A device for measuring the characteristics of a seawater moving magnetic field

Abstract

The application provides a seawater motion magnetic field characteristic measuring device, which comprises a composite structure pipeline, an excitation system and a measuring system; the composite structure pipeline is used for improving the motion speed of seawater; the excitation system adopts square Helmholtz coils which are arranged on the upper and lower sides of the composite structure pipeline respectively and generate a uniform magnetic field between the coils, and the magnetic field direction is perpendicular to the cuboid pipeline; the measuring system adopts a ring-shaped magnetic core induction coil, the resonant frequency of the ring-shaped magnetic core induction coil is tuned to the fundamental wave frequency of a PWM square wave signal through parallel matching capacitors, the in-band output response is improved, and the out-of-band interference is suppressed; a preamplifier, an analog switch and a low-pass filter jointly constitute a phase-locked amplification circuit, the analog switch is controlled by a PWM signal, and the seawater motion magnetic field signal under the action of an alternating field is extracted from the interference environment.

Description

Technical Field

[0001] This invention relates to the field of magnetic field measurement technology, and in particular to a device for measuring the magnetic field characteristics of seawater motion. Background Technology

[0002] When a submersible travels underwater, it causes movement in the surrounding seawater, creating a large wake behind it. This seawater movement generated by the submersible produces a magnetic field. This is because the conductive seawater moves through the background Earth's magnetic field, cutting magnetic field lines and inducing currents within the seawater, thus generating a magnetic field. This magnetic field has a wide range and long duration, and is independent of whether the submersible's materials are magnetic or whether it has undergone demagnetization. Understanding the measurement methods for this magnetic field characteristic of seawater movement is of significant research importance and potential application value for the development of new submersible detection technologies.

[0003] Compared to the magnetic field of the submersible itself, the magnetic field generated by the submersible's movement in the seawater is relatively weak. Research on this field is still in its early stages both domestically and internationally, and there are few publicly reported measurement methods. Although magnetic sensing technology has made significant progress, with ultra-low noise magnetic sensors now reaching the femtometer level both domestically and internationally—such as vector magnetic sensors represented by superconducting magnetic sensors, inductive magnetic sensors, and diamond color center magnetic sensors, and scalar magnetic sensors represented by optically pumped magnetic sensors and atomic magnetic sensors—directly applying these advanced magnetic sensors to measure the characteristics of the seawater's moving magnetic field still presents some challenges. There are three main reasons: First, the seawater's velocity is relatively low during submersible navigation, resulting in a weak magnetic field. Second, because the Earth's magnetic field is a steady-state field, and the seawater's velocity changes slowly, the seawater's moving magnetic field under the Earth's magnetic background is concentrated in the extremely low-frequency range, but the magnetic sensor exhibits significant low-frequency noise. Third, environmental noise from the platform along the sensitive axis of the magnetic sensor also affects the measurement of the seawater's moving magnetic field.

[0004] Against the backdrop of geomagnetism, the magnetic field generated by the movement of seawater during the operation of a submersible is not only very weak and has a very low frequency, but is also affected by the magnetic noise of the platform during measurement. Currently, directly applying advanced magnetic sensors to the measurement of the magnetic field characteristics of seawater movement faces many challenges. Summary of the Invention

[0005] This invention provides a device for measuring the magnetic field characteristics of seawater motion, the device comprising a composite structure pipeline, an excitation system, and a measurement system;

[0006] The composite structure pipeline includes two symmetrical trumpet-shaped pipelines on both sides and a cuboid pipeline in the middle of the two trumpet-shaped pipelines.

[0007] The excitation system includes a square Helmholtz coil and an excitation circuit; the square Helmholtz coil is a pair of identical square coils that are parallel to each other and are respectively arranged on the upper and lower sides of the cuboid pipe; the output current of the excitation circuit flows through the two coils in the same direction and generates a uniform magnetic field between the coils, with the magnetic field direction perpendicular to the cuboid pipe.

[0008] The measurement system includes a ring-shaped magnetic core induction coil and a measurement circuit. The measurement circuit is used to detect the magnetic field characteristics inside the cuboid pipe. The measurement circuit includes two electrodes, which are respectively set on the left and right sides of the cuboid pipe. One of the electrodes is connected to the side of its opposite electrode by a wire to form an equipotential electrode. The ring-shaped magnetic core induction coil is set between the equipotential electrode and the electrode on the same side.

[0009] Furthermore, the large opening of the trumpet-shaped pipe has a cross-section larger than that of the cuboid pipe; the small opening has a cross-section equal to that of the cuboid pipe and is connected to the cuboid pipe.

