CN115718273B - Device for measuring object magnetic susceptibility based on magnetic induction intensity and measuring method thereof - Google Patents
Device for measuring object magnetic susceptibility based on magnetic induction intensity and measuring method thereof Download PDFInfo
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Abstract
The invention discloses a device and a method for measuring the magnetic susceptibility of an object based on magnetic induction intensity, wherein the device comprises: the magnetic field mechanism comprises a magnetic field modulation coil, a background magnetic field compensation coil and a gradient magnetic field compensation coil, wherein the magnetic field modulation coil is used for applying a magnetic field to an object to be detected so as to generate an induction magnetic moment; the background magnetic field compensation coil is used for reducing background magnetic field and background noise of a measuring point, so that a magnetic field measuring mechanism works within a measuring range and improves the measuring signal-to-noise ratio; the gradient magnetic field compensation coil is used for compensating the gradient magnetic field at the magnetometer measurement point, so that the background magnetic field gradient of the magnetic field measurement point is reduced. The displacement mechanism is used for changing the distance between coils and adjusting the space position of the magnetometer to move the magnetometer to a region with a background magnetic field of zero, so that the measurement of an induced magnetic field is realized, and the measurement of the magnetic susceptibility of an object to be measured is further realized. The invention has the characteristics of simple and convenient measurement, quick measurement process and small influence on the magnetization of the object.
Description
Technical Field
The invention belongs to the field of magnetic precision measurement, and particularly relates to a device and a method for measuring the magnetic susceptibility of an object based on magnetic induction intensity.
Background
At present, space gravitational wave detection plans are developed in competition in countries of the world. The core component of the space gravitational wave detection is an ultra-low magnetic inspection quality. In the development and ground test stage of the inspection quality, the prepared susceptibility index of the inspection quality needs to be measured continuously to judge whether the susceptibility of the inspection quality meets the requirement of space gravitational wave detection. The prior art adopts the traditional and very mature torsion pendulum technology for measuring the magnetic susceptibility of the inspection mass, and the torsion pendulum has the characteristics of high measurement precision and low measurement noise. However, the torsion pendulum test process is complicated, and vacuum environment needs to be maintained during measurement, so that quick replacement of objects is not facilitated; the period of the signal to be measured is longer, so that the total measurement time is longer; the need to apply a large magnetic field to the object in order to achieve a certain accuracy may result in the object being magnetized.
Therefore, a new magnetic susceptibility measuring device and method are needed to be provided to solve the problems of complicated measuring process, long period and easy magnetization of the object in the prior art.
Disclosure of Invention
Aiming at the defects of the prior art, the invention aims to provide a device for measuring the magnetic susceptibility of an object based on magnetic induction intensity and a measuring method thereof, and the device has the characteristics of simple and convenient measurement, quick measuring process and small influence on the magnetization of the object.
To achieve the above object, in a first aspect, the present invention provides an apparatus for measuring susceptibility of an object based on magnetic induction, comprising a magnetic field mechanism, a magnetic field measuring mechanism, a displacement mechanism, and a processing mechanism, wherein,
The magnetic field mechanism comprises a current generating unit and coils C1-C4 which are coaxially arranged in sequence and have the same structural dimension, an object to be measured is placed between the coils C1 and C2, and the magnetic field measuring mechanism is arranged at the position where the midpoint of the connecting line of the axes of the coils C2 and C3 and the midpoint of the connecting line of the axes of the coils C1 and C4 coincide; the current generation unit is used for respectively providing alternating current modulation current with a certain frequency for the coils C1-C4; the coil C1 is used for generating an alternating magnetic field B s to enable an object to be tested to generate an alternating induction magnetic moment m i;
The displacement mechanism comprises a multi-degree-of-freedom displacement platform and a single-degree-of-freedom displacement platform, the multi-degree-of-freedom displacement platform is used for adjusting the spatial position of the magnetic field measurement mechanism to enable the axial magnetic field at the measurement point where the magnetic field measurement mechanism is located to be the lowest, and the single-degree-of-freedom displacement platform is used for adjusting the interval between coils to enable the axial magnetic field gradient at the measurement point to be the lowest; the coil C4 is used for generating a magnetic field which is in the same large reverse direction as the alternating-current magnetic field B s and counteracts the axial magnetic field generated by the coil C1 at the measuring point; the coils C2 and C3 are used for generating equal-magnitude reverse magnetic fields to counteract axial magnetic field gradients at the measuring points;
The processing mechanism is used for collecting a section of axial magnetic field data measured by the magnetic field measuring mechanism when the axial magnetic field and the axial magnetic field gradient at the measuring point are regulated to be the lowest, and carrying out spectrum analysis on the data to find a peak point B zf of the axial magnetic field data, wherein the peak point is an induction magnetic field B iz excited by alternating-current induction magnetic moment in space; according to the volume of the object to be measured, different setting formulas are selected, and the relative position vector R of the alternating-current magnetic field B s and the magnetic field measuring mechanism to the object to be measured is combined, so that the magnetic susceptibility of the object to be measured is calculated.
According to the device for measuring the magnetic susceptibility of the object based on the magnetic induction intensity, the object to be measured is placed on one side of the modulation coil, and alternating modulation current with a certain frequency is introduced into the modulation coil, so that the object can generate an induction magnetic moment signal with the same frequency, and a sample does not need to be hung, vibrated or periodically moved, and the device has the characteristics of high measurement speed and short measurement period; when the applied modulated magnetic field is of uT magnitude, the centimeter-level object with extremely low magnetic rate such as copper can generate a 10pT magnitude induction magnetic field signal, so that the measurement resolution of the high-precision magnetic sensor such as an atomic magnetometer is met, and the uT magnitude magnetic field is far smaller than the average level of the geomagnetic field, so that the problem that a sample is magnetized in the measurement process is basically not considered.
In one embodiment, the processing mechanism is further configured to collect a section of axial magnetic field data measured by the magnetic field measuring mechanism after the object to be measured is removed when both the axial magnetic field and the axial magnetic field gradient at the measuring point are adjusted to be the lowest, and perform spectrum analysis on the data to find a peak point B zf0 of the data; and then, the peak point B zf and the peak point B zf0 are subjected to difference to obtain an induced magnetic field B iz, different setting formulas are selected according to the volume of the object to be measured, and the relative position vector R of the alternating magnetic field B s and the magnetic field measuring mechanism to the object to be measured is combined, so that the magnetic susceptibility of the object to be measured is calculated.
