A gravity gradient measurement device based on an elastic pendulum

Abstract

The application discloses a gravity gradient measuring device based on elastic pendulums, characterized in that the device comprises a support structure, a gravity gradient probe and a rotating motion measuring unit; the gravity gradient probe is located in the support structure and is a pair of elastic pendulums; each of the elastic pendulums comprises a reed, a hard light rod, a mass block and a fiber ring skeleton; one end of the hard light rod is connected with the support structure through the reed, the other end is connected with the mass block, and the fiber ring skeleton is installed on the mass block; the rotating motion measuring unit comprises two angle measuring instruments with the same modulation signal, and the fiber rings of the two angle measuring instruments are arranged on the fiber ring skeletons of the mass blocks respectively; when measuring the diagonal component gravity gradient Γ zz , the two gravity gradient probes are symmetrically arranged in parallel along the z direction; when measuring the non-diagonal component gravity gradient Γ zy , the sensitive z-axis gravity acceleration of the two probes is adjusted, and the two probes are arranged in a head-to-tail mode along the y direction.

Description

Technical Field

[0001] This invention relates to a gravity gradient measurement method based on an elastic pendulum, which can be used in the field of gravity gradient field observation, such as Earth tectonic interpretation, geophysical exploration, geological resource exploration, earthquake early warning, passive navigation, and military anti-submarine warfare. Realizing gravity gradient measurement is of great significance. Background Technology

[0002] The Earth's gravitational field contains rich density distribution information. Gravitational acceleration is the first derivative of gravitational potential, and gravitational gradient is the spatial tensor of gravitational acceleration. Its expression is shown in equation (1).

[0003]

[0004] Due to the properties of the gravitational field, the three components Γ on the main diagonal of the Γ matrix... xx ,Γ yy ,Γ zz The sum is 0, and all tensor elements are symmetric about the main diagonal. The relationships between the Γ components are summarized in equations (2) and (3). The SI unit of the gravitational gradient is / s. 2 The commonly used unit is the E (symbol E). The conversion relationship between E and the International System of Units (SI) is 1 E = 10⁻⁶. -9 / s 2 .

[0005] Γ xx +Γ yy +Γ zz =0 (2)

[0006]

[0007] The gravity gradient is the spatial derivative of gravitational acceleration, containing more high-frequency information about geological space. Geological observation methods based on gravity gradient observation can achieve higher resolution in geological structure detection.

[0008] Gravity gradient detection has a history of several decades, primarily employing two methods: ground-based and airborne. A representative ground-based method is the cold atom interferometric gravity gradiometer. Representative airborne gravity gradient measurements include linear accelerometer differential gravity gradiometers and cryogenic superconducting gravity gradiometers. The measurement methods used include laser interferometers, linear accelerometers, and superconducting circuits. However, existing methods all have limitations. Ground-based cold atom interferometric gravity gradiometers and airborne cryogenic superconducting gravity gradiometers require complex cryogenic maintenance and magnetic shielding devices, significantly reducing their engineering practicality. Airborne linear accelerometer differential gravity gradiometers have extremely high requirements for accelerometer consistency and installation accuracy. Summary of the Invention

[0009] To address the challenges of complex structures and demanding operating conditions in current gravity gradient detection systems, this invention provides a gravity gradient measurement device based on a combination of an elastic pendulum and a Sagnac interferometer. This scheme converts gravity field information into the angular motion of a mass block, and uses the Sagnac interferometer to calculate the gravity gradient from the differences in the angular motion of a group of mass blocks.

[0010] This invention provides a gravity gradient measurement device based on an elastic pendulum and a Sagnac interferometer. The device includes a housing, an elastic spring, a rigid lightweight rod, a high-density mass block, a fiber optic ring frame, a polarization-maintaining fiber optic ring, an ASE light source, an optical circulator, a multifunctional waveguide chip, a photodetector, data processing hardware, and software. This invention optimizes the gravity gradient detection scheme in both the mechanical device installation and configuration and the optical path setup, effectively reducing the instrument's requirements for equipment and environment, and improving its measurement accuracy.

