CN114217248B - Active and passive hybrid magnetic shielding system and design method - Google Patents
Active and passive hybrid magnetic shielding system and design method Download PDFInfo
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Abstract
The invention discloses an active and passive hybrid magnetic shielding system and a design method thereof, wherein firstly, a passive magnetic shielding cylinder is designed and built to reduce the influence of the geomagnetic environment; secondly, designing an active magnetic compensation coil to comprise a three-axis shimming coil and a longitudinal gradient coil so as to expand the uniform region of the magnetic field in the cylinder and reduce the residual magnetism in the cylinder; and finally, an active magnetic compensation system is set up to eliminate external interference and realize that the magnetic field in the uniform area in the cylinder is close to zero. The invention takes the gradient compensation into consideration when designing the coil, and can solve the problem of larger axial gradient when the single end of the magnetic shielding cylinder is opened; when the active magnetic compensation coil is designed, the interaction between the coil and the shielding material is taken into consideration, so that the problem of reduced uniformity of a magnetic field generated by the coil due to coupling is solved; by adopting the scheme of a passive magnetic shielding cylinder, an active magnetic compensation coil and an active magnetic compensation closed-loop control system, the weak magnetic environment with large uniform area and high stability is effectively created. The invention provides practical and effective guidance for the construction of the extremely weak magnetic environment required by the magnetic measurement device based on the SERF effect.
Description
Technical Field
The invention belongs to the technical field of magnetic shielding, and particularly relates to an active and passive hybrid magnetic shielding system and a design method thereof, which meet the requirements on an extremely weak magnetic field based on SERF measurement and ensure that the magnetic field can be stabilized near zero magnetism for a long time.
Background
SERF atomic magnetometers are widely used for measuring biomagnetic signals due to their ultra-high sensitivity. And the realization of SERF state requires that the spin exchange frequency is far more than the Larmor precession frequency, and the three conditions of low magnetism, high temperature and laser pumping are required. The increase of the external magnetic field can cause the relaxation increase caused by spin exchange, so that the relaxation time is reduced, the realization of a narrow linewidth SERF state cannot be ensured, and the sensitivity of a magnetometer is reduced. An extremely weak and uniform magnetic field is a necessary working environment for atomic sensors.
In order to realize an extremely weak magnetic field environment, the amplitude and fluctuation of the geomagnetism can be attenuated by a passive magnetic shielding mode of a high-permeability shielding layer, and a magnetic field environment which is as clean as possible is provided. Because the performance of the passive magnetic shielding is limited, a magnetic field and a gradient are also introduced into the single-end opening of the magnetic shielding cylinder, and therefore a three-dimensional magnetic compensation coil needs to be designed to compensate the environmental residual magnetism to obtain a larger uniform magnetic field. However, the coupling between the high permeability material of the shielding layer and the coil amplifies the magnetic field of the coil and influences the distribution of the magnetic field. Therefore, when designing the magnetic compensation coil, the interaction of the high-permeability passive shielding material is considered, and the shim coil and the gradient coil are designed. And magnetic field disturbance can be caused due to the influence of an environmental geomagnetic field, personnel walking and other electromagnetic equipment, and an active magnetic compensation closed-loop control system needs to be designed to compensate the static residual magnetic field and inhibit the magnetic field disturbance. Therefore, it is crucial to combine passive magnetic shielding and active magnetic compensation to realize a weak magnetic environment with large uniform area and high stability.
Disclosure of Invention
The technical problem of the invention is solved: the defects in the prior art are overcome, and an active and passive hybrid shielding system and a design method are provided, so that the problem of a very weak magnetic field environment of a large uniform area required by SERF atomic magnetometer during measurement is solved.
