A dual Helmholtz control device for dynamic magnetic field compensation

Abstract

The present invention discloses a dual Helmholtz control device for dynamic magnetic field compensation, comprising: a reverse Helmholtz coil, a forward Helmholtz coil, a fluxgate sensor, a uniform field compensation module, and a gradient compensation module; the reverse Helmholtz coil and the forward Helmholtz coil are parallel and opposite to each other, and are used to generate an induced magnetic field; the fluxgate sensor is used to convert the magnetic induction intensity of the induced magnetic field into a voltage signal; the uniform field compensation module is used to provide a gradient magnetic field for the reverse Helmholtz coil to cancel the non-uniform term in the geomagnetic field; the gradient compensation module is used to make the forward Helmholtz coil generate a corresponding bias uniform magnetic field to cancel the uniform term in the geomagnetic field. The dual Helmholtz coil structure adopted by the present invention compensates the uniform term and the non-uniform term in the geomagnetic field while reducing the size of the shielding device, and can provide an extremely weak magnetic environment with a large uniform area.

Description

Technical Field

[0001] The present invention belongs to the field of heart and brain magnetic control, and particularly relates to a double Helmholtz control device for dynamic magnetic field compensation. Background Art

[0002] The measurement of heart and brain magnetism is a very important research direction in biomedicine. Brain magnetism is the local magnetic field generated during the transmission of nerve signals controlled by brain nerve tissues. The magnitude of this extremely weak magnetic field is usually below 100 fT. Cardiac magnetism is the magnetic field generated during the cardiac beating process accompanied by activation currents, and the amplitude of the corresponding magnetic field is one order of magnitude higher than that of the brain magnetic field. The main measurement methods for both are superconducting quantum interference devices (SQUIDs).

[0003] The superconducting quantum interference device sensor array magnetic imaging technology is a widely accepted technology for measuring the weak magnetic fields generated by human physiological phenomena. However, since the superconducting quantum interference device can only achieve ultra-high sensitivity under low-temperature conditions, its wide application is limited. The spin-exchange relaxation-free atomic magnetometer (SERF) is a solution to replace the superconducting quantum interference device at room temperature, but it requires a very low ambient magnetic field, and the intensity of the geomagnetic field is 30000 - 50000 nT. Therefore, a dynamic magnetic field compensation device is required to offset the geomagnetic field so that the magnetic induction intensity in the central region of the device reaches the working range of the atomic magnetometer. The high-permeability shielding materials used in general magnetic shielding devices are expensive, and the gradient coils used increase the volume of the active magnetic compensation device, resulting in an increase in magnetic shielding costs. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to overcome the deficiencies of general magnetic shielding devices in terms of cost and volume, and construct a dynamic magnetic field compensation device with miniaturization, low cost, and capable of providing an extremely weak magnetic environment with a large uniform area.

[0005] To achieve the above object, the present invention provides the following solution: A double Helmholtz control device for dynamic magnetic field compensation, comprising:

[0006] A reverse Helmholtz coil, a forward Helmholtz coil, a fluxgate sensor, a uniform field compensation module, and a gradient compensation module;

[0007] The reverse Helmholtz coil and the forward Helmholtz coil are parallel and opposite to each other, and are used to generate an induced magnetic field;

[0008] The fluxgate sensor is used to convert the magnetic induction intensity of the induced magnetic field into a voltage signal;

[0009] The uniform field compensation module is used to provide a gradient magnetic field for the reverse Helmholtz coil to offset the non-uniform term in the geomagnetic field;

[0010] The gradient compensation module is used to make the forward Helmholtz coil generate a corresponding bias uniform magnetic field to cancel the uniform term in the geomagnetic field.

[0011] Preferably, the uniform field compensation module includes a first PID controller, a first digital-to-analog converter, a first voltage-controlled current source, and a first analog-to-digital converter connected in sequence;

[0012] The first PID controller is used to generate a control quantity to act on the first voltage-controlled current source to provide current for the forward Helmholtz coil and generate a uniform magnetic field.

[0013] Preferably, the gradient compensation module includes a differential operation circuit, a second analog-to-digital converter, a second PID controller, a second digital-to-analog converter, and a second voltage-controlled current source connected in sequence;

[0014] The second analog-to-digital converter is used to convert the voltage signal into a digital signal;

[0015] The second PID controller is used to generate a control quantity to act on the second voltage-controlled current source to provide current for the reverse Helmholtz coil and generate a gradient magnetic field.

[0016] Preferably, the fluxgate sensor includes a first fluxgate sensor and a second fluxgate sensor;

[0017] The first fluxgate sensor and the second fluxgate sensor are respectively connected to the differential operation circuit;

[0018] The first fluxgate sensor is connected to the first analog-to-digital converter.

