Ultra-low frequency vector hydrophone calibration device based on liquid metal magnetic fluid exciter

Abstract

The liquid metal magnetic fluid exciter-based ultra-low frequency vector hydrophone calibration device belongs to the field of low frequency vector hydrophone calibration devices, adopts the magnetic fluid propulsion technology and the annular overflow channel to realize excitation of the underwater sound standing wave field, uses the high electrical conductivity gallium indium tin alloy and the like normal temperature liquid metal as the working medium of the magnetic fluid propeller, has simple overall structure, no relative motion mechanical parts, can eliminate the electrolysis side reaction of the water-based working medium from the root, solves the problems of sound field waveform distortion, uniformity deterioration and safety hidden trouble of the sealed environment caused by electrolysis gas, and the like; the high electrical conductivity characteristics of the liquid metal can significantly improve the thrust density of the device under the same excitation condition, thereby reducing the volume and weight of the device, and improving the integration performance of the equipment; the present application effectively breaks through the displacement stroke limitation of the existing mechanical exciter, and stably excites the ultra-low frequency underwater sound standing wave field which can cover the sub-Hz frequency band, so as to meet the ultra-low frequency band expansion and high-precision calibration requirements of the low frequency vector hydrophone calibration device.

Description

Technical Field

[0001] This invention belongs to the field of low-frequency vector hydrophone calibration devices, specifically relating to an ultra-low frequency vector hydrophone calibration device based on a liquid metal magnetohydrodynamic exciter. Background Technology

[0002] Vector hydrophones are underwater acoustic vector receivers. Low-frequency vector hydrophones, relying on the natural characteristics of low-frequency sound waves—low attenuation and long coverage distance underwater—have become a core support for long-distance underwater target detection, deep-sea biological acoustic monitoring, and seabed resource exploration. Vector hydrophone calibration is a crucial step in ensuring the accuracy and consistency of underwater vector acoustic detection data, directly determining the reliability and effectiveness of the detection results.

[0003] Currently, the latest international standard IEC 63305:2024, "Underwater acoustics - Calibration of acoustic wave vector receivers in the frequency range 5 Hz to 10 kHz," specifies commonly used calibration methods for vector hydrophones, including the free-field method, the vibrating liquid column method, and the standing wave tube method. The free-field method is generally suitable for calibration at frequencies above 1 kHz. Its core applicability condition is that the measurement environment must meet the requirements of a planar free-field. However, indoor anechoic tanks are limited by factors such as space size and sound field reflection suppression capabilities, making it difficult to meet the planar free-field requirements in the frequency band below 500 Hz. Therefore, the applicability of this method in the low-frequency band is limited.

[0004] The vibrating liquid column method and its improved standing wave tube method are devices specifically developed for the calibration of low-frequency vector hydrophones, capable of meeting calibration requirements in frequency bands below 1kHz. Both share the same core principle: achieving calibration by constructing an underwater acoustic standing wave field within a limited area. The vibrating liquid column method calibration device mainly consists of a water-filled cylindrical acoustic tube and an exciter. The exciter is placed outside the acoustic tube, exciting the entire tube and the internal water body to vibrate synchronously, thus forming a standing wave in the water column. To ensure the uniformity of the sound field within the acoustic tube, its inner diameter must be greater than twice the maximum size of the hydrophone being calibrated; to meet the rigidity requirements of "overall vibration," the acoustic tube wall thickness must be greater than one-quarter of its inner diameter. Due to the significant increase in the overall size of the vector hydrophone as the operating frequency decreases, the volume and mass of the acoustic tube in low-frequency calibration scenarios increase substantially, placing extremely high demands on the load-bearing capacity of the matching exciter. To reduce the high load on the exciter in the vibrating liquid column method, the standing wave tube calibration device uses a flexible deformable thin plate sealed at the bottom (vertical standing wave tube) or both ends (horizontal standing wave tube) of a water-filled cylindrical acoustic tube. The exciter drives the flexible deformable thin plate to vibrate, thereby exciting the water column inside the acoustic tube and forming a stable standing wave field. The standing wave tube method only excites the water column without exciting the acoustic tube itself, thus effectively eliminating the load effect of the acoustic tube itself. Standing wave tube calibration devices, which typically use electrostriction or magnetostriction driving methods, can achieve vector hydrophone calibration in the frequency band above 5Hz and are currently the mainstream technical solution in the field of vector hydrophone calibration devices.

