude proportional to the output of the other SQUID drive circuit to at least one SQUID of the three or more SQUIDs. A means for applying the SQUID via the feedback coil and a feedback magnetic flux having a magnitude proportional to the output of the SQUID drive circuit other than these SQUIDs to the at least one SQUID via the SQUID feedback coil. Characterized by comprising means for applying additively UID magnetometer.   2. The SQUID magnetometer according to claim 1, wherein a plurality of feedback coils are provided for the at least one SQUID, and at least one of the feedback coils is connected in series with the feedback coil of the other SQUID. A SQUID magnetometer.   2. The SQUID magnetometer according to claim 1, wherein an output terminal of each of the SQUID drive circuits is connected to each of the three or more SQUID feedback coils via a constant current circuit, and a plurality of feedback coils among the plurality of feedback coils. A specific feedback coil is connected in series with the feedback coil of another SQUID, and the connection point of the drive circuit connected to one SQUID of the other series-connected feedback coil pairs is connected to this series connection point. A SQUID magnetometer in which an output end is connected via another constant current circuit.   4. The SQUID magnetometer according to claim 3, wherein the another constant current circuit connected to the connection point is a circuit that supplies a current proportional to an output voltage of the drive circuit.   4. The SQUID magnetometer according to claim 3, wherein a feedback coil of another SQUID is connected to the connection point.   2. The SQUID magnetometer according to claim 1, further comprising: an adder that adds a signal having a magnitude proportional to an output value of a plurality of other drive circuits to an output of one of the plurality of drive circuits. The SQUID magnetometer is characterized in that the conversion coefficients can be individually changed.   2. The SQUID magnetometer according to claim 1, wherein means for converting the output signals of the plurality of drive circuits into digital values and arithmetic means for weighting and adding the output signals of the drive circuits converted into digital values to each other. A SQUID magnetometer, characterized in that the weighting factor can be changed.   In a SQUID magnetometer comprising a plurality of SQUIDs, a drive circuit that independently drives each of the plurality of SQUIDs, and a feedback coil attached to each of the plurality of SQUIDs, each of the plurality of SQUIDs has its SQUID Means for applying a feedback magnetic flux having a magnitude proportional to the output of the drive circuit via the feedback coil of the SQUID, and a plurality of SQUIDs among the plurality of SQUIDs, and another SQUID Means for additively applying a feedback current supplied to the feedback coil, and at least one SQUID feedback coil of the plurality of SQUIDs to which the feedback current is additively applied. Proportional to output voltage of other SQUID drive circuit Means for further additive applied to the magnitude of the feedback current has, SQUID magnetometer, characterized in that a changeable means a conversion factor to the current from the output voltage.   9. The SQUID magnetometer according to claim 6, further comprising a magnetic field generating coil, and means for determining the changeable conversion coefficient based on a measured value of the magnetic field generated by the magnetic field generating coil. A SQUID magnetometer characterized by that.

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