[0019] Preferably, the control device further includes a compensation control module, and the compensation control module is used to prevent coupling between gradient compensation and uniform field compensation;

[0020] The compensation control module includes a first control unit and a second control unit;

[0021] The first control unit is used to control the gradient compensation module to perform gradient compensation;

[0022] The second control unit is used to judge whether the magnetic field output by the gradient compensation module is stable, and when the magnetic field is stable, control to turn on the uniform field compensation module.

[0023] Preferably, the induced magnetic fields generated by the reverse Helmholtz coil and the forward Helmholtz coil are superimposed magnetic fields, and the relationship between the magnetic induction intensity of the superimposed magnetic field and the Z-axis direction is:

[0024] B sum =kz + B h

[0025] Among them, k is the change rate of the magnetic field generated by the reverse Helmholtz coil along the z direction, and B h is the magnitude of the bias uniform magnetic field generated by the square Helmholtz coil.

[0026] Preferably, the magnetic induction intensity component of the single-stage coil of the forward Helmholtz coil on the Z-axis is obtained based on the vacuum permeability, the current passing through the coil, the spacing between the two-pole coils of the forward Helmholtz coil, and the side length of the forward Helmholtz coil.

[0027] Preferably, the calculation formula for the magnetic induction intensity of the gradient magnetic field is:

[0028]

[0029] Among them, B Zi is the magnetic induction intensity component of the single-stage coil of the reverse Helmholtz coil on the Z-axis, and I is the current passing through the coil.

[0030] The present invention discloses the following technical effects:

[0031] A dual Helmholtz control device for dynamic magnetic field compensation provided by the present invention uses a square Helmholtz coil to generate a uniform magnetic field and a reverse Helmholtz coil to generate a gradient magnetic field. The magnetic fields generated by the two pairs of coils are superimposed in the central region, thereby canceling the uniform term and the non-uniform term in the geomagnetic field. The voltage signal generated by the fluxgate sensor is used as a negative feedback signal to form a closed-loop stable control of the coil current. It not only reduces the volume of the magnetic shielding device and the material cost, but also can generate a very weak magnetic environment with a large uniform area in the central region of the device. Description of the Drawings

[0032] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required to be used in the embodiments. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0033] Figure 1 It is a schematic diagram of the system structure of the embodiment of the present invention;

[0034] Figure 2 It is a schematic diagram of the principle of the dual Helmholtz coil of the embodiment of the present invention;

[0035] Figure 3 It is a COMSOL simulation model diagram of the embodiment of the present invention;

[0036] Figure 4 It is a magnetic field simulation result diagram generated by the embodiment of the present invention. Detailed Embodiments

[0037] Next, in combination with the accompanying drawings in the embodiments of the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without making creative efforts shall fall within the protection scope of the present invention.

[0038] To make the above objects, features, and advantages of the present invention more obvious and understandable, the present invention will be further described in detail below in combination with the accompanying drawings and specific embodiments.

[0039] The double Helmholtz control device for dynamic magnetic field compensation of the present invention mainly includes a uniform field compensation module and a gradient compensation module, a pair of square Helmholtz coils, and a pair of reverse Helmholtz coils. Both modules include an analog-to-digital converter, a digital-to-analog converter, a PID controller, a voltage-controlled current source, and a fluxgate sensor. The device converts the magnetic induction intensity into a voltage signal through the fluxgate sensor. Among them, the gradient compensation module first passes the voltage signals output by the two fluxgate sensors through a differential circuit, and then converts them into digital signals through an analog-to-digital converter and enters the PID controller. The generated control quantity acts on the voltage-controlled current source to provide current for the reverse Helmholtz coil, so as to generate a gradient magnetic field. The uniform field compensation module directly uses the output voltage signal of a single fluxgate sensor, enters the PID controller through the analog-to-digital converter, and the control quantity generated by the controller acts on the voltage-controlled current source to provide current for the square Helmholtz coil to generate a uniform magnetic field. Finally, a magnetic field with a first-order linear relationship with the axial position is generated in the central region of the double Helmholtz coil to offset the geomagnetic field and achieve an extremely weak magnetic environment in the central region.

[0040] As Figure 1 shown, the present invention provides a double Helmholtz control device for dynamic magnetic field compensation, including:

[0041] Gradient compensation module:

[0042] Taking the gradient ΔB ref = 0 as a reference quantity, the differential signals of the two fluxgate sensors are converted into digital quantities through an analog-to-digital converter and enter the controller to be compared with the reference quantity to obtain a deviation quantity. The PID control algorithm is used in the controller to generate a control quantity, which is converted into an analog quantity through a digital-to-analog converter as the reference quantity of the voltage-controlled voltage source. Finally, the reverse Helmholtz coil is made to provide an appropriate gradient magnetic field to offset the non-uniform term in the geomagnetic field.