In one embodiment, when the volume of the object to be measured is less than or equal to 5mm 3, the set formula is:
when the volume of the object to be measured is greater than 5mm 3, the set formula is:
Wherein μ 0 represents vacuum permeability; the alternating current induction magnetic moment m i generated by the object to be measured is equivalent to a single magnetic dipole positioned at the centroid of the object to be measured, and the relation between the alternating current induction magnetic moment m i and the magnetic susceptibility χ of the object to be measured is as follows: v represents the volume of the object to be measured.
In one embodiment, the magnetic field measurement mechanism employs a magnetometer; the single-degree-of-freedom displacement table comprises four displacement tables, coils C1-C4 are correspondingly arranged on the four displacement tables, wherein the two displacement tables for arranging the coils C1 and C4 and the two displacement tables for arranging the coils C2 and C3 are respectively connected to two ends of a precise screw rod, and screw pitches of screw threads at two ends of the two precise screw rods are identical but opposite in direction.
In one embodiment, the distance D 12 between the coils C1 and C2 is determined according to the size of the object to be measured; the distance D 23 between the coils C2 and C3 is determined according to the sizes of the magnetometer and the multi-degree-of-freedom displacement table; the distance between the object to be measured and the measuring point is determined according to the magnetic field measuring resolution of the magnetometer and the attenuation condition of the induction magnetic field along with the distance between the object to be measured and the magnetometer; the minimum axial magnetic field gradient at the measurement point is achieved by adjusting the ac modulation current ratio to coils C1 and C2, and the spacing D 14 of coils C1 and C4.
In one embodiment, the ac magnetic field B s is determined according to the minimum detectable induced magnetic moment δ mi of the article to be measured, the susceptibility measurement resolution, and the volume V of the article to be measured.
In one embodiment, the current generating unit includes a current generator, two varistors and two ampere meters, wherein the coil C1, one varistor, one ampere meter and the coil C4 are connected to form a first current branch, the coil C2, the other varistor, the other ampere meter and the coil C3 are connected to form a second current branch, and the first current branch and the second current branch are connected in parallel and then connected in series with the current generator; the connection sequence of the input and output interfaces of the coils C1-C4 is adjusted, so that the alternating current modulation currents which are introduced into the coils C1 and C4, the coils C1 and C2 and the coils C2 and C3 are equal and large in reverse directions, and the two varistors are used for adjusting the current ratio of the coils C1 and C2, so that the magnetic field gradient at the measuring point is the lowest.
In one embodiment, the frequency f.ltoreq.f s/2,fs of the alternating current modulation current is the sampling frequency of the magnetometer.
In one embodiment, the device further comprises a magnetic shielding mechanism, wherein the magnetic shielding mechanism adopts a magnetic shielding chamber, and the magnetic shielding mechanism, the magnetic field measuring mechanism and the displacement mechanism are all arranged in the magnetic shielding chamber; the object to be measured is arranged between the coils C1 and C2 through the objective table, and the objective table adopts a non-magnetic organic glass bracket and a table top.
In a second aspect, the present invention provides a method for measuring magnetic susceptibility by using the device for measuring magnetic susceptibility of an object based on magnetic induction intensity, including the following steps:
(1) Placing all parts including a magnetic field mechanism and a displacement mechanism in a magnetic shielding chamber, fixing a magnetometer probe on a table top of a multi-degree-of-freedom displacement table, finding the position of a coil axis through a laser measuring instrument, adjusting the z-axis of the magnetometer to coincide with the coil axis, fixing other degrees of freedom of adjustment, only keeping the degree of freedom in the z-direction, and only enabling the magnetometer to move on the z-axis in the subsequent adjustment process;
(2) Opening the magnetometer, recording and checking magnetic field measurement results in real time, wherein the magnetic field fluctuation in the magnetic shielding room and noise floor caused by measurement noise of the magnetometer are measured by the magnetometer, and represent the magnetic field resolution limit which can be measured by the measuring device;
(3) According to the volume size of an object to be measured and the required magnetic susceptibility measurement precision, determining proper coil spacing and the size of an alternating current magnetic field B s, wherein after the spacing D 23 of the coils C2 and C3 is determined, keeping the spacing unchanged, and changing the spacing ratio only by changing the spacing D 14 of the coils C1 and C4 in the subsequent operation, so as to adjust the size of the magnetic field gradient at the point P of a measurement point where a magnetometer is positioned; then determining the current of the coils according to the alternating current magnetic field B s applied as required, and electrifying all the four coils;
(4) Measuring a magnetic field B z at a point P along the axis direction of a coil by using a magnetometer, slowly moving the magnetometer along the z axis by using a multi-degree-of-freedom displacement table below the magnetometer, observing the indication of the magnetometer, if the absolute value of the magnetic field becomes large in the moving process, moving the magnetometer in the opposite direction, if the absolute value of the magnetic field becomes small in the moving process, moving the magnetometer in the same direction until the indication of the magnetometer reaches the minimum, representing that the magnetometer has reached the point with the lowest magnetic field intensity on the z axis, and then fixing the magnetometer at the point;
(5) Slowly adjusting the size of the coil interval D 14 by utilizing the rotation of the precise screw rod, observing the change of the absolute value of B z, and continuously adjusting the precise screw rod towards the direction if B z is gradually reduced; if B z is bigger, the screw rod is moved towards the opposite direction until B z obtains the minimum value, and at the moment, the distance adjustment of the coils is completed, so that the magnetic field gradient at the point P reaches the minimum value;
(6) Slowly moving the magnetometer again, observing the size of B z, repeating the operation steps of the step 2 until B z obtains the minimum value, and completing the operation of searching the minimum background magnetic field measuring point; then recording magnetic field data for a period of time, and if the magnitude of the background magnetic field signal and the background noise meet the requirement of measuring the magnetic susceptibility of the object, namely the background magnetic field noise is smaller than the magnitude of the induction magnetic field signal calculated according to the magnetic susceptibility measuring resolution, indicating that experimental conditions are already provided, and measuring the magnetic susceptibility of the object can be started;
(7) After the experimental conditions are provided, placing an object to be tested on the objective table, so that the center of the object to be tested is positioned between the position of a magnetic field peak value generated by the coil C1 and the position of the magnetometer; after fixing an object to be measured, recording magnetic field measurement data in real time, applying a sinusoidal current signal with a certain frequency to a coil, making a noise spectrum density curve according to the magnetic field data measured in the step 6, finding a frequency band with the lowest noise, and setting a magnetic field modulation frequency f in the frequency band;
(8) After recording magnetic field data for a period of time, removing the object to be measured from the objective table, and then continuously recording the data for a period of time; after the experiment is finished, processing the data, firstly removing unstable data generated in the process of removing the object to be detected, then dividing the data into two sections, performing spectrum analysis on the data before removing the object to be detected, finding that a peak value appears at a modulation frequency f, setting the size of the peak value as B zf, then performing spectrum analysis on the data after removing the object to be detected, marking the amplitude value at the frequency f as B zf0, wherein the smaller the peak value is, the better the compensation effect on the background magnetic field signal is, and ideally no signal peak appears at the position; thus, the magnitude of the magnetic field signal generated by the magnetic susceptibility of the object to be measured is: b iz＝Bzf－Bzf0, selecting different setting formulas according to the volume of the object to be measured, and combining the magnetic field B s and the relative position vector R from the magnetic field measuring mechanism to the object to be measured, and calculating to obtain the magnetic susceptibility of the object to be measured.
