CN110568384B - Active magnetic compensation method for ultra-sensitive atomic magnetometer - Google Patents

Abstract

The invention discloses an active magnetic compensation method for an ultra-sensitive atomic magnetometer, which obtains a magnetic field value

And linearly varying voltage

Slope of the fit

And intercept of

(ii) a Setting the output voltage range, the damping factor and the sampling rate of the DAC conversion unit; calculating power frequency magnetic field noise amplitude

And phase

The DAC conversion unit outputs the compensation voltage

For the voltage-controlled current source module, the invention is suitable for the compensation of static and power frequency magnetic field noise, can realize the compensation of high-frequency magnetic field noise, and has the compensation effect limited only by the magnetic field resolution and the sampling rate of the magnetic measurement unit. Compared with the traditional PID magnetic compensation method, the compensation speed is higher, and the compensation effect on the power frequency magnetic field noise is more obvious.

Description

Active magnetic compensation method for ultra-sensitive atomic magnetometer

Technical Field

The invention belongs to the fields of software algorithms, precise magnetism measurement and the like, and particularly relates to an active magnetic compensation method for an ultra-sensitive atomic magnetometer, which is suitable for researches in relevant fields of atomic physical experiments, biomedicine, precise measurement and the like.

Background

The atomic magnetometer is a super-sensitive magnetic measurement tool that can be compared with a Superconducting quantum interference device (SQUID) and does not require a low-temperature environment, and has been currently applied to measurement of Nuclear Magnetic Resonance (NMR), Magnetocardiography (MCG), Magnetoencephalography (MEG), magnetic materials, and the like according to some related literature reports. Generally, most atomic magnetometers passively shield magnetic fields in the environment using multi-layer high permeability alloys, however, techniques and methods of passive shielding limit the range of applications of atomic magnetometers, such as geomagnetic measurements, spatial magnetic field measurements, magnetic anomaly measurements, and the like. Therefore, it is necessary to develop a novel active magnetic compensation atomic magnetometer and invent a novel active magnetic compensation method, so as to reduce or eliminate power frequency magnetic field noise which is one of the main factors limiting the sensitivity of the active magnetic compensation type atomic magnetometer, and lay a foundation for realizing the high-precision active magnetic compensation type atomic magnetometer.

There are many common Active magnetic compensation methods, for example, in the document "Active shielding to reduce low frequency interference in direct current biological magnetic measurements" [ rev. sci. instrum.70,2465(1999) ], a magnetic field in the environment is measured by a magnetic sensor, a PID controller generates a feedback signal according to the measured magnetic field and enters a current source, and then the current source outputs a current and enters a helmholtz coil to generate a uniform magnetic field, thereby realizing the external environment magnetic field compensation. The method can realize the compensation of the static magnetic field, but has no obvious compensation effect on the power frequency magnetic field noise. The literature, "Active cancellation of linear fields in a Bose-insertion condensation experiment" [ rev.sci.instrum.78,024703(2007) ] reports that the compensation of power frequency magnetic field noise is realized by using an independent inductance coil, and the method needs to select a coil with proper capacitive reactance, and has certain limitations. In the literature, "Magnetic field stabilization system for atomic physical experiments" [ rev.sci.instrum.90,044702(2019) ], it is reported that the compensation of power frequency Magnetic field noise is realized by using calcium ions as a Magnetic measurement sensor and combining a positive feedback circuit, however, the method is only suitable for compensating the power frequency Magnetic field noise with a fixed phase.

The invention provides an active magnetic compensation method for an ultra-sensitive atomic magnetometer, which comprises the compensation of static and power frequency magnetic field noise. The magnetic field in the environment is measured by using the magnetic measurement unit, the magnetic compensation program in the computer is used for analyzing and measuring magnetic field signals, the phase and the amplitude of the signals are extracted rapidly in real time, a single current source and a single group of Helmholtz coils are used for generating opposite magnetic fields, and the magnetic field value after compensation is less than 20 nT. Compared with the traditional magnetic compensation method by utilizing a Proportion-integration-differentiation (PID) control mode, the method has better attenuation effect on power frequency magnetic field noise.

