Underwater all-optical six-axis force sensor based on diamagnetic levitation principle

Abstract

The application discloses an underwater all-optical six-axis force sensor based on the anti-magnetic suspension principle, which comprises a low-frequency magnetic potential well unit, a suspended mechanical oscillator unit and a displacement detection unit. The low-frequency magnetic potential well unit suspends the suspended mechanical oscillator unit in the water medium based on the anti-magnetic suspension principle. The force movement displacement of the suspended mechanical oscillator unit is converted into an optical intensity signal by the displacement detection unit, and the force result is calculated to realize force sensing. The force sensor is a suspended oscillator without physical contact, the detection mode is all-optical signal detection without physical contact based on an optical fiber, and the force signal measurement can be directly carried out in the water medium. Compared with the optical fiber underwater acoustic sensor, the anti-magnetic suspension force sensor has the advantages of simple structure and low system noise, and the six-axis force signal measurement can be realized by a single suspended oscillator, and the underwater acoustic detection sensitivity is higher.

Description

Technical Field

[0001] This invention belongs to the field of underwater force sensors and is an underwater all-optical six-axis force sensor based on the principle of antimagnetic levitation. Background Technology

[0002] Underwater high-precision force sensing technology is one of the core technologies in marine resource exploration, underwater operation equipment, and marine scientific research. With the deepening of human marine development activities, the performance requirements for underwater force sensors are increasing, especially in terms of accuracy, stability, and multi-dimensional force measurement. Six-axis force sensors, as high-end sensors capable of simultaneously detecting forces Fx, Fy, and Fz in three directions and torques Mx, My, and Mz in three directions, have become core components of underwater robots, marine equipment monitoring, and precision underwater operations.

[0003] Traditional strain gauge and capacitive force sensors, while widely used in industry, struggle to adapt to complex underwater environments, facing challenges such as sealing and protection, circuit corrosion, signal transmission, and interference resistance. In the field of underwater sensors, fiber optic hydrophones are representative, but these systems are complex and expensive. They require high-precision demodulation of weak acoustic signals from optical signals, and their sensitivity in the low-frequency range below 100 Hz remains to be improved due to system noise. Underwater multi-axis vector force measurement has high application value but has also consistently been a technical challenge.

[0004] In recent years, with the emergence of new principles and technologies, the antimagnetic levitation principle has received widespread attention from the academic community as a novel force sensing scheme. This technology utilizes the stable confinement of antimagnetic materials within a constructed magnetic confinement potential well to achieve highly sensitive measurement of external forces through non-contact detection via an all-optical path. Antimagnetic levitation force measurement technology is a precision measurement method developed in recent years, and it has become one of the most sensitive force measurement technologies currently available. However, current magnetic levitation force measurement systems mainly operate in vacuum environments and are limited by the structure and size of the levitation oscillator unit and magnetic potential well unit, thus only enabling uniaxial force measurement. Further research is needed on how to apply this technology to high-precision vector detection of underwater force signals to explore its application potential and technological advantages in fields such as underwater target detection, marine biological research, and marine environmental monitoring. Summary of the Invention

[0005] The technical problem this invention aims to solve is that traditional underwater sensors, such as fiber optic hydrophones and microelectromechanical systems, suffer from insufficient sensitivity and complex structures. Existing antimagnetic levitation force measurement technologies achieve uniaxial force measurement in a vacuum environment, but their measurement structures cannot meet the requirements for multi-degree-of-freedom force sensing underwater. Multi-axial force sensing cannot be obtained by simply superimposing uniaxial force measurements. Firstly, it is necessary to design a force-bearing structure to effectively sense multi-axial force signals transmitted in the water medium; secondly, it is necessary to design the geometric structures of the levitation oscillator unit and the magnetic potential trap unit to meet the stable levitation requirements of the underwater environment.

[0006] The technical solution of this invention is: an underwater all-optical six-axis force sensor based on the principle of antimagnetic levitation, comprising:

[0007] The low-frequency magnetic potential trap unit is determined by numerical simulation calculation of the magnetic field distribution generated by different magnet combinations and sizes. Combined with the potential energy curve of the levitation oscillator unit, the magnetic potential trap configuration that can stably bind the levitation oscillator unit in three dimensions is determined, and the eigenfrequency generated by the three-dimensional binding potential trap and the position of the levitation oscillator unit when stably bound are determined.