Drawings
FIG. 1 is a schematic structural diagram of an apparatus for measuring susceptibility of an object based on magnetic induction according to an embodiment of the present invention;
FIG. 2 is a schematic diagram of a magnetic field mechanism according to an embodiment of the present invention;
FIG. 3 is a graph of magnetic field gradient at point P as a function of current I 1 of coil C1 provided by an embodiment of the present invention; in the figure, configuration I is that only a coil C1 is electrified, configuration II is that only coils C1 and C2 are electrified, and configuration III is that all coils C1 to C4 are electrified;
FIG. 4 is a graph of magnetic field gradient at point P as a function of current ratio I 1/I2 provided by an embodiment of the present invention;
FIG. 5 is a graph of magnetic field gradient at point P as a function of the spacing D 14 between coils C1 and C4, as provided by an embodiment of the present invention.
Detailed Description
The present invention will be described in further detail with reference to the drawings and examples, in order to make the objects, technical solutions and advantages of the present invention more apparent. It should be understood that the specific embodiments described herein are for purposes of illustration only and are not intended to limit the scope of the invention.
In order to solve the problems of complicated measuring process, long measuring period and easy magnetization of an object in the traditional torsion pendulum technology measuring the magnetic susceptibility of the object, the invention provides a device for measuring the magnetic susceptibility of a weak magnetic object based on magnetic induction intensity, which is used for measuring the magnetic susceptibility of a weak magnetic object, such as testing quality, biological material or biological tissue, and the measuring principle is as follows:
An object to be measured is mounted on a platform made of a weak magnetic material, then alternating current is applied to a magnetic field modulation coil to generate an alternating current magnetic field B s, the alternating current magnetic field B s enables the object to be measured to generate an alternating current magnetic moment m i, an induction magnetic field B iz is excited in space by the magnetic moment, and the magnetic field B iz is measured through a high-precision magnetometer.
When the volume of the object to be measured is less than or equal to 5mm 3, the relationship between the induced magnetic field B iz generated by the object to be measured and the induced magnetic moment m i is:
in the formula, the induction magnetic moment m i of the object to be detected is equivalent to a single magnetic dipole positioned at the mass center of the object to be detected, and the relation between the induction magnetic moment m i and the magnetic susceptibility χ of the object is as follows: wherein V represents the volume of an object, mu 0 is vacuum magnetic permeability, R is a relative position vector from a magnetometer to the object to be measured, and B s is an external magnetic field applied to the object, namely a magnetic field generated by a coil, and the external magnetic field can be calculated by using the Bioshal law.
When the volume of the object to be measured is larger than 5mm 3, the spatial distribution scale of the magnetic dipoles is larger, so that the object to be measured cannot be equivalent to a single magnetic dipole, otherwise, larger calculation errors are brought, and the contribution of each magnetic dipole is integrated, so that an accurate result can be obtained. The moment generated by the single magnetic dipole is: The relationship between the induced magnetic field B iz generated by the object to be measured and the induced magnetic moment m i can be written as:
that is, as long as the induced magnetic field B iz is measured, the relative position vector R of the magnetometer and the alternating magnetic field B s generated by the coil is substituted into the above formula to calculate, so as to obtain the magnetic susceptibility χ of the object to be measured.
The invention provides a device for measuring the magnetic susceptibility of an object based on magnetic induction intensity according to the magnetic susceptibility measuring principle. The magnetic field measuring mechanism can adopt a high-precision magnetometer, and the processing mechanism can adopt a processor commonly used in the field. The invention utilizes the magnetometer to measure the induction magnetic field B iz generated by the object to be measured, then utilizes the processor to acquire the magnetic field B iz, and selects a corresponding formula (1) or formula (2)) according to the volume of the object to be measured, thereby calculating and obtaining the magnetic susceptibility of the object to be measured.
When the magnetic field modulation coil is used to generate the induction magnetic field B iz, the magnetic field modulation coil also generates the alternating magnetic field B s under the action of the alternating modulation current, so that the magnetic field measured by the magnetometer at the measurement point (point P) contains the alternating magnetic field B s besides the induction magnetic field B iz, and the measurement accuracy is affected. For this, in order to make the magnetic field measured by the magnetometer be the induced magnetic field B iz generated by the actual object to be measured, other magnetic fields are introduced, and the position of the magnetometer is adjusted to offset the axial magnetic field and the axial magnetic field gradient of the magnetic field modulation coil at the point P, so that the background magnetic field and the magnetic field noise floor measured by the magnetometer after being placed at the point P are the lowest, and the induced magnetic field B iz generated by the object is measured under a higher signal-to-noise ratio.