[0005] As the application of vector hydrophones in ultra-low frequency (UHF) detection scenarios continues to expand, their UHF coverage capability is constantly improving, leading to an increasingly urgent need for UHF calibration of hydrophones. There is a need to expand and improve the vibrating liquid column method or standing wave tube method in the Hz and sub-Hz UHF range. The core technical challenge is that existing exciters struggle to stably excite an UHF underwater acoustic standing wave field within the acoustic tube. Due to the longer period and slower velocity change of UHF signals, the exciter must have a larger excitation displacement stroke to form an effective and stable standing wave field within the acoustic tube. Simultaneously, the output vibration signal must meet high standards of amplitude stability, frequency accuracy, and long-term operational reliability.

[0006] Existing drive solutions for exciting ultra-low frequency underwater acoustic standing wave fields all have significant technical limitations: the elongation of the excitation unit in electrostrictive and magnetostrictive drive methods is usually limited to less than 1%, making it impossible to achieve stable ultra-low frequency excitation with a certain vibration amplitude; large load-carrying capacity drive systems, represented by hydraulic cylinders, are constrained by the "stick-slip effect" induced by static friction, and the output ultra-low frequency vibration waveform is easily distorted from a continuous sine wave to a stepped or sawtooth shape, making it difficult to meet the waveform purity requirements of ultra-low frequency vibration; linear motor-type electromagnetic drive systems rely on air-bearing guide rails or hydrostatic support technology to achieve near-frictionless, large-stroke, and large-load excitation characteristics, which is an effective solution for ultra-low frequency vibration signal excitation, but there are technical problems such as complex mechanical structure of the drive system, stringent requirements for parts processing and assembly precision, and very high system hardware and engineering implementation costs.

[0007] Magnetohydrodynamic (MHD) propulsion technology is a novel ship propulsion solution. Its core principle utilizes the electromagnetic force generated by the interaction of current and magnetic field in seawater to drive the seawater's movement, thereby propelling the ship. This technology has no moving parts, fundamentally solving the problem of limited displacement stroke in conventional exciters. It also boasts advantages such as simple structure, flexible operation, low noise, and convenient placement, making it highly compatible with the design requirements of ultra-low frequency underwater acoustic standing wave field exciters. However, directly applying seawater-based MHD propulsion technology to ultra-low frequency underwater acoustic standing wave field excitation presents significant technical bottlenecks: the electrode surface of the propeller is highly susceptible to seawater electrolysis, generating large amounts of hydrogen and chlorine gas. These gases rise and create strong noise interference within the standing wave tube, severely damaging the waveform purity and sound field uniformity of the ultra-low frequency standing wave field. Furthermore, in indoor environments, it poses safety hazards such as poisoning, explosion, and fire.

[0008] Based on the above technical background, researchers in this field urgently need to solve the following problems in the field of existing low-frequency vector hydrophone calibration devices: insufficient coverage in the ultra-low frequency band.

[0009] 1. Existing underwater acoustic standing wave field excitation schemes combining exciters and acoustic tubes generally suffer from insufficient lower frequency band coverage when applied to ultra-low frequency calibration scenarios for hydrophones, making it difficult to meet the application requirements of Hz-level and sub-Hz-level underwater acoustic standing wave field excitation.