Disclosure of Invention

The invention aims to provide an active magnetic compensation method for an ultra-sensitive atomic magnetometer, aiming at solving the problems in the prior art and method, and solving the compensation of power frequency magnetic field noise and static magnetic field noise.

The aim of the invention is achieved by the following technical measures:

an active magnetic compensation method for an ultrasensitive atomic magnetometer, comprising the steps of:

step 1, controlling a DAC conversion unit to output a linearly-changed voltage V by a computer_linLinearly varying voltage V_linActing on the Helmholtz coil through the voltage-controlled current source module, placing the magnetism measuring unit at the central position of the Helmholtz coil, and measuring magnetismThe unit measures the magnetic field value B_magWith a linearly varying voltage V_linPerforming linear fitting to obtain a magnetic field value B_magWith a linearly varying voltage V_linThe slope k and intercept a of the fit;

step 2, setting the rotation speed of the non-magnetic rotating platform, enabling the Helmholtz coil and the magnetism measuring unit to rotate in a horizontal plane along with the non-magnetic rotating platform, enabling the magnetism measuring unit to measure a magnetic field in real time, obtaining an environmental magnetic field distribution data range, setting an output voltage range of the DAC conversion unit to be (an environmental magnetic field distribution data range-a)/k, and setting a damping factor and a sampling rate;

step 3, the Helmholtz coil is not electrified;

step 4, measuring the magnetic field value B by the magnetic measuring unit_NMagnetic field value B_NIncluding power frequency magnetic field noise a_NAnd static magnetic field noise b_NWherein N represents the number of current cycles of step 4 and step 5;

B_N＝a_N+b_N

step 5, power frequency magnetic field noise amplitude | a_NI and phase θ_NAdopting the following calculation method, wherein Re is power frequency magnetic field noise a_NThe real part after the frequency spectrum conversion is carried out, Im is power frequency magnetic field noise a_NThe imaginary part after the frequency spectrum transformation is carried out, f represents the power frequency magnetic field noise a needing to be compensated_NI ∈ {0 to (n-1) }, t being the time difference between the sample point sequence number and the corresponding associated output sequence number;

step 6, outputting the compensation voltage V 'by the DAC conversion unit'_NTo the voltage-controlled current source module, compensating the voltage V'_NObtained by the following formula, v_NOutputting a compensation voltage, V, for static magnetic field noise_NOutputting compensation voltage for power frequency magnetic field noise, and damming is a damping factor;

V′_N＝v_N+V_N

and 7, repeating the steps 4-6.

Compared with the prior art and the method, the invention has the following beneficial effects:

the invention is suitable for the compensation of static and power frequency magnetic field noise, can realize the compensation of other high-frequency magnetic field noise in the magnetic measuring frequency band of the magnetic measuring unit, and the final limiting factors of the compensation effect are the magnetic field resolution and the sampling rate of the magnetic measuring unit. Compared with the traditional PID magnetic compensation method, the compensation speed is higher, and the compensation effect on the power frequency magnetic field noise is more obvious.

Drawings

FIG. 1 is a general workflow block diagram of the present invention;

FIG. 2 is a schematic diagram of a magnetic compensation device used in an active magnetic compensation method for an ultrasensitive atomic magnetometer;

1-helmholtz coil; 2-a magnetic measuring unit; 3, supporting the table; 4-non-magnetic rotating platform.

FIG. 3 is a graph showing the results of the measured magnetic field before compensation;

FIG. 4 is a graph showing the results of measuring the magnetic field after magnetic compensation.

Detailed Description

The present invention will be described in further detail with reference to examples for the purpose of facilitating understanding and practice of the invention by those of ordinary skill in the art, and it is to be understood that the present invention has been described in the illustrative embodiments and is not to be construed as limited thereto.