Therefore, the magnetic field mechanism provided by the embodiment comprises the current generating unit and coils C1-C4 which are coaxially arranged in sequence and have the same structural dimension, an object to be measured is placed between the coils C1 and C2, and the magnetometer is placed at the position where the midpoint of the connecting line of the axes of the coils C2 and C3 and the midpoint of the connecting line of the axes of the coils C1 and C4 coincide.
The current generating unit is used for providing alternating current modulation current with a certain frequency to the coils C1-C4 respectively. The coil C1 is a magnetic field modulation coil and is used for generating an alternating magnetic field B s under the action of alternating modulation current, so that an object to be measured generates an alternating induction magnetic moment m i. The coil C4 is a background magnetic field compensation coil, and is connected with the coil C1 by an equal-large reverse alternating current modulation current to generate a background magnetic field which is equal-large reverse to the alternating current magnetic field B s so as to offset the axial magnetic field generated by the coil C1 at the P point where the magnetometer is located, so that the axial magnetic field at the P point tends to zero. The coils C2 and C3 are gradient magnetic field compensation coils, and are used for introducing equal-magnitude reverse alternating current to generate equal-magnitude reverse magnetic fields so as to offset axial magnetic field gradients generated at the P point, so that the magnetic field gradients at the P point tend to zero, the magnetic field in the space near the P point is more uniform, the background magnetic field and the magnetic field noise floor measured by the magnetometer after the magnetometer is placed at the P point are lower, and the measurement of the induction magnetic field generated by an object is realized under higher signal-to-noise ratio.
It should be noted that, the current generating unit provided in this embodiment may use 4 current generators to provide the required ac modulation currents for the 4 coils respectively; it is also possible to use 2 current generators, one of which supplies ac modulation current to the coils C1 and C4, and the other current generator supplies ac modulation current to the coils C2 and C3 by adjusting the connection order of the input/output interfaces of the coils C1 and C4 when connecting, thereby changing the current flow direction and changing the direction of the generated magnetic field.
Preferably, in order to increase the measurement speed and simplify the measurement operation, the current generating unit provided in this embodiment may use 1 current generator, where the coils C1 and C4 are connected to form a branch, the coils C2 and C3 are connected to form a branch, and the two branches are connected in parallel and then connected in series with the current generator, and in order to facilitate adjustment of the current ratios of the two sets of coils, a varistor may be further disposed on each branch. In this embodiment, the alternating current modulation current is provided to the four coils by using 1 current generator, so that the currents in the four coils have the same phase, and the absolute magnitude of the magnetic field generated by the coils can be counteracted, and the fluctuation of the magnetic field caused by the fluctuation noise of the current source can be counteracted, so that the fluctuation noise of the modulation magnetic field is reduced.
The displacement mechanism provided by the embodiment comprises a multi-degree-of-freedom displacement table and a single-degree-of-freedom displacement table, wherein the multi-degree-of-freedom displacement table is used for adjusting the spatial position of the magnetometer so as to enable the axial magnetic field at the P point where the magnetometer is positioned to be the lowest; the single-degree-of-freedom displacement table is used for adjusting the distance between the coils C1-C4, so that the axial magnetic field gradient at the point P is the lowest.
The processing mechanism provided in this embodiment is configured to execute the following flow: (1) When the axial magnetic field and the axial magnetic field gradient at the point P are regulated to be the lowest, acquiring a section of axial magnetic field data measured by a magnetometer, and carrying out spectrum analysis on the data to find a peak point B zf of the data, wherein the peak point is an induction magnetic field B iz of alternating current induction magnetic moment excited in space; (2) And selecting a corresponding calculation formula (1) or formula (2)) according to the volume of the object to be measured, and calculating the magnetic susceptibility of the object to be measured by combining the alternating magnetic field B s and the relative position vector R of the magnetic field measuring mechanism to the object to be measured.
Further, in order to provide accuracy and reliability of the measurement result, the processing mechanism provided in this embodiment may be further configured to collect a section of axial magnetic field data measured by the magnetometer after the object to be measured is removed when both the axial magnetic field and the axial magnetic field gradient at the P point are adjusted to be the lowest, and perform spectrum analysis on the data to find a peak point B zf0 of the data, where the smaller the peak value is, the better the compensation effect on the background magnetic field signal is represented, and ideally no signal peak occurs at the peak point; then, the peak point B zf and the peak point B zf0 are subjected to difference to obtain an induced magnetic field B iz, a corresponding calculation formula (1) or formula (2) is selected according to the volume of the object to be measured, and the relative position vector R of the alternating magnetic field B s and the magnetic field measuring mechanism to the object to be measured is combined to calculate the magnetic susceptibility of the object to be measured.
According to the device for measuring the magnetic susceptibility of the object based on the magnetic induction intensity, the object to be measured is placed on one side of the modulation coil, and alternating modulation current with a certain frequency is introduced into the modulation coil, so that the object can generate an induction magnetic moment signal with the same frequency, and a sample does not need to be hung, vibrated or periodically moved, and the device has the characteristics of high measurement speed and short measurement period; when the applied modulated magnetic field is of uT magnitude, the centimeter-level object with extremely low magnetic rate such as copper can generate a 10pT magnitude induction magnetic field signal, so that the measurement resolution of the high-precision magnetic sensor such as an atomic magnetometer is met, and the uT magnitude magnetic field is far smaller than the average level of the geomagnetic field, so that the problem that a sample is magnetized in the measurement process is basically not considered.
The device for measuring the magnetic susceptibility of the object based on the magnetic induction intensity provided by the invention is described in detail below with reference to the specific embodiments.
As shown in FIG. 1, the present embodiment provides a magnetometer-based device for measuring the susceptibility of an object, comprising a magnetic field shielding mechanism, an object stage 10, a magnetic field mechanism, a single degree of freedom displacement stage, a mounting plate 50, a magnetometer 30 and a multiple degree of freedom displacement stage 40.
The magnetic field shielding mechanism comprises a magnetic shielding chamber and is used for shielding background magnetic fields such as geomagnetic fields and the like and reducing magnetic field measurement noise floor.
The objective table 10 is a non-magnetic organic glass bracket and a table top, and is used for placing an object to be measured.
