CN115718273A - Device and method for measuring object magnetic susceptibility based on magnetic induction intensity - Google Patents
Device and method for measuring object magnetic susceptibility based on magnetic induction intensity 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 the object to be detected to enable the object to be detected to generate an induced magnetic moment; the background magnetic field compensation coil is used for reducing the background magnetic field and the background noise of the measuring point, so that the magnetic field measuring mechanism works within a measuring range and the measuring signal-to-noise ratio is improved; the gradient magnetic field compensation coil is used for compensating the gradient magnetic field at the measurement point of the magnetometer so that the background magnetic field gradient of the magnetic field measurement point is reduced. And the displacement mechanism is used for changing the distance between the coils, adjusting the spatial position of the magnetometer and moving the magnetometer to a region with zero background magnetic field, so that the measurement of the induction magnetic field is realized, and the measurement of the magnetic susceptibility of the object to be measured is further realized. The invention has the characteristics of simple and convenient measurement, fast measurement process and small influence on the magnetization of an object.
Description
Technical Field
The invention belongs to the field of magnetic precision measurement, and particularly relates to a device for measuring the magnetic susceptibility of an object based on magnetic induction intensity and a measuring method thereof.
Background
At present, all countries in the world compete to develop a space gravitational wave detection plan. The core component of the space gravitational wave detection is an ultralow-magnetic check mass. In the stages of the development and the ground test of the inspection quality, the magnetic susceptibility index of the prepared inspection quality needs to be continuously measured so as to judge whether the magnetic susceptibility of the inspection quality meets the detection requirement of the space gravitational wave. At the present stage, the traditional and mature torsion pendulum technology is adopted 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 a vacuum environment needs to be maintained during measurement, so that the object is not easy to quickly replace; the period of the signal to be measured is longer, so that the total measurement time is longer; to achieve a certain accuracy, a large magnetic field needs to be applied to the object, which may result in the object being magnetized.
Therefore, a new magnetic susceptibility measuring device and method need to be provided to solve the problems of the prior art that the measuring process is complicated, the period is long, and the object is easily magnetized.
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.
In order to achieve the above object, the present invention provides, in a first aspect, an apparatus for measuring magnetic 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 at intervals in sequence and have the same structure and size, 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 middle points of the axis connecting lines of the coils C2 and C3 and the middle point of the axis connecting line of the coils C1 and C4 coincide; the current generating unit is used for respectively providing alternating current modulation current with certain frequency to the coils C1-C4; coil C1 for generating an alternating magnetic field B s So that the object to be measured generates an AC induction magnetic moment m i ；
The displacement mechanism comprises a multi-degree-of-freedom displacement table and a single-degree-of-freedom displacement table, the multi-degree-of-freedom displacement table is used for adjusting the spatial position of the magnetic field measuring mechanism to enable the axial magnetic field at the measuring point where the magnetic field measuring mechanism is located to be the lowest, and the single-degree-of-freedom displacement table is used for adjusting the distance between the coils to enable the axial magnetic field gradient at the measuring point to be the lowest; wherein the coil C4 is used for generating an alternating current magnetic field B s The equal-magnitude reverse magnetic field counteracts the axial magnetic field generated by the coil C1 at the measuring point; the coils C2 and C3 are used for generating equal-size reverse magnetic fields and offsetting the axial magnetic field gradient at the measuring point;
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 both adjusted to be minimum, and carrying out spectrum analysis on the data to find a peak point B of the data zf The peak point is an induced magnetic field B excited by the AC induced magnetic moment in space iz (ii) a Selecting different set formulas according to the volume of the object to be measured, and combining with an alternating-current magnetic field B s And calculating the relative position vector R from the magnetic field measuring mechanism to the object to be measured to obtain the magnetic susceptibility of the object to be measured.
According to the device for measuring the object magnetic susceptibility based on the magnetic induction intensity, provided by the invention, the object to be measured is placed on one side of the modulation coil, and the alternating modulation current with a certain frequency is introduced into the modulation coil, so that the object can generate induced magnetic moment signals with the same frequency without suspending, vibrating or periodically moving a sample, and the device has the characteristics of high measurement speed and short measurement period; meanwhile, when the applied modulation magnetic field is of uT magnitude, centimeter-level objects with extremely low magnetic susceptibility such as copper and the like can generate 10pT magnitude induced magnetic field signals, so that the measurement resolution of high-precision magnetic sensors such as an atomic magnetometer is met, and the uT magnitude magnetic field is far smaller than the average level of the earth magnetic 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, when the axial magnetic field and the axial magnetic field gradient at the measurement point are both adjusted to be minimum, collect a section of axial magnetic field data measured by the magnetic field measurement mechanism after the object to be measured is removed, and perform spectrum analysis on the data to find a peak point B thereof zf0 (ii) a Then the peak point B is calculated zf And B zf0 The difference is made to obtain an induced magnetic field B iz Selecting different set formulas according to the volume of the object to be measured, and combining with the alternating current magnetic field B s And calculating the relative position vector R from the magnetic field measuring mechanism to the object to be measured to obtain the magnetic susceptibility of the object to be measured.
In one embodiment, when the volume of the object to be measured is less than or equal to 5mm 3 Then, the set formula is:
when the volume of the object to be measured is more than 5mm 3 Then, the set formula is:
in the formula, mu 0 Represents the vacuum permeability; AC induction magnetic moment m generated by object to be measured i The relationship between the single magnetic dipole equivalent to the mass center of the single magnetic dipole and the magnetic susceptibility χ of the object to be measured is as follows:
v denotes the volume of the object to be measured.