[0011] This invention mainly includes a support structure, a gravity gradient probe, and a rotational motion measurement unit. The support structure and gravity gradient probe are as follows: Figure 1 As shown, the two reed-mass structures form a pair of elastic pendulums. The main supporting structure includes: an outer shell. The gravity gradient probe components include elastic reeds, rigid lightweight rods, high-density mass blocks, and fiber optic ring frames.

[0012] The reed provides elastic restoring force, the lightweight rod transmits stress, and the high-density mass acts as a sensor for gravitational acceleration. For example... Figure 1 As shown, under the influence of gravitational acceleration in the z-direction, the end face connecting the reed and the light rod experiences a bending moment, thus producing a bending deformation around the x-axis. This deformation is then transmitted to the mass block through the light rod, causing the mass block to rotate and oscillate around the x-axis under the influence of the reed's deformation. This motion contains information about the gravitational acceleration at the mass block's location.

[0013] Figure 1 The structural diagram shown contains two identical reed-mass structures. The angular motion of each mass contains information about the gravitational acceleration at its location. Due to the existence of the gravitational gradient, the spatial distribution of gravitational acceleration differs, resulting in a slight difference in gravitational acceleration at the locations of the two mass blocks. By using a Sagnac interferometer to precisely measure the angular velocities of the two mass blocks, the gravitational gradient can be calculated from the difference in their angular motions. Mass block A and mass block B have the same mass and volume.

[0014] Specifically, one end of the elastic spring is mounted on the housing, and the other end is fastened to the light rod with screws. The end of the light rod is connected to the test mass. The performance of the spring directly affects the stability and elastic modulus of the measurement system; therefore, materials with a tensile strength greater than 200 MPa and an elastic modulus less than 40 GPa should be selected. Notches are designed on the spring to increase its deformation capacity, such as... Figure 2(a) The light rod is a torque transmission device and a connecting mechanism. Its deformation can cause the position of the sensing mass to shift, thus affecting the accuracy of gravity gradient measurement. Simultaneously, to avoid affecting the mass distribution of the system, the density of the light rod should be as low as possible; therefore, a high-strength, low-density material is required. Furthermore, to further reduce the deformation of the light rod, this invention fabricates it as a double-beam structure, such as... Figure 2 (b) The mass block is a component in the instrument that directly senses the gravitational gradient and also affects the system's natural vibration frequency f. n Therefore, high-strength, high-density materials are required. The shape parameters and density of the two mass blocks should be as similar as possible. The fiber optic ring frame is a crucial component connecting the mechanical and optical structures. Its central threaded hole facilitates connection to the mass blocks, while external grooves allow for the winding of the optical fiber. Figure 2 (c)

[0015] This invention employs a pair of Sagnac interferometers sharing a common light source as the rotational motion measurement unit. The main components include an ASE light source, a 50 / 50 polarization-maintaining coupler, a circulator, a multifunctional waveguide chip, an optical fiber ring, and a photodetector. A detailed schematic diagram is shown below. Figure 3 Two fiber loops, A and B, with identical parameters, are wound around fiber loop frames on two mass blocks, respectively.

[0016] Two Sagnac interferometers sharing a common light source measure the angular motion of two mass blocks, serving as the signal acquisition front-end for gravity gradient measurement. More importantly, relative intensity noise is a common-mode noise component in both Sagnac interferometers. The shared light source system can suppress relative intensity noise while calculating the angular motion difference, improving the accuracy of gravity gradient measurement. Simultaneously, environmental vibration noise can also be suppressed through this differential configuration, reducing the environmental requirements of the gravity gradient measurement scheme.

[0017] The technical solution of this invention is as follows:

[0018] A gravity gradient measurement device based on an elastic pendulum, characterized in that it includes a support structure, a gravity gradient probe, and a rotational motion measurement unit; the gravity gradient probe is located within the support structure, and the gravity gradient probe is a pair of elastic pendulums, each of the elastic pendulums including a reed, a rigid light rod, a mass block, and an optical fiber ring frame; one end of the rigid light rod is connected to the support structure through the reed, and the other end is connected to the mass block; the optical fiber ring frame is mounted on the mass block; the mass block on the first elastic pendulum is denoted as mass block A, and the mass block on the second elastic pendulum is denoted as mass block B;

[0019] The rotational motion measurement unit includes two angle measuring instruments with the same modulation signal. The fiber optic ring of the first angle measuring instrument is set on the fiber optic ring skeleton of mass block A, and the fiber optic ring of the second angle measuring instrument is set on the fiber optic ring skeleton of mass block B.