As shown in fig. 2, the magnetic field mechanism comprises an electric current generator, two varistors, two ampere meters and two pairs of coaxial circular coils (C1, C4, C2 and C3) with equal radius, wherein the coil C1, one varistor, one ampere meter and the coil C4 are connected to form a first electric current branch, the coil C2, the other varistor, the other ampere meter and the coil C3 are connected to form a second electric current branch, and the first electric current branch and the second electric current branch are connected in parallel and then are connected with the electric current generator in series.
By adjusting the connection sequence of the input and output interfaces of the coils C1 and C4, the coils C1 and C4 are led with equal and opposite currents, namely I 1＝﹣I4, and a point with zero axial magnetic field Bz is generated at the midpoint of the connecting line of the axes of the two coils, which is called point P. Coil C2 is placed on the right side of coil C1, coil C3 is placed on the left side of coil C4, and the midpoint of the axis line between C2 and C3 coincides with the above-mentioned point P. Thus, a magnetic field modulation mechanism with functions of magnetic field generation, background magnetic field compensation and gradient magnetic field compensation is formed.
The four coils from left to right in fig. 2 are C1, C2, C3, C4, respectively. Wherein, C1 is a magnetic field modulation coil for generating a magnetic field required by an object to generate an induced magnetic moment. C4 is a background magnetic field compensation coil for canceling the axial magnetic field generated by the coil C1 at the point P, so that the magnetic field at the point P tends to zero. The coils C2 and C3 are gradient magnetic field compensation coils, and the connection sequence of the input and output interfaces of the coils C2 and C3 is adjusted to enable the coils to be fed with equal-magnitude reverse current, namely I 2＝﹣I3, and the coils are used for counteracting the axial magnetic field gradient generated at the P point, so that the magnetic field gradient at the P point tends to zero, the magnetic field in the space near the P point is more uniform, the background magnetic field and the magnetic field background noise measured by the magnetometer after the magnetometer is placed at the P point are lower, and the measurement of the induction magnetic field generated by an object is realized under the condition of higher signal-to-noise ratio.
The two varistors are used to vary the ratio of the currents of the outer and inner coils, i.e., I 1/I2, and when the coil spacing is determined, the magnetic field gradient at point P is determined entirely by the magnitude of I 1 and the value of I 1/I2. Assuming I 2 is 1A, then with I 1/I2＝I1, the magnetic field gradient at point P can be plottedAs shown in fig. 3, a curve of the change of the current ratio according to I 1 shows that a zero point exists in the gradient, and the current value I 1＝I10 at this time is set. The current I 1 was regulated by a rheostat while the magnitude I 1t of I 1 was checked in real time with an ammeter. Let δ=i 1t-I10, the higher the tuning accuracy of the varistor, the smaller δ will be, and the closer the magnetic field gradient at point P can be made to be zero.
The single-degree-of-freedom displacement table comprises four displacement tables, coils C1-C4 are correspondingly arranged on the four displacement tables, wherein the two displacement tables for arranging the coils C1 and C4 and the two displacement tables for arranging the coils C2 and C3 are respectively connected to two ends of a precise screw rod 20, and screw pitches of screw threads at two ends of the two precise screw rods 20 are identical but opposite in direction. Therefore, when the screw rod rotates, the two displacement tables move simultaneously, the distances from the center of the screw rod are always equal, and the coil can always keep coaxiality and symmetry to the P point in the moving process. The spacing of the magnetic fields C1 and C4 and the spacing of the coils C2 and C3 can thus be adjusted. Theoretical calculations show that when the ratio of the currents of the two sets of coils, i.e., I 1/I2, is determined, what value the magnetic field gradient at point P takes is determined by the ratio of the spacing D 14 of coils C1, C4 to the spacing D 23 of coils C2, C3, i.e., D 14/D23. Firstly, theoretical calculation can give the ratio of the coil spacing when the magnetic field gradient at the P point is zero, but because of errors in measurement of experimental parameters, the actual ratio of the spacing deviates from a theoretical value, after the coil spacing is adjusted to a theoretical reference value, fine adjustment is further needed by means of a screw rod, a magnetic field measurement result is observed in real time in the adjustment process, and when the actually measured background magnetic field and noise value are reduced to meet expected indexes, the adjustment of the coil spacing is completed. The higher the distance adjustment precision of the screw rod is, the lower the magnetic field gradient value at the P point can be. By precisely adjusting the ratio of the intervals between the magnetic field compensation coil and the gradient compensation coil, the magnetic field gradient value at the magnetic field measurement point can be infinitely close to zero, so that a target with sufficiently small magnetic field gradient at the magnetic field measurement point is realized, and the magnetic field intensity at the measurement point P is sufficiently low and the signal to noise ratio is sufficiently high.
The mounting base 50 is used to fix the displacement stage on which the coil and magnetometer 30 are placed and the stage 10 on which the object to be measured is placed.
The magnetometer 30 is used for measuring the magnetic induction intensity excited by the magnetic moment of the object to be measured, the magnetometer 30 is placed on the multi-degree-of-freedom displacement table 40, the spatial position of the magnetometer is finely adjusted through the multi-degree-of-freedom displacement table 40, a measuring point with sufficiently small background magnetic field and magnetic field noise floor near the P point is found, then the magnetometer 30 is fixed at the measuring point, and the magnetic susceptibility of the object can be measured without changing the positions of the coil and the magnetometer in the subsequent process.
In the embodiment, the magnetic susceptibility of the object is determined by measuring the induction magnetic moment generated by the object to be measured under the action of the modulated magnetic field by using a magnetometer. In the actual measurement process, according to the magnetic field measurement resolution delta Bz of the magnetometer and the distance d between the object and the magnetometer, the minimum detectable induction magnetic moment delta mi (calculated according to the general formula) generated by the object can be calculated according to the formula (the general formula is not explained here) for generating the magnetic field by the magnetic dipoleSubstituting the required magnetic susceptibility measurement resolution delta χ to obtain the required minimum external magnetic field B s, thereby determining the minimum current required to be input to the coil. And then according to the size of the object to be measured, the range of values of parameters such as coil spacing, radius and the like can be determined, and then a set of satisfied magnetic field modulation device is processed according to the parameters.
