Electricity: measuring and testing – Magnetic – Magnetometers
Reexamination Certificate
2000-08-10
2002-10-08
Strecker, Gerard R. (Department: 2862)
Electricity: measuring and testing
Magnetic
Magnetometers
C324S225000, C324S261000, C505S846000, C600S409000, C702S191000
Reexamination Certificate
active
06462540
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a magnetic field measuring apparatus using Superconducting Quantum Interference Device (SQUID) magnetometers (gradiometers) to sense extremely weak magnetic fields such as biomagnetic fields originating from a portion of a living body, e.g., heart, brain, etc., geomagnetism, and those involved in nondestructive testing, and particularly relates to such apparatus that achieves the cancellation of external magnetic noise that causes interference.
2. Description of Related Art
Magnetic field measuring apparatus using SQUID magnetometers (gradiometers) is applied to measurements of extremely weak magnetic fields in such a manner that magnetic fields in the brain, heart, etc. are measured in a magnetically shielded room featuring a rate of attenuating magnetic fields of 40 to 50 decibels (dB) or more. As a combination of pickup coils to sense biomagnetic fields, first-order gradiometers are often used that detect a differential signal occurring between a first one-turn pickup coil and a second coil wound in a direction opposite to the first coil. The first-order gradiometers are characterized in that they cancel external magnetic noise from a remote noise source, while can detect signals from a magnetic field generated in the vicinity of the object to be inspected, such as, heart, brain, etc., without causing significant cancellation, and therefore they can easily decrease the effect of the external magnetic noise. The first-order gradiometers generally attenuate uniform magnetic fields by about 40 to 50 dB.
The above-mentioned magnetically shielded room and gradiometers in combination can cancel external magnetic noise by about 80 to 100 dB or more. However, if a moving body such as an electric train or a motor vehicle passes rather near the magnetically shielded room, 50 to 100 meters away from the room, external magnetic noise several tens of times as large as a biomagnetic field originating from a living body may be observed. In order to cancel strong magnetic noise of this order, various methods have been attempted.
For example, a method has been proposed by which a flux-gate magnetometer is placed outside the magnetically shielded room and signals detected by it from a magnetic field are used to allow a feedback current to flow through compensating coils, or noise-canceling coils wound around the outside wall of the room, aiming at compensation to adjust the output of the flux-gate magnetometer theoretically to 0 (related art 1: Meas. Sci. Technol. Vol. 2, pp. 596-601 [1991]).
Another noise cancellation method by using SQUID sensors (high-order gradiometers) formed through software has been devised, wherein signals detected by reference sensor coils are used (related art 2: Clin. Phys. Physiol. Meas., Vol. 12, Suppl. B, pp. 81-86 [1991]). This method uses a low-order, hardware gradient detector which is then processed relative to a common reference system to form a higher-order gradient response, though the detail of the reference system is not stated.
Japanese Patent Prepublication No. Hei 11-47108 (related art 3) describes a biomagnetic field measuring apparatus for decreasing the error of environmental magnetic noise elimination due to the lost balance of the coils employed. According to this publication, the biomagnetic field measuring apparatus is summarized below. A plurality of coils that artificially generate an incoming magnetic field are installed in known position on, for example, the external spherical surface of a dewar. Driving current is allowed to flow through the coils to drive them. By using the values that the pickup coils detected as the strength of this magnetic field generated by the coils and the driving current value, the pickup coils sensitivities and their balance are measured with high accuracy. The biomagnetic measurements obtained from the values detected by the pickup coils are compensated with high accuracy for the effect of the lost coil balance. As a result, the error of environmental magnetic noise elimination caused by the lost balance of the coils is decreased and the accuracy of biomagnetic measurements is improved. In the vacuum adiabatic enclosure, a plurality of biomagnetic field measuring pickup coils that primarily measure biomagnetic fields are installed near the physical object to be inspected and a plurality of reference sensor coils that primarily measure environmental magnetic fields are installed farther than the pickup coils away from the object.
Japanese Patent Prepublication No. Hei 11-83965 (related art 4) describes a complex of an environmental noise canceling system and a magnetic measurement apparatus that enables highly precise measurement of extremely weak magnetic fields from the physical object of interest without being affected by environmental noise in different frequency bands. According to this publication, a plurality of SQUID gradiometers with different dynamic ranges and through rates sense different environmental magnetic fields with different field strengths in different frequency bands and measure electric signals corresponding to the sensed environmental magnetic fields. The noise canceling system can cancel noise of environmental magnetic fields with different field strengths in different frequency bands, by generating a magnetic field, through a pair of actively-shielded coils, in an opposite direction of environmental magnetic fields, based on addition electric signals obtained by adding all measured electric signals, or, by subtracting the addition electric signals from the magnetic signals measured by the SQUID gradiometers for magnetic measurement.
Japanese Patent Prepublication No. Hei 9-84777 (related art 5) describes a biomagnetic field measuring apparatus that can precisely probe a current dipole in deeper position among a plurality of current dipoles put together with one laid on top of another downward. According to this publication, the apparatus uses an array of pickup coils to detect magnetic flux originating from biomagnetic fields, the array formed by combining a plurality types of pickup coils that differ in gradient order or/and baseline.
Related art 1 aims at compensation to adjust the output of the flux-gate magnetometer placed outside the magnetically shielded room theoretically to 0 by flowing the feedback current through the compensating coils, or noise-canceling coils installed on the outside wall of the room. When this compensation method is applied to a multi-channel magnetic field measuring apparatus having SQUID magnetometers consisting of a plurality of differential-type pickup coils, or namely, first-order gradiometers, the coils of all channels may vary in the rate of noise cancellation. A problem arises that this variation of noise cancellation rate is difficult to rectify.
The document disclosing related art 2 simply describes the relevant data in a case where a magnetically shielded room is not used, but does not describe a specific configuration of the reference system using reference sensor coils.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a magnetic field measuring apparatus that enables external magnetic noise cancellation with precision, allows for variation in the noise cancellation rate and baseline among the differential-type pickup coils employed to make the channels of a multi-channel magnetic field measuring apparatus when executing noise cancellation, and makes it possible to eliminate distorted magnetic signal waveforms induced by the frequency property of a magnetically shielded room in which the apparatus is installed.
Terminology used herein is explained below:
“Differential-type pickup coils” mean the coils constituting a first-order or second-order gradiometer.
(1) Providing differential type pickup coils constitute a first-order gradiometer:
(1.1) A “gradiometer for detection” (first SQUID gradiometer) is a first-order SQUID gradiometer that detects the component in Z direction of magnetic noise from an external magne
Kandori Akihiko
Miyashita Tsuyoshi
Tsukada Keiji
Mattingly, Stanger & Malur
Strecker Gerard R.
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