The technical solution of the invention is as follows: an active-passive hybrid magnetic shield system comprising: a passive magnetic shielding cylinder and an active magnetic compensation part; the magnetic compensation part comprises a magnetic compensation coil and a magnetic compensation closed-loop control system; the magnetic compensation coil comprises a three-axis shimming coil and a longitudinal gradient coil; the magnetic compensation closed-loop control system comprises a signal acquisition unit, a signal processing unit and a magnetic field control unit; the signal acquisition unit comprises a magnetometer and is used for accurately measuring the magnetic field and detecting the real-time distribution of the space magnetic field; the signal processing unit comprises a preamplifier, an AD module and a filter, wherein the preamplifier receives a weak voltage signal output by the magnetometer, converts the voltage signal into an analog-to-digital signal through the AD module, and enters the controller after being processed by the filter; the magnetic field control unit comprises a controller, a DA module, a voltage-controlled current source and a magnetic compensation coil, wherein the controller forms a PID control signal according to a processed voltage signal fed back and an input set value, the voltage-controlled current source is input after digital-to-analog conversion is carried out through the DA module, the output current of the voltage-controlled current source drives the magnetic compensation coil to generate a corresponding compensation magnetic field, closed-loop control is formed, magnetic field fluctuation is restrained, and external interference is reduced.
The invention discloses a design method of an active and passive hybrid magnetic shielding system, which comprises the following steps:
the method comprises the steps of (1) determining the influence of the length, the radius and the thickness of the magnetic shielding cylinder on the performance of the magnetic shielding, designing and building a magnetic shielding cylinder; the method comprises the following 2 steps:
in the step (11), a cylindrical magnetic shielding cylinder is adopted for passive magnetic shielding to attenuate an external stray magnetic field, the magnetic shielding cylinder adopts a structural form of a detachable end cover, the inner layer is formed by nesting four mutually independent coaxial cylindrical magnetic shielding cylinders, the outermost layer is an aluminum alloy outer cylinder, the alternating magnetic field and the supporting effect of the shielding part are realized, each layer of shielding cylinder is composed of a cylinder body and an end cover, PEEK plastic linings are adopted between the layers for isolation, and the four layers of magnetic shielding cylinders are all made of permalloy materials;
step (12) according to the Laplace equation and the boundary condition, obtaining an axial shielding factor which does not contain a demagnetization factor and does not consider demagnetization treatment by a magnetic potential method
Comprises the following steps:
wherein, mu i Relative magnetic permeability of material of i-th layer cylindrical magnetic shielding cylinder, t i Thickness, R, of the i-th cylindrical magnetic shield tube i Is the average radius, L, of the i-th layer cylindrical magnetic shield cylinder i Is the average length of the i-th layer of cylindrical magnetic shield cylinders;
the axial shielding factor of the obtained multilayer cylindrical magnetic shielding cylinder is as follows:
wherein n is the number of layers of the magnetic shield,
axial shielding factors of the ith, j and n layers of cylindrical magnetic shielding cylinders; l is i ,L j ,L k Is the average length of the i, j and k-th layer cylindrical magnetic shielding cylinders; four layers of permalloy magnetic shielding cylinders are adopted, and the axial shielding factor is as follows:
and finally, calculating the influence of each factor, namely the length of each layer, the radius of each layer and the thickness of each layer, on the axial shielding factor of the magnetic shielding cylinder by adopting simulation, and selecting the optimal solution of each factor to design and build the passive magnetic shielding system.
Designing an active magnetic compensation coil to comprise a three-axis shimming coil and a longitudinal gradient coil; the method comprises the following 3 steps:
step (21) is based on the influence of ferromagnetic boundary coupling on the reduction of a uniform area, and a magnetic shielding layer with high magnetic permeability (the magnetic shielding layer adopts permalloy, and the relative magnetic permeability is 10) 4 ～10 5 Magnitude) and the coupling of an active current-carrying coil on a cylinder, modifying a green function of magnetic vector potential, matching boundary conditions of a hollow magnetic shielding cylinder, reestablishing a magnetic field formula generated by the three-axis shim coil, optimizing the position of the three-dimensional shim coil according to the requirement of uniformity in the magnetic shielding cylinder, and designing the three-dimensional shim coil;
wherein b is the radius of the magnetic shielding cylinder, a is the radius of the current-carrying cylindrical surface, mu 0 Is a vacuum permeability, F φ m Is the Fourier transform of the components of the current density of the three-dimensional shim coil in the phi direction, r (rho, phi, z) is the target field point in a cylindrical coordinate system,I m and K m Respectively representing a first type and a second type of modified Bessel function;
step (22) based on the influence of ferromagnetic boundary coupling on gradient descent, analyzing and calculating the coupling of the high-permeability magnetic shielding layer and the gradient coil by using the formula in the step (21), reestablishing a magnetic field formula generated by the gradient coil, optimizing the position of the longitudinal gradient coil according to the gradient requirement in the magnetic shielding cylinder, and designing the longitudinal gradient coil;
and (23) processing an active compensation coil slot on the hollow cylinder to serve as a magnetic compensation coil framework, multiplexing the three-dimensional shimming coil and the longitudinal gradient coil with the same coil framework, and manufacturing the magnetic compensation coil framework by adopting a nylon material to ensure the non-magnetism.