In order to make the coil C4 achieve the purpose of counteracting the axial magnetic field and the magnetic field fluctuation generated by the coil C1 at the point P, the magnitudes of the magnetic fields generated by the coils C4 and C1 need to be equal and the directions are opposite, so the coils C1 and C4 are connected in series, and the direction of the magnetic field can be changed by changing the flow direction of the current by adjusting the connection sequence of the input-output interfaces of the coils. In order to meet the measurement requirements, it is necessary to ensure that the gap between the coils, i.e. between the coils C1 and C2, can accommodate the object, so that the minimum distance of D 12 is determined according to the size of the object to be measured. The device for placing the magnetometer and moving the magnetometer is reserved between the coils C2 and C3, and meanwhile, the resolution of magnetic field measurement of the magnetometer and the attenuation of the induced magnetic field along with the distance are considered, so that the object cannot be far away from the point P, otherwise, the induced magnetic field generated by the object cannot be detected due to the fact that the resolution is smaller than that at the point P, and in general, the closer the object is to the point P, the higher the resolution of magnetic moment measurement is. In this way, the distance of the coils C2, C3 from the object point is substantially determined. After the spacing of the coils is determined, the ratio of the currents of the outer and inner coils, i.e., I 1/I2, needs to be equal to a determined value in order for the magnetic field gradients produced by coils C2, C3 and coils C1, C4 to cancel each other at point P. Therefore, the coils C1 and C4 are connected in series, the coils C2 and C3 are connected in series, then the coils C2 and C3 are connected in parallel, and a rheostat is respectively connected to the two current branches for changing the current ratio of the two groups of coils. However, in practical operation, the ratio of the currents needs to be very precisely adjusted, so that the magnetic field gradient at the point P can be theoretically set to zero. The ratio of the coil spacing is easily adjusted by the screw rod compared with the ratio of the current by adopting a rheostat. We therefore achieve this goal by adjusting the current ratio and coil spacing simultaneously. Theoretical analysis shows that the larger the ratio I 1/I2 of the current, the magnetic field gradient at the P point isThe larger the spacing D 14 of the gradient compensation coils C1, C4 required for zero. Meanwhile, as can be seen from FIG. 4, the greater D 14 is taken, the greater is the/>The smaller the gradient of the curve is, the less sensitive the magnetic field gradient is to the change of the coil spacing, so that the larger the error range of the D 14 value required by the magnetic field gradient value of zero is, and the positioning accuracy requirement of the screw rod can be reduced. However, taking too large a D 14 will result in a large current ratio I 1/I2, whereas I 2 is mainly used to generate the modulated magnetic field, and the magnitude of the modulated magnetic field needs to meet certain requirements, which means that the current I 1 needs to be increased to make D 14 larger, but the maximum current in the coil is limited by the maximum power that the coil can work normally, so in practical design, the value of D 14 should be taken to be larger under comprehensive consideration.
Specifically, the measuring device provided in this embodiment measures the magnetic susceptibility of an object by:
Step 1: each part comprising a coil mechanism, a single-degree-of-freedom displacement table, a multi-degree-of-freedom displacement table, a mounting base and a magnetometer is placed in a magnetic shielding room, then a magnetometer probe is fixed on the table top of the multi-degree-of-freedom displacement table, the position of a coil axis is found through a laser measuring instrument, then the z-axis of the magnetometer is adjusted to coincide with the coil axis, then other degrees of freedom of adjustment are fixed, only the degree of freedom of the z-direction is reserved, and the magnetometer is only required to move on the z-axis in the subsequent adjustment process.
Step 2: and opening the magnetometer, and recording and checking the magnetic field measurement result in real time. The magnetic field fluctuation in the magnetic shielding room and the noise floor caused by the measuring noise of the magnetometer represent the magnetic field resolution limit which can be measured by the measuring device.
Step 3: according to the volume size of an object to be measured and the required magnetic susceptibility measurement precision, a proper coil interval and the size of an alternating magnetic field B s are determined, wherein after the interval D 23 between the C2 and the C3 is determined, the interval is kept unchanged, and the interval ratio can be changed only by changing the interval D 14 between the coils C1 and C4 in the follow-up operation, so that the size of the magnetic field gradient at the point P is adjusted. The magnitude of the current flowing into the coils is determined according to the magnitude of the alternating current magnetic field B s which needs to be applied, so that all the four coils are electrified.
Step 4: and measuring a magnetic field B z at the point P along the axis direction of the coil by using a magnetometer, slowly moving the magnetometer along the z axis by using a displacement table below the magnetometer, observing the indication of the magnetometer, moving the magnetometer in the opposite direction if the absolute value of the magnetic field becomes large during movement, and moving the magnetometer in the same direction if the absolute value of the magnetic field becomes small during movement until the indication of the magnetometer reaches the minimum, representing that the magnetometer has reached the point with the lowest magnetic field intensity on the z axis, and then fixing the magnetometer at the point.
Step 5: the size of the coil interval D 14 is slowly adjusted by utilizing the rotation of the screw rod, the change of the size (absolute value) of B z is observed, and if B z is gradually reduced, the screw rod is continuously adjusted towards the direction; if B z becomes large, the screw is moved in the opposite direction until B z assumes a minimum. At this time, the distance between the coils is adjusted, so that the magnetic field gradient at the P point reaches the minimum value.
Step 6: the magnetometer is slowly moved again, the size of B z is observed, and the operation steps of step 2 are repeated until B z attains a minimum value. The operation of finding the minimum background magnetic field measurement point is completed. And then recording magnetic field data for a period of time, and if the magnitude of the background magnetic field signal and the background noise meet the requirement of measuring the magnetic susceptibility of the object, namely the background magnetic field noise is smaller than the magnitude of the induction magnetic field signal calculated according to the magnetic susceptibility measuring resolution, indicating that experimental conditions are already provided, and measuring the magnetic susceptibility of the object can be started.