[0010] Furthermore, the side length of the square Helmholtz coil is 2a, and the distance between the two square Helmholtz coils is 1.089a; the magnetic field amplitude B s Represented as:

[0011]

[0012] Where N is the number of coil turns, I is the output current of the excitation circuit, and a is half the side length of the square Helmholtz coil.

[0013] Furthermore, the measurement circuit includes a matching capacitor, a preamplifier, an analog switch, and a low-pass filter. The matching capacitor first tunes the toroidal magnetic core induction coil to make the coil work in a resonant state and obtain the maximum output response. The preamplifier then enhances the coil output voltage. Finally, the analog switch and low-pass filter are used for signal detection.

[0014] Furthermore, the signal input of the measurement circuit is:

[0015] x(t)=V s cos(ω0t+θ)

[0016] Among them, V s The voltage amplitude of the signal input is represented by t, time is represented by θ, the initial phase is represented by ω0, and the angular frequency is represented by ω0.

[0017] And assume that the reference input r(t) is a square wave with an amplitude of ±Vr, a period of T, and an angular frequency of ω0=2π / T;

[0018] According to the Fourier analysis method, the Fourier series expression of the square wave r(t) is:

[0019]

[0020] Among them, V r This represents the reference voltage amplitude, where n is a positive integer, n = 1, 2, 3, ...;

[0021] The product of r(t) and x(t) is:

[0022]

[0023]

[0024]

[0025] In the formula result, df(t) is the difference frequency term and af(t) is the sum frequency term. After passing through a low-pass filter, the difference frequency terms with n>1 and all sum frequency terms are filtered out, leaving only the difference frequency term with n=1.

[0026]

[0027] When the square wave amplitude Vr = 1, the multiplication process between the square wave signal and the preamplified signal can be realized using an analog switch. That is, when r(t) is +1, the output of the analog switch is connected to x(t); when r(t) is -1, the output of the analog switch is connected to -x(t). The output of the low-pass filter is:

[0028]

[0029] The acquired seawater motion magnetic field signal is converted from analog to digital using a high-resolution, high-precision data acquisition device, and then the magnetic field characteristics are measured using signal processing software. The signal processing methods include time domain measurement, frequency domain measurement, and time-frequency domain measurement.

[0030] The beneficial effects achieved by this invention are:

[0031] This invention employs a flow velocity amplification structure, with trumpet-shaped pipes at both ends and a rectangular pipe in the middle, all interconnected. Regardless of whether external seawater flows in from the left or right trumpet-shaped pipes, the flow velocity of the seawater inside the rectangular pipe is greater than the inflow velocity of the external seawater. By increasing the seawater movement velocity, the detection capability of the seawater's magnetic field is enhanced.

[0032] This invention employs an excitation chopping method to measure the magnetic field of seawater motion. When an alternating excitation magnetic field exists within a pipe through which conductive seawater flows, an induced magnetic field is generated on the pipe's cross-section. This principle transforms the measurement of the seawater motion magnetic field from extremely low-frequency detection to high-frequency detection. Since magnetic sensors are characterized by high noise at low frequencies and low noise at high frequencies, and the induced magnetic field exhibits distinct line spectrum characteristics, the excitation chopping method can solve the problem of measuring extremely low-frequency, weak, moving magnetic fields.

[0033] This invention employs a ring-shaped magnetic core inductive magnetic sensor for measurement. Under excitation chopping mode, according to Maxwell-Ampère's law, the induced magnetic field of seawater motion inside the pipe is distributed in a ring shape, and its direction is different from that of the external magnetic field. Therefore, the ring-shaped magnetic core inductive magnetic sensor can conveniently measure the magnetic field of seawater motion with minimal influence from external magnetic interference. Attached Figure Description

[0034] Figure 1 This is a schematic diagram of a device for measuring the magnetic field characteristics of seawater motion.

[0035] Figure 2 This is a schematic diagram of an embodiment of a device for measuring the magnetic field characteristics of seawater motion.

[0036] Figure 3 This is a perspective schematic diagram of an embodiment of a device for measuring the magnetic field characteristics of seawater motion.

[0037] Figure 4 This is a schematic diagram of the seawater velocity field distribution inside a pipe, representing an embodiment of a seawater motion magnetic field characteristic measurement device.

[0038] Figure 5 This is a schematic diagram of the excitation system in an embodiment of a seawater motion magnetic field characteristic measurement device.

[0039] Figure 6 This is a schematic diagram of the structure of a measurement system in an embodiment of a seawater motion magnetic field characteristic measurement device.