As shown in fig. 2, the active magnetic compensation device includes a helmholtz coil 1, a magnetism measuring unit 2, a support 3, and a nonmagnetic turntable 4.

The helmholtz coil 1 is used to generate a uniform magnetic field to compensate for magnetic field noise in the external environment.

The magnetism measuring unit 2 is used for measuring a magnetic field and magnetic field noise in an external environment in real time.

The support base 3 is used to support the magnet measuring unit 2 and is fixed to the magnet measuring unit 2.

The non-magnetic rotating platform 4 is used for realizing the distribution of the magnetic field and the magnetic field noise in the measuring environment of the magnetic measuring unit 2, thereby determining the optimal magnetic compensation parameters.

As shown in fig. 2, the magnetism measuring unit 2 is placed at the center of the helmholtz coil 1, in this embodiment, the length L of the side of the helmholtz coil 1 is 400mm, and the coil distance D is 218 mm. The magnetism measuring unit 2 is fixed on the non-magnetic rotating platform 4 through the bottom end of the supporting platform 3 in a threaded mode, and the Helmholtz coil 1 and the non-magnetic rotating platform 4 are tightly connected and fixed through Teflon screws.

As shown in fig. 1, the computer is connected to the input terminal of the DAC conversion unit, the output terminal of the DAC conversion unit is connected to the input terminal of the voltage-controlled current source, the output terminal of the voltage-controlled current source is connected to the helmholtz coil 1, the output terminal of the magnetism measurement unit 2 is connected to the input terminal of the ADC conversion unit, and the output terminal of the ADC conversion unit is connected to the computer.

An active magnetic compensation method for an ultrasensitive atomic magnetometer, comprising the steps of:

step 1, controlling a DAC conversion unit to output a linearly-changed voltage V by a computer_linLinearly varying voltage V_linActing on the Helmholtz coil 1 through the voltage-controlled current source module, and placing the magnetism measuring unit 2 in the center of the Helmholtz coil 1Position, magnetic field value B measured by magnetic measuring unit 2_magLeading the magnetic field into a computer, and obtaining a magnetic field value B through linear fitting_magWith a linearly varying voltage V_linSatisfies B_mag＝k*V_lin+ a relationship, k and a representing the magnetic field value B_magWith a linearly varying voltage V_linSlope and intercept of the fit;

and 2, setting the rotation speed of the non-magnetic rotating platform 4, wherein the Helmholtz coil 1 and the magnetism measuring unit 2 rotate in the horizontal plane along with the non-magnetic rotating platform 4. The magnetism measuring unit 2 measures a magnetic field in real time, obtains an environment magnetic field distribution data range, sets an output voltage range of the DAC conversion unit as (the environment magnetic field distribution data range-a)/k, and sets a damping factor and a sampling rate.

Step 3, no current is introduced into the Helmholtz coil 1;

step 4, the magnetic field measuring unit 2 measures a magnetic field value B_NMagnetic field value B_NIncluding power frequency magnetic field noise a_NAnd static magnetic field noise b_NWherein N represents the number of current cycles of step 4 and step 5;

B_N＝a_N+b_N

step 5, power frequency magnetic field noise amplitude | a_NI and phase θ_NThe following calculation method is adopted, wherein Re is power frequency magnetic field noise a_NThe real part after the frequency spectrum conversion is carried out, Im is power frequency magnetic field noise a_NThe imaginary part after the frequency spectrum transformation is carried out, f represents the power frequency magnetic field noise a needing to be compensated_NN represents the total number of samples, fs represents the sampling rate, i ∈ {0 to (n-1) };

step 6, outputting the compensation voltage V 'by the DAC conversion unit'_NWhen the voltage-controlled current source module is supplied with N ═ 0, namely, the compensation voltage V 'of the initial value'₀Is 0, v_NOutputting a compensation voltage, V, for static magnetic field noise_NOutputting compensation voltage for power frequency magnetic field noise and initially outputting compensation voltage v for static magnetic field noise₀Initial output compensation voltage V with power frequency magnetic field noise₀All are 0, k is the magnetic field value B obtained in step 1_magWith a linearly varying voltage V_linThe slope of the fit, damping factor (damming), is set to 0.6, mainly to reduce the low frequency magnetic field noise after magnetic compensation.