Step 7: after the experimental conditions were met, the object was placed on the stage such that the center of the object was located between the position of the peak of the magnetic field generated by coil C1 and the position of the magnetometer. After the object is fixed, magnetic field measurement data are recorded in real time, and then a sine current signal with a certain frequency is applied to the coil, wherein the magnetic field signal generated by the coil can be expressed as: Assuming that the sampling rate of the magnetometer is f s, the frequency of the alternating current signal needs to meet f which is less than or equal to f s/2, simultaneously, a noise spectrum density curve is made according to the data measured in the step 6, a frequency band with the lowest noise is found, and the magnetic field modulation frequency f is selected in the frequency band.
Step 8: after recording the magnetic field data for a period of time, the object is removed from the stage and the recording of the data for a period of time is continued. Processing of the data begins after the experiment is completed. Firstly, removing unstable data generated in the object removing process, then dividing the data into two sections, carrying out spectrum analysis on the data before the object is removed, finding that a peak value appears at a modulation frequency f, setting the size of the peak value as B zf, then carrying out spectrum analysis on the data after the object is removed, marking the amplitude value at the frequency f as B zf0, wherein the smaller the peak value is, the better the compensation effect on a background magnetic field signal is represented, and ideally no signal peak appears at the position. Thus, the magnitude of the magnetic field signal generated by the susceptibility of the object is: and B iz＝Bzf－Bzf0, substituting the data into a related calculation formula of the magnetic susceptibility and the induced magnetic field, and obtaining the magnetic susceptibility of the measured object.
It will be readily appreciated by those skilled in the art that the foregoing description is merely a preferred embodiment of the invention and is not intended to limit the invention, but any modifications, equivalents, improvements or alternatives falling within the spirit and principles of the invention are intended to be included within the scope of the invention.
Claims (9)
1. The device for measuring the magnetic susceptibility of the object based on the magnetic induction intensity is characterized by comprising a magnetic field mechanism, a magnetic field measuring mechanism, a displacement mechanism and a processing mechanism, wherein,
The magnetic field mechanism comprises a current generating unit and coils C1-C4 which are coaxially arranged in sequence and have the same structural dimension, an object to be measured is placed between the coils C1 and C2, and the magnetic field measuring mechanism is arranged at the position where the midpoint of the connecting line of the axes of the coils C2 and C3 and the midpoint of the connecting line of the axes of the coils C1 and C4 coincide; the current generation unit is used for respectively providing alternating current modulation current with a certain frequency for the coils C1-C4; the coil C1 is used for generating an alternating magnetic field B s to enable an object to be tested to generate an alternating induction magnetic moment m i;
The displacement mechanism comprises a multi-degree-of-freedom displacement platform and a single-degree-of-freedom displacement platform, the multi-degree-of-freedom displacement platform is used for adjusting the spatial position of the magnetic field measurement mechanism to enable the axial magnetic field at the measurement point where the magnetic field measurement mechanism is located to be the lowest, and the single-degree-of-freedom displacement platform is used for adjusting the interval between coils to enable the axial magnetic field gradient at the measurement point to be the lowest; the coil C4 is used for generating a magnetic field which is in the same large reverse direction as the alternating-current magnetic field B s and counteracts the axial magnetic field generated by the coil C1 at the measuring point; the coils C2 and C3 are used for generating equal-magnitude reverse magnetic fields to counteract axial magnetic field gradients at the measuring points;
The processing mechanism is used for collecting a section of axial magnetic field data measured by the magnetic field measuring mechanism when the axial magnetic field and the axial magnetic field gradient at the measuring point are regulated to be the lowest, and carrying out spectrum analysis on the data to find a peak point B zf of the axial magnetic field data, wherein the peak point is an induction magnetic field B iz of the alternating current induction magnetic moment m i excited in space; selecting different set formulas according to the volume of the object to be measured, and combining the alternating-current magnetic field B s and the relative position vector R of the magnetic field measuring mechanism to the object to be measured to calculate and obtain the magnetic susceptibility of the object to be measured;
when the volume of the object to be measured is less than or equal to 5mm 3, the set formula is:
when the volume of the object to be measured is greater than 5mm 3, the set formula is:
Wherein μ 0 represents vacuum permeability; the alternating current induction magnetic moment m i generated by the object to be measured is equivalent to a single magnetic dipole positioned at the centroid of the object to be measured, and the relation between the alternating current induction magnetic moment m i and the magnetic susceptibility χ of the object to be measured is as follows: v represents the volume of the object to be measured.
2. The device for measuring magnetic susceptibility of an object based on magnetic induction according to claim 1, wherein the processing mechanism is further configured to collect a piece of axial magnetic field data measured by the magnetic field measuring mechanism after the object to be measured is removed when both the axial magnetic field and the axial magnetic field gradient at the measuring point are adjusted to be minimum, and perform spectrum analysis on the data to find a peak point B zf0; and then, the peak point B zf and the peak point B zf0 are subjected to difference to obtain an induced magnetic field B iz, different setting formulas are selected according to the volume of the object to be measured, and the relative position vector R of the alternating magnetic field B s and the magnetic field measuring mechanism to the object to be measured is combined, so that the magnetic susceptibility of the object to be measured is calculated.
3. The device for measuring the magnetic susceptibility of an object based on magnetic induction according to claim 2, wherein the magnetic field measuring mechanism employs a magnetometer; the single-degree-of-freedom displacement table comprises four displacement tables, coils C1-C4 are correspondingly arranged on the four displacement tables, wherein the two displacement tables for arranging the coils C1 and C4 and the two displacement tables for arranging the coils C2 and C3 are respectively connected to two ends of a precise screw rod, and screw pitches of screw threads at two ends of the two precise screw rods are identical but opposite in direction.
4. A device for measuring magnetic susceptibility of an object based on magnetic induction according to claim 3, wherein the distance D 12 between the coils C1 and C2 is determined according to the size of the object to be measured; the distance D 23 between the coils C2 and C3 is determined according to the sizes of the magnetometer and the multi-degree-of-freedom displacement table; the distance between the object to be measured and the measuring point is determined according to the magnetic field measuring resolution of the magnetometer and the attenuation condition of the induction magnetic field along with the distance between the object to be measured and the magnetometer; the minimum axial magnetic field gradient at the measurement point is achieved by adjusting the ac modulation current ratio to coils C1 and C2, and the spacing D 14 of coils C1 and C4.
