A high-precision MOEMS relative gravimeter based on self-traceable grating

Abstract

The present invention discloses a high-precision MOEMS relative gravimeter based on a self-traceable grating, which belongs to the field of detection instruments, and includes a MEMS resonator, a self-traceable grating interferometer and a data processing module, wherein the MEMS resonator includes a folding beam, a fixed frame and a mass block, wherein both ends of the folding beam are connected to the inner side wall of the fixed frame, and the mass block is suspended in the center of the fixed frame through the support of the folding beam, and the self-traceable grating interferometer includes a self-traceable grating and an interference measurement reading head, and the self-traceable grating is arranged on the mass block. The present invention adopts the above-mentioned high-precision MOEMS relative gravimeter based on a self-traceable grating, which has extremely high measurement accuracy, and the displacement measurement can be traced in real time. Compared with the traditional MOMES relative gravimeter, it can improve the accuracy of its displacement measurement and reduce the nonlinear effect of gravity acceleration measurement.

Description

Technical Field

[0001] The invention relates to the field of detection instruments, and in particular to a high-precision MOEMS relative gravimeter based on a self-traceable grating. Background Art

[0002] The distribution of gravity acceleration on the Earth is affected by many factors, including location and time. The location-related part is mainly determined by the local mass distribution, latitude and longitude. The time-related part is affected by tides, crustal movement and other time-related phenomena. Accurate measurement of gravity acceleration is crucial in many fields and provides important data support for scientific research, resource exploration, engineering design and environmental monitoring. Therefore, continuously improving the accuracy of gravity measurement technology and methods will help to better understand the physical properties of the Earth and promote the development of research and practice in related fields.

[0003] Gravimeters are divided into absolute gravimeters and relative gravimeters. Relative gravimeters calculate the change in gravitational acceleration by measuring the displacement of the mass block. Therefore, accurately measuring the displacement of the mass block is a key factor in improving the resolution and measurement accuracy of relative gravimeters. Traceability is the basis of measurement accuracy. The shorter the traceability chain, the smaller the value transfer error, and the higher the corresponding measurement accuracy. Grating interferometer is a high-precision displacement measurement instrument. Since traditional gratings do not have traceability, they must be traced back to the instrument setting of the metrology agency. Therefore, its traceability chain in actual measurement: "gravimeter resonator displacement-grating line spacing-metrological instrument-laser wavelength" is too long, so it is difficult to ensure accurate values. In order to improve the performance of gravimeters, it is necessary to solve the problems of long traceability chain and large error accumulation in traditional gravity sensor displacement measurement, and improve the displacement measurement capability and level of gravimeters. Summary of the invention

[0004] The purpose of the present invention is to provide a high-precision MOEMS relative gravimeter based on a self-traceable grating, which has extremely high measurement accuracy and displacement measurement that can be traced in real time. Compared with traditional MOEMS relative gravimeters, it can improve the accuracy of its displacement measurement and reduce the nonlinear effect of gravitational acceleration measurement.

[0005] To achieve the above-mentioned purpose, the present invention provides a high-precision MOEMS relative gravimeter based on a self-traceable grating, comprising a MEMS resonator, a self-traceable grating interferometer and a data processing module, wherein the MEMS resonator comprises a folded beam, a fixed frame and a mass block, wherein both ends of the folded beam are connected to the inner side wall of the fixed frame, and the mass block is suspended in the center of the fixed frame through the support of the folded beam, and the self-traceable grating interferometer comprises a self-traceable grating and an interference measurement reading head, and the self-traceable grating is arranged on the mass block.

[0006] Preferably, the sensitive axis direction of the folding beam coincides with the gravity sensing direction.

[0007] Preferably, the MEMS resonator is obtained by processing through a deep silicon etching process.

[0008] Preferably, the self-traceable grating is a deposited grating prepared by adopting a laser convergence atom deposition method, heating and sublimating a metal material to a gas state in a vacuum environment, and drawing out a metal atom beam in an effusion manner.

[0009] Preferably, the metal atom species used in the self-traceable grating is one of chromium, aluminum, iron or barium.