[0008] A levitation oscillator unit, suspended above a low-frequency magnetomotive force trap unit, includes a pyrolytic graphite sheet for providing levitation force, a force-receiving structure for sensing forces in the aqueous medium, and a light-blocking component fixed to the force-receiving structure. The pyrolytic graphite sheet is positioned above the low-frequency magnetomotive force trap unit to achieve passive three-dimensional confinement. The force-receiving structure is fixed to the pyrolytic graphite sheet. The light-blocking component is positioned along the six axes of the force-receiving structure for optical measurement of low-frequency force signals in different axes. The total density of the levitation oscillator unit is 2.3 ± 0.1 g / cm³. 3 When simulating the configuration of the low-frequency magnetomotive force trap unit, a through hole is reserved for the light-blocking component to pass through in the downward direction.

[0009] The displacement detection unit includes a laser, a lens group, and a photoelectric converter. The low-frequency magnetomotive force trap unit and the displacement detection unit are fixed by a bracket, and the light-blocking component of the levitation mechanical oscillator unit is placed in the optical path inside the lens group. When the light-blocking component moves with the force-bearing structure, the light intensity signal generated by the relative motion of the levitation mechanical oscillator unit with respect to the lens group is obtained through optical measurement. The displacement of the levitation mechanical oscillator unit is calculated from the light intensity signal, and the corresponding force result is calculated to realize force sensing.

[0010] Furthermore, a low-frequency magnetomotive force trap unit is composed of four square permanent magnets, with opposite magnetization directions between each pair of permanent magnets. A through hole is retained in the center. In the magnetomotive force trap, one magnet has a susceptibility of... The potential energy expression for an object in a magnetically confined potential well is:

[0011]

[0012] Where V is the volume of the object, and r is the three-dimensional spatial coordinate of the object. Let be the magnetic permeability in vacuum, B(r) be the distribution of magnetic flux density in space, m be the mass of the object, g be the gravitational acceleration at the Earth's surface, and z be the vertical coordinate of the object. When the object is a diamagnetic material, i.e. Its potential energy curve is the shape of a stable, confined potential well. When an object moves near the lowest point of the confined potential well, the restoring force it experiences is approximately proportional to the distance the object deviates from its equilibrium position. The potential energy expression for the object near the equilibrium point is obtained as follows:

[0013]

[0014] A levitation oscillator element is constructed based on the principle of antimagnetic levitation. The eigenfrequency of the levitation oscillator element is: , The elastic coefficient represents a certain translational motion mode of the oscillator element in the suspension mechanics, and i represents the three-dimensional coordinates x, y, z.

[0015] The antimagnetic levitation force sensor designed in this invention has a very low intrinsic frequency (2 ~ 20 Hz), and its density is adjusted to be close to that of the surrounding water medium. In the low-frequency range (below 100 Hz), its displacement response to underwater force signals is much greater than that of traditional solid force sensors, thus resulting in higher detection sensitivity. Addressing the issues of sealing protection, circuit corrosion, signal transmission, and anti-interference faced by strain gauge and capacitive six-dimensional force sensors in underwater applications, this invention utilizes an antimagnetic levitation force sensor that is a non-physically contacting levitation oscillator. The detection method is based on fiber optic non-physically contacting, fully optical signal detection, and it can be directly placed in the water medium to measure force signals.

[0016] The antimagnetic levitation force sensor used in this invention has the advantages of multi-axis force sensing, simple structure, and low system noise. It has higher low-frequency underwater acoustic detection sensitivity, and a single levitation oscillator can realize six-axis force signal measurement.

[0017] The present invention has the following advantages:

[0018] 1. An underwater antimagnetic levitation mechanical oscillator structure was designed:

[0019] a. The antimagnetic levitation oscillator features non-physical contact levitation, simple and stable structure, and long lifespan. When placed in an aqueous medium for measurement, it avoids the problem of sealing and protection.

[0020] b. For underwater environments, the structure and dimensions of the magnetomotive force trap and levitation oscillator unit are designed collaboratively, including their intrinsic frequencies and force-bearing structures. This results in a force sensor with a large response to underwater force and torque signals in the 1–1 kHz low-frequency range, exhibiting high measurement sensitivity. Compared to fiber optic hydrophones, which are currently the mainstream low-frequency hydrophones, this design offers a precision advantage.

[0021] c. By combining a multi-axis force-bearing structure with a multi-axis displacement measurement structure, a single suspended oscillator can achieve six-axis force signal measurement, solving the technical challenge of multi-axis vector measurement.

[0022] 2. High-precision six-axis all-optical force sensing:

[0023] The fiber-optic-based optical detection scheme achieves decoupling and high-precision measurement of the six degrees of freedom motion of the oscillator, realizes vector sensing of underwater force signals, and avoids problems such as circuit corrosion and electromagnetic signal interference. The signal transmission is stable and easy to deploy on a large scale. Attached Figure Description

[0024] Figure 1 This is a schematic diagram of the underwater all-optical six-axis force sensor based on the antimagnetic levitation principle of the present invention.

[0025] Figure 2 This is a schematic diagram of the structure of a low-frequency magnetomotive force trap unit in an embodiment of the present invention.

[0026] Figure 3 This is a schematic diagram of the structure of the levitation oscillator unit in an embodiment of the present invention. Figure 1 .

[0027] Figure 4 This is a schematic diagram of the structure of the levitation oscillator unit in an embodiment of the present invention. Figure 2 .

[0028] Figure 5 This is a schematic diagram of the displacement detection unit in an embodiment of the present invention.

[0029] Figure 6 The displacement power spectral density diagram is a measurement of a specific embodiment of the present invention.

[0030] Figure 7 The power spectral density diagram of the force measured in a specific embodiment of the present invention is shown. Detailed Implementation

[0031] This invention proposes a high-precision six-axis force sensor based on the antimagnetic levitation principle, with a detection frequency range of 1-1 kHz, using all-optical detection. A low-frequency mechanical oscillator with a density close to the surrounding water environment is constrained using the antimagnetic levitation principle to sense underwater force signals, converting them into motion signals of the suspended mechanical oscillator; and the displacement signal is measured with high precision using optical detection methods, thereby achieving high-precision six-axis measurement of underwater low-frequency force signals.

[0032] The main difficulty in multi-axis displacement measurement lies in the need for coordinated design of the geometry of the levitation oscillator unit and the magnetomotive force trap unit. This invention reveals that, on the one hand, the buoyancy of the water medium reduces the requirement for the magnetomotive force trap to provide vertical resistance to gravity. Therefore, the magnetomotive force trap unit can be designed to be thinner and lighter, with vertical through-holes, allowing for the proper placement of the light-blocking components used for multi-axis displacement detection. Simultaneously, this invention coordinates the design of the levitation oscillator unit, employing a rationally sized force-bearing structure to ensure multi-axis stability while sensing multi-axis force signals. Figure 1 As shown, the force sensor of the present invention includes:

[0033] The low-frequency magnetomotive force well unit is designed based on numerical simulation, using the geometry of the permanent magnet to control the eigenfrequency and equilibrium position of the magnetically confined potential well. Specifically, based on numerical simulation calculations of the magnetic field distribution generated by different magnet combinations and sizes, and combined with the potential energy curve of the levitation oscillator unit, the magnetomotive force well configuration capable of three-dimensionally stabilizing the levitation oscillator unit is determined, along with the eigenfrequency generated by the three-dimensional confinement potential well and the position of the levitation oscillator unit when stably confined.

[0034] The levitation mechanical oscillator unit, which is suspended above the low-frequency magnetic potential trap unit, includes a pyrolytic graphite sheet that provides stable levitation capability, a triaxially placed plate for sensing force signals in the water medium, and a light-blocking component fixed on the oscillator. Together, they constitute a low-frequency mechanical oscillator that can be stably confined in the magnetic confinement potential trap at a density close to the surrounding water environment.

[0035] The displacement detection unit includes a laser, a lens group, and a photoelectric converter. The light-blocking component on the levitation mechanical oscillator unit is placed in the optical path inside the lens group, thereby realizing the conversion of the motion signal of the levitation mechanical oscillator unit into a light intensity signal.