In one embodiment, the magnetic field measuring mechanism employs a magnetometer; the single-degree-of-freedom displacement platform comprises four displacement platforms, coils C1-C4 are correspondingly arranged on the four displacement platforms, the two displacement platforms provided with the coils C1 and C4 and the two displacement platforms provided with the coils C2 and C3 are respectively connected to two ends of a precise screw rod, and the thread pitches of the two ends of the two precise screw rods are the same but opposite in direction.
In one embodiment, the distance D between the coils C1 and C2 12 Determining according to the size of the object to be detected; spacing D of coils C2 and C3 23 Determining 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 condition that the induction magnetic field is attenuated along with the distance between the object to be measured and the magnetometer; the axial magnetic field gradient at the measuring point is minimized by adjusting the ratio of the alternating modulation currents fed into the coils C1 and C2 and the distance D between the coils C1 and C4 14 And (5) realizing.
In one embodiment, the alternating magnetic field B s Induced magnetic moment delta capable of being detected according to minimum of object to be detected mi Magnetic susceptibility measurement resolution and volume V determination of the object to be measured.
In one embodiment, the current generating unit comprises a current generator, two varistors and two current meters, wherein a coil C1, a varistor, one current meter and a coil C4 are connected to form a first current branch, a coil C2, the other varistor, the other current meter and a 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 with the current generator in series; and the connecting sequence of the input and output interfaces of the coils C1-C4 is adjusted to ensure that alternating modulation currents introduced into the coils C1 and C4, the coils C1 and C2 and the coils C2 and C3 are equal and opposite, and the two varistors are used for adjusting the ratio of the currents of the coil C1 and the coil C2, so that the magnetic field gradient at a measuring point is lowest.
In one embodiment, the frequency f ≦ f of the AC modulated current s /2，f s Is the sampling frequency of the magnetometer.
In one embodiment, the magnetic field shielding mechanism is a magnetic shielding chamber, and the magnetic field mechanism, the magnetic field measuring mechanism and the displacement mechanism are arranged in the magnetic shielding chamber; the object to be measured passes through the objective table setting between coil C1 and C2, just the objective table adopts nonmagnetic organic glass support and mesa.
In a second aspect, the present invention provides a method for measuring magnetic susceptibility by using the above-mentioned apparatus for measuring magnetic susceptibility of an object based on magnetic induction, comprising the following steps:
(1) Placing all components including a magnetic field mechanism and a displacement mechanism in a magnetic shielding room, fixing a magnetometer probe on the table top of a multi-freedom-degree displacement table, finding the position of a coil axis through a laser measuring instrument, adjusting the z axis of a magnetometer to be coincident with the coil axis, fixing other adjustment degrees of freedom, only keeping the degree of freedom in the z direction, and only moving the magnetometer on the z axis in the subsequent adjustment process;
(2) Opening the magnetometer, and recording and checking the magnetic field measurement result in real time, wherein the magnetic field fluctuation in the magnetic shielding room and the background noise caused by the measurement noise of the magnetometer represent the resolution limit of the magnetic field which can be measured by the measuring device;
(3) According to the volume of the object to be measured and the required magnetic susceptibility measurement accuracy, determining the proper coil spacing and the proper alternating-current magnetic field B s Wherein the distance D between the coils C2 and C3 23 After determination, the spacing is kept constant, and only the spacing D of the coils C1 and C4 needs to be changed in subsequent operations 14 The space ratio can be changed, so that the size of the magnetic field gradient at the point P where the magnetometer is located is adjusted; then an alternating magnetic field B applied as required s The current passing through the coils is determined according to the size of the coil, so that all four coils are electrified;
(4) Measuring the magnetic field B along the coil axis at point P with a magnetometer z Slowly moving the magnetometer along the z axis by using the multi-degree-of-freedom displacement table below the magnetometer to observe the readings of the magnetometer, moving the magnetometer in the opposite direction if the absolute value of the magnetic field in the moving process is increased, and moving the magnetometer in the same direction until the absolute value of the magnetometer is decreased in the moving processThe index reaches a minimum, indicating that the magnetometer has reached the point on the z-axis where the magnetic field strength is at its lowest, and then the magnetometer is fixed at that point;
(5) Utilize accurate lead screw to rotate and slowly adjust coil interval D 14 Size of (1), observe B z If B changes the absolute value of z Gradually reducing, and continuously adjusting the precision screw rod towards the direction; if B is present z Increasing the diameter of the screw rod, moving the screw rod in the opposite direction until B z Obtaining the minimum value, and finishing the adjustment of the distance of the coils at the moment to ensure that the magnetic field gradient at the point P reaches the minimum value;
(6) The magnetometer was again slowly moved, observe B z Until B, repeating the operation steps of step 2 z Obtaining a minimum value, and finishing the operation of searching a minimum background magnetic field measuring point; then recording magnetic field data for a period of time, if the size 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 size of the induced magnetic field signal calculated according to the magnetic susceptibility measurement resolution, indicating that the experimental conditions are met, and starting to measure the magnetic susceptibility of the object;
(7) After the experimental conditions are met, placing the object to be tested on an objective table, and enabling the center of the object to be tested to be located between the position of the magnetic field peak value generated by the coil C1 and the position of the magnetometer; after the object to be measured is fixed, recording magnetic field measurement data in real time, applying a sinusoidal current signal with a certain frequency to the coil, making a noise spectral density curve according to the magnetic field data measured in the step 6, finding a frequency band with the lowest noise, and setting the magnetic field modulation frequency f in the frequency band;
(8) After recording the magnetic field data for a period of time, moving the object to be measured away from the objective table, and then continuously recording the data for a period of time; processing data after the experiment is finished, firstly removing unstable data generated in the removing process of the object to be detected, then dividing the data into two sections, carrying out spectrum analysis on the data before the object to be detected is removed, finding that a peak value appears at the modulation frequency f, and setting the size of the peak value as B zf Then, the data after the object to be measured is removed is subjected to frequency conversionSpectral analysis, the amplitude at frequency f is recorded as B zf0 The smaller the peak value is, the better the compensation effect on the 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 to be measured is: b iz ＝B zf －B zf0 Then selecting different set formulas according to the volume of the object to be measured and combining the magnetic field B s And calculating the relative position vector R from the magnetic field measuring mechanism to the object to be measured to obtain the magnetic susceptibility of the object to be measured.
Drawings
Fig. 1 is a schematic structural diagram of an apparatus for measuring magnetic susceptibility of an object based on magnetic induction according to an embodiment of the present invention;
FIG. 2 is a schematic diagram of the electrical circuit of the magnetic field mechanism provided by one embodiment of the present invention;
FIG. 3 is a graph of magnetic field gradient versus coil C1 current I at point P in accordance with an embodiment of the present invention 1 A graph of the variation; in the figure, configuration I is that only the coil C1 is electrified, configuration II is that only the coils C1 and C2 are electrified, and configuration III is that the coils C1-C4 are electrified;
FIG. 4 is a graph of the magnetic field gradient at point P versus current I provided by an embodiment of the present invention 1 /I 2 A graph of the variation;
FIG. 5 is a graph of magnetic field gradient at point P as a function of separation D of coils C1 and C4, according to an embodiment of the present invention 14 Graph of the variation.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and do not limit the invention.
In order to solve the problems that the traditional torsion pendulum technology has complicated measuring process, long measuring period and easy magnetization of an object, the invention provides a device for measuring the object magnetic susceptibility based on magnetic induction intensity, which is used for measuring the magnetic susceptibility of a weak magnetic object, such as the magnetic susceptibility measurement of inspection mass, biological materials or biological tissues, and the measuring principle is as follows:
mounting an object to be measured on a platform made of weak magnetic material, and applying alternating current modulation current to a magnetic field modulation coil to generate an alternating current magnetic field B s The alternating magnetic field B s Will make the object to be measured generate an AC magnetic moment m i The magnetic moment can excite an induction magnetic field B in space iz Measuring the magnetic field B by a high-precision magnetometer iz 。
Wherein, when the volume of the object to be measured is less than or equal to 5mm 3 While the object to be measured generates an induced magnetic field B iz And induced magnetic moment m i The relationship of (1) is:
in the formula, the induced magnetic moment m of the object to be measured i A single magnetic dipole, equivalent to be at its centroid, is related to the object's magnetic susceptibility χ by:
wherein V represents the volume of the object, μ 0 Is the vacuum permeability, R is the relative position vector from the magnetometer to the object to be measured, B s The external magnetic field applied to the object, i.e. the magnetic field generated by the coil, can be calculated using the biot savart law.
When the volume of the object to be measured is more than 5mm 3 In the process, because the spatial distribution scale of the magnetic dipoles is large, the magnetic dipoles can not be equivalent to a single magnetic dipole, otherwise, a large calculation error is caused, and the contribution of each magnetic dipole is integrated so as to obtain an accurate result. The magnetic moment generated by a single magnetic dipole is:
induced magnetic field B generated by object to be measured iz Induce magnetic moment m with it i The relationship of (c) can be written as:
that is, as long as the induced magnetic field B is measured iz Then the alternating current magnetic field B generated by the coil is used s And substituting the vector R of the relative position of the magnetometer and the object to be measured into the formula to calculate, thus obtaining 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 measurement principle, which comprises a magnetic field mechanism, a magnetic field measurement mechanism, a displacement mechanism and a processing mechanism. Wherein, the magnetic field measuring mechanism can adopt a magnetometer with high precision, and the processing mechanism can adopt a processor commonly used in the field. The invention utilizes magnetometer to measure the induction magnetic field B generated by the object to be measured iz Then using a processor to acquire the magnetic field B iz And selecting 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.
Because the object to be measured generates an induced magnetic field B by using the magnetic field modulation coil iz The magnetic field modulation coil can generate an alternating current magnetic field B under the action of alternating current modulation current s The magnetic field measured by the magnetometer at the measurement point (point P) is made to be other than the induced magnetic field B iz Besides, it also contains an AC magnetic field B s Affecting the measurement accuracy. In this respect, the magnetic field measured by the magnetometer is the induced magnetic field B generated by the actual object to be measured iz Other magnetic fields are required to be introduced, and the position of the magnetometer is adjusted to counteract 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 background noise measured by the magnetometer are lowest after the magnetometer is placed at the point P, and the induced magnetic field B generated on the object is generated under the condition of higher signal-to-noise ratio iz The measurement is performed.
Therefore, the magnetic field mechanism provided by this embodiment includes a current generating unit and coils C1 to C4 that are coaxially arranged in sequence at intervals and have the same structural size, the object to be measured is placed between the coils C1 and C2, and the magnetometer is placed at the center point of the connection line between the axes of the coils C2 and C3 and the center point of the connection line between the axes of the coils C1 and C4.
The current generating unit is used for respectively providing alternating current modulation current with certain frequency to the coils C1-C4. The coil C1 is a magnetic field modulation coil and is used for generating an alternating magnetic field B under the action of alternating current modulation current s So that the object to be measured generates an AC induction magnetic moment m i . The coil C4 is a background magnetic field compensation coil, and alternating current modulation current with equal magnitude and reverse direction is introduced into the coil C1 to generate an alternating current magnetic field B s The background magnetic field is equal and opposite to offset the axial magnetic field generated by the coil C1 at the point P where the magnetometer is located, so that the axial magnetic field at the point P tends to be zero. The coils C2 and C3 are gradient magnetic field compensation coils and are used for introducing alternating current modulation current with equal and reverse directions to generate magnetic fields with equal and reverse directions so as to offset the axial magnetic field gradient generated at the point P and enable the magnetic field gradient at the point P to tend to zero, so that the magnetic field in the space near the point P is more uniform, the background magnetic field and the magnetic field background noise measured by the magnetometer after the magnetometer is placed at the point P are lower, and the measurement of the induced magnetic field generated by the object is realized under the higher signal-to-noise ratio.
It should be noted that the current generating unit provided in this embodiment may adopt 4 current generators, and respectively provide the required alternating current modulation current for the 4 coils; the current required by the coils C1 and C4 is in the same direction, so that the connection sequence of the input and output interfaces of the coils C1 and C4 needs to be adjusted during connection to change the current flow direction, thereby changing the direction of the generated magnetic field, and similarly, another current generator is used to provide the alternating current modulation current to the coils C2 and C3.
Preferably, in order to increase the measurement speed and simplify the measurement operation, the current generating unit provided in this embodiment may employ 1 current generator, 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 conveniently adjust the current ratio of the two groups of coils, a rheostat may be further disposed on each branch. In this embodiment, 1 current generator is used to provide ac modulation current to the four coils, so that the currents in the four coils have the same phase, and the absolute magnitude of the magnetic field generated by the coils is cancelled, and the fluctuation of the magnetic field due to the fluctuation noise of the current source is also cancelled, thereby reducing the fluctuation noise of the modulation magnetic field.
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 that the axial magnetic field of the magnetometer at a point P is the lowest; the single degree of freedom displacement table is used for adjusting the distance between the coils C1-C4 to enable the axial magnetic field gradient at the point P to be the lowest.
The processing mechanism provided by the embodiment is used for executing the following processes: (1) The method is used for collecting a section of axial magnetic field data measured by the magnetometer when the axial magnetic field and the axial magnetic field gradient at the P point are both adjusted to be the lowest, and carrying out spectrum analysis on the data to find a peak point B of the data zf The peak point is the induced magnetic field B excited by the AC induced magnetic moment in space iz (ii) a (2) Selecting corresponding calculation formula (1) or formula (2)) according to the volume of the object to be measured, and combining with the alternating-current magnetic field B s And calculating the relative position vector R from the magnetic field measuring mechanism to the object to be measured to obtain the magnetic susceptibility of 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, and perform spectrum analysis on the data to find the peak point B when the axial magnetic field and the axial magnetic field gradient at the point P are both adjusted to the lowest values zf0 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; then the peak point B is added zf And B zf0 The difference is made to obtain an induced magnetic field B iz Selecting corresponding calculation formula (1) or formula (2)) according to the volume of the object to be measured, and combining with the alternating-current magnetic field B s And calculating the relative position vector R from the magnetic field measuring mechanism to the object to be measured to obtain the magnetic susceptibility of the object to be measured.
According to the device for measuring the object magnetic susceptibility based on the magnetic induction intensity, the object to be measured is placed on one side of the modulation coil, and the alternating modulation current with a certain frequency is introduced into the modulation coil, so that the object can generate induced magnetic moment signals with the same frequency, a sample does not need to be suspended, vibrated or periodically moved, and the device has the characteristics of high measuring speed and short measuring period; meanwhile, when the applied modulation magnetic field is of uT magnitude, centimeter-level objects with extremely low magnetic susceptibility such as copper and the like can generate induced magnetic field signals of 10pT magnitude, so that the measurement resolution of high-precision magnetic sensors such as an atomic magnetometer is met, and the magnetic field of uT magnitude 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 following describes the apparatus for measuring the magnetic susceptibility of an object based on magnetic induction according to the present invention in detail with reference to specific embodiments.
As shown in fig. 1, the present embodiment provides a magnetometer-based apparatus for measuring magnetic susceptibility of an object, which includes a magnetic shielding mechanism, an object stage 10, a magnetic mechanism, a single degree-of-freedom displacement stage, a mounting base plate 50, a magnetometer 30, and a multiple degree-of-freedom displacement stage 40.
The magnetic field shielding mechanism comprises a magnetic shielding room for shielding background magnetic fields such as a geomagnetic field and the like and reducing background noise of magnetic field measurement.
The objective table 10 is a non-magnetic organic glass support and table top for placing an object to be measured.
As shown in fig. 2, the magnetic field mechanism includes a current generator, two varistors, two ammeters, and two pairs of coaxial circular coils (C1, C4, C2, and C3) with equal radius, wherein the coil C1, the one rheostat, the one ammeter, and the coil C4 are connected to form a first current branch, the coil C2, the other rheostat, the other ammeter, 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.
By adjusting the connection sequence of the input and output interfaces of the coils C1 and C4, the equal and reverse currents, i.e. I, are introduced into the coils C1 and C4 1 ＝﹣I 4 A point where the axial magnetic field Bz is zero is generated at the midpoint of the line connecting the axes of the two coils, and is called a point P. A coil C2 is placed on the right side of the coil C1, a coil C3 is placed on the left side of the coil C4, and the midpoint of the axis connecting line of the coils C2 and C3 is made to be in contact withThe above-mentioned points P coincide. Thus, the magnetic field modulation mechanism with the 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 used for generating a magnetic field required by the object to generate the induced magnetic moment. And C4 is a background magnetic field compensation coil and is used for counteracting the axial magnetic field generated by the coil C1 at the point P so as to enable the magnetic field at the point P to tend to be zero. The coils C2 and C3 are gradient magnetic field compensation coils, and are introduced with equal reverse current, i.e. I, by adjusting the connection sequence of the input and output interfaces of the coils C2 and C3 2 ＝﹣I 3 The device is used for offsetting the axial magnetic field gradient generated at the point P and enabling the magnetic field gradient at the point P to tend to zero, so that the magnetic field in the space near the point P is more uniform, the background magnetic field and the magnetic field background noise measured by the magnetometer after the magnetometer is placed at the point P are lower, and the measurement of the induced magnetic field generated by the object is realized under the condition of higher signal-to-noise ratio.
Two varistors are used to vary the ratio of the currents in the outer and inner coils, i.e. I 1 /I 2 When the coil spacing is determined, the magnetic field gradient at point P is represented by I 1 Size and I of 1 /I 2 The value of (b) is completely determined. Suppose to take I 2 Is 1A, then has I 1 /I 2 ＝I 1 The magnetic field gradient at point P can be plotted
With I 1 I.e. the current ratio variation curve, as shown in fig. 3, it can be found that there is a zero point in the gradient, and the current value I is set at this time 1 ＝I 10 . Regulating the current I by means of a varistor 1 Simultaneously checking I in real time by ammeter 1 Size I of 1t . Let δ = I 1t -I 10 The higher the adjustment accuracy of the rheostat, the smaller δ will be, and the closer to zero the magnetic field gradient at point P can be made.
The single-degree-of-freedom displacement table comprises four displacement tables, wherein the coils C1-C4 are correspondingly arranged on the four displacement tables, and the two displacement tables provided with the coils C1 and C4 and the two displacement tables provided with the coils C2 and C3 are respectively arranged on the four displacement tablesThe two ends of the two precision screw rods 20 are connected with the same thread pitch but in opposite directions. 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 coils can always keep the coaxiality and the symmetry to a point P 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 calculation shows that when the ratio of the currents of the two groups of coils is I 1 /I 2 It is determined what the value of the magnetic field gradient at point P is given by the separation D of the coils C1, C4 14 And the distance D between the coils C2 and C3 23 Ratio of (D) 14 /D 23 And (6) determining. The method comprises the steps of firstly, theoretically calculating the ratio of the distances between coils when the magnetic field gradient at a P point is zero, wherein due to the fact that errors exist in measurement of experimental parameters, the actual distance ratio deviates from a theoretical value, after the distance between the coils is adjusted to a theoretical reference value, fine adjustment of the distance is needed by means of a lead screw, magnetic field measurement results are observed in real time in the adjustment process, and when the actually measured background magnetic field and noise values are reduced to meet a preset index, adjustment of the distance between the coils is completed. The higher the distance adjustment accuracy of the screw rod is, the lower the magnetic field gradient value at the P point can be. The magnetic field gradient value at the magnetic field measuring point can be infinitely close to zero by accurately adjusting the distance ratio of the magnetic field compensation coil to the gradient compensation coil, so that the target that the magnetic field gradient at the magnetic field measuring point is small enough is realized, and the measuring point P has enough low background magnetic field strength and enough high signal-to-noise ratio.
The mounting base 50 is used to fix the displacement table on which the coil and the 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 induction 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 measurement point with a background magnetic field and a magnetic field background noise small enough near a point P is searched, then the magnetometer 30 is fixed at the measurement point, and the magnetic susceptibility of the object can be measured without changing the positions of a coil and the magnetometer in the subsequent process.
This embodiment is to use a magnetometerThe magnitude of the object magnetic susceptibility is determined by measuring the induced magnetic moment generated by the object to be measured under the action of the modulation magnetic field. During the actual measurement, the resolution delta is measured according to the magnetic field of the magnetometer Bz And the distance d between the object and the magnetometer, the minimum detectable induced magnetic moment δ generated by the object can be calculated according to a formula (a general formula is not illustrated) of the magnetic field generated by the magnetic dipole mi (calculated according to the general formula) and
substitution of the required susceptibility measurement resolution δ χ The required minimum external magnetic field B can be obtained s To determine the minimum current required to be input to the coil. And then, according to the size of the object to be measured, the value ranges of parameters such as the coil spacing, the radius and the like can be determined, and then a set of magnetic field modulation device meeting the requirements is processed according to the parameters.