[0010] Preferably, the interferometric measurement reading head adopts a homodyne structure or a heterodyne structure.

[0011] Preferably, the optical frequency in the interference optical path of the homodyne structure is the same as the reference optical frequency, and the optical interference signal is detected and demodulated by using specific optical devices and optical structure detection.

[0012] Preferably, the optical frequency in the interference optical path of the heterodyne structure is different from the reference optical frequency, and specific optical devices and optical structures are used for detection to achieve mixing modulation of the optical signals of the two, and then detection is performed.

[0013] Therefore, the present invention adopts the above-mentioned high-precision MOEMS relative gravimeter based on self-traceable grating, which has the following beneficial effects:

[0014] 1. The present invention uses a self-traceable grating as a displacement measurement reference element, so that the displacement sensing of the relative gravity instrument directly depends on the ultra-high period uniformity and ultra-high accuracy of the grating, as well as the higher resolution of the self-traceable grating interferometer, and the flattened traceability chain, which greatly improves its displacement sensing performance. Therefore, compared with the traditional relative gravimeter gravity measurement, it has higher linearity, higher accuracy and higher resolution, and solves the problem of poor gravity measurement accuracy caused by inaccurate displacement sensing measurement of the traditional relative gravimeter.

[0015] 2. The self-traceable grating interferometer of the present invention uses the self-traceable grating as the reference element for displacement measurement, so that the displacement sensor of the gravimeter can be directly traced to the pitch value of the self-traceable grating, and then indirectly traced to the atomic transition energy level constant. This method not only greatly simplifies the measurement value traceability chain, but also gives full play to the high period uniformity of the self-traceable grating in displacement measurement, which helps to achieve accurate measurement of mass block displacement, thereby obtaining high-precision gravitational acceleration values.

[0016] 3. The self-traceable grating of the present invention is prepared by one-dimensional laser convergence atomic deposition technology, which is an atomic deposition grating with the advantages of small period uncertainty, good uniformity and excellent environmental stability. The Cr atomic self-traceable grating currently developed has been certified by the world's authoritative metrology organization PTB and has a period uncertainty of picometers.

[0017] 4. The present invention has extremely high measurement accuracy, and the displacement measurement can be traced in real time. Compared with the traditional MOMES relative gravimeter, it can improve the accuracy of its displacement measurement and reduce the nonlinear effect of gravity acceleration measurement.

[0018] The technical solution of the present invention is further described in detail below through the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 A schematic structural diagram of a high-precision MOEMS relative gravimeter based on a self-traceable grating according to an embodiment of the present invention;

[0020] Figure 2 A schematic diagram of the structure of a MEMS resonator of a high-precision MOEMS relative gravimeter based on a self-traceable grating according to an embodiment of the present invention;

[0021] Figure 3 A schematic diagram of a self-traceable grating preparation process of a high-precision MOEMS relative gravimeter based on a self-traceable grating according to an embodiment of the present invention;

[0022] Figure 4 A static noise spectral density diagram of a high-precision MOEMS relative gravimeter based on a self-traceable grating according to an embodiment of the present invention;

[0023] Reference numerals

[0024] 1. Folding beam; 2. Fixed frame; 3. Mass block; 4. Self-traceable grating; 5. Interferometric reading head; 6. Data processing module; 7. Cr atoms; 8. Laser cooling; 9. Laser standing wave field; 10. Substrate;

[0025] M1, reflector one; M2, reflector two; PBS1, polarization beam splitter prism one; PBS2, polarization beam splitter prism two; PBS3, polarization beam splitter prism three; NPBS, depolarization beam splitter prism; PD1, photodetector one; PD2, photodetector two; PD3, photodetector three; PD4, photodetector four; Laser, laser. DETAILED DESCRIPTION

[0026] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, rather than all of the embodiments. The components of the embodiments of the present invention generally described and shown in the drawings here can be arranged and designed in various different configurations. Therefore, the following detailed description of the embodiments of the present invention provided in the drawings is not intended to limit the scope of the claimed invention, but merely represents selected embodiments of the present invention. Based on the embodiments in the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0027] It should be noted that similar reference numerals and letters denote similar items in the following drawings, and therefore, once an item is defined in one drawing, further definition and explanation thereof is not required in subsequent drawings.