[0043] Uniform field compensation module:

[0044] Taking the magnetic induction intensity B refTaking 0 as the reference quantity, the signal of a fluxgate sensor is converted into a digital quantity through an analog-to-digital converter and enters the controller. The obtained deviation quantity enters the PID controller. The control quantity generated by the PID controller is converted into a digital quantity through a digital-to-analog converter and acts on the voltage-controlled current source to output current, ultimately enabling the square Helmholtz coil to generate a corresponding bias uniform magnetic field to cancel the uniform term in the geomagnetic field.

[0045] To prevent coupling between gradient compensation and uniform field compensation and simplify the parameter adjustment process, gradient compensation is first performed. After the magnetic field output by the gradient compensation module stabilizes, the uniform field compensation module is then turned on. The magnetic fields generated by the two sets of Helmholtz coils in the central region are superimposed, and the total generated magnetic induction intensity has a first-order linear relationship with z:

[0046] B sum = kz + B h (1)

[0047] In the above formula, k is the rate of change of the magnetic field generated by the reverse Helmholtz coil along the z direction, and B h is the magnitude of the bias uniform magnetic field generated by the square Helmholtz coil.

[0048] As Figure 2 shown, first, according to the Biot-Savart law, the magnitude of the component of the magnetic induction intensity generated by a wire l1 with a length of L in space through which a current of magnitude I is passed at a point M(x, y, z) in space in the Z-axis direction is as follows:

[0049]

[0050] In the above formula, μ0 is the vacuum permeability, I is the current passed through the coil, h is the distance between the two coils of the square Helmholtz coil, and the length L is used as the side length of the square Helmholtz coil. From formula (2), the magnitudes of the components of the magnetic induction intensity in the Z-axis direction generated by the other three sides l2, l3, and l4 of the single-pole coil in the square Helmholtz coil at point M(x, y, z) are as follows:

[0051]

[0052]

[0053]

[0054] From formulas (2), (3), (4), and (5), the magnitude of the component of the magnetic induction intensity in the Z-axis direction generated by the other pole coil of the square Helmholtz coil at point M(x, y, z) is:

[0055]

[0056] When the square Helmholtz coils are energized with the same-direction current I, the magnitude of the component of the magnetic induction intensity in the Z-axis direction generated at the spatial point M(x, y, z) is as follows:

[0057]

[0058] To make the magnetic induction intensity generated by the square Helmholtz coils in the central region the largest, the following conditions need to be satisfied:

[0059]

[0060] To make the magnetic field generated by the square Helmholtz coils in the central region uniform, the following conditions need to be satisfied:

[0061]

[0062] Solving equations (8) and (9), it can be obtained that to make the magnetic induction intensity generated by the square Helmholtz coils in the central region the largest and the formed magnetic field uniform, the side length of the square Helmholtz coils and the distance between the two pole coils need to satisfy:

[0063] h = 0.54L (10)

[0064] When the side length of the square Helmholtz coils and the distance between the two pole coils satisfy equation (10), the magnitude of the magnetic induction intensity generated by the square Helmholtz coils at the center point is:

[0065]

[0066] Since the geomagnetic field not only contains a uniform magnetic field but also a gradient magnetic field. To cancel the gradient magnetic field and achieve a zero magnetic space, opposite-direction currents can be passed through the two pole coils of the square Helmholtz coils, and the generated gradient magnetic field is shown in the following formula:

[0067]

[0068] From equation (12), it can be obtained that for the reverse Helmholtz coils to generate the largest gradient in the central region and the generated gradient change to be uniform, the following conditions need to be satisfied:

[0069]

[0070] According to the calculation of equation (13), the relationship between the side length L2 of the reverse Helmholtz coils and the coil distance h2 is:

[0071] h2 = 1.064L2 (14)

[0072] From equations (10) and (14), it can be seen that to make the distances between the square Helmholtz coils and the reverse Helmholtz coils equal, that is, h = h2, the side lengths of the two types of coils can be obtained as:

[0073] L = 1.97L2 (15)

[0074] As Figure 3 shown, the COMSOL simulation model of the present invention, where the side length of the outer square Helmholtz coil is 550 mm, the number of turns of the coil is 120 turns, and the current passing through is 0.42 A. The side length of the inner reverse Helmholtz coil is 282 mm, the number of turns of the coil is 40 turns, and the current passing through is 0.1 A. The distance between the two pairs of coils is 300 mm for both.

[0075] As Figure 4 shown, it can be seen that the magnetic field simulation results generated by the present invention are consistent with the design conclusions. The superimposed magnetic field of the square Helmholtz coil and the reverse Helmholtz coil in the central region has a first-order linear relationship with the axial position, and can generate enough magnetic field to offset the geomagnetic field.