[0020] When measuring the diagonal component of the gravity gradient Γ zz When measuring the off-diagonal component of the gravity gradient Γ, the two gravity gradient probes are placed parallel and symmetrically along the z-direction; zy At that time, the gravitational acceleration of the two probes along the z-axis is adjusted, and they are placed end to end opposite each other along the y-direction; the distance between the mass block A and the mass block B is h;

[0021] The rotational motion measuring unit is used to measure the rotation angle of the mass block relative to its initial position and utilizes... Γ was calculated zz Or Γ zy Where m is the mass of the mass block, L is the length of the rigid light rod, and θ A Let θ represent the rotation angle of mass block A relative to its initial position. B This represents the rotation angle of mass block B relative to its initial position; Δθ = θ A -θ B I is the moment of inertia of the mass block-rigid lightweight rod-fiber ring skeleton structure, γ is the damping coefficient, and k is the elastic coefficient of the spring.

[0022] Furthermore, the spring is made of a material with a tensile strength greater than 200 MPa and an elastic modulus less than 40 GPa; the spring is designed with notches to increase its deformation capacity.

[0023] Furthermore, the rigid lightweight bar is a double-beam lightweight bar.

[0024] Furthermore, the angle measuring instrument is a Sagnac interferometer.

[0025] Furthermore, the angle measuring instrument is an angular accelerometer, an angle sensor, or an angular motion sensor.

[0026] Furthermore, both angle measuring instruments share the same light source.

[0027] Compared with the prior art, the positive effects of the present invention are as follows:

[0028] This invention provides a gravity gradient measurement scheme based on an elastic pendulum and a Sagnac interferometer, with a simple overall structure. An added notch design enhances the elastic deformation capability of the spring, while a double-beam structure increases the stiffness of the lightweight rod. The scheme uses a fiber optic ring frame to connect the mechanical and optical components, protecting the fiber optic ring while ensuring the responsiveness of the mechanical system. Compared with existing gravity gradient measurement schemes, this scheme offers advantages such as suppressing environmental vibration noise and eliminating the need for complex superconducting control equipment. Furthermore, thanks to the high scalability of the Sagnac interferometer, this scheme has good potential for accuracy improvement. By configuring sensors in different orientations, full-component spatial gravity gradient detection can be achieved. Attached Figure Description

[0029] Figure 1 This is an assembly drawing of the mechanical structure for the gravity gradient scheme.

[0030] (a) Schematic diagram of the diagonal component gravity gradient measurement assembly, (b) Schematic diagram of the off-diagonal component gravity gradient measurement assembly.

[0031] Figure 2 Drawings of mechanical structure parts for the gravity gradient scheme;

[0032] (a) Spring, (b) Double beam light rod, (c) Fiber optic ring skeleton.

[0033] Figure 3 This is a schematic diagram of the optical structure of the gravity gradient scheme. Detailed Implementation

[0034] The gravity gradient measurement scheme based on an elastic pendulum and a Sagnac interferometer requires at least one pair of Sagnac interferometers and one pair of sensing probes. A schematic diagram of the sensing probes is shown below. Figure 1 As shown, the structure of the Sagnac interferometer is as follows: Figure 3 As shown.

[0035] Figure 1 The intermediate sensing probe components include a mechanical housing, a spring, a double-beam lightweight rod, a mass block, and a fiber optic ring frame. The mechanical housing is made of a high-strength alloy to ensure the internal structure is not damaged; a groove is added to its side wall, into which the spring is embedded. To improve sensitivity, the spring is designed to be as thin as possible. Notches can be added to the spring to further enhance the system's sensitivity. The spring and the double-beam lightweight rod are tightly connected by screws, and the double-beam lightweight rod is tightly connected to the mass block, on which the fiber optic ring frame is mounted. During use, both sets of probes require strict adjustment of their center of mass distribution to ensure stable operation.