[0028] Example

[0029] like Figure 1 As shown, the present invention provides a high-precision MOEMS relative gravimeter based on a self-traceable grating, including a MEMS resonator, a self-traceable grating interferometer and a data processing module 6. The MEMS resonator converts the gravity signal to be measured into a relative displacement, and the displacement is measured using the self-traceable grating interferometer to obtain a corresponding interference light signal, which is then transmitted to the data processing module 6 and the change in the gravity value to be measured is obtained through the data processing module 6.

[0030] like Figure 2 As shown, the MEMS resonator includes a folding beam 1, a fixed frame 2 and a mass block 3. The two ends of the folding beam 1 are connected to the inner wall of the fixed frame 2. The mass block 3 is suspended in the center of the fixed frame 2 through the support of the folding beam 1. The fixed frame 2 is used to fix the mass block 3. The fixing process should adopt a rigid connection to reduce the measurement error caused by the deformation of the fixed frame 2. When using this instrument, the measurement sensitive direction should be parallel to the gravity field direction, and its measurement range is 9.8m / s 2 Fluctuations near gravitational acceleration, specifically (9.76~9.83) m / s 2 When the instrument is in use, the sensitive axis of the resonator is along the direction of the earth's gravity, and external vibration isolation is usually required to ensure measurement accuracy.

[0031] The self-traceable grating interferometer includes a self-traceable grating 4 and an interferometric reading head 5. The self-traceable grating 4 is arranged on the mass block 3. The self-traceable grating 4 is a deposited grating prepared by heating and sublimating a metal material to a gas state in a vacuum environment and drawing out a metal atom beam in an effusion manner by using a laser convergence atom deposition method. The metal atom types used in the self-traceable grating 4 are chromium Cr, aluminum Al, iron Fe or barium Ba, etc.

[0032] The interferometric reading head 5 adopts a homodyne or heterodyne structure. The optical frequency in the interference optical path of the homodyne structure is the same as the reference optical frequency, and the optical interference signal is detected and demodulated by using a specific optical device and optical structure detection. The optical frequency in the interference optical path of the heterodyne structure is different from the reference optical frequency, and the optical signals of the two are mixed and modulated by using a specific optical device and optical structure detection, so as to perform detection.

[0033] like Figure 1 Taking the homodyne structure as an example, the interferometric reading head 5 includes a reflector 1, a reflector 2, a polarization beam splitter prism 1, a polarization beam splitter prism 2, a polarization beam splitter prism 3, a depolarization beam splitter prism, a photodetector 1, a photodetector 2, a photodetector 3, a photodetector 4 and a laser. The interferometric reading head 5 is based on the frequency shift effect of the grating Doppler. The photoelectric signal reference of the grating displacement is d multiples of the grating period. The displacement signal of the self-traceable grating 4 is measured by means of interference structures such as homodyne structure, heterodyne and self-mixing.

[0034] The grating line direction of the self-traceable grating 4 deposited on the MEMS resonator is perpendicular to the sensitive direction of the MEMS resonator motion, and the grating period can be directly traced back to the natural constant of the atomic transition frequency. The interferometric reading head 5 obtains a photoelectric signal carrying displacement information by incident and emitting a laser on the self-traceable grating 4. The signal processing module 6 collects the photoelectric signal carrying the displacement, and uses the corresponding algorithm to solve the displacement of the mass block 3 according to its interference structure, and then obtains the corresponding relative gravitational acceleration.

[0035] The following takes the MOEMS relative gravimeter using a homodyne four-way Cr atom 7 self-traceable grating 4 as an example to explain its working process in detail:

[0036] The sensitive axis direction of the folded beam 1 of the MEMS resonator is the gravity sensing direction. When the relative gravimeter is placed in the vertical direction, gravity is loaded on the mass block 3 along the sensitive axis. When the gravity acceleration changes, the mass block 3 will produce a corresponding displacement Δx. Correspondingly, the self-traceable grating 4 deposited on the mass block 3 also produces the same displacement Δx. At this time, when the two lasers emitted by the laser are diffracted on the grating surface, the laser frequency will change due to the grating Doppler effect, which will eventually affect the phase of the combined beam. The phase change satisfies the following relationship:

[0037]

[0038] In the formula, is the phase change of the interference light, Δx is the displacement of the self-traceable grating, and d is the grating period of the self-traceable grating. Therefore, the displacement measurement is completely based on the grating period.