[0010] 2. Existing electrostrictive and magnetostrictive exciters have limited strokes, making it difficult to meet the excitation requirements of ultra-low frequency underwater acoustic standing wave fields for large amplitude displacement. Large amplitude exciters, such as hydraulic cylinders, have inherent "stick-slip effects," which easily cause distortion of ultra-low frequency vibration waveforms and cannot meet the stringent requirements for amplitude stability, frequency accuracy, and long-term operational reliability in the field of vector hydrophone calibration. Linear motor-type electromagnetic drive systems rely on auxiliary technologies such as air-bearing guides or hydrostatic supports, which present technical problems such as complex mechanical structures and stringent requirements for parts processing and assembly precision, resulting in high system hardware and engineering implementation costs.

[0011] 3. When existing seawater-based marine magnetohydrodynamic propulsion technology is directly applied to ultra-low frequency underwater acoustic standing wave field excitation scenarios, although it solves the problem of the large stroke limitation of the exciter, the water-based working medium used in its old solutions has serious side reactions of seawater electrolysis. A large number of floating bubbles will seriously degrade the waveform purity and acoustic field spatial uniformity of the ultra-low frequency standing wave field. The flammable, explosive, toxic and harmful gases such as hydrogen and chlorine produced by electrolysis are very easy to accumulate in the indoor closed test environment, which will bring dual safety hazards of personnel poisoning and explosion accidents. Summary of the Invention

[0012] To address the problems existing in current low-frequency vector hydrophone calibration devices, this invention, combining the characteristics of magnetohydrodynamic propulsion technology, proposes the following further optimization scheme:

[0013] The ultra-low frequency vector hydrophone calibration device based on a liquid metal magnetohydrodynamic exciter includes: a computer and an ultra-low frequency standing wave field excitation system. The ultra-low frequency standing wave field excitation system includes a motion control system, a magnetohydrodynamic exciter, a power amplifier, a standing wave field vibration signal acquisition unit, and a laser vibrometer. The computer is electrically connected to the parameter input terminal of the motion control system. The standing wave field vibration signal acquisition unit acquires the standing wave field vibration signal, and the laser vibrometer measures the vibration signal from the acquisition unit and feeds it back to the feedback signal input terminal of the motion control system via a signal cable. The output terminal of the motion control system is electrically connected to the power amplifier. The power amplifier outputs a low-frequency voltage power signal to the magnetohydrodynamic exciter. The device is characterized by further including a standing wave tube device with an annular overflow channel, a flow equalizer, a top layer fluid, and a bottom layer fluid.

[0014] The standing wave tube device with an annular overflow channel includes a vertical open tank and a double-ended open standing wave tube; the vertical open tank is a cylindrical container with an open top, and the double-ended open standing wave tube is a straight cylindrical structure with open ends, and the two are coaxially fixed together; the length of the double-ended open standing wave tube is less than the length of the vertical open tank, and the double-ended open standing wave tube is located in the middle of the vertical open tank; the gap between the outer diameter sidewall of the double-ended open standing wave tube and the inner diameter sidewall of the vertical open tank forms an overflow channel; the inner sidewall of the vertical open tank and the entire body of the double-ended open standing wave tube are coated with an insulating and corrosion-resistant coating.

[0015] The vertical open-top tank is filled with a bottom fluid and a top fluid. The bottom fluid is a high-density, high-conductivity liquid metal, and the top fluid is a liquid that carries the underwater acoustic standing wave field. The two are immiscible and naturally separate into two distinct layers after settling. The top of the double-ended open-top standing wave tube is completely submerged below the surface of the top fluid. The flow equalizer and the magnetohydrodynamic exciter are coaxially nested in the lower part of the double-ended open-top standing wave tube in a top-to-bottom order, and the magnetohydrodynamic exciter is always submerged in the bottom fluid. A fixed gap is provided between the flow equalizer and the magnetohydrodynamic exciter.

[0016] The output of the power amplifier is connected to the magnetohydrodynamic exciter via a control cable. Under the control of the low-frequency voltage power signal output by the power amplifier, the magnetohydrodynamic exciter drives the bottom fluid inside the standing wave tube to reciprocate along the axis of the standing wave tube at a low frequency, thereby forming an underwater acoustic standing wave field in the top fluid.

[0017] The standing wave field vibration signal acquisition unit includes a co-vibrating sphere, a co-vibrating sphere mounting mechanism, and an elastic element. The density of the co-vibrating sphere is close to that of the top layer fluid, and the co-vibrating sphere moves synchronously with the top layer fluid. The co-vibrating sphere mounting mechanism is rigidly connected to the upper section of the double-ended open standing wave tube near the central axis via a rigid support rod. The elastic element flexibly connects the co-vibrating sphere to the center of the co-vibrating sphere mounting mechanism in a manner known in the field of co-vibrating vector hydrophone design. The standing wave field vibration signal acquisition unit is completely immersed in the top layer fluid to measure the standing wave field vibration signal of the top layer fluid. A laser vibrometer is vertically fixed to the top of the vertical open tank and located directly above the co-vibrating sphere. The laser vibrometer uses a non-contact measurement method to measure the vibration signal of the co-vibrating sphere through the top layer fluid and feeds back the measurement data to the feedback signal input terminal of the motion control system via a signal cable.

[0018] The bottom fluid is a room-temperature liquid gallium indium tin alloy or metallic mercury, preferably a liquid gallium indium tin alloy; the top fluid is transparent pure water.

[0019] The ultra-low frequency vector hydrophone calibration device based on a liquid metal magnetohydrodynamic exciter is characterized in that: the magnetohydrodynamic exciter is coaxially fixed to the inner diameter wall of the bottom of a double-ended open standing wave tube; the magnetohydrodynamic exciter includes multiple bundled magnetohydrodynamic excitation tubes, and the gaps between the tube bundles are sealed with solid filler; the magnetohydrodynamic excitation tube includes a magnetic field emitting pole pair, an electric field emitting pole pair, and a tubular fixing base; the two magnets of the magnetic field emitting pole pair are fixed outside the side wall of the tubular fixing base or embedded in the side wall of the tubular fixing base; the two electrode plates of the electric field emitting pole pair are parallel to each other and fixed to the inner diameter side wall of the tubular fixing base; the line connecting the centers of the two electrode plates in the electric field emitting pole pair is orthogonal to the line connecting the geometric centers of the two magnets in the magnetic field emitting pole pair.

[0020] The present invention relates to an ultra-low frequency vector hydrophone calibration device based on a liquid metal magnetohydrodynamic exciter, the outstanding beneficial effects of which are mainly reflected in the following aspects:

[0021] 1. The design of combining magnetohydrodynamic propulsion technology with an annular overflow channel to excite underwater acoustic standing wave fields effectively breaks through the displacement stroke limitation of existing mechanical exciters. It can stably excite ultra-low frequency underwater acoustic standing wave fields that can cover the sub-Hz frequency band and meet the ultra-low frequency band expansion and high-precision calibration requirements of low frequency vector hydrophone calibration devices.

[0022] 2. By using high-conductivity gallium indium tin alloy and other room-temperature liquid metals as the working medium of the magnetohydrodynamic thruster, the occurrence of electrolytic side reactions in water-based working media is eliminated at the source. This solves problems such as acoustic waveform distortion, uniformity degradation, and safety hazards in confined environments caused by electrolysis gas generation. The high conductivity of liquid metals can also significantly increase the thrust density of the device under the same excitation conditions, thereby reducing the overall size and weight of the device and improving the integration performance of the equipment.

[0023] 3. Combining magnetohydrodynamic (MHD) thruster technology with liquid metal working medium for the drive system design of ultra-low frequency vector hydrophone calibration device results in a simple overall structure with no relatively moving mechanical parts. This effectively avoids the technical defects of existing vibrators, such as complex mechanical structure and high maintenance costs. It can significantly reduce the production and manufacturing costs of vibrators and the complexity of on-site installation and commissioning, and effectively extend the continuous service life of the equipment. Attached Figure Description

[0024] Figure 1 This is a schematic diagram of the longitudinal section of the ultra-low frequency vector hydrophone calibration device based on a liquid metal magnetohydrodynamic exciter according to the present invention.

