Ultrahigh-linearity MOEMS accelerometer based on self-tracing grating

Abstract

The invention discloses an ultrahigh-linearity MOMES accelerometer based on a self-tracing grating, and belongs to the field of detection instruments, the ultrahigh-linearity MOMES accelerometer comprises an MEMS harmonic oscillator, a self-tracing grating interferometer and a data processing module, the MEMS harmonic oscillator comprises a folding beam, a fixed outer frame and a mass block, the outer side wall of the folding beam is connected with the inner side wall of the fixed outer frame, and the mass block is connected with the MEMS harmonic oscillator. The mass block is arranged at the bottom of the folded beam, the self-traceability grating interferometer comprises a self-traceability grating and an interference measurement reading head, and the self-traceability grating is arranged on the mass block. The ultrahigh-linearity MOMES accelerometer based on the self-traceability grating has extremely high measurement linearity, displacement measurement can be traced in real time, and compared with a traditional MOMES accelerometer, the accuracy of displacement measurement can be improved, and the nonlinear effect of acceleration measurement is reduced.

Description

Technical Field

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

[0002] Acceleration, as a key physical quantity that relates to time and displacement, can reflect many key parameters of the carrier system and has important value in production and life. As a direct measurement device for acceleration, accelerometers have been widely used: in the industrial field, they are used for vibration monitoring to reduce safety hazards; in the consumer electronics field, they provide inertial information for mobile phones, smart wearable devices, etc.; in conjunction with gyroscopes, they can achieve positioning without external dependence and provide inertial navigation services for rockets and missiles. It can be said that the process of intelligent and unmanned human industry is the process of continuous improvement of accelerometer measurement performance.

[0003] The high-precision measurement of the accelerometer ensures the high-precision operation of various intelligent sensors and control systems. The measured acceleration value can be obtained by multiplying the accelerometer output voltage by the accelerometer sensitivity. Therefore, the linearity of the accelerometer output determines the key factor in the measurement accuracy of the accelerometer.

[0004] The measurement of accelerometer is mainly divided into two parts: inertial sensing and displacement sensing. The nonlinear deviation of the displacement sensing part is the main factor causing the nonlinearity of accelerometer measurement. Taking the MEMS capacitive displacement sensor accelerometer, which is the most widely used in smart wearable devices and medical monitoring fields, as an example, due to factors such as capacitor edge effect, capacitor structure non-uniformity, circuit nonlinearity and temperature effect, a large nonlinear error will be introduced into the displacement measurement, thereby affecting the measurement accuracy of the accelerometer. Therefore, solving the nonlinear problem of accelerometer in displacement sensing is the key to improving the measurement accuracy of accelerometer.

[0005] In addition, traditional displacement sensing reference gratings use interference lithography gratings, ruled gratings, etc. The accuracy of these grating lines depends on the calibration of the upper-level measuring instruments, the traceability chain is long, and the accumulated error as a displacement measurement benchmark is large. Summary of the invention

[0006] The purpose of the present invention is to provide an ultra-high linearity MOEMS accelerometer based on a self-traceable grating, which has extremely high measurement linearity and displacement measurement that can be traced in real time. Compared with traditional MOMES accelerometers, it can improve the accuracy of its displacement measurement and reduce the nonlinear effect of acceleration measurement.

[0007] To achieve the above-mentioned purpose, the present invention provides an ultra-high linearity MOEMS accelerometer 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 outer frame and a mass block, the outer wall of the folded beam is connected to the inner wall of the fixed outer frame, the mass block is arranged at the bottom of the folded beam, 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.

[0008] Preferably, the sensitive axis direction of the MEMS resonator coincides with the measurement sensitive direