CN118330525A - A device and method for measuring AC magnetic susceptibility of large-size objects - Google Patents

Abstract

The present application discloses a device and method for measuring the AC magnetic susceptibility of a large-sized object, wherein the measuring device comprises: a magnetic field generating mechanism, comprising two pairs of Helmholtz coils with overlapping geometric centers and different radii, the two pairs of Helmholtz coils being fed with reverse sinusoidal currents with varying frequency intervals, for generating a uniform magnetic field covering the area where the sample is located and a magnetic field amplitude zero point away from the sample; a phase reference mechanism, for providing and measuring a reference phase in phase with the uniform magnetic field; a control mechanism, for calculating the phase and modulus of the magnetic susceptibility of the sample at different sinusoidal current frequencies based on the reference phase and the amplitude and phase of the coil residual background magnetic field and superposition field measured by the magnetic field measuring mechanism. The present application can measure the AC magnetic susceptibility of large-sized objects above the centimeter level, can provide the uniform magnetic field required for the AC magnetic susceptibility measurement, and can reduce the measurement error caused by the uneven distribution of the AC magnetic moment.

Description

A device and method for measuring AC magnetic susceptibility of large-size objects

Technical Field

The present application belongs to the field of material magnetic property measurement in space gravitational wave detection, and more specifically, to a device and method for measuring the alternating current magnetic susceptibility of a large-sized object.

Background technique

In space gravitational wave detection, it is necessary to detect gravitational wave signals in the range of 1mHz to 10Hz. Since the gravitational wave signal is extremely weak, strict requirements are placed on the magnitude of the interference force on the test mass. Among the interference forces on the test mass, the magnetic noise generated by the coupling of the magnetic susceptibility of the test mass with the external magnetic field is the main source of noise in gravitational wave detection. According to relevant research at home and abroad, high-frequency magnetic fields above 10Hz can couple magnetic force to low-frequency signals by difference frequency, thereby interfering with gravitational wave detection. And because the test mass is an alloy material, its magnetic susceptibility will change with the frequency of the magnetic field. Therefore, the frequency characteristic measurement of the magnetic susceptibility of the test mass, that is, the AC magnetic susceptibility measurement, is of great significance for evaluating the magnetic noise of the test mass.

Since the size of the test mass reaches several centimeters, current commercial instruments such as VSM and SQUID cannot measure its AC magnetic susceptibility. The torsion balance measurement method was recently applied to the measurement of AC magnetic susceptibility, but the torsion balance has the disadvantages of cumbersome measurement process, long measurement cycle, high vacuum environment required for measurement, inconvenient sample replacement, etc., which is not conducive to efficient systematic research on the magnetism of materials. Chinese patent CN 115718273 A proposes a magnetic susceptibility measurement device and method based on magnetic induction intensity, but the device applies a gradient magnetic field to the sample. The AC magnetic susceptibility is defined as the ratio of the total magnetic moment generated by the sample under the action of a uniform magnetic field to the external magnetic field. Because in the case of a high-frequency AC magnetic field, due to the presence of factors such as the skin effect, the magnetic moment of the sample is no longer uniformly distributed, so that the AC magnetic susceptibility of the sample obtained by the gradient field measurement no longer has a clear physical meaning, so the device is not suitable for the measurement of AC magnetic susceptibility.

Summary of the invention

In view of the defects of the prior art, the purpose of the present application is to provide a device and method for measuring the AC magnetic susceptibility of large-sized objects, which can measure the AC magnetic susceptibility of large-sized objects above the centimeter level.

To achieve the above-mentioned purpose, in a first aspect, the present application provides a device for measuring the AC magnetic susceptibility of a large-sized object, comprising a magnetic field generating mechanism, a magnetic field measuring mechanism, a phase reference mechanism and a control mechanism;

The magnetic field generating mechanism comprises two pairs of Helmholtz coils with overlapping geometric centers and different radii, wherein the Helmholtz coil with a smaller radius is arranged inside the Helmholtz coil with a larger radius, and reverse sinusoidal currents with varying frequency intervals are passed through the two pairs of Helmholtz coils to generate a uniform magnetic field covering the area where the sample is located and a magnetic field amplitude zero point away from the sample;

The magnetic field measuring mechanism is arranged at the zero point of the magnetic field amplitude, and the distance between the magnetic field measuring mechanism and the geometric center of the two pairs of Helmholtz coils is greater than 10 times the size of the sample along the magnetic field direction, and is used to measure the magnetic field and phase at the position point when the sample moves away from and approaches; wherein, when the sample approaches, the sample is located in the uniform magnetic field area generated by the magnetic field generating mechanism;

The phase reference mechanism is used to provide and measure a reference phase that is in phase with the uniform magnetic field;

The control mechanism is used to calculate the phase and modulus of the magnetic susceptibility of the sample at different sinusoidal current frequencies based on the reference phase and the magnetic field and phase measured by the magnetic field measurement mechanism when the sample is far away from and close to it, so as to obtain the magnetic susceptibility spectrum distribution of the sample within a certain frequency range.

The present application provides a device for measuring the AC magnetic susceptibility of a large-sized object, which uses two pairs of Helmholtz coils to generate a magnetic field amplitude zero point far away from the sample. The position of the magnetic field amplitude zero point can be adjusted by adjusting the radius and number of turns of the coil, thereby enabling the measurement of the AC magnetic susceptibility of samples of different sizes; and a magnetic field measuring mechanism is set at the magnetic field amplitude zero point, and the radius and number of turns of the coil are adjusted to make the distance from the magnetic field measuring mechanism to the geometric center of the two pairs of Helmholtz coils much larger than the size of the sample along the direction of the uniform magnetic field, which can effectively reduce the measurement error caused by the uneven distribution of the AC magnetic moment.

As further preferred, the sample is for inspection quality.

As further preferred, the phase reference mechanism comprises a phase reference coil arranged far away from the sample and a magnetometer for measuring the magnetic field generated by the phase reference coil, and the phase reference coil is connected in series with two pairs of Helmholtz coils.

As a further preferred embodiment, when the sample is far away, the magnetic field measurement mechanism measures the amplitude B _z0 and phase of the residual background magnetic field of the coil. When the sample approaches, the magnetic field measurement mechanism measures the amplitude B _z2 and phase of the superposition field of the residual background magnetic field of the coil and the induced magnetic field generated by the sample under the action of the uniform magnetic field.

The calculation formula of the modulus value of the magnetic susceptibility of the sample at the sinusoidal current frequency ω |χ(ω)| is:

The amplitude B _z1 and phase of the induced magnetic field The calculation formula is:

The phase of the magnetic susceptibility of the sample at a sinusoidal current frequency of ω The calculation formula is:

In the formula, The reference phase measured by the phase reference mechanism; is the phase value of the coil residual background magnetic field, is the phase value of the superposition field, K is a constant matrix related to the relative positions of the magnetic field measurement mechanism and the geometric centers of the two pairs of Helmholtz coils; V is the volume of the sample; and H is the size of the uniform magnetic field.

As a further preferred embodiment, it also includes a displacement mechanism and a sample moving mechanism;

The displacement mechanism is used to adjust the orientation of the sample so that the sample is away from or close to the magnetic field measuring mechanism; the sample moving mechanism is used to adjust the spatial position of the magnetic field measuring mechanism so that the sensitive element in the magnetic field measuring mechanism is located at the zero point of the magnetic field amplitude.

As a further preferred embodiment, the displacement mechanism includes a magnetometer mounting platform, a magnetometer bracket and a displacement table, the magnetic field measuring mechanism is fixed on the magnetometer mounting platform, the magnetometer mounting platform is rigidly connected to the magnetometer bracket, the magnetometer bracket is mounted on the displacement table, and the displacement table is used to receive and adjust the spatial position of the magnetic field measuring mechanism according to the first control signal sent by the control mechanism.

As a further preferred embodiment, the sample moving mechanism includes a conveyor belt, a sample stage bracket, a conveyor belt mounting platform and a motor. The sample stage bracket is installed on the ground, the conveyor belt mounting platform is fixed on the sample stage bracket, two bearings are provided on the conveyor belt mounting platform, the conveyor belt is sleeved on the two bearings, the sample is placed on the conveyor belt, the motor is connected to the bearings through a transmission belt, and the motor is used to receive and drive the bearings to rotate according to the second control signal sent by the control mechanism, thereby driving the conveyor belt to move, so that the sample is away from or close to the magnetic field measurement mechanism.

As a further preference, the magnetic field generating mechanism is fixed on a coil mounting platform, the coil mounting platform is made of non-magnetic non-metallic material, and the coil mounting platform and the sample moving mechanism are mounted on a vibration isolation platform.

As a further preference, a rigid connection is adopted between the two pairs of Helmholtz coils and the coil mounting platform, and between the coil mounting platform and the vibration isolation platform.

In a second aspect, the present application provides a method for measuring the AC magnetic susceptibility of a large-sized object based on the above, comprising the following steps:

The magnetic field amplitude zero point is calculated according to the parameters of the two pairs of Helmholtz coils, and the magnetic field measurement mechanism is placed near the magnetic field amplitude zero point;

Pass square wave currents in opposite directions through the two pairs of Helmholtz coils, and move the magnetic field measuring mechanism toward the axial direction of the coils, while observing the change of the magnetic field measured by the magnetic field measuring mechanism, until the measured magnetic field amplitude decreases within the set range, and then keep the magnetic field measuring mechanism at this position;

Place the phase reference mechanism away from the sample and connect it in series with the two pairs of Helmholtz coils;

The sample is moved away from the magnetic field measurement mechanism, and then a reverse sinusoidal current with a frequency of ω is passed through the two pairs of Helmholtz coils, and the magnetic field measurement mechanism measures the residual background magnetic field of the coils; after measuring for a period of time, the sample is moved close to the magnetic field measurement mechanism and kept for a period of time, and the magnetic field measurement mechanism measures the superposition field of the residual background magnetic field of the coils and the induced magnetic field generated by the sample under the action of the uniform magnetic field;

The control mechanism calculates the phase and modulus of the magnetic susceptibility of the sample at the frequency ω according to the reference phase measured by the phase reference mechanism and the amplitude and phase of the coil residual background magnetic field and the superposition field;

By changing the frequency of the sinusoidal current, the phase and modulus of the magnetic susceptibility of the sample at other frequencies are measured, and finally the spectral distribution of the magnetic susceptibility of the sample within a certain frequency range is obtained.

It can be understood that the beneficial effects of the second aspect mentioned above can be found in the relevant description of the first aspect mentioned above, and will not be repeated here.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG1 is a structural diagram of a device for measuring AC magnetic susceptibility of a large-sized object provided in one embodiment of the present application;

FIG2 is an axial distribution diagram of an external magnetic field provided in an embodiment of the present application;

FIG3 is a time domain curve diagram at 1500 Hz measured in a specific embodiment of the present application;

FIG4 is a frequency domain curve diagram at 1500 Hz measured in a specific embodiment of the present application;

FIG5 is a mode value spectrum of AC magnetic susceptibility in the range of 1 Hz to 1500 Hz measured in a specific embodiment of the present application;

FIG. 6 is a phase spectrum diagram of AC magnetic susceptibility in the range of 1 Hz to 1500 Hz measured in a specific embodiment of the present application.

In all the drawings, the same figure numbers are used to represent the same elements or structures, wherein 1 is a pair of large Helmholtz coils, 1 is a pair of small Helmholtz coils, 3 is a magnetometer in a magnetic field measurement mechanism, 4 is a magnetometer mounting platform, 5 is a magnetometer bracket, 6 is a displacement table, 7 is a phase reference coil, 8 is a magnetometer in a phase reference mechanism, 9 is a vibration isolation platform, 10 is a coil mounting platform, 11 is a stepper motor, 12 is a sample stage bracket, 13 is a transmission belt, 14 is a bearing, 15 is a conveyor belt mounting platform, and 16 is a conveyor belt.

Detailed ways

In order to make the purpose, technical solution and advantages of the present application more clearly understood, the present application is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and are not used to limit the present application.

It should be understood that, in the description of the present application, the term "several" means at least one, such as one, two, etc., unless otherwise clearly and specifically defined; the term "plurality" means two or more, unless otherwise clearly and specifically defined; the terms "first" and "second" etc. are used to distinguish different objects rather than to describe a specific order of objects; the term "and/or" includes any and all combinations of one or more related listed items.

In addition, references throughout this specification to "one embodiment," "one embodiment," "an example," or similar language indicate that a particular feature, structure, or characteristic described in conjunction with the embodiment is included in at least one embodiment of the present application. Thus, appearances of the phrase "in one embodiment," "in one embodiment," and similar language throughout this specification may, but do not necessarily, all refer to the same embodiment.

The present application provides a device for measuring the AC magnetic susceptibility of a large-sized object, which can be used to measure the AC magnetic susceptibility of a sample with a size of centimeters or above, such as to inspect the quality. FIG1 is a structural diagram of a device for measuring the AC magnetic susceptibility of a large-sized object provided by an embodiment of the present application. As shown in FIG1 , the measuring device includes a magnetic field generating mechanism, a magnetic field measuring mechanism, a phase reference mechanism, and a control mechanism (not shown in the figure).

The magnetic field generating mechanism includes two pairs of Helmholtz coils with different radii (i.e., a pair of large Helmholtz coils 1 and a pair of small Helmholtz coils 2), the geometric centers of the two pairs of Helmholtz coils coincide, and the small Helmholtz coil 2 is arranged on the inner side of the large Helmholtz coil 1.

During operation, an external variable frequency sinusoidal power supply is used to pass reverse sinusoidal currents with varying frequency intervals through two pairs of Helmholtz coils to generate two external magnetic fields with different uniform regions and opposite directions. This magnetic field distribution places the sample in a uniform magnetic field and generates a point with zero magnetic field amplitude at a certain distance from the sample.

According to the magnetic field distribution of the two pairs of Helmholtz coils, the position of the magnetic field amplitude zero point and the volume of the magnetic field uniform area provided in this embodiment are jointly determined by the ratio of the radius of the two pairs of coils and the ratio of the number of turns. Therefore, the position of the magnetic field amplitude zero point and the volume of the magnetic field uniform area provided in this embodiment can be adjusted by changing the radius and the number of turns of the two pairs of coils, thereby realizing the AC magnetic susceptibility measurement of samples of different sizes.

The magnetic field measuring mechanism may adopt a magnetometer 3 that is insensitive to magnetic field gradient, such as a fluxgate magnetometer. The sensitive element of the magnetometer 3 is arranged at the zero point of the magnetic field amplitude generated by the magnetic field generating mechanism, which can reduce the residual background magnetic field generated by the Helmholtz coil at the magnetometer 3.

The magnetometer 3 provided in this embodiment is used to measure the amplitude and phase of the magnetic field at the location point when the sample is far away from or close to it. That is, when the sample is far away from the magnetometer 3, that is, when measuring at the far end, the induced magnetic field generated by the sample under the action of the external magnetic field is almost 0, and the signal measured by the magnetometer 3 is only the amplitude and phase of the residual background magnetic field of the coil; when the sample is close to the magnetometer 3, that is, when measuring at the near end, the sample is located in the magnetic field uniform area generated by the magnetic field generating mechanism, and the signal measured by the magnetometer 3 at this time is the amplitude and phase of the superposition field of the residual background magnetic field of the coil and the induced magnetic field generated by the sample under the external magnetic field.

It can be understood that the calculation formula for the induced magnetic field generated by a sample with a volume of V at point r' is:

Where r is the position coordinate vector of a magnetic dipole m(r) in the sample, m(r) is the magnetic moment at point r, and r′ represents the position coordinate vector of the measurement point (magnetometer 3). The integral is over the entire volume V of the sample.

Taking a cubic sample as an example, the condition for determining the magnetic dipole approximation is that the influence of the sample's geometric parameter V ^1/3 on the induced magnetic field measurement value B(r') can be ignored, that is, the magnetic field generated by the magnetic moments at different positions on the sample at the measurement point is almost equal. To meet this condition, the distance from the sample to the center of the Helmholtz coil must be much larger than the size of the sample along the direction of the external magnetic field.

In order to better equate the sample to a magnetic dipole, so that the magnetic susceptibility measurement error caused by the uneven distribution of the AC magnetic moment can be ignored, it is generally necessary to satisfy D/L≥10, where L represents the size of the sample along the direction of the external magnetic field, D represents the distance from the magnetometer 3 to the center of the Helmholtz coil, and D is related to the ratio of the number of turns of the two pairs of Helmholtz coils. When the number of turns of the large coil is smaller than the number of turns of the small coil, D is larger, and vice versa.

Therefore, in order to reduce the magnetic susceptibility measurement error caused by the uneven distribution of AC magnetic moment, the present embodiment needs to adjust the ratio of the number of turns of the two pairs of Helmholtz coils according to the size of the sample, so that the distance between the magnetic field amplitude zero point, that is, the placement point of the magnetometer, and the center point of the Helmholtz coil is far enough. Under the premise that the number of turns of the large Helmholtz coil is fixed, generally, the larger the size of the sample, the more turns the small Helmholtz coil needs. Specifically, the distance from the magnetometer 3 to the geometric center point of the two pairs of Helmholtz coils needs to be greater than 10 times the size of the sample along the magnetic field direction.

The phase reference mechanism may include a phase reference coil 7 that is far enough away from the sample and a magnetometer 8 for measuring the magnetic field generated by the coil. The phase reference coil 7 is connected in series with two pairs of Helmholtz coils in the magnetic field generating mechanism, so that the magnetic field generated by the phase reference coil 7 has the same phase as the external magnetic field applied to the sample. The phase of the magnetic field generated by the phase reference coil 7 is measured by the magnetometer 8 that is far away from the sample area and is not disturbed by the magnetic field generated by the sample, so that the reference phase measured by the magnetometer 8 can always be kept in phase with the external magnetic field, thereby providing a phase reference signal for the magnetometer 3, and then obtaining the phase information generated by the sample.

The control mechanism provided in this embodiment can adopt a controller commonly used in the art, which is used to calculate the phase and modulus of the magnetic susceptibility of the sample at different sinusoidal current frequencies based on the reference phase measured by the magnetometer 8 and the amplitude and phase of the coil residual background magnetic field and the superposition field measured by the magnetometer 3, and finally obtain the magnetic susceptibility spectrum distribution of the sample within a certain frequency range.

The device for measuring the AC magnetic susceptibility of a large-sized object provided in this embodiment uses two pairs of Helmholtz coils to generate a magnetic field amplitude zero point far away from the sample. The position of the magnetic field amplitude zero point can be adjusted by adjusting the radius and the number of turns of the coil, thereby enabling the measurement of the AC magnetic susceptibility of samples of different sizes; and the magnetic field measuring mechanism is set at the magnetic field amplitude zero point, and the radius and the number of turns of the coil are adjusted to make the distance from the magnetic field measuring mechanism to the geometric center of the two pairs of Helmholtz coils much larger than the size of the sample along the uniform magnetic field direction, which can effectively reduce the measurement error caused by the uneven distribution of the AC magnetic moment.

In one embodiment, as shown in FIG. 1 , the two pairs of Helmholtz coils provided above may be arranged on a coil mounting platform 10, and the coil mounting platform 10 may preferably be made of a non-magnetic non-metallic material with a relatively high hardness, such as PMMA, etc., so as to avoid the effects of residual magnetism and eddy currents affecting the measurement results.

In one embodiment, the measuring device provided by the present application may also include a displacement mechanism and a sample moving mechanism. The displacement mechanism is used to accurately adjust the spatial position of the magnetometer 3 so that the sensitive unit of the magnetometer 3 is close to the zero point of the magnetic field amplitude, thereby making the residual background magnetic field of the coil reach the minimum value, meeting the need to measure the magnetic field signal generated by the sample.

The sample moving mechanism is used to control the sample to move away from or close to the magnetometer 3, so as to obtain the magnetic field information generated by the sample, that is, when the sample is far away from the magnetometer 3, the induced magnetic field generated by the sample under the action of the external magnetic field is almost 0, and the signal measured by the magnetometer 3 is the amplitude and phase of the residual background magnetic field of the coil; when the sample is close to the magnetometer 3, the signal measured by the magnetometer 3 is the amplitude and phase of the superposition field of the residual background magnetic field of the coil and the induced magnetic field generated by the sample under the external magnetic field.

Specifically, the displacement mechanism may include a magnetometer mounting platform 4 , a magnetometer bracket 5 and a displacement platform 6 .

In this embodiment, the magnetometer mounting platform 4 is used to fix the magnetometer 3 and is connected to the magnetometer bracket 5. The two are rigidly connected as much as possible to avoid shaking of the position of the magnetometer 3. The magnetometer bracket 5 is used to connect the displacement stage 6 and the magnetometer mounting platform 4. The bracket has a suitable height, so as to ensure that the displacement stage 6 is far enough away from the magnetometer 3 to avoid the magnetism of the displacement stage 6 affecting the magnetic field to be measured, and also to avoid the magnetometer 3 from shaking too much due to the magnetometer 3 being too high from the ground. The displacement stage 6 is used to receive the control signal sent by the control mechanism to adjust the position of the magnetometer 3. The smaller the moving step of the displacement stage 6, the smaller the residual background magnetic field that can be obtained.

Furthermore, the measuring device provided in the present application may also include a vibration isolation platform 9, on which the coil mounting platform 10 and the displacement platform 6 are mounted. The vibration isolation platform 10 is used to reduce the relative position change between the Helmholtz coil and the magnetometer 3 caused by ground vibration. The relative position change δz between the magnetometer and the Helmholtz coil will introduce magnetic field noise. Therefore, reducing ground vibration noise is beneficial to reducing magnetic field noise. Preferably, the two pairs of Helmholtz coils and the coil mounting platform 10 and the coil mounting platform 10 and the vibration isolation platform 9 should be connected as rigidly as possible, which can further reduce the position fluctuation of the Helmholtz coils.

The sample moving mechanism may include a conveyor belt 16, a sample stage bracket 12, a conveyor belt mounting platform 15 and a motor 11. The sample stage bracket 12 is installed on the ground, the conveyor belt mounting platform 15 is fixed on the sample stage bracket 12, two bearings 14 are provided on the conveyor belt mounting platform 15, the conveyor belt 16 is sleeved on the two bearings 14, the sample is placed on the conveyor belt 16, and the motor 11 is connected to the bearings 14 through a transmission belt 13.

The working principle of the sample moving mechanism provided in this embodiment is: during measurement, the sample is placed on the end of the conveyor belt 16 farthest from the sample, that is, placed at the far end of the conveyor belt 16, the motor 11 is connected to the bearing 14 through the transmission belt 13, thereby driving the bearing 14 to rotate, and the rotation of the bearing 14 drives the movement of the conveyor belt 16, and the control mechanism controls the motor to rotate forward or reverse to make the sample approach or move away from the magnetometer, thereby obtaining the magnetic field information generated by the sample.

Preferably, the bearing 14 provided in this embodiment can adopt non-magnetic bearings, such as plastic bearings or ceramic bearings, and the motor 11 can adopt a weak magnetic stepping motor to avoid the magnetic field generated by its magnetism or conductivity from affecting the magnetic field to be measured. The length of the conveyor belt 16 should be long enough so that the sample is almost not generated. The magnetic field signal is not generated when the sample is away from the magnetometer. The length of the transmission belt 13 should be long enough to ensure that the motor 11 can be away from the magnetometer to avoid the magnetic field generated by the motor coupling into the magnetic field to be measured. Since the sample will cause the conveyor belt mounting platform 15 to shake or deform during movement, and the relative position change of the Helmholtz coil and the magnetometer is very sensitive, the sample stage bracket 12 should be installed separately on the ground, without contact with the coil and the coil mounting platform 10 and the vibration isolation platform 9, so as to avoid the vibration generated during the sample movement from being transmitted to the magnetometer and generating magnetic field noise.

The present application also provides a measurement method based on the above-mentioned measurement device, which comprises the following steps:

(1) The axial distribution (Z direction) of the applied magnetic field is calculated based on the parameters such as the radius, spacing and number of turns of the two pairs of Helmholtz coils. As shown in FIG2 , it can be obtained that there are two intersection points between the variation curve of the magnetic field in the Z direction and the horizontal coordinate. The intersection point is the zero point of the magnetic field amplitude. The magnetometer 3 is placed near the zero point of the magnetic field amplitude and fixed on the magnetometer mounting platform 4 with a clamp.

(2) Square wave currents in opposite directions are passed through the two pairs of Helmholtz coils. At this time, the magnetic field amplitude measured by the magnetometer 3 is relatively large. Then the magnetometer 3 is moved toward the axial direction of the coil (Z direction), and the change of the magnetic field measured by the magnetometer is observed. If the magnetic field amplitude decreases, the magnetometer 3 is continued to be moved in this direction. Otherwise, the magnetometer 3 is moved in the opposite direction until the measured magnetic field amplitude decreases within the set range, so that the magnetic field noise δB is lower than the magnetic field measurement resolution requirement ΔB. Then, the magnetometer 3 is stopped from being moved, and the magnetometer 3 is kept at this position and does not change.

In step (2), when the measured magnetic field amplitude decreases to within the set range, it indicates that the measurement conditions are met and the AC magnetic susceptibility can be measured.

(3) The phase reference coil 7 and the magnetometer 8 in the phase reference mechanism are placed away from the sample, and the phase reference coil 7 is connected in series with two pairs of Helmholtz coils to measure the phase of the external magnetic field.

It should be noted that step (3) provided in this embodiment may be performed before step (1) or after step (1) or (2), and this embodiment does not impose any limitation thereto.

(4) The sample is placed at the far end of the conveyor belt 16 and fixed, and then a reverse sinusoidal current with an amplitude of I ₀ and a frequency of ω is passed through the two pairs of Helmholtz coils. The magnetometer 3 measures the magnetic field at a sampling frequency higher than 2ω. At this time, since the sample is far enough away from the magnetometer, the signal generated by the sample under the action of the external magnetic field is almost 0. The signal measured by the magnetometer 3 is the amplitude B _{z0 and phase B z0} of the residual background magnetic field B _z0t of the coil.

After measuring for a period of time, the controller controls the motor 11 to move the sample to the near end of the conveyor belt 16 and keep it there for a period of time. At this time, the signal measured by the magnetometer 3 is the amplitude B _z2 and phase of the superposition field B _z2t of the coil residual background magnetic field B _z0t and the induced magnetic field B _z1t generated by the sample under the action of the external magnetic field. The three satisfy the following relationship:

Where _Bz1 is the amplitude of the induced magnetic field _Bz1t , is the phase of the induced magnetic field B _z1t .

(5) The controller measures the reference phase according to the magnetometer 8 The amplitude B _z0 and phase of the coil residual background magnetic field measured by magnetometer 3 and the amplitude B _z2 and phase of the superposition field B _z2t Calculate the phase and magnitude of the magnetic susceptibility of the sample at frequency ω.

Using the auxiliary angle formula of trigonometric functions, we can get:

It should be noted that since the phase is a relative quantity, not an absolute quantity, it must be subtracted from a constant reference phase to obtain the phase magnitude. is the phase value of the coil residual background magnetic field B _z0t , is the phase value of the superposition field B _z2t , is the phase value of the induced magnetic field B _z1t generated by the sample under the action of the external magnetic field, that is, the phase of the magnetic susceptibility of the sample at the frequency ω, in, is the phase of the external magnetic field measured by the magnetometer 8.

according to and The calculation formula can be obtained:

B _z0 , B _z2 , and Substituting into formulas (1) and (2), we can obtain the magnetic field B _z1 generated by the sample and the phase difference between the magnetic field generated by the sample and the external magnetic field, that is, the phase of the magnetic susceptibility of the sample at frequency ω:

When the magnetometer is far enough away from the sample, the magnetic field B _z1 generated by the sample at the magnetometer 3 and the total magnetic moment M of the sample satisfy the following relationship:

In the formula, R represents the relative position vector between the magnetometer and the geometric center of the two pairs of Helmholtz coils; K is a constant matrix related only to the relative position of the two. Based on the above known measured and calculated quantities, the modulus of the AC magnetic susceptibility of the sample is:

In the formula, H represents the magnitude of the external magnetic field applied to the sample; V represents the volume of the sample. So far, the modulus value |χ(ω)| and phase of the sample at frequency ω are Measurement completed.

(6) The frequency of the sinusoidal current is changed, and the phase and modulus of the magnetic susceptibility of the sample at other frequencies are measured, and finally the spectral distribution of the magnetic susceptibility of the sample within a certain frequency range is obtained.

The measurement method provided in the above embodiment is described below in conjunction with a specific embodiment:

The AC magnetic susceptibility of a rectangular (4cm×4cm×1cm) pure copper sample in the range of 1Hz to 1500Hz was measured using the measurement method provided above. The time domain curve measured at 1500Hz is shown in Figure 3, and the frequency domain curve is shown in Figure 4. The modulus spectrum of the AC magnetic susceptibility is shown in Figure 5, and the phase spectrum is shown in Figure 6. The measurement results are consistent with the conclusion proposed in this application that the modulus of the AC magnetic susceptibility increases continuously with increasing frequency and eventually tends to saturation, proving the accuracy and reliability of the measurement method provided in this embodiment.

It will be easily understood by those skilled in the art that the above is only a preferred embodiment of the present application and is not intended to limit the present application. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present application shall be included in the scope of protection of the present application.

Claims (10)

1. The measuring device of the alternating-current magnetic susceptibility of the large-size object is characterized by comprising a magnetic field generating mechanism, a magnetic field measuring mechanism, a phase reference mechanism and a control mechanism;

The magnetic field generating mechanism comprises two pairs of Helmholtz coils with coincident geometric centers and different radiuses, the Helmholtz coils with small radiuses are arranged on the inner sides of the Helmholtz coils with large radiuses, and the two pairs of Helmholtz coils are fed with reverse sinusoidal currents with frequency interval changes and used for generating a uniform magnetic field covering the area where a sample is located and a magnetic field amplitude zero point far away from the sample;

The magnetic field measuring mechanism is arranged at the zero point of the magnetic field amplitude, the distance from the geometric centers of the two pairs of Helmholtz coils is more than 10 times of the dimension of the sample along the magnetic field direction, and the magnetic field measuring mechanism is used for measuring the magnetic field and the phase at the position point when the sample is far away from or near to the sample; when the sample approaches, the sample is positioned in a magnetic field uniform area generated by the magnetic field generating mechanism;

The phase reference mechanism is used for providing and measuring a reference phase which is in phase with the uniform magnetic field;

the control mechanism is used for calculating the phase and the modulus of the magnetic susceptibility of the sample at different sinusoidal current frequencies according to the reference phase, the magnetic field and the phase measured by the magnetic field measuring mechanism when the sample is far away from or near to obtain the magnetic susceptibility spectrum distribution of the sample in a certain frequency range.

2. The apparatus for measuring ac susceptibility of a large object according to claim 1, wherein the sample is a proof mass.

3. A device for measuring ac susceptibility of large objects according to claim 1 or2, wherein the phase reference means comprises a phase reference coil arranged remote from the sample and a magnetometer for measuring the magnetic field generated by the phase reference coil, the phase reference coil being connected in series with two pairs of helmholtz coils.

4. The measuring device for alternating current magnetic susceptibility of large object according to claim 1 or 2, wherein the magnetic field measuring means measures amplitude B _z0 and phase of residual background magnetic field of coil when the sample is far awayWhen the sample is close, the magnetic field measuring mechanism measures and obtains the residual background magnetic field of the coil and the amplitude B _z2 and the phase of the superimposed field of the induced magnetic field generated by the sample under the action of the uniform magnetic field

The modulus |χ (ω) | of the susceptibility of the sample at the sinusoidal current frequency ω is calculated as:

Amplitude B _z1 and phase of the induced magnetic field The calculation formula of (2) is as follows:

phase of susceptibility of sample at sinusoidal current frequency ω The calculation formula of (2) is as follows:

in the method, in the process of the invention, A reference phase measured for the phase reference mechanism; for the phase value of the residual background magnetic field of the coil, In order to superimpose the phase values of the fields,K is a constant matrix related to the relative positions of the magnetic field measuring mechanism and the geometric centers of the two pairs of Helmholtz coils; v is the volume of the sample; h is the magnitude of the uniform magnetic field.

5. The device for measuring alternating current magnetic susceptibility of a large object according to claim 1, further comprising a displacement mechanism and a sample moving mechanism;

The displacement mechanism is used for adjusting the azimuth of the sample to enable the sample to be far away from or close to the magnetic field measuring mechanism; the sample moving mechanism is used for adjusting the spatial position of the magnetic field measuring mechanism, so that the magnetic field measuring mechanism is positioned at the zero point of the amplitude of the magnetic field.

6. The device for measuring ac magnetic susceptibility of large-sized object according to claim 5, wherein the displacement mechanism includes a magnetometer mounting platform, a magnetometer support and a displacement table, the magnetic field measuring mechanism is fixed on the magnetometer mounting platform, the magnetometer mounting platform is rigidly connected to the magnetometer support, the magnetometer support is mounted on the displacement table, and the displacement table is used for receiving and adjusting the spatial position of the magnetic field measuring mechanism according to the first control signal sent by the control mechanism.

7. The device for measuring ac magnetic susceptibility of large-sized object according to claim 5, wherein the sample moving mechanism includes a conveyor belt, a sample table support, a conveyor belt mounting platform and a motor, the sample table support is mounted on the ground, the conveyor belt mounting platform is fixed on the sample table support, two bearings are provided on the conveyor belt mounting platform, the conveyor belt is sleeved on the two bearings, the sample is placed on the conveyor belt, the motor is connected with the bearings through the conveyor belt, and the motor is used for receiving and driving the bearings to rotate according to a second control signal sent by the control mechanism, so as to drive the conveyor belt to move, so that the sample is far away from or near the magnetic field measuring mechanism.

8. The device for measuring ac susceptibility of large-sized objects according to claim 5, wherein the magnetic field generating mechanism is fixed on a coil mounting platform, the coil mounting platform is made of non-magnetic nonmetallic material, and the coil mounting platform and the sample moving mechanism are mounted on a vibration isolation platform.

9. The device for measuring ac susceptibility of a large object according to claim 8, wherein the two pairs of helmholtz coils are rigidly connected to the coil mounting platform and the coil mounting platform is rigidly connected to the vibration isolation platform.

10. A measuring method based on the measuring device for alternating current magnetic susceptibility of large-sized object according to claim 1, comprising the steps of:

Calculating a magnetic field amplitude zero point according to parameters of the two pairs of Helmholtz coils, and placing a magnetic field measuring mechanism near the magnetic field amplitude zero point;

square wave currents with opposite directions are introduced into the two pairs of Helmholtz coils, the magnetic field measuring mechanism is moved towards the axial direction of the coils, meanwhile, the magnetic field change condition measured by the magnetic field measuring mechanism is observed, and the magnetic field measuring mechanism is kept at the position point until the measured magnetic field amplitude is reduced within a set range;

The phase reference mechanism is placed far away from the sample and is connected in series with two pairs of helmholtz coils;

The method comprises the steps that a sample is far away from a magnetic field measuring mechanism, then, reverse sinusoidal current with the frequency omega is introduced into two pairs of Helmholtz coils, and the magnetic field measuring mechanism measures the residual background magnetic field of the coils; after measuring for a period of time, the sample is close to the magnetic field measuring mechanism and kept for a period of time, and the magnetic field measuring mechanism measures the residual background magnetic field of the coil and the superimposed field of the induced magnetic field generated by the sample under the action of the uniform magnetic field;

The control mechanism calculates the phase and the mode value of the magnetic susceptibility of the sample at the frequency omega according to the reference phase measured by the phase reference mechanism and the amplitude and the phase of the coil residual background magnetic field and the superimposed field;

And (3) changing the frequency of the sinusoidal current, measuring the phase and the modulus of the magnetic susceptibility of the sample at other frequencies, and finally obtaining the magnetic susceptibility spectrum distribution of the sample in a certain frequency range.