[0025] Figure 2 This is a schematic diagram of the cross-sectional structure of the magnetohydrodynamic vibrator of the present invention;

[0026] Figure 3 This is a schematic diagram of the cross-sectional structure of the magnetohydrodynamic excitation tube of the present invention;

[0027] Figure 4 This is a schematic diagram illustrating the principle of the underlying fluid being acted upon by alternating electromagnetic force in this invention.

[0028] Figure 5 This is a schematic diagram of the working state of the ultra-low frequency vector hydrophone calibration device based on the liquid metal magnetohydrodynamic exciter of the present invention. Detailed Implementation

[0029] like Figures 1 to 4As shown, the following scheme is adopted when manufacturing the ultra-low frequency vector hydrophone calibration device of the present invention: the laser vibrometer L is rigidly fixed to the top of the fixed bracket, and the vector hydrophone F to be calibrated is detachably fixed to the bottom of the fixed bracket and located near the central axis of the upper section of the double-ended open standing wave tube D. Both the laser vibrometer L and the vector hydrophone F to be calibrated are connected to the computer through the motion control system. The vector hydrophone F to be calibrated is completely immersed in the top layer fluid J and is arranged in a staggered manner with the standing wave field vibration signal pickup unit G in the horizontal direction to receive the underwater acoustic standing wave field signal in the top layer fluid J.

[0030] The motion control system can employ technologies known in the fields of automatic control and signal processing, including but not limited to signal generation units, data acquisition units, and data communication and high-speed processing units. The parameter input terminals of the data communication and high-speed processing unit are connected to and interact with a computer to enable signal presets by the signal generation unit and to generate preset single-frequency or multi-frequency sinusoidal reference signals. The feedback input port of the data acquisition unit acquires the standing wave field vibration feedback signal measured by the laser vibrometer L. The output terminal of the data acquisition unit is electrically connected to the corresponding input terminal of the data communication and high-speed processing unit.

[0031] The data communication and high-speed processing unit performs dynamic characteristic feedforward compensation processing on the sinusoidal reference signal output by the signal generation unit, and sequentially performs low-pass filtering, phase lead compensation, and smoothing filtering processing on the standing wave field vibration signal measured by the laser vibrometer L acquired by the data acquisition unit to complete the noise reduction and phase calibration of the signal. The data communication and high-speed processing unit also performs differential operation on the sinusoidal reference signal after feedforward compensation processing and the standing wave field vibration feedback signal after filtering and phase compensation processing. The high-speed data processing unit also performs conventional PID control operation on the difference signal and uses the operation result as the low-frequency voltage power signal output by the motion control system.

[0032] The tubular mounting base E-3 is a hollow, insulated, non-magnetic circular tube, comprising an axial central cavity and two symmetrically arranged magnet fixing slots on the left and right sides of the central cavity; the extension direction of the magnet fixing slots is parallel to the axis of the central cavity; the opposing end faces of the two electrode plates of the electric field emitter pair E-2 are both flat and directly fixed to the inner diameter sidewall of the central cavity, parallel to each other; the two magnets of the magnetic field emitter pair E-1 are respectively embedded in corresponding magnet fixing slots, forming an NS magnetic pole pair. The inner and outer diameter surfaces of the double-ended open standing wave tube D are both smoothed; the inner cavity sidewall of the vertical open tank C is smoothed and insulated, both using acoustically stable, high-strength, and corrosion-resistant non-magnetic stainless steel circular tubes and tank structures; the outer surface of the vertical open tank C is covered with sound-absorbing and insulating materials and is equipped with electrical grounding protection.

[0033] The flow equalizer Q is a common single-layer flow equalizer plate or flow equalizer tube sheet, made of polytetrafluoroethylene or polyethylene to avoid the corrosive effect of liquid metal on other common metals. The shape, number, density, and distribution pattern of its flow equalization holes can be designed according to the manufacturer's specifications or by fluid dynamics simulation software using known methods.