[0036] In numerical simulation, the low-frequency magnetomotive force trap unit and the levitation oscillator unit are designed collaboratively. Under the action of magnetic levitation force, the levitation oscillator unit is relatively close to the low-frequency magnetomotive force trap unit. The downward light-blocking component needs to pass through the magnetomotive force trap to facilitate the displacement detection unit's measurement. The light-blocking component should not be too long, otherwise there may be errors in the displacement generated by the force transmission of the stressed structure. In the past, the magnetomotive force traps in vacuum environments were very thick. When designing the low-frequency magnetomotive force trap unit in this invention, the underwater environment provides a certain buoyancy, which is conducive to designing more magnetomotive force traps. This invention proposes to open a central hole in the magnetomotive force trap design so that the downward light-blocking component can pass through. While meeting the requirements of stable binding of the levitation oscillator unit and the intrinsic frequency, the thickness of the magnetomotive force trap is reduced, which is conducive to the light-blocking component passing through the through hole at an appropriate length to realize the force measurement of the downward axis. The size and structure of the levitation oscillator unit need to consider whether the overall levitation oscillator unit can be stably bound after the stressed structure is added to the pyrolytic graphite sheet, and whether the resonant frequency meets the requirements.

[0037] The implementation of this invention is described in detail below.

[0038] The basic principle of diamagnetism is that when a diamagnetic material is placed in a magnetic field, it will naturally be repelled to the position where the magnetic field is weakest. Based on this, this invention designs an ideal magnetic confinement potential well composed of four permanent magnets through finite element simulation, where a minimum point exists in the spatial distribution of the magnetic field, such as... Figure 2 As shown, the permanent magnets are magnetized in opposite directions in pairs. In the constructed magnetic potential well, one magnet has a susceptibility of... The potential energy expression for an object in a magnetically confined potential well is:

[0039]

[0040] Where V is the volume of the object, and r is the three-dimensional spatial coordinate of the object. Let be the magnetic permeability in vacuum, B(r) be the distribution of magnetic flux density in space, m be the mass of the object, g be the gravitational acceleration at the Earth's surface, and z be the numerical direction coordinate of the object. When the object is a diamagnetic material, i.e. The potential energy curve represents a stable, securely bound potential well. The position of the levitation oscillator element under stable binding can be calculated using the potential energy expression above. When an object moves near the lowest point of the bound potential well, the restoring force it experiences is approximately proportional to the distance the object deviates from its equilibrium position. The potential energy expression near the equilibrium point is:

[0041]

[0042] Therefore, this invention calculates the eigenfrequency of the levitation oscillator element based on the principle of antimagnetic levitation. , The elastic coefficient represents a certain translational motion mode of the oscillator element in the suspension mechanics, and i represents the three-dimensional coordinates x, y, z.

[0043] There are other designs for the magnetic potential trap configuration, but the four square permanent magnet groups used in this invention are a relatively simple configuration. Numerical simulation is performed, specifically calculating the magnetic field distribution generated by magnet groups of specific sizes and configurations, and then calculating the potential energy curve of the suspended oscillator according to the formula, thereby determining whether the oscillator can be stably bound in three dimensions, the eigenfrequency generated by the three-dimensional bound potential trap, and the specific position of the suspended oscillator when stably bound.

[0044] The stable equilibrium position and intrinsic frequency of the oscillator can be controlled by adjusting the structure and remanence of the permanent magnet. As a specific example, the parameters are: the permanent magnet is a 5mm*5mm*5mm neodymium iron boron (N52) cubic magnet; the pyrolytic graphite sheet is a square sheet with dimensions of 4mm*4mm*0.4mm and a material density of... g / cm 3 The load-bearing structure consists of three 7mm*7mm*0.05mm silicon wafers with a material density of approximately [missing information]. g / cm 3 The total mass of the levitation oscillator unit is approximately 30 mg. The density of water is generally 1 g / cm³. 3, Seawater density, even in the deep sea, generally does not exceed 1.1 g / cm³. 3 The total density of the levitation oscillator unit designed in this invention is... g / cm 3 It is suitable for various water bodies and will not affect the stability of the magnetic levitation oscillator. If there is a water body with a special density, the geometric dimensions of the pyrolytic graphite sheets and the force-bearing structure in the oscillator can be adjusted through finite element calculations so that the total density of the levitation oscillator unit is close to the density of the surrounding water environment, thus achieving levitation and force sensing of the water medium environment.

[0045] In this invention, the main body of the levitation oscillator unit consists of pyrolytic graphite and a force-bearing structure fixed above it. A light-blocking component is mounted on the force-bearing structure, positioned along six axes for optical measurements in different directions. In this embodiment, a thin rod is used as the light-blocking component. The force-bearing structure senses the force in the water medium, generating displacement, which in turn causes the light-blocking component to produce a change in light intensity within the displacement detection unit. This displacement then yields a force sensing signal. The force-bearing structure can employ various structures, such as a sphere or a flat plate; preferably, two perpendicular flat plates form a triaxial plate.

[0046] like Figure 3As shown, the main body of the levitation oscillator unit consists of pyrolytic graphite and a small sphere with a diameter of approximately 3 mm fixed above it. Graphite is the most diamagnetic material, and when placed in a magnetic potential well, it can achieve stable passive three-dimensional confinement. The small sphere is used to sense force signals in the water medium. The density of pyrolytic graphite is higher than that of water, while the density of the small sphere is slightly lower than that of water. Therefore, the relative dimensions of the pyrolytic graphite and the small sphere can be designed so that the overall density of the levitation oscillator is close to that of the water medium, thereby achieving effective sensing of force signals in the water medium. Furthermore, when the overall density of the levitation oscillator is close to that of the water medium, the magnetic confinement potential well can stably confine the levitation oscillator. As a completely passive and stable confinement potential well, it does not introduce additional noise interference. The eigenfrequency of the levitation oscillator in this invention is approximately 5 Hz. A thin rod with a diameter of approximately 50 micrometers is connected above the levitation oscillator to block the detection laser, enabling optical measurement of the multi-axis motion of the levitation oscillator.

[0047] like Figure 4 As shown, the main body of the suspended mechanical oscillator unit consists of pyrolytic graphite and a triaxially placed plate fixed above it. The triaxial plate is used to effectively sense force signals in the water medium. The mass of the suspended mechanical oscillator mainly comes from the pyrolytic graphite, while the mass of the triaxial plate and the light-blocking structure is very small. When the overall density of the suspended mechanical oscillator is close to that of the water medium, the magnetic confinement potential well can stably bind the suspended oscillator in the water. As a completely passive and stable confinement potential well, the system does not introduce additional noise interference. The eigenfrequency of the suspended mechanical oscillator in this invention is approximately 5 Hz. A thin rod with a diameter of approximately 50 micrometers is connected above the suspended mechanical oscillator as a light-blocking component, which can block the detection laser and realize optical measurement of the multi-axis motion of the suspended mechanical oscillator.

[0048] A schematic diagram of the detection optical path of the displacement detection unit of this invention is shown below. Figure 5 In the displacement detection unit, a lens group is provided for the light-blocking component on each axis. The lens group includes a first lens and a second lens placed confocally. A first optical fiber is connected to a laser for inputting laser light. The laser light input into the first optical fiber passes sequentially through the first and second confocal lenses and is then collected into the second optical fiber. The second optical fiber is connected to a photodetector for detecting the laser light intensity. When the thin rod on the levitation oscillator unit is placed near the beam waist between the first and second lenses, it will block part of the laser light, and the intensity of the blocked light changes with its position.

[0049] Within the range of the oscillator's minute motion, the voltage measured by the photodetector With the amplitude of the oscillator displacement Proportional to voltage; however, in actual measurements, it is necessary to calibrate the specific voltage-displacement conversion coefficient. ,in It represents the magnitude of displacement in a certain direction. This indicates that the displacement causes a change in the measured voltage. This represents the voltage displacement conversion coefficient in that direction. The voltage displacement conversion coefficient is calibrated by directly moving the light-blocking rod using a precision displacement stage and measuring the light intensity at different positions of the rod.

[0050] This invention relates to a fully optical six-axis force sensor that senses the movement of a suspended mechanical oscillator unit through light-blocking components along the six axes, and then calculates the force signal in the water medium. When the suspended mechanical oscillator unit is placed in the water medium, it will produce a displacement response when subjected to force. Near its equilibrium position, the oscillator is located within the linear response range. For a translational vibration mode in a certain direction of the six axes or a rotational vibration mode about a certain axis, when there is a frequency... force or torque The displacement response of the levitation oscillator element when the excitation signal is applied. Or rotation angle response for:

[0051]

[0052]

[0053] in The eigenfrequency of the oscillator. The frequency of the force signal to be measured. Let M be the dissipation coefficient of the levitation oscillator element, M be the mass of the levitation oscillator element, and I be the moment of inertia of the levitation oscillator about a specific axis.

[0054] The objective of this invention is to detect low-frequency underwater force signals: 1Hz–1kHz. A low oscillator frequency is beneficial for improving the response coefficient of the mechanical system to external acceleration signals. Furthermore, the magnetic levitation system of this invention is a completely passive restraint system, introducing no additional noise. Other factors affecting the sensitivity of underwater acoustic detection include background environmental vibration noise and the measurement noise of the optical detection unit.

[0055] Each motion mode of a mechanical oscillator has its own eigenfrequency. These eigenfrequencys are generally different for different modes, and this is related to the geometry of the magnetic potential trap and the magnetic levitation oscillator unit. In this invention, when designing the magnetic potential trap unit and the levitation oscillator unit based on finite element numerical simulation, the eigenfrequency of the levitation oscillator is designed according to the measurement requirements. The eigenfrequency of each motion mode of the levitation oscillator unit is described below. The frequency is relatively low, preferably between 2 and 20 Hz, thus it is more sensitive to low-frequency excitation signals and produces a larger displacement or angle response, resulting in higher detection sensitivity for low-frequency underwater force signals of 1 Hz–1 kHz.

[0056] A suspended mechanical oscillator placed in water can be used for underwater force signal measurement in all six degrees of freedom motion modes. To achieve high-precision detection of the six degrees of freedom, each magnetically suspended mechanical oscillator is equipped with six sets of displacement detection units, such as... Figure 1 As shown, detectors x1, x2, y1, y2, z1, and z2 are used. The six-axis motion signals of the mechanical oscillator can be obtained by subtracting or summing the signals measured by each detector unit. For example, adding detector x1 to detector x2 yields the translational displacement signal in the x-direction; subtracting detector x2 from detector x1 and dividing by the distance between detector x1 and detector x2 yields the rotational angle signal around the y-axis. The corresponding force / torque signal can then be obtained using the displacement / angle response formula of the mechanical oscillator. Figure 6 The image shows a test embodiment of the force sensor of this invention. The force sensor was placed at the bottom of a 0.4m x 0.4m x 0.4m water tank filled with water, and the displacement and force power spectral density were measured. Currently, the measured signal is mainly background vibration noise in the water medium. Figure 6 The voltage signal of the force sensor ( ) and the corresponding displacement signal ( The power spectral density of a signal, with the horizontal axis representing the signal frequency; Figure 7 The power spectral density of the force signal obtained by converting the force response function of the force sensor. The power spectral density reached... The corresponding acceleration power spectral density is at the level of . This represents an improvement of more than an order of magnitude in detection sensitivity compared to existing accelerometers.

[0057] This invention, taking into account the characteristics of the underwater environment, researches and designs a low-frequency magnetic potential trap unit and a multi-degree-of-freedom force sensing suspension oscillator unit. Compared with fiber optic underwater acoustic sensors, the force sensing system designed based on magnetic levitation measurement technology in this invention inverses the magnitude of external force by measuring the position change of the suspended body in the magnetic field. Combined with the all-optical path detection scheme, it can effectively avoid the problems of circuit corrosion, electromagnetic interference and signal attenuation faced by traditional electrical sensors in the underwater environment. The sensor of this invention has the characteristics of extremely high sensitivity and no physical contact, and the passive levitation technology has a simple structure and long life.

Claims

1. An underwater all-optical six-axis force sensor based on the principle of diamagnetic levitation, characterized in that The application relates to a low-frequency magnetic potential well unit, a displacement detection unit and a force sensor. The low-frequency magnetic potential well unit and the displacement detection unit are fixed through a support, and the light-blocking part of the suspended mechanical oscillator unit is arranged in the light path inside the lens group. The suspended mechanical oscillator unit is suspended above the low-frequency magnetic potential well unit, and includes a pyrolytic graphite sheet for providing suspension force, a force sensing structure for sensing force in the water medium, and a light blocking component fixed on the force sensing structure, the pyrolytic graphite sheet is placed above the low-frequency magnetic potential well unit to obtain passive three-dimensional binding, the force sensing structure is fixed on the pyrolytic graphite sheet, and the light blocking component is arranged in the six-axis direction of the force sensing structure and used for optical measurement of low-frequency force signals in different axial directions, and the total density of the suspended mechanical oscillator unit is 2.3±0.1 g / cm 3 ; wherein when the low-frequency magnetic potential well unit is simulated and calculated in a configuration, a through hole for the light blocking component in the downward direction is reserved; In the numerical simulation, the appropriate permanent magnet structure, size and residual magnetism are calculated and selected to control the stable equilibrium position and the intrinsic frequency of the suspended mechanical oscillator unit.