And (3) performing coarse compensation on residual magnetism in the cylinder by using the active magnetic compensation coil, building a magnetic compensation closed-loop control system, controlling the current in real time, driving the magnetic compensation coil to generate a corresponding magnetic field to perform fine compensation on the residual magnetism and disturbance in the cylinder, eliminating the interference of the residual magnetism and the external environment, and realizing that the magnetic field in the uniform area of the magnetic shielding cylinder is close to zero.
The method comprises the following 2 steps:
the active magnetic compensation system comprises a signal acquisition unit, a signal processing unit and a magnetic field control unit; the signal acquisition unit comprises a magnetometer; the signal processing unit comprises a preamplifier, an AD module and a filter; the magnetic field control unit comprises a controller, a DA module and a voltage-controlled current source;
step (31) coarse compensation: firstly, accurately measuring a magnetic field, and acquiring magnetic field data of a plurality of points by adopting a magnetometer so as to fit a triaxial uniform magnetic field component and a first-order gradient component in a space region; secondly, measuring a magnetic field generated by preset current in each coil by using the fluxgate to obtain a coil constant of the coil; finally, calculating to obtain the current required to be introduced according to the triaxial uniform magnetic field component, the first-order gradient component and the coil constant, and introducing corresponding current by using a current source to realize coarse compensation;
fine compensation in step (32): detecting the real-time distribution of the space magnetic field through a magnetometer, and receiving a weak voltage signal output by the magnetometer and amplifying the weak voltage signal by a preamplifier; the AD module performs analog-to-digital conversion on the voltage signal to obtain a digital voltage signal, and the AD module adopts differential input voltage, so that common-mode noise can be effectively inhibited; the signal enters a controller after being processed by a filter, and the filter adopts a Butterworth low-pass filter; the controller forms a PID control signal according to the processed voltage signal fed back and an input set value, the PID control signal is subjected to digital-to-analog conversion through the DA module and then is input into the voltage-controlled current source, the output current of the voltage-controlled current source drives the shimming coil to generate a corresponding compensation magnetic field, a closed-loop control system is formed, magnetic field fluctuation is restrained, and external interference is reduced.
Compared with the prior art, the invention has the advantages that:
(1) The invention designs the gradient magnetic compensation coil, solves the problem of larger axial gradient of the magnetic shielding cylinder under the condition of single-end opening, further reduces the remanence and achieves the near-zero magnetic environment;
(2) The invention takes the interaction of the active current-carrying coil and the high-permeability passive shielding material into consideration when designing the magnetic compensation coil, solves the problem of reduced uniformity of a magnetic field generated by the coil under the coupling action and ensures that the range of a uniform area in the magnetic shielding cylinder is larger;
(3) The invention adopts four layers of magnetic shielding barrels, magnetic compensation coils and closed-loop magnetic compensation control, realizes the extremely weak magnetic environment of a large uniform area, can inhibit the fluctuation of a magnetic field in real time and reduces the external interference.
Drawings
FIG. 1 is a schematic diagram of an active and passive hybrid shielding system according to the present invention;
FIG. 2 is a block diagram of the active magnetic compensation closed-loop control system of the present invention.
Detailed Description
The invention is further described with reference to the following figures and detailed description.