[0034] During operation, the magnetohydrodynamic exciter E generates an alternating electromagnetic force conforming to a preset operating frequency under the control of the motion control system and the low-frequency voltage power signal output by the power amplifier T. The bottom fluid U, located inside each magnetohydrodynamic excitation tube E0, undergoes axial reciprocating motion along the magnetohydrodynamic exciter E under the action of this alternating electromagnetic force. The top fluid J, under the thrust and gravity of the bottom fluid U, moves synchronously with the bottom fluid U, generating a sub-Hz frequency ultra-low frequency underwater acoustic standing wave field in the upper and middle sections of the double-ended open standing wave tube D. This achieves the standing wave field excitation effect on the vector hydrophone F and the co-vibrating sphere G immersed in the top fluid J, meeting the calibration requirements of the vector hydrophone in the ultra-low frequency band.

[0035] like Figure 5 As shown, during the upward movement of the fluid inside the double-ended open-ended standing wave tube D, the bottom fluid U inside the magnetohydrodynamic (MHD) excitation and flow equalization device is driven upward by electromagnetic force, thereby exciting the top fluid J to move synchronously. The top fluid J located in the upper section of the inner cavity of the double-ended open-ended standing wave tube D overflows from the top opening end of the double-ended open-ended standing wave tube D and flows into the specially designed overflow channel K of this invention. Through liquid pressure conduction, a small amount of bottom fluid U near the bottom port of the overflow channel K is pushed into the MHD exciter E through the bottom of the overflow channel K. The interface liquid level between the top fluid J and the bottom fluid U inside the double-ended open-ended standing wave tube D rises from the initial S0 position to the S1 position; the interface liquid level between the top fluid J and the bottom fluid U inside the overflow channel K falls from the initial S0 position to the S2 position. During the downward movement of the fluid inside the double-ended open-ended standing wave tube D, the interface liquid level between the overflow channel K and the fluid inside the double-ended open-ended standing wave tube D will undergo opposite position changes.

[0036] Best practice:

[0037] When manufacturing and applying this device, the data communication and high-speed processing unit can use the cRIO-9030 product manufactured by National Instruments, Inc.; the signal generation unit can use the 9263 product manufactured by National Instruments, Inc.; and the data acquisition unit can use the 9234 product manufactured by National Instruments, Inc.

[0038] The bottom fluid U is a room-temperature liquid eutectic gallium-indium-tin alloy, formulated with Ga: 68.5%, In: 21.5%, and Sn: 10%; the top fluid J is degassed pure water. The electric field emitter pair E-2 uses titanium TC4 flat plates, with polytetrafluoroethylene or polyethylene insulating and corrosion-resistant coatings on all but the opposite ends; the two magnets of the magnetic field emitter pair E-1 are neodymium iron boron permanent magnets.

[0039] The vertical open-top tank C is a stainless steel tank with a total length L1 of 1500mm, an inner diameter of 800mm, a wall thickness of 65mm, and an axial length of 1400mm. Its outer surface is covered with sound-absorbing material, preferably 305 type non-magnetic stainless steel. The double-ended open-top standing wave tube D is a round tube made of 305 type non-magnetic stainless steel with an inner diameter of 300mm, a wall thickness of 100mm, and a length L2 of 1100mm. The magnetohydrodynamic exciter E has an axial length L3 of 160mm. The flow equalizer Q has an axial thickness L4 of 65mm. The single-layer flow equalization tube sheet; the total length L5 from the top of the flow equalizer Q to the bottom of the magnetohydrodynamic exciter E is 260mm; the distance from the initial position S0 of the bottom fluid U when the liquid surface is at rest to the bottom of the double-ended open standing wave tube D is 380mm; the distance L6 from the vibrating ball G to the upper port of the double-ended open standing wave tube D is 250mm; the distance L7 between the liquid surface of the top fluid J and the top of the double-ended open standing wave tube D is not less than 100mm; the inner wall of the vertical open tank C and the entire body of the double-ended open standing wave tube D are coated with polytetrafluoroethylene or polyethylene.

Claims

1. A calibration device for an ultra-low frequency vector hydrophone based on a liquid metal magnetohydrodynamic exciter, comprising a computer and an ultra-low frequency standing wave field excitation system, wherein the ultra-low frequency standing wave field excitation system comprises a motion control system, a magnetohydrodynamic exciter (E), a power amplifier (T), a standing wave field vibration signal acquisition unit (G), a laser vibrometer (L), and a current equalizer (Q); the computer is electrically connected to the parameter input terminal of the motion control system; the standing wave field vibration signal acquisition unit (G) acquires the standing wave field vibration signal, the laser vibrometer (L) measures the vibration signal of the standing wave field vibration signal acquisition unit (G), and feeds it back to the feedback signal input terminal of the motion control system through a signal cable; the output terminal of the motion control system is electrically connected to the power amplifier (T); the power amplifier (T) outputs a low-frequency voltage power signal to the magnetohydrodynamic exciter (E); characterized in that: The device also includes a standing wave tube device (A) with an annular overflow channel, a top layer fluid (J) and a bottom layer fluid (U). The standing wave tube device (A) with an annular overflow channel includes a vertical open tank (C) and a double-ended open standing wave tube (D); the vertical open tank (C) is a cylindrical container with an open top, and the double-ended open standing wave tube (D) is a straight cylindrical structure with open ends, and the two are coaxially fixedly connected; the length of the double-ended open standing wave tube (D) is less than the length of the vertical open tank (C), and the double-ended open standing wave tube (D) is located in the middle of the vertical open tank (C); the gap between the outer diameter sidewall of the double-ended open standing wave tube (D) and the inner diameter sidewall of the vertical open tank (C) forms an overflow channel (K); the inner sidewall of the vertical open tank (C) and the entire body of the double-ended open standing wave tube (D) are provided with a polymer anti-corrosion membrane layer; The vertical open tank (C) is filled with a bottom fluid (U) and a top fluid (J); the bottom fluid (U) is a high-density, high-conductivity liquid metal, and the top fluid (J) is a liquid that carries the underwater acoustic standing wave field. The two are immiscible and naturally separate into two distinct layers after settling; the top of the double-ended open standing wave tube (D) is completely submerged below the surface of the top layer fluid (J); the flow equalizer (Q) and the magnetohydrodynamic exciter (E) are coaxially nested in the lower part of the inner cavity of the double-ended open standing wave tube (D) in a top-to-bottom order, and the magnetohydrodynamic exciter (E) is always submerged in the bottom layer fluid (U); a fixed gap is provided between the flow equalizer (Q) and the magnetohydrodynamic exciter (E); The output of the power amplifier (T) is connected to the magnetohydrodynamic exciter (E) via a control cable. Under the control of the low-frequency voltage power signal output by the power amplifier (T), the magnetohydrodynamic exciter (E) drives the bottom fluid (U) inside the standing wave tube device to reciprocate along the axis of the standing wave tube device at a low frequency, thereby forming an underwater acoustic standing wave field in the top fluid (J). The standing wave field vibration signal acquisition unit (G) includes a co-vibrating sphere, a co-vibrating sphere mounting mechanism, and an elastic element. The density of the co-vibrating sphere is close to that of the top fluid (J), and the co-vibrating sphere moves synchronously with the top fluid (J). The co-vibrating sphere mounting mechanism is fixed to the upper section of the double-ended open standing wave tube (D) near the central axis via a rigid support rod. The elastic element flexibly fixes the co-vibrating sphere to the center of the co-vibrating sphere mounting mechanism in a manner known in the field of co-vibrating vector hydrophone design. The standing wave field vibration signal acquisition unit (G) is completely immersed in the top fluid (J) to measure the standing wave field vibration signal of the top fluid (J). The laser vibrometer (L) is vertically fixed to the top of the vertical open tank (C) and located directly above the co-vibrating sphere. The laser vibrometer (L) uses a non-contact measurement method to measure the standing wave field vibration signal of the co-vibrating sphere through the top fluid (J) and feeds the measurement data back to the feedback signal input terminal of the motion control system via a signal cable.