2. The underwater all-optical six-axis force sensor based on diamagnetic levitation principle according to claim 1, characterized in that The low-frequency magnetic potential well unit is composed of four square permanent magnets, the magnetization directions of the two permanent magnets are opposite, a through hole is reserved in the center, and the potential energy expression of an object with a magnetic susceptibility of in the magnetic confinement potential well is: where V is the volume of the object, r is the three-dimensional coordinate of the object, is the permeability in vacuum, B(r) is the distribution of magnetic induction in space, m is the mass of the object, g is the acceleration of gravity on the earth's surface, z is the vertical coordinate of the object, and when the object is a diamagnetic substance, i.e. , the potential energy curve is a stable bound potential well shape, and when the object moves near the lowest point of the bound potential well, the restoring force it experiences is approximately proportional to the distance of the object from the equilibrium position , and the potential energy expression of the object near the equilibrium point is obtained as: The intrinsic frequency of the levitation mechanical resonator unit is , The elastic coefficient represents a certain translational motion mode of the levitation mechanical resonator unit, and i represents three coordinates x, y and z.

3. The underwater all-optical six-axis force sensor based on diamagnetic levitation principle according to claim 2, characterized in that In the suspended mechanical oscillator unit, a ball is used as the force receiving structure, and the light-blocking part is a thin rod arranged in six directions perpendicular to the ball.

4. The underwater all-optical six-axis force sensor based on diamagnetic levitation principle according to claim 1, characterized in that In the suspended mechanical oscillator unit, two pairs of vertical plates are used to form a three-axis plate as the force receiving structure, and the light-blocking part is a thin rod arranged in six directions corresponding to the plates.

5. The underwater all-optical six-axis force sensor based on diamagnetic levitation principle according to claim 1, characterized in that In the displacement detection unit, a lens group is arranged for each light-blocking part on each axis, the lens group comprises a first lens and a second lens arranged in a confocal manner, a laser is transmitted through a first optical fiber, the laser is collected into a second optical fiber after passing through the first lens and the second lens in sequence, the second optical fiber is connected with a photoelectric converter, and the photoelectric converter is used for detecting the laser intensity; when the light-blocking part is located at the waist position of the light beam between the first lens and the second lens, part of the laser is blocked, and the position of the light-blocking part is changed, so that the blocked light intensity is changed; and the displacement of the suspended mechanical oscillator unit is calculated according to the optical measurement result.

6. The underwater all-optical six-axis force sensor based on diamagnetic levitation principle according to claim 4 or 5, characterized in that The light-blocking thin rod is moved through a precision displacement stage, and the light intensity measured at different positions of the light-blocking thin rod is measured to calibrate the voltage-displacement conversion coefficient.

7. The underwater all-optical six-axis force sensor based on diamagnetic levitation principle according to claim 6, wherein the voltage measured by the photodetector is proportional to the displacement amplitude of the levitated mechanical resonator unit. ​​​​​​ 8. The underwater all-optical six-axis force sensor based on diamagnetic levitation principle according to claim 7, characterized in that ​ 9. The underwater all-optical six-axis force sensor based on diamagnetic levitation principle according to claim 1, characterized in that The suspended mechanical resonator unit is placed in a water medium and makes displacement response according to force. In the vicinity of the equilibrium position of the suspended mechanical resonator unit, the suspended mechanical resonator unit is in a linear response interval. For a translational vibration mode in a certain direction or a rotational vibration mode around a certain axis in the six-axis, when there is an excitation signal of a force or a moment of force , the displacement response or the rotation angle response of the suspended mechanical resonator unit is: wherein is the eigenfrequency of the suspended mechanical resonator unit, is the frequency of the external force signal to be measured, is the dissipation coefficient of the suspended mechanical resonator unit, M is the mass of the suspended mechanical resonator unit, I is the moment of inertia of the suspended mechanical resonator unit about the axis of rotation.

10. The underwater all-optical six-axis force sensor based on diamagnetic levitation principle according to claim 1, characterized in that Eigenfrequencies of each mode of motion of a levitated mechanical oscillator unit Designed between 2 to 20 Hz to be sensitive to low frequency excitation signals, improving the detection sensitivity to low frequency underwater force signals from 1 Hz - 1 kHz.

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