As shown in fig. 1, the active-passive hybrid shielding system of the present invention comprises: a passive magnetic shielding cylinder 1, an active magnetic compensation part 2; the passive magnetic shielding cylinder 1 comprises an inner magnetic shielding cylinder made of 4 layers of permalloy and an outer cylinder made of outer aluminum alloy, and the active magnetic compensation part 2 comprises a magnetic compensation coil and a magnetic compensation closed-loop control system.
As shown in fig. 2, the magnetic compensation closed-loop control system includes a signal acquisition unit, a signal processing unit, and a magnetic field control unit. The signal acquisition unit comprises a magnetometer; the signal processing unit comprises a preamplifier, an AD module and a filter; the magnetic field control unit comprises a controller, a DA module, a voltage-controlled current source and a magnetic compensation coil.
As can be seen from the above schematic diagrams 1 and 2, the active and passive hybrid magnetic shielding system of the present invention includes a passive magnetic shielding cylinder and an active magnetic compensation part; the passive magnetic shielding cylinder comprises an inner magnetic shielding cylinder made of 4 layers of permalloy and an outer cylinder made of outer aluminum alloy; the magnetic compensation part comprises a magnetic compensation coil and a magnetic compensation closed-loop control system; the magnetic compensation coil comprises a three-axis shimming coil and a longitudinal gradient coil; the magnetic compensation closed-loop control system comprises a signal acquisition unit, a signal processing unit and a magnetic field control unit; the signal acquisition unit comprises a magnetometer and is used for accurately measuring the magnetic field and detecting the real-time distribution of the space magnetic field; the signal processing unit comprises a preamplifier, an AD module and a filter, wherein the preamplifier receives a weak voltage signal output by the magnetometer, converts the voltage signal into an analog-to-digital signal through the AD module, and enters the controller after being processed by the filter; the magnetic field control unit comprises a controller, a DA module, a voltage-controlled current source and a magnetic compensation coil, wherein the controller forms a PID control signal according to a processed voltage signal fed back and an input set value, the voltage-controlled current source is input after digital-to-analog conversion is carried out through the DA module, the output current of the voltage-controlled current source drives the magnetic compensation coil to generate a corresponding compensation magnetic field, closed-loop control is formed, magnetic field fluctuation is restrained, and external interference is reduced.
The specific design method comprises the following steps:
determining the influence of the length, the radius and the thickness of a magnetic shielding cylinder on the magnetic shielding performance, and designing and building the magnetic shielding cylinder to reduce the influence of the external geomagnetic environment; the method comprises the following 2 steps:
in the step (11), a cylindrical magnetic shielding cylinder is adopted for passive magnetic shielding to attenuate an external stray magnetic field, the magnetic shielding cylinder adopts a structural form of a detachable end cover, the inner layer is formed by nesting four mutually independent coaxial cylindrical magnetic shielding cylinders, the outermost layer is an aluminum alloy outer cylinder, the alternating magnetic field and the supporting effect of the shielding part are realized, each layer of shielding cylinder is composed of a cylinder body and an end cover, PEEK plastic linings are adopted between the layers for isolation, and the four layers of magnetic shielding cylinders are all made of permalloy materials;
step (12) according to the Laplace equation and the boundary condition, obtaining an axial shielding factor which does not contain a demagnetization factor and does not consider demagnetization treatment by a magnetic potential method
Comprises the following steps:
wherein, mu i Relative magnetic permeability of material of i-th layer cylindrical magnetic shielding cylinder, t i Thickness, R, of the i-th cylindrical magnetic shield tube i Is the average radius, L, of the i-th layer cylindrical magnetic shield cylinder i Is the average length of the ith layer of cylindrical magnetic shielding cylinder;
the axial shielding factor of the obtained multilayer cylindrical magnetic shielding cylinder is as follows:
wherein n is the number of layers of the magnetic shield,
axial shielding factors of the ith, j and n layers of cylindrical magnetic shielding cylinders; l is i ,L j ,L k Is the average length of the i, j and k-th layer cylindrical magnetic shielding cylinders; four layers of permalloy magnetic shielding cylinders are adopted, and the axial shielding factor is as follows:
finally, calculating the influence of each factor, namely the length of each layer, the radius of each layer and the thickness of each layer on the axial shielding factor of the magnetic screen cylinder by adopting simulation, and selecting the optimal solution design of each factor to construct a passive magnetic shielding system;
designing an active magnetic compensation coil comprising a three-axis shimming coil and a longitudinal gradient coil to further reduce residual magnetism in the cylinder; the method comprises the following 3 steps:
step (21) is based on the influence of ferromagnetic boundary coupling on the reduction of a uniform area, and a magnetic shielding layer with high magnetic permeability (the magnetic shielding layer adopts permalloy, and the relative magnetic permeability is 10) 4 ～10 5 Magnitude) and the coupling of an active current-carrying coil on a cylinder, modifying a green function of magnetic vector potential, matching boundary conditions of a hollow magnetic shielding cylinder, reestablishing a magnetic field formula generated by the three-axis shim coil, optimizing the position of the three-dimensional shim coil according to the requirement of uniformity in the magnetic shielding cylinder, and designing the three-dimensional shim coil;
wherein, b is the radius of the magnetic shielding cylinder, a is the radius of the current-carrying cylindrical surface, mu 0 Is a vacuum permeability, F φ m Is the Fourier transform of the current density of the three-dimensional shim coil in the phi direction component, r (rho, phi, z) is the target field point in a cylindrical coordinate system, I m And K m Respectively representing a first type and a second type of modified Bessel function;
step (22) based on the influence of ferromagnetic boundary coupling on gradient descent, carrying out analytical calculation on the coupling of the high-permeability magnetic shielding layer and the gradient coil by using the formula in the step (21), reestablishing a magnetic field formula generated by the gradient coil, optimizing the position of the longitudinal gradient coil according to the gradient requirement in the magnetic shielding cylinder, and designing the longitudinal gradient coil;
and (23) processing an active compensation coil slot on the hollow cylinder to serve as a magnetic compensation coil framework, multiplexing the three-dimensional shimming coil and the longitudinal gradient coil with the same coil framework, and manufacturing the magnetic compensation coil framework by adopting a nylon material to ensure the non-magnetism.
And (3) performing coarse compensation on residual magnetism in the cylinder by using the active magnetic compensation coil, building a magnetic compensation closed-loop control system, controlling the current in real time, driving the magnetic compensation coil to generate a corresponding magnetic field to perform fine compensation on the residual magnetism and disturbance in the cylinder, eliminating the interference of the residual magnetism and the external environment, and realizing that the magnetic field in the uniform area of the magnetic shielding cylinder is close to zero.
The method comprises the following 2 steps:
the active magnetic compensation system comprises a signal acquisition unit, a signal processing unit and a magnetic field control unit; the signal acquisition unit comprises a magnetometer; the signal processing unit comprises a preamplifier, an AD module and a filter; the magnetic field control unit comprises a controller, a DA module and a voltage-controlled current source;
step (31) coarse compensation: firstly, accurately measuring a magnetic field, and acquiring magnetic field data of a plurality of points by adopting a magnetometer so as to fit a three-axis uniform magnetic field component and a first-order gradient component in a spatial region; secondly, measuring a magnetic field generated by preset current in each coil by using the fluxgate to obtain a coil constant of the coil; finally, calculating to obtain the current required to be introduced according to the triaxial uniform magnetic field component, the first-order gradient component and the coil constant, and introducing corresponding current by using a current source to realize coarse compensation;
fine compensation in step (32): detecting the real-time distribution of the space magnetic field through a magnetometer, and receiving a weak voltage signal output by the magnetometer and amplifying the weak voltage signal by a preamplifier; the AD module performs analog-to-digital conversion on the voltage signal to obtain a digital voltage signal, and the AD module adopts differential input voltage, so that common-mode noise can be effectively inhibited; the signal enters a controller after being processed by a filter, and the filter adopts a Butterworth low-pass filter; the controller forms a PID control signal according to the processed voltage signal fed back and an input set value, the PID control signal is subjected to digital-to-analog conversion through the DA module and then is input into the voltage-controlled current source, the output current of the voltage-controlled current source drives the shimming coil to generate a corresponding compensation magnetic field, a closed-loop control system is formed, magnetic field fluctuation is restrained, and external interference is reduced.