In order to make the coil C4 achieve the purpose of canceling the axial magnetic field and the magnetic field fluctuation generated by the coil C1 at the point P, the magnetic fields generated by the coils C4 and C1 need to be equal in magnitude and opposite in direction, so the coils C1 and C4 are connected in series, and the direction of the magnetic field can be changed by changing the current flow direction by adjusting the connection sequence of the input and 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, is able to receive an object, and therefore D 12 Is determined according to the size of the object to be measured. And a device for placing the magnetometer and moving the magnetometer needs to be reserved between the coils C2 and C3, and meanwhile, the magnetic field measurement resolution of the magnetometer and the attenuation of the induced magnetic field along with the distance are considered, so that the distance from the object to the point P cannot be too far, otherwise, the induced magnetic field generated by the object cannot be detected because the induced magnetic field is smaller than the resolution at the point P, and generally, the closer the object is to the point P, the higher the magnetic moment measurement resolution is. Thus, the spacing of the coils C2, C3 and the distance of the object from the point are substantially determined. After the distances between the coils are determined, the ratio of the currents in the outer and inner coils, i.e., I, is determined so that the magnetic field gradients generated by the coils C2, C3 and the coils C1, C4 cancel each other at point P 1 /I 2 Needs to be equal to a certain value. Therefore, the coils C1 and C4 are connected in series, the coils C2 and C3 are connected in series, then the coils and the coils are connected in parallel, and a rheostat is respectively connected to the two current branches to change the current ratio of the two groups of coils. However, in practice, the ratio of the currents needs to be adjusted very precisely in order to theoretically be able to null the magnetic field gradient at point P. And the ratio of accurately adjusting the coil spacing through the lead screw is easier to realize than the ratio of accurately adjusting the current through the rheostat. We therefore achieve this goal by adjusting the current ratio and the coil spacing simultaneously. Theoretical analysis can find that the ratio of the currents I 1 /I 2 The larger, the larger the magnetic field gradient at point P
Zero required gradient compensation coil C1, C4 separation D 14 The larger the size. Meanwhile, as can be seen from FIG. 4, D 14 The larger the amount to be taken is,
the smaller the gradient, i.e. the smaller the slope of the curve, the less sensitive the magnetic field gradient is to changes in coil spacing, and thus the D required to bring the magnetic field gradient to zero 14 The larger the error range of the value is, the requirement on the positioning accuracy of the screw rod can be reduced. But take too large a D 14 Will make the ratio of the currents I 1 /I 2 Is very large, and I 2 Primarily for generating a modulated magnetic field, the magnitude of which has to meet certain requirements, which means that the current I needs to be increased 1 Can D make D 14 Become large, but the maximum current in the coil is limited by the maximum power at which the coil can operate properly, so in practical design, D should be considered 14 The value of (c) is greater.
Specifically, the step of measuring the magnetic susceptibility of the object by the measuring device provided by this embodiment is as follows:
step 1: the method comprises the steps of placing all components including a coil mechanism, a single-degree-of-freedom and multi-degree-of-freedom displacement table, an installation base and a magnetometer in a magnetic shielding room, fixing a magnetometer probe on the table top of the 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 be coincident with the coil axis, fixing other adjusting degrees of freedom, only keeping the z-direction degree of freedom, and only enabling the magnetometer to move on the z axis in the subsequent adjusting process.
Step 2: and (4) opening the magnetometer, and recording and checking the magnetic field measurement result in real time. The magnetometer measures the background noise caused by the magnetic field fluctuation in the magnetic shielding room and the measurement noise of the magnetometer, and represents the limit of the resolution of the magnetic field which can be measured by the measuring device.
And step 3: according to the volume of the object to be measured and the required magnetic susceptibility measurement accuracy, determining the proper coil spacing and the proper alternating-current magnetic field B s Wherein the distance D between C2 and C3 23 After the spacing is determined, the spacing is kept constant, and only the spacing D of the coils C1 and C4 needs to be changed in subsequent operations 14 The pitch ratio can be varied to adjust the magnitude of the magnetic field gradient at point P. Alternating magnetic field B applied as required s Determines the magnitude of the current passing through the coil, so that all four coils are energized.
And 4, step 4: measuring the magnetic field B along the coil axis at point P with a magnetometer z And slowly moving the magnetometer along the z axis by using a displacement table below the magnetometer, observing the indication number of the magnetometer, moving the magnetometer in the opposite direction if the absolute value of the magnetic field in the moving process is increased, and moving the magnetometer in the same direction until the indication number of the magnetometer reaches the minimum value which represents that the magnetometer reaches the point with the lowest magnetic field intensity on the z axis if the absolute value of the magnetic field in the moving process is decreased, and then fixing the magnetometer at the point.
And 5: utilize lead screw to rotate and slowly adjust coil interval D 14 Size of (1), observation B z If B is a change in the magnitude (absolute value) of z Gradually reducing, and continuously adjusting the screw rod towards the direction; if B is present z Increasing the size of the screw rod, moving the screw rod in the opposite direction until B z The minimum value is obtained. At this time, the adjustment of the spacing of the coils is completed, so that the magnetic field gradient at the point P reaches the minimum value.
And 6: slowly moving magnetometer againObservation of B z Repeating the operation steps of step 2 until B z The minimum value is obtained. This completes the operation of finding the minimum background magnetic field measurement point. And then recording magnetic field data for a period of time, and if the size 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 size of the induced magnetic field signal calculated according to the magnetic susceptibility measurement resolution, indicating that the experimental conditions are met, and starting to measure the magnetic susceptibility of the object.