2. The ultra-low frequency vector hydrophone calibration device based on a liquid metal magnetohydrodynamic exciter as described in claim 1, characterized in that: The bottom fluid (U) is a room temperature liquid gallium indium tin alloy or metallic mercury, preferably a liquid gallium indium tin alloy; the top fluid (J) is transparent pure water.

3. The ultra-low frequency vector hydrophone calibration device based on a liquid metal magnetohydrodynamic exciter as described in claim 2, characterized in that: The magnetohydrodynamic exciter (E) is fixed to the inner diameter wall of the bottom of the double-ended open standing wave tube (D). The magnetohydrodynamic exciter (E) includes multiple bundled magnetohydrodynamic excitation tubes (E0), and the gaps between the tube bundles are sealed with solid filler. The magnetohydrodynamic excitation tube (E0) includes a magnetic field emitting pole pair (E-1), an electric field emitting pole pair (E-2), and a tubular fixing seat (E-3). The two magnets of the magnetic field emitting pole pair (E-1) are fixed to the outside of the side wall of the tubular fixing seat (E-3) or embedded in the side wall of the tubular fixing seat (E-3). The two electrode plates of the electric field emitting pole pair (E-2) are fixed parallel to each other on the inner diameter side wall of the tubular fixing seat (E-3). The line connecting the centers of the two electrode plates in the electric field emitting pole pair (E-2) is orthogonal to the line connecting the geometric centers of the two magnets in the magnetic field emitting pole pair (E-1).

4. The ultra-low frequency vector hydrophone calibration device based on a liquid metal magnetohydrodynamic exciter as described in claim 3, characterized in that: The tubular fixing base (E-3) is a hollow, insulated, non-magnetic circular tube, including an axial central cavity tube and two magnet fixing slots symmetrically opened on the left and right sides of the central cavity tube; the extension direction of the magnet fixing slots is parallel to the axis of the central cavity tube; the two electrode plates of the electric field emitting pole pair are directly and parallel to each other on the inner diameter sidewall of the central cavity tube; the two magnets of the magnetic field emitting pole pair (E-1) are respectively embedded in the corresponding magnet fixing slots and form an N-S magnetic pole pair.

5. The ultra-low frequency vector hydrophone calibration device based on a liquid metal magnetohydrodynamic exciter as described in claim 4, characterized in that: The excitation magnet pairs of the magnetic field emitting pole pair (E-1) are all permanent magnet pole pairs or electromagnet pole pairs with strong magnetic force; the opposite end faces of the two plates of the electric field emitting pole pair (E-2) are both flat; the electric field emitting pole pair (E-2) is preferably made of corrosion-resistant metal material, such as titanium; except for the opposite end faces, the electric field emitting pole pair (E-2) is coated with an insulating and corrosion-resistant coating, preferably made of polytetrafluoroethylene or polyethylene.

6. The ultra-low frequency vector hydrophone calibration device based on a liquid metal magnetohydrodynamic exciter as described in claim 5, characterized in that: Both the double-ended open standing wave tube (D) and the vertical open tank (C) are non-magnetic stainless steel round tubes and tank structures with stable acoustic performance, high strength, and corrosion resistance. The inner and outer diameter surfaces of the double-ended open standing wave tube (D) are smoothed. The inner sidewall of the vertical open tank (C) is smoothed and insulated. The inner sidewall of the vertical open tank (C) and the entire double-ended open standing wave tube (D) are coated with an insulating and corrosion-resistant coating, preferably made of polytetrafluoroethylene or polyethylene. The outer surface of the vertical open tank (C) is covered with sound-absorbing and insulating materials and is equipped with electrical grounding protection.

7. The ultra-low frequency vector hydrophone calibration device based on a liquid metal magnetohydrodynamic exciter as described in claim 6, characterized in that: The flow equalizer (Q) is made of polytetrafluoroethylene or polyethylene.

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