[0040] Figure 7 This is a schematic diagram of the distribution of the magnetic field of seawater motion inside a cuboid pipe in an embodiment of a seawater motion magnetic field characteristic measuring device.

[0041] Figure 8 A schematic diagram of a square Helmholtz coil in an embodiment of a seawater motion magnetic field characteristic measuring device;

[0042] Figure 9 This is a schematic diagram of the structure of a toroidal magnetic core coil in an embodiment of a device for measuring the magnetic field characteristics of seawater motion. Detailed Implementation

[0043] The technical solution of the present invention will be described in more detail below with reference to the accompanying drawings. The present invention includes, but is not limited to, the following embodiments.

[0044] As attached Figure 1-2 As shown, this invention provides a device for measuring the magnetic field characteristics of seawater motion. This device comprises three main parts: a composite structure pipeline, an excitation system, and a measurement system. The measurement in this patent is for detecting the motion of submersibles. The overall solution is as follows:

[0045] As attached Figure 3 As shown, the composite structure pipeline consists of three parts: two symmetrical trumpet-shaped pipes on both sides, and a rectangular pipe located between the two trumpet-shaped pipes, with both ends of the rectangular pipe connected to the trumpet-shaped pipes. The larger cross-section of the trumpet-shaped pipe is larger than the cross-section of the rectangular pipe; the smaller cross-section is equal to the cross-section of the rectangular pipe and is connected to the rectangular pipe.

[0046] Seawater flows in through the wide opening of the funnel-shaped pipe and out through the narrow opening, then flows through the rectangular pipe. Because the velocity of the seawater inside the rectangular pipe is greater than the velocity at the entrance of the funnel-shaped pipe, this pipe structure can increase the speed of seawater movement.

[0047] Regardless of whether seawater flows in from the left or right funnel-shaped pipe, the motion of the seawater can be characterized by the balance of mass and momentum. This balance is described by the continuity equation and the Navier-Stokes equation, as shown in equation (1):

[0048]

[0049] Where u is the seawater velocity, p is the seawater pressure, ρ is the seawater density, and μ is the seawater dynamic viscosity. The left side of the Navier-Stokes equations corresponds to inertial forces, while the terms on the right side correspond to pressure, viscous forces, and external forces acting on the seawater, respectively. Through analysis and calculation of the seawater velocity field, [the following is a continuation of the previous sentence]. Figure 4 The results show that the velocity of seawater inside the cuboid pipe is greater than the inflow velocity of the external seawater, thus achieving a velocity enhancement effect.

[0050] The excitation system consists of a square Helmholtz coil and an excitation circuit. The system block diagram is attached. Figure 5 As shown.

[0051] As attached Figure 8As shown, the square Helmholtz coils are a pair of identical square coils parallel to each other, respectively placed on the upper and lower sides of the cuboid pipe. The side length of each square Helmholtz coil is 2a, and the coils are wound together with a spacing of 1.089a between them. The output current of the excitation circuit flows through the two coils in the same direction. They are close to the outside of the cuboid pipe and generate a uniform magnetic field between the coils. The direction of the magnetic field is perpendicular to the cuboid pipe. The magnetic field amplitude Bs can be expressed by equation (2):

[0052]

[0053] Where N is the number of coil turns, I is the output current of the excitation circuit, and a is half the side length of the square Helmholtz coil.

[0054] The excitation circuit consists of a microprocessor, a digital isolator, and a driver. The driver includes a constant current source and an H-bridge, and is controlled by the microprocessor. A digital isolator is added between the microprocessor and the driver, providing a safe interface between low-level control signals and the high-power bridge, preventing damage to the microprocessor. The output current of the constant current source is applied to the H-bridge. At this time, the microprocessor's timer generates two complementary pulse-width modulation (PWM) signals. This control signal opens and closes the switches of the H-bridge, causing the excitation circuit to output an alternating current, which generates an alternating magnetic field in the measuring tube via a square Helmholtz coil. To prevent excessive voltage across the H-bridge from damaging the microprocessor, a digital isolator is added between them, ensuring the microprocessor can drive the H-bridge normally.

[0055] When the excitation circuit is working, the microprocessor's I / O interface outputs a PWM control signal. By opening and closing different switch pairs on the H-bridge, the driver changes the direction of the excitation current, and thus the magnetic field changes direction accordingly. To eliminate environmental magnetic interference, the alternation frequency is an integer fraction of the power frequency.