And 7: after the experimental conditions are met, the object is placed on the stage so that the center of the object is located between the position of the peak of the magnetic field generated by the coil C1 and the position of the magnetometer. After the object is fixed, the magnetic field measurement data is recorded in real time, then a sinusoidal current signal with a certain frequency is applied to the coil, and the magnetic field signal generated by the coil can be expressed as:
let the sample rate of the magnetometer be f s The frequency of the alternating current signal is required to satisfy f ≦ f s And/2, simultaneously drawing a noise spectral density curve according to the data measured in the step 6, finding a frequency band with the lowest noise, and selecting the magnetic field modulation frequency f in the frequency band.
And 8: after recording the magnetic field data for a period of time, the object is removed from the stage and recording of the data continues for a period of time. The data processing was started after the experiment was finished. Firstly, removing unstable data generated in the process of removing an object, then dividing the data into two sections, carrying out spectrum analysis on the data before removing the object, finding that a peak value appears at a modulation frequency f, and setting the size of the peak value as B zf Then, the data after object removal is subjected to spectral analysis, and the amplitude at frequency f is recorded as B zf0 The smaller the peak value, the better the compensation effect on the background magnetic field signal, and ideally no signal peak appears there. Thus, the magnitude of the magnetic field signal generated by the susceptibility of the object is: b iz ＝B zf －B zf0 Substituting the data into a correlation calculation formula of magnetic susceptibility and induced magnetic fieldThe magnetic susceptibility of the measured object can be obtained.
It will be understood by those skilled in the art that the foregoing is only a preferred embodiment of the present invention, and is not intended to limit the invention, and that any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the scope of the present invention.
Claims (10)
1. A device for measuring the magnetic susceptibility of an object based on 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 at intervals in sequence and have the same structure and size, 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 middle point of the axis connecting line of the coils C2 and C3 and the middle point of the axis connecting line of the coils C1 and C4 coincide; the current generating unit is used for respectively providing alternating current modulation current with certain frequency to the coils C1-C4; coil C1 for generating an alternating magnetic field B s So that the object to be measured generates an AC induction magnetic moment m i ；
The displacement mechanism 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 magnetic field measuring mechanism to enable the axial magnetic field at the measuring point where the magnetic field measuring mechanism is located to be the lowest, and the single-degree-of-freedom displacement table is used for adjusting the distance between the coils to enable the axial magnetic field gradient at the measuring point to be the lowest; wherein the coil C4 is used for generating an alternating current magnetic field B s The equal-magnitude reverse magnetic field 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 and offsetting the axial magnetic field gradient at the measuring point;
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 both adjusted to be minimum, and carrying out spectrum analysis on the data to find a peak point B of the data zf The peak point is the AC induced magnetic moment m i Induced magnetic field B excited in space iz (ii) a According to the body of the object to be measuredDifferent setting formulas are selected and combined with an alternating magnetic field B s And calculating the relative position vector R from the magnetic field measuring mechanism to the object to be measured to obtain the magnetic susceptibility of the object to be measured.
2. The device according to claim 1, wherein the processing means is further configured to collect a section of axial magnetic field data measured by the magnetic field measuring means after the object to be measured is removed, and perform a spectrum analysis on the data to find its peak point B when the axial magnetic field and the axial magnetic field gradient at the measurement point are both adjusted to the minimum zf0 (ii) a Then the peak point B is calculated zf And B zf0 The difference is made to obtain an induced magnetic field B iz Selecting different set formulas according to the volume of the object to be measured, and combining with the alternating current magnetic field B s And calculating the relative position vector R from the magnetic field measuring mechanism to the object to be measured to obtain the magnetic susceptibility of the object to be measured.
3. The apparatus according to claim 2, wherein the volume of the object is less than or equal to 5mm 3 Then, the set formula is:
when the volume of the object to be measured is more than 5mm 3 Then, the set formula is:
in the formula, mu 0 Represents the vacuum permeability; AC induction magnetic moment m generated by object to be measured i The relationship between the single magnetic dipole equivalent to the mass center of the single magnetic dipole and the magnetic susceptibility χ of the object to be measured is as follows:
v denotes the volume of the object to be measured.
4. The device for measuring the magnetic susceptibility of an object based on magnetic induction according to claim 2, wherein the magnetic field measuring means employs a magnetometer; the single-degree-of-freedom displacement platform comprises four displacement platforms, coils C1-C4 are correspondingly arranged on the four displacement platforms, the two displacement platforms provided with the coils C1 and C4 and the two displacement platforms provided with the coils C2 and C3 are respectively connected to two ends of a precision screw rod, and the thread pitches of the two ends of the two precision screw rods are the same but the directions of the two ends of the two precision screw rods are opposite.
5. Device for measuring the magnetic susceptibility of objects based on magnetic induction according to claim 4, characterized in that the distance D between the coils C1 and C2 12 Determining according to the size of the object to be detected; spacing D of coils C2 and C3 23 Determining 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 measurement resolution of the magnetometer and the condition that the induction magnetic field is attenuated along with the distance between the object to be measured and the magnetometer; the axial magnetic field gradient at the measuring point is lowest by adjusting the ratio of the alternating modulation currents introduced into the coils C1 and C2 and the distance D between the coils C1 and C4 14 And (4) realizing.
6. The apparatus for measuring magnetic susceptibility of an object based on magnetic induction according to claim 5, wherein the AC magnetic field B s Induced magnetic moment delta capable of being detected according to minimum of object to be detected mi Magnetic susceptibility measurement resolution and volume V determination of the object to be measured.