[0036] Figure 2The optical structure shown is a rotating motion measurement unit. Light emitted from the ASE light source is transmitted to two fiber optic gyroscopes with identical parameters after passing through a 50 / 50 coupler. Fiber optic loop A and fiber optic loop B are respectively wound around... Figure 1 On the two fiber optic ring frames, two fiber optic gyroscopes use synchronized and consistent modulation signals. In this configuration, the relative intensity noise of the light source and the ambient vibration noise are both common-mode noise, while the difference in angular motion of the mass block caused by the gravity gradient is a differential-mode signal. By subtracting the outputs of the two fiber optic gyroscopes, the difference in angular motion can be obtained, and the gravity gradient can then be calculated.

[0037] The complete measurement procedure for gravity gradient is as follows: According to... Figure 1 The two sensor probes are arranged as shown. Figure 2 The two fiber optic loops are respectively wound around Figure 1 The two fiber optic rings are located on the frame. Due to the gravitational gradient, the gravitational acceleration in the z-direction experienced by the two mass blocks differs slightly, leading to a difference in their rotational motion around the x-axis. Fiber optic ring A is used to measure the angular displacement θ of mass block A. A The fiber optic loop B is used to measure the angular displacement θ of the mass block B. B The difference between the angular displacements of A and B is calculated to obtain the angular displacement difference Δθ = θ. A -θ B . Figure 1 The diagonal component of the gravity gradient Γ shown in (a) zz and Figure 1 The off-diagonal component gravity gradient Γ shown in (b) zy All can be achieved using formulas Find the value of I, where I is the moment of inertia of the mass block-rigid light rod-fiber ring skeleton, γ is the system damping coefficient, k is the spring elastic coefficient, m is the mass of the mass block, h is the center distance between the two mass blocks, and L is the length of the light rod.

[0038] In the above case, the Sagnac interferometer used to measure angular motion difference can be replaced by other rotational motion measurement units such as angular acceleration sensors and angle sensors.

[0039] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A gravity gradient measuring device based on an elastic pendulum, characterized in that, The system includes a support structure, a gravity gradient probe, and a rotational motion measurement unit. The gravity gradient probe is located within the support structure and consists of a pair of elastic pendulums. Each elastic pendulum includes a spring, a rigid light rod, a mass block, and an optical fiber ring frame. One end of the rigid light rod is connected to the support structure via the spring, and the other end is connected to the mass block. The optical fiber ring frame is mounted on the mass block. The mass block on the first elastic pendulum is designated as mass block A, and the mass block on the second elastic pendulum is designated as mass block B. The rotational motion measurement unit includes two angle measuring instruments with the same modulation signal. The fiber optic ring of the first angle measuring instrument is set on the fiber optic ring skeleton of mass block A, and the fiber optic ring of the second angle measuring instrument is set on the fiber optic ring skeleton of mass block B. When measuring the diagonal component of the gravity gradient Γ zz When measuring the off-diagonal component of the gravity gradient Γ, the two gravity gradient probes are placed parallel and symmetrically along the z-direction; zy At that time, the gravitational acceleration of the two probes along the z-axis is adjusted, and they are placed end to end opposite each other along the y-direction; the distance between the mass block A and the mass block B is h; The rotational motion measuring unit is used to measure the rotation angle of the mass block relative to its initial position and utilizes... Γ was calculated zz Or Γ zy Where m is the mass of the mass block, L is the length of the rigid light rod, and θ A Let θ represent the rotation angle of mass block A relative to its initial position. B This represents the rotation angle of mass block B relative to its initial position; Δθ = θ A -θ B I is the moment of inertia of the mass block-rigid lightweight rod-fiber ring skeleton structure, γ is the damping coefficient, and k is the elastic coefficient of the spring.

2. The gravity gradient measuring device according to claim 1, characterized in that, The spring is made of a material with a tensile strength greater than 200 MPa and an elastic modulus less than 40 GPa; the spring is designed with notches to increase its deformation capacity.

3. The gravity gradient measuring device according to claim 1, characterized in that, The rigid lightweight bar is a double-beam lightweight bar.

4. The gravity gradient measuring device according to claim 1, 2, or 3, characterized in that, The angle measuring instrument is a Sagnac interferometer.

5. The gravity gradient measuring device according to claim 1, 2, or 3, characterized in that, The angle measuring instrument is an angular accelerometer, an angle sensor, or an angular motion sensor.

6. The gravity gradient measuring device according to claim 1, 2, or 3, characterized in that, Both angle measuring instruments share the same light source.

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