[0056] The measurement system consists of a toroidal magnetic core induction coil and a measurement circuit. A block diagram of the system is attached. Figure 6 As shown. Under the action of an alternating excitation magnetic field, the induced electric field E and induced magnetic field B of seawater satisfy Maxwell's electromagnetic theory:

[0057]

[0058]

[0059] Equations (3) and (4) are expressions of Maxwell's equations. The seawater current conduction density is J = σ(E + u × Bs), where σ is the seawater conductivity, Bs represents the alternating excitation magnetic field, and u is the seawater velocity.

[0060] It is generally believed that the conduction current density of seawater is much greater than the displacement current density, so the second term on the right-hand side of equation (4) can be ignored. Then the magnetic field of seawater motion can be expressed as:

[0061]

[0062] The measuring circuit includes two electrodes, which are respectively set on the left and right sides of the cuboid pipe. One of the electrodes is connected to the side of the opposite electrode by a wire to form an equipotential electrode. The ring-shaped magnetic core induction coil is set between the equipotential electrode and the electrode on the same side.

[0063] According to the appendix Figure 7 The magnetic field analysis and calculation results inside the rectangular pipe show that the magnetic field of seawater motion is distributed in a ring shape. Therefore, a ring-shaped magnetic core induction coil is suitable for measuring it because the sensitive axis of the coil is in the loop direction. The ring-shaped magnetic core coil, as shown... Figure 9 As shown, the placement is as follows Figure 2 As shown. When the excitation magnetic field is generated, the seawater passing through will create an induced electric field E between the two electrodes. This electric field is also an alternating electric field, thus forming a ring-shaped induced magnetic field between the electrodes. By adding another electrode connected to one of the electrodes on the pipe wall to form an equipotential, the measurement site can be moved from inside the pipe to outside the pipe. The ring-shaped induced magnetic field can be measured by setting a ring-shaped magnetic core coil between the two electrodes.

[0064] The measurement circuit consists of a matching capacitor, a preamplifier, an analog switch, and a low-pass filter. The resonant frequency of the toroidal magnetic core induction coil is tuned to the fundamental frequency of the PWM square wave signal by a parallel matching capacitor, improving the in-band output response while suppressing out-of-band interference. The preamplifier, analog switch, and low-pass filter together form a lock-in amplifier circuit, which uses the PWM signal to control the analog switch, extracting the magnetic field signal of seawater motion under AC field conditions from the interference environment.

[0065] The matching capacitor first tunes the toroidal magnetic core induction coil to make it operate in a resonant state, obtaining the maximum output response; then the preamplifier amplifies the coil's output voltage; finally, analog switches and low-pass filters are used for signal detection, as follows:

[0066] Let the signal input be

[0067] x(t)=V s cos(ω0t+θ) (6)

[0068] Let the reference input r(t) be a square wave with an amplitude of ±Vr, a period of T, and an angular frequency of ω0=2π / T.

[0069] According to the Fourier analysis method, the Fourier series expression of the square wave r(t) is:

[0070]

[0071] The product of r(t) and x(t) is:

[0072]

[0073]

[0074]

[0075] In the formula result, af(t) is the difference frequency term and af(t) is the sum frequency term. After passing through the low-pass filter, the difference frequency terms with n>1 and all the sum frequency terms are filtered out, leaving only the difference frequency term with n=1:

[0076]

[0077] When the square wave amplitude Vr = 1, the multiplication process between the square wave signal and the preamplified signal can be achieved using an analog switch. Specifically, when r(t) is +1, the output of the analog switch is connected to x(t); when r(t) is -1, the output of the analog switch is connected to -x(t). The output of the low-pass filter is then:

[0078]

[0079] The acquired seawater motion magnetic field signal is converted from analog to digital using a high-resolution, high-precision data acquisition device. Then, signal processing software is used to measure the magnetic field characteristics. Signal processing methods include time-domain measurement, frequency-domain measurement, and time-frequency-domain measurement. Time-domain measurement refers to measuring the characteristics of the seawater motion magnetic field at different times, treating it as a function of time. Frequency-domain measurement refers to measuring the characteristics of the seawater motion magnetic field at different frequencies, treating it as a function of frequency. Time-frequency-domain measurement is a tool that can simultaneously observe the time and frequency domain information of the seawater motion magnetic field, suitable for analyzing the characteristics of magnetic field spectrum changes over time.

[0080] To address the challenge of measuring the magnetic field of seawater motion caused by submersibles, this invention enhances the detection capability of seawater motion magnetic fields in three ways: First, it employs a water flow compression structure to increase the velocity of seawater near the measurement unit, thereby strengthening the seawater motion magnetic field. Second, it applies an artificial alternating magnetic field near the measurement unit, changing the measurement target from a coupled magnetic field of "geomagnetic field-seawater motion" to a coupled magnetic field of "alternating current field-seawater motion," thus shifting the frequency of the seawater motion magnetic field from the extremely low frequency band to the high frequency band, utilizing the low noise characteristic of high-frequency magnetic sensors for measurement. Third, it uses a toroidal magnetic core induction coil to measure the seawater motion magnetic field. Because the sensitive axis of the magnetic sensor is a loop structure, the platform environment noise in this direction is very low. Furthermore, the toroidal magnetic core induction coil can be designed to be sensitive only to high-frequency alternating magnetic fields of specific frequencies, thus improving the signal-to-noise ratio of the measurement unit. In summary, the measurement method proposed in this invention can effectively improve the detection capability of seawater motion magnetic field characteristics, providing a new technical means for magnetic field detection related to submersibles, and has good application prospects in the field of marine target detection.

[0081] The invention is not limited to the specific embodiments described above. Those skilled in the art can implement the invention using other specific embodiments based on the disclosed content of the embodiments and accompanying drawings. Therefore, any design that adopts the design structure and concept of the invention and makes some simple changes or modifications falls within the protection scope of the invention.

Claims

1. A device for measuring the magnetic field characteristics of seawater motion, characterized in that, The seawater motion magnetic field characteristic measuring device includes a composite structure pipeline, an excitation system, and a measuring system; The composite structure pipeline includes two symmetrical trumpet-shaped pipelines on both sides and a cuboid pipeline in the middle of the two trumpet-shaped pipelines. The excitation system includes a square Helmholtz coil and an excitation circuit; the square Helmholtz coil is a pair of identical square coils that are parallel to each other and are respectively arranged on the upper and lower sides of the cuboid pipe; the output current of the excitation circuit flows through the two coils in the same direction and generates a uniform magnetic field between the coils, with the magnetic field direction perpendicular to the cuboid pipe. The measurement system includes a ring-shaped magnetic core induction coil and a measurement circuit; the measurement circuit is used to detect the magnetic field characteristics inside the cuboid pipe. The measurement circuit includes two electrodes, which are respectively disposed on the left and right sides of the cuboid pipe. One of the electrodes is connected to the side of its opposite electrode by a wire to form an equipotential electrode; the ring-shaped magnetic core induction coil is disposed between the equipotential electrode and the electrode on the same side. The large opening of the trumpet-shaped pipe has a cross-section larger than that of the rectangular pipe; the small opening has a cross-section equal to that of the rectangular pipe and is connected to the rectangular pipe. The signal input of the measurement circuit is: ； in, Indicates the voltage amplitude of the signal. Indicates time, Indicates the initial phase. Indicates angular frequency; Let the reference input r(t) be a square wave with amplitude ±Vr, period T, and angular frequency ω. =2π / T; According to the Fourier analysis method, the Fourier series expression of the square wave r(t) is: ； in, Indicates the amplitude of the reference voltage. Represents an integer, n = 1, 2, 3…; The product of r(t) and x(t) is: ； In the formula results For the difference frequency term, As the sum-frequency term, after passing through a low-pass filter, the difference-frequency terms (n>1) and all sum-frequency terms are filtered out, leaving only the difference-frequency term (n=1): ； When the square wave amplitude Vr=1, the multiplication process between the square wave signal and the preamplified signal can be realized using an analog switch. That is, when r(t) is +1, the output of the analog switch is connected to x(t); when r(t) is -1, the output of the analog switch is connected to -x(t). The output of the low-pass filter is: ； The acquired seawater motion magnetic field signal is converted from analog to digital using a high-resolution, high-precision data acquisition device, and then the magnetic field characteristics are measured using signal processing software. The signal processing methods include time domain measurement, frequency domain measurement, and time-frequency domain measurement.

2. The seawater motion magnetic field characteristic measuring device according to claim 1, characterized in that, The square Helmholtz coil has a side length of 2a, and the two square Helmholtz coils are spaced 1.089a apart; magnetic field amplitude Represented as: ； Where N is the number of coil turns, and I is the output current of the excitation circuit. It is half the side length of a square Helmholtz coil.

3. The seawater motion magnetic field characteristic measuring device according to claim 1, characterized in that, The measurement circuit includes a matching capacitor, a preamplifier, an analog switch, and a low-pass filter. The matching capacitor first tunes the toroidal magnetic core induction coil to make the coil work in a resonant state and obtain the maximum output response. The preamplifier then enhances the coil output voltage. Finally, the analog switch and low-pass filter are used for signal detection.

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