V′_N＝v_N+V_N

From step 5, it can be learned that_NThe compensation voltage V 'is obtained from step 6 for a series of values arranged by the sampling point number'_NAlso a series of values arranged according to the output serial number, and the sampling point serial number is associated with the output serial number in a one-to-one correspondence. T in step 5 is set as the time difference between the sampling point sequence number and the corresponding associated output sequence number.

And 7, repeating the operations of the steps 4 to 6, and finally realizing the real-time compensation of the magnetic field noise in the external environment. After the above steps are completed, the magnetic compensation operation is suspended and the post-compensation magnetic field data is derived, the result being shown in fig. 4 and compared with the pre-compensation magnetic field data shown in fig. 3. The conclusion can be drawn that the compensation method realizes the great attenuation of static and power frequency magnetic field noise, and the static and power frequency magnetic field noise is respectively attenuated by 40 dB and 20 dB.

The specific embodiments described herein are merely illustrative of the spirit of the invention. Various modifications or additions may be made to the described embodiments or alternatives may be employed by those skilled in the art without departing from the spirit or ambit of the invention as defined in the appended claims.

Claims (1)

1. An active magnetic compensation method for an ultrasensitive atomic magnetometer, comprising the steps of:

step 1, controlling a DAC conversion unit to output a linearly-changed voltage V by a computer_linLinearly varying voltage V_linThe voltage-controlled current source module acts on the Helmholtz coil (1), the magnetism measuring unit (2) is placed at the central position of the Helmholtz coil (1), and the magnetism measuring unit (2) measures a magnetic field value B_magWith a linearly varying voltage V_linPerforming linear fitting to obtain a magnetic field value B_magWith a linearly varying voltage V_linThe slope k and intercept a of the fit;

step 2, setting the rotation speed of the non-magnetic rotating platform (4), enabling the Helmholtz coil (1) and the magnetism measuring unit (2) to rotate in the horizontal plane along with the non-magnetic rotating platform (4), enabling the magnetism measuring unit (2) to measure a magnetic field in real time, obtaining an environmental magnetic field distribution data range, setting an output voltage range of the DAC conversion unit to be (an environmental magnetic field distribution data range-a)/k, and setting a damping factor and a sampling rate;

step 3, the Helmholtz coil (1) is not electrified;

step 4, measuring the magnetic field value B by the magnetic measuring unit (2)_NMagnetic field value B_NIncluding power frequency magnetic field noise a_NAnd static magnetic field noise b_NWherein N represents the number of current cycles of step 4 and step 5;

B_N＝a_N+b_N

step 5, power frequency magnetic field noise amplitude | a_NI and phase θ_NAdopting the following calculation method, wherein Re is power frequency magnetic field noise a_NReal part after spectral transformation, Im isPower frequency magnetic field noise a_NThe imaginary part after the frequency spectrum transformation is carried out, f represents the power frequency magnetic field noise a needing to be compensated_NI ∈ {0 to (n-1) }, t being the time difference between the sample point sequence number and the corresponding associated output sequence number;

step 6, outputting the compensation voltage V 'by the DAC conversion unit'_NTo the voltage-controlled current source module, compensating the voltage V'_NObtained by the following formula, v_NOutputting a compensation voltage, V, for static magnetic field noise_NOutputting compensation voltage for power frequency magnetic field noise, and damming is a damping factor;

V′_N＝v_N+V_N

and 7, repeating the steps 4-6.

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