5. The device for measuring magnetic susceptibility of an object based on magnetic induction according to claim 4, wherein the ac magnetic field B s is determined according to the minimum detectable induced magnetic moment δ mi of the object to be measured, the resolution of measuring magnetic susceptibility, and the volume V of the object to be measured.
6. The device for measuring magnetic susceptibility of an object based on magnetic induction according to claim 5, wherein the current generating unit comprises a current generator, two varistors and two ampere meters, wherein the coil C1, one varistor, one ampere meter and the coil C4 are connected to form a first current branch, the coil C2, the other varistor, the other ampere meter and the coil C3 are connected to form a second current branch, and the first current branch and the second current branch are connected in parallel and then connected in series with the current generator; the connection sequence of the input and output interfaces of the coils C1-C4 is adjusted, so that the alternating current modulation currents which are introduced into the coils C1 and C4, the coils C1 and C2 and the coils C2 and C3 are equal and large in reverse directions, and the two varistors are used for adjusting the current ratio of the coils C1 and C2, so that the magnetic field gradient at the measuring point is the lowest.
7. The device for measuring magnetic susceptibility of an object based on magnetic induction according to claim 6, wherein the frequency f.ltoreq.f s/2,fs of the ac modulation current is a sampling frequency of a magnetometer.
8. The device for measuring the magnetic susceptibility of an object based on magnetic induction according to claim 7, further comprising a magnetic shielding mechanism, wherein the magnetic shielding mechanism adopts a magnetic shielding chamber, and wherein the magnetic shielding mechanism, the magnetic measuring mechanism and the displacement mechanism are all arranged in the magnetic shielding chamber; the object to be measured is arranged between the coils C1 and C2 through the objective table, and the objective table adopts a non-magnetic organic glass bracket and a table top.
9. A method of measuring magnetic susceptibility using the apparatus for measuring magnetic susceptibility of an object based on magnetic induction according to claim 8, comprising the steps of:
(1) Placing all parts including a magnetic field mechanism and a displacement mechanism in a magnetic shielding chamber, fixing a magnetometer probe on a table top of a multi-degree-of-freedom displacement table, finding the position of a coil axis through a laser measuring instrument, adjusting the z-axis of the magnetometer to coincide with the coil axis, fixing other degrees of freedom of adjustment, only keeping the degree of freedom in the z-direction, and only enabling the magnetometer to move on the z-axis in the subsequent adjustment process;
(2) Opening the magnetometer, recording and checking magnetic field measurement results in real time, wherein the magnetic field fluctuation in the magnetic shielding room and noise floor caused by measurement noise of the magnetometer are measured by the magnetometer, and represent the magnetic field resolution limit which can be measured by the measuring device;
(3) According to the volume size of an object to be measured and the required magnetic susceptibility measurement precision, determining proper coil spacing and the size of an alternating current magnetic field B s, wherein after the spacing D 23 of the coils C2 and C3 is determined, keeping the spacing unchanged, and changing the spacing ratio only by changing the spacing D 14 of the coils C1 and C4 in the subsequent operation, so as to adjust the size of the magnetic field gradient at the point P of a measurement point where a magnetometer is positioned; then determining the current of the coils according to the alternating current magnetic field B s applied as required, and electrifying all the four coils;
(4) Measuring a magnetic field B z at a point P along the axis direction of a coil by using a magnetometer, slowly moving the magnetometer along the z axis by using a multi-degree-of-freedom displacement table below the magnetometer, observing the indication of the magnetometer, if the absolute value of the magnetic field becomes large in the moving process, moving the magnetometer in the opposite direction, if the absolute value of the magnetic field becomes small in the moving process, moving the magnetometer in the same direction until the indication of the magnetometer reaches the minimum, representing that the magnetometer has reached the point with the lowest magnetic field intensity on the z axis, and then fixing the magnetometer at the point;
(5) Slowly adjusting the size of the coil interval D 14 by utilizing the rotation of the precise screw rod, observing the change of the absolute value of B z, and continuously adjusting the precise screw rod towards the direction if B z is gradually reduced; if B z is bigger, the screw rod is moved towards the opposite direction until B z obtains the minimum value, and at the moment, the distance adjustment of the coils is completed, so that the magnetic field gradient at the point P reaches the minimum value;
(6) Slowly moving the magnetometer again, observing the size of B z, repeating the operation steps of the step 2 until B z obtains the minimum value, and completing the operation of searching the minimum background magnetic field measuring point; then recording magnetic field data for a period of time, and if the magnitude of the background magnetic field signal and the background noise meet the requirement of measuring the magnetic susceptibility of the object, namely the background magnetic field noise is smaller than the magnitude of the induction magnetic field signal calculated according to the magnetic susceptibility measuring resolution, indicating that experimental conditions are already provided, and measuring the magnetic susceptibility of the object can be started;
(7) After the experimental conditions are provided, placing an object to be tested on the objective table, so that the center of the object to be tested is positioned between the position of a magnetic field peak value generated by the coil C1 and the position of the magnetometer; after fixing an object to be measured, recording magnetic field measurement data in real time, applying a sinusoidal current signal with a certain frequency to a coil, making a noise spectrum density curve according to the magnetic field data measured in the step 6, finding a frequency band with the lowest noise, and setting a magnetic field modulation frequency f in the frequency band;
(8) After recording magnetic field data for a period of time, removing the object to be measured from the objective table, and then continuously recording the data for a period of time; after the experiment is finished, processing the data, firstly removing unstable data generated in the process of removing the object to be detected, then dividing the data into two sections, performing spectrum analysis on the data before removing the object to be detected, finding that a peak value appears at a modulation frequency f, setting the size of the peak value as B zf, then performing spectrum analysis on the data after removing the object to be detected, marking the amplitude value at the frequency f as B zf0, wherein the smaller the peak value is, the better the compensation effect on the background magnetic field signal is, and ideally no signal peak appears at the position; thus, the magnitude of the magnetic field signal generated by the magnetic susceptibility of the object to be measured is: b iz＝Bzf－Bzf0, selecting different setting formulas according to the volume of the object to be measured, and combining the magnetic field B s and the relative position vector R from the magnetic field measuring mechanism to the object to be measured, and calculating to obtain the magnetic susceptibility of the object to be measured.
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