[0039] The interference light is modulated by the wave plate and polarization beam splitter prisms (polarization beam splitter prism 1, polarization beam splitter prism 2 and polarization beam splitter prism 3), so that the phases of the corresponding outgoing lights differ by 90° in sequence. The light intensity voltage signals measured by the four photodetectors are:

[0040]

[0041] In the formula, u out1 is the light intensity voltage signal measured by the photodetector, u out2 is the light intensity voltage signal measured by photodetector 2, u out3 is the light intensity voltage signal measured by photodetector 3, u out4 is the light intensity voltage signal measured by photodetector 4, u0 is the sinusoidal light intensity voltage amplitude detected by the photodetector, is the initial phase of the interference signal.

[0042] The data processing module 6 collects the signal for processing, and two adjacent orthogonal interference signals with phase difference are selected, and the two can be demodulated to obtain the displacement information including the motion direction. The phase of the interference signal is calculated by the inverse tangent phase unwrapping algorithm:

[0043]

[0044] Among them, u′ out1 (t) is the first signal of the standard orthogonal interference signal, u′ out2 (t) is the second signal of the standard orthogonal interference signal, is the interference signal phase.

[0045] The corresponding displacement signal can be obtained:

[0046]

[0047] According to the design sensitivity of the oscillator, the acceleration value measured by the gravimeter is inverted:

[0048] a=kΔx

[0049] Where k is the corresponding test sensitivity of the gravimeter, Δx is the demodulated relative displacement, and a is the gravity change value measured by the MOEMS relative gravimeter.

[0050] like Figure 2As shown, the self-traceable grating 4 uses laser convergence atomic deposition technology to heat the grating material to gas in a vacuum environment and use a metal atom beam as a raw material to prepare the deposited grating. The type of metal atom used in the self-traceable grating 4 can be one of Cr, Al, Ba, Fe, etc., and its grating period can be directly traced back to a natural constant. Taking Cr atom 7 as an example, the preparation process of the self-traceable grating 4 is introduced, specifically: a crucible filled with high-purity Cr powder is heated to a sublimation state in a vacuum environment, and a metal atom beam is ejected from a furnace tube. The atomic beam is collimated by a slit and laser cooling 8 to reduce its lateral velocity. The collimated Cr atomic beam passes through a one-dimensional laser standing wave field orthogonal to it, and is deposited on the template under the action of a dipole force to form a one-dimensional deposition grating structure. The wavelength of the one-dimensional laser standing wave field is 425.553nm, and the frequency is adjusted to a positive detuning position of +250MHz or a negative detuning position of -250MHz. At this time, the pitch of the one-dimensional Cr atom 7 deposition grating formed is half the wavelength of the one-dimensional laser standing wave field 9 used, that is, 212.8nm. Since the grating prepared by the atomic lithography method needs to form a structure, the wavelength of the laser standing wave field 9 must be exactly equal to the wavelength of the atomic transition energy level. At the same time, the wavelength of the Cr atom 7 deposition grating formed by the structure is exactly equal to half the wavelength of the laser standing wave field 9. Therefore, the Cr atom 7 deposition grating is the materialization of the laser wavelength and is a self-traceable grating 4 with metrological characteristics.

[0051] Compared with the grating used in general MOEMS relative gravimeters, the self-traceable grating 4 used in this scheme has its own traceability capability, does not require additional settings, and has extremely high linearity. It can effectively avoid the nonlinear effects of traditional capacitive displacement measurement and improve the linearity of gravimeter displacement sensing.