7. The device for measuring the magnetic susceptibility of an object based on magnetic induction according to claim 6, wherein the current generating unit comprises a current generator, two varistors and two current meters, wherein the coil C1, the rheostat, the current meter and the coil C4 are connected to form a first current branch, the coil C2, the other rheostat, the other current 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; and the connecting sequence of the input and output interfaces of the coils C1-C4 is adjusted to ensure that alternating modulation currents introduced into the coils C1 and C4, the coils C1 and C2 and the coils C2 and C3 are equal in magnitude and reverse, and the two varistors are used for adjusting the ratio of the currents of the coils C1 and C2, so that the magnetic field gradient at a measuring point is lowest.
8. The device for measuring the magnetic susceptibility of an object based on magnetic induction according to claim 7, wherein the frequency f of the AC modulated current is ≦ f s /2，f s Is the sampling frequency of the magnetometer.
9. The apparatus for measuring the magnetic susceptibility of an object based on magnetic induction according to claim 8, further comprising a magnetic shielding mechanism, wherein the magnetic shielding mechanism employs a magnetic shielding chamber, and the magnetic field mechanism, the magnetic field measuring mechanism and the displacement mechanism are disposed in the magnetic shielding chamber; the object to be measured passes through the objective table setting between coil C1 and C2, just the objective table adopts nonmagnetic organic glass support and mesa.
10. A method for measuring magnetic susceptibility using the magnetic induction based apparatus for measuring magnetic susceptibility of an object according to claim 9, comprising the steps of:
(1) Placing all components including a magnetic field mechanism and a displacement mechanism in a magnetic shielding room, fixing a magnetometer probe on a table board of a displacement table with multiple degrees of freedom, finding the position of a coil axis through a laser measuring instrument, adjusting the z axis of a magnetometer to be coincident with the coil axis, fixing other adjustment degrees of freedom, only keeping the z-direction degree of freedom, and only enabling the magnetometer to move on the z axis in the subsequent adjustment process;
(2) Opening the magnetometer, and recording and checking the magnetic field measurement result in real time, wherein the magnetic field fluctuation in the magnetic shielding room and the background noise caused by the measurement noise of the magnetometer represent the resolution limit of the magnetic field which can be measured by the measuring device;
(3) According to the volume of the object to be measured and the required magnetic susceptibility measurement accuracy, determining the proper coil spacing and the proper alternating-current magnetic field B s Wherein the distance D between the coils C2 and C3 23 After determination, the spacing is kept constant, and only the spacing D of the coils C1 and C4 needs to be changed in subsequent operations 14 The space ratio can be changed, so that the size of the magnetic field gradient at the point P where the magnetometer is located is adjusted; then an alternating magnetic field B applied as required s The size of the current led into the coils is determined, so that the four coils are all electrified;
(4) Measuring the magnetic field B along the coil axis at point P with a magnetometer z Slowly moving the magnetometer along the z axis by using a multi-degree-of-freedom displacement table below the magnetometer, observing the readings of the magnetometer, moving the magnetometer in a reverse direction if the absolute value of the magnetic field in the moving process is increased, and moving the magnetometer in the same direction until the readings of the magnetometer reach the minimum value which represents that the magnetometer reaches the point with the lowest magnetic field intensity on the z axis if the absolute value of the magnetic field in the moving process is decreased, and then fixing the magnetometer at the point;
(5) Utilize accurate lead screw to rotate and slowly adjust coil interval D 14 Size of (1), observation B z If B changes in the absolute value of z Gradually reducing, and continuously adjusting the precision screw rod towards the direction; if B is present z Increasing the size of the screw rod, moving the screw rod in the opposite direction until B z Obtaining the minimum value, and finishing the adjustment of the distance between the coils at the moment to ensure that the magnetic field gradient at the point P reaches the minimum value;
(6) Slowly moving magnetometer again, observe B z Until B, repeating the operation steps of step 2 z Obtaining a minimum value, and finishing the operation of searching a minimum background magnetic field measuring point; then recording magnetic field data for a period of time, if the size 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 size of the induced magnetic field signal calculated according to the magnetic susceptibility measurement resolution, indicating that the experimental conditions are met, and starting to measure the magnetic susceptibility of the object;
(7) After the experimental conditions are met, placing the object to be tested on an objective table, and enabling the center of the object to be tested to be located between the position of the magnetic field peak value generated by the coil C1 and the position of the magnetometer; after the object to be measured is fixed, recording magnetic field measurement data in real time, applying a sinusoidal current signal with a certain frequency to the coil, making a noise spectral density curve according to the magnetic field data measured in the step 6, finding a frequency band with the lowest noise, and setting the magnetic field modulation frequency f in the frequency band;
(8) After recording the magnetic field data for a period of time, moving the object to be measured away from the objective table, and then continuously recording the data for a period of time; processing data after the experiment is finished, firstly removing unstable data generated in the removing process of the object to be detected, then dividing the data into two sections, carrying out spectrum analysis on the data before the object to be detected is removed, finding that a peak value appears at the modulation frequency f, and setting the size of the peak value as B zf Then, performing spectrum analysis on the data of the object to be detected after the object to be detected is removed, and marking the amplitude at the frequency f as B zf0 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 susceptibility of the object to be measured is: b is iz ＝B zf －B zf0 Then, different set formulas are selected according to the volume of the object to be measured and combined with the magnetic field B s And calculating the relative position vector R from the magnetic field measuring mechanism to the object to be measured to obtain the magnetic susceptibility of the object to be measured.
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