Those skilled in the art will appreciate that the invention may be practiced without these specific details.
Claims (3)
1. A design method of an active and passive hybrid magnetic shielding system is characterized in that: the active and passive hybrid magnetic shielding system adopted in the method comprises the following components: a passive magnetic shielding cylinder and an active magnetic compensation part; the magnetic compensation part comprises a magnetic compensation coil and a magnetic compensation closed-loop control system; the magnetic compensation coil comprises a three-axis shimming coil and a longitudinal gradient coil; the magnetic compensation closed-loop control system comprises a signal acquisition unit, a signal processing unit and a magnetic field control unit; the signal acquisition unit comprises a magnetometer and is used for accurately measuring the magnetic field and detecting the real-time distribution of the spatial magnetic field; the signal processing unit comprises a preamplifier, an AD module and a filter, wherein the preamplifier receives a weak voltage signal output by the magnetometer, converts the voltage signal into an analog-to-digital signal through the AD module, and enters the controller after being processed by the filter; the magnetic field control unit comprises a controller, a DA module, a voltage-controlled current source and a magnetic compensation coil, wherein the controller forms a PID control signal according to a processed voltage signal fed back and an input set value, the PID control signal is subjected to digital-to-analog conversion through the DA module and then is input into the voltage-controlled current source, and the output current of the voltage-controlled current source drives the magnetic compensation coil to generate a corresponding compensation magnetic field to form closed-loop control, inhibit magnetic field fluctuation and reduce external interference;
the method implementation comprises the following steps:
determining the influence of the length, the radius and the thickness of the magnetic shielding cylinder on the magnetic shielding performance, and designing and building the magnetic shielding cylinder;
designing an active magnetic compensation coil to comprise a three-axis shimming coil and a longitudinal gradient coil;
step (3) using the active magnetic compensation coil to perform residual magnetism coarse compensation in the cylinder, building a magnetic compensation closed-loop control system, controlling the current in real time, driving the magnetic compensation coil to generate a corresponding magnetic field to perform fine compensation on the residual magnetism and disturbance in the cylinder, eliminating the interference of the residual magnetism and the external environment, and realizing that the magnetic field in the uniform area of the magnetic shielding cylinder is close to zero;
in the step (1), the process of designing and building the magnetic shielding cylinder comprises the following steps:
step (11): the passive magnetic shielding adopts a cylindrical magnetic shielding cylinder to attenuate an external stray magnetic field, the magnetic shielding cylinder adopts a structural form of a detachable end cover, the inner layer is formed by nesting four mutually independent coaxial cylindrical magnetic shielding cylinders, the outermost layer is an aluminum alloy outer cylinder, the alternating magnetic field and the supporting action of the shielding part are realized, each layer of shielding cylinder consists of a cylinder body and an end cover, the layers are isolated by adopting a PEEK plastic lining, and the four layers of magnetic shielding cylinders are all made of permalloy materials;
step (12): obtaining an axial shielding factor which does not contain a demagnetization factor and does not consider demagnetization treatment by a magnetic potential calibration method according to a Laplace equation and boundary conditions
Comprises the following steps:
wherein, mu i Relative magnetic permeability of material of i-th layer cylindrical magnetic shielding cylinder, t i Thickness, R, of the ith cylindrical magnetic shield tube i Is the average radius, L, of the i-th layer cylindrical magnetic shield cylinder i Is the average length of the i-th layer of cylindrical magnetic shield cylinders;
the axial shielding factor of the obtained multilayer cylindrical magnetic shielding cylinder is as follows:
wherein n is the number of layers of the magnetic shield bodies,
axial shielding factors of the ith, j and n layers of cylindrical magnetic shielding cylinders; l is i ,L j ,L k Is the average length of the i, j and k-th layer cylindrical magnetic shielding cylinders; four layers of permalloy magnetic shielding cylinders are adopted, and the axial shielding factor is as follows:
and finally, calculating the influence of each factor, namely the length of each layer, the radius of each layer and the thickness of each layer on the axial shielding factor of the magnetic screen cylinder by adopting simulation, and selecting the optimal solution design of each factor to construct a passive magnetic shielding system.