[0052] On the basis of the above-mentioned atomic lithography grating preparation, in order to meet the high-precision and large dynamic range measurement requirements of the MOEMS relative gravimeter, the scanning and splicing atomic lithography method can be used to prepare a large-area atomic lithography grating without splicing errors, thereby realizing gravity measurement with a large dynamic range.

[0053] In order to ensure the consistency of the grating period direction on the resonator and the displacement direction of the mass block 3, a right-angle reflector on the right side of the laser standing wave field 9 is used to generate a standing wave field, and the resonator is placed and deposited perpendicular to the reflecting surface of the right-angle reflector.

[0054] The sensitivity calibration test method of the MOEMS relative gravimeter in this scheme generally adopts the standard gravity point tilt method for calibration. The specific method is: at the gravity point where the absolute gravimeter is calibrated, change the inclination angle between the relative gravimeter and the gravity field, and use the change in gravity component to calibrate the sensitivity parameters of the gravimeter.

[0055] The MEMS resonator of the present solution is manufactured by using a deep silicon etching process, so that the folded beam 1 can achieve an extremely high aspect ratio, thereby achieving better mechanical properties.

[0056] like Figure 4 As shown, the noise spectral density within the test bandwidth of the MOEMS relative gravimeter of the present invention is 0.0625 mGal / √Hz, which has excellent gravity measurement performance and improves the measurement level of existing gravity measurement instruments.

[0057] Therefore, the high-precision MOEMS relative gravimeter based on the self-traceable grating disclosed in the present invention has extremely high measurement accuracy, and the displacement measurement can be traced in real time. Compared with the traditional MOMES relative gravimeter, it can improve the accuracy of its displacement measurement and reduce the nonlinear effect of gravitational acceleration measurement.

[0058] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that they can still modify or replace the technical solution of the present invention with equivalents, and these modifications or equivalent replacements cannot cause the modified technical solution to deviate from the spirit and scope of the technical solution of the present invention.

Claims

1. A high-precision MOEMS relative gravimeter based on a self-traceable grating, characterized by: It includes a MEMS resonator, a self-traceable grating interferometer and a data processing module. The MEMS resonator includes a folded beam, a fixed frame and a mass block. The two ends of the folded beam are connected to the inner wall of the fixed frame. The mass block is supported by the folded beam and suspended in the center of the fixed frame. The self-traceable grating interferometer includes a self-traceable grating and an interference measurement reading head. The self-traceable grating is arranged on the mass block.

2. A high-precision MOEMS relative gravimeter based on a self-traceable grating according to claim 1, characterized in that: The sensitive axis direction of the folding beam coincides with the gravity sensing direction.

3. The high-precision MOEMS relative gravimeter based on self-traceable grating according to claim 1, characterized in that: The MEMS resonator is obtained by deep silicon etching process.

4. The high-precision MOEMS relative gravimeter based on self-traceable grating according to claim 1, characterized in that: The self-traceable grating is a deposited grating prepared by adopting a laser convergence atom deposition method, heating and sublimating a metal material to a gas state in a vacuum environment, and then drawing out a metal atom beam in an effusion manner.

5. The high-precision MOEMS relative gravimeter based on self-traceable grating according to claim 4, characterized in that: The metal atom type used in the self-traceable grating is one of chromium, aluminum, iron or barium.

6. The high-precision MOEMS relative gravimeter based on self-traceable grating according to claim 1, characterized in that: The grating period of the self-traceable grating can be directly traced back to a constant in nature.

7. The high-precision MOEMS relative gravimeter based on self-traceable grating according to claim 1, characterized in that: The interferometric measurement reading head adopts a homodyne structure or a heterodyne structure.

8. The high-precision MOEMS relative gravimeter based on self-traceable grating according to claim 7, characterized in that: The optical frequency in the interference optical path of the homodyne structure is the same as the reference optical frequency, and the optical interference signal is detected and demodulated by using specific optical devices and optical structure detection.

9. The high-precision MOEMS relative gravimeter based on self-traceable grating according to claim 7, characterized in that: The optical frequency in the interference optical path of the heterodyne structure is different from the reference optical frequency, and the optical signals of the two are mixed and modulated by using specific optical devices and optical structures for detection, and then detection is performed.

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