[0076] The single-axis dynamic magnetic field compensation designed by the present invention can be extended to three-axis dynamic magnetic field compensation, so as to realize the dynamic compensation of the geomagnetic field in three directions. It is a magnetic field compensation device with universality.

[0077] A double Helmholtz control device for dynamic magnetic field compensation provided in this embodiment can perform real-time automatic compensation for the geomagnetic field and can generate an extremely weak magnetic environment in the central region of the device. It mainly includes a uniform field compensation module and a gradient magnetic field compensation module. Both modules include a PID controller, a digital-to-analog converter, an analog-to-digital converter, and a voltage-controlled current source. The control quantity generated by the PID controller acts on the square Helmholtz coils with equal distances between the two pairs of coils through the voltage-controlled current source. Among them, the outer Helmholtz coil provides a bias uniform magnetic field, and the inner Helmholtz coil provides a gradient magnetic field. The double Helmholtz coil structure adopted by the present invention compensates the uniform term and the non-uniform term in the geomagnetic field while reducing the size of the shielding device, and can provide a large uniform area and an extremely weak magnetic environment.

[0078] The above-described embodiments are only used to describe the preferred mode of the present invention, and do not limit the scope of the present invention. Without departing from the design spirit of the present invention, various deformations and improvements made by those of ordinary skill in the art to the technical solution of the present invention should fall within the protection scope determined by the claims of the present invention.

Claims

1. A dual Helmholtz control device for dynamic magnetic field compensation, characterized in that, Including: A reverse Helmholtz coil, a forward Helmholtz coil, a fluxgate sensor, a uniform field compensation module, and a gradient compensation module; The forward Helmholtz coil is a square Helmholtz coil; The reverse Helmholtz coil and the forward Helmholtz coil are parallel and opposite to each other, and are used to generate an induced magnetic field; The fluxgate sensor is used to convert the magnetic induction intensity of the induced magnetic field into a voltage signal; The uniform field compensation module is used to make the forward Helmholtz coil generate a corresponding bias uniform magnetic field to cancel the uniform term in the geomagnetic field; The gradient compensation module is used to provide a gradient magnetic field for the reverse Helmholtz coil to cancel the non-uniform term in the geomagnetic field; The uniform field compensation module includes a first PID controller, a first digital-to-analog converter, a first voltage-controlled current source, and a first analog-to-digital converter connected in sequence; The first PID controller is used to generate a control quantity to act on the first voltage-controlled current source to provide current for the forward Helmholtz coil to generate a uniform magnetic field; The gradient compensation module includes a differential operation circuit, a second analog-to-digital converter, a second PID controller, a second digital-to-analog converter, and a second voltage-controlled current source connected in sequence; The second analog-to-digital converter is used to convert the voltage signal into a digital signal; The second PID controller is used to generate a control quantity to act on the second voltage-controlled current source to provide current for the reverse Helmholtz coil to generate a gradient magnetic field; The fluxgate sensor includes a first fluxgate sensor and a second fluxgate sensor; The first fluxgate sensor and the second fluxgate sensor are respectively connected to the differential operation circuit; The first fluxgate sensor is connected to the first analog-to-digital converter; The control device further includes a compensation control module, and the compensation control module is used to prevent coupling between gradient compensation and uniform field compensation; The compensation control module includes a first control unit and a second control unit; The first control unit is used to control the gradient compensation module to perform gradient compensation; The second control unit is used to judge whether the magnetic field output by the gradient compensation module is stable, and when the magnetic field is stable, control to turn on the uniform field compensation module.

2. The dual Helmholtz control device for dynamic magnetic field compensation according to claim 1, wherein The induced magnetic field generated by the reverse Helmholtz coil and the forward Helmholtz coil is a superimposed magnetic field, and the relationship between the magnetic induction intensity of the superimposed magnetic field and the Z-axis direction is: B sum = kz + B h where k is the rate of change of the magnetic field generated by the anti-Helmholtz coils along the z direction, and B h is the magnitude of the bias uniform magnetic field generated by the square Helmholtz coils.

3. The dual Helmholtz control device for dynamic magnetic field compensation according to claim 2, wherein The magnetic induction intensity component of a single-stage coil of the forward Helmholtz coil on the Z-axis is obtained based on the vacuum permeability, the current passing through the coil, the distance between the two-pole coils of the forward Helmholtz coil, and the side length of the forward Helmholtz coil.

4. The dual Helmholtz control device for dynamic magnetic field compensation according to claim 1, wherein The calculation formula for the magnetic induction intensity of the gradient magnetic field is: Among them, B Zi is the magnetic induction intensity component of a single-stage coil of the reverse Helmholtz coil on the Z-axis, and I is the current passing through the coil.

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