2. The design method of active and passive hybrid magnetic shield system according to claim 1, characterized in that: the step (2) of designing the active magnetic compensation coil comprises a process of a three-axis shimming coil and a longitudinal gradient coil, and the process comprises the following 3 steps:
step (21) analyzing and calculating the coupling of the high-permeability magnetic shielding layer and the active current-carrying coil on the cylinder based on the influence of ferromagnetic boundary coupling on the reduction of a uniform area, modifying the Green function of magnetic vector potential, matching the boundary condition of the hollow magnetic shielding cylinder, reestablishing a magnetic field formula generated by the three-axis shimming coil, optimizing the position of the three-dimensional shimming coil according to the requirement of uniformity in the magnetic shielding cylinder, and designing the three-dimensional shimming coil;
wherein b is the radius of the magnetic shielding cylinder, a is the radius of the current-carrying cylindrical surface, mu 0 Is a vacuum permeability, F φ m Is the Fourier transform of the current density of the three-dimensional shim coil in the phi direction component, r (rho, phi, z) is the target field point in a cylindrical coordinate system, I m And K m Respectively representing a first type and a second type of modified Bessel function;
step (22) based on the influence of ferromagnetic boundary coupling on gradient descent, analyzing and calculating the coupling of the high-permeability magnetic shielding layer and the gradient coil by using the formula in the step (21), reestablishing a magnetic field formula generated by the gradient coil, optimizing the position of the longitudinal gradient coil according to the gradient requirement in the magnetic shielding cylinder, and designing the longitudinal gradient coil;
and (23) processing an active compensation coil slot on the hollow cylinder to serve as a magnetic compensation coil framework, multiplexing the three-dimensional shimming coil and the longitudinal gradient coil with the same coil framework, and manufacturing the magnetic compensation coil framework by adopting a nylon material to ensure the non-magnetism.
3. The design method of active and passive hybrid magnetic shield system according to claim 1, characterized in that: the process of building the magnetic compensation closed-loop control system in the step (3) comprises the following 2 steps:
the active magnetic compensation system comprises a signal acquisition unit, a signal processing unit and a magnetic field control unit; the signal acquisition unit comprises a magnetometer; the signal processing unit comprises a preamplifier, an AD module and a filter; the magnetic field control unit comprises a controller, a DA module and a voltage-controlled current source;
step (31) coarse compensation: firstly, accurately measuring a magnetic field, and acquiring magnetic field data of a plurality of points by adopting a magnetometer so as to fit a triaxial uniform magnetic field component and a first-order gradient component in a space region; secondly, measuring a magnetic field generated by preset current in each coil by using the fluxgate to obtain a coil constant of the coil; finally, calculating to obtain the current required to be introduced according to the triaxial uniform magnetic field component, the first-order gradient component and the coil constant, and introducing corresponding current by using a current source to realize coarse compensation;
fine compensation in step (32): detecting the real-time distribution of the space magnetic field through a magnetometer, and receiving a weak voltage signal output by the magnetometer and amplifying the weak voltage signal by a preamplifier; the voltage signal is subjected to analog-to-digital conversion through the AD module to obtain a digital voltage signal, and the AD module effectively suppresses common-mode noise by adopting differential input voltage; the signal enters a controller after being processed by a filter, and the filter adopts a Butterworth low-pass filter; the controller forms a PID control signal according to the processed voltage signal fed back and an input set value, the PID control signal is subjected to digital-to-analog conversion through the DA module and then input into the voltage-controlled current source, the output current of the voltage-controlled current source drives the shimming coil to generate a corresponding compensation magnetic field, a closed-loop control system is formed, magnetic field fluctuation is restrained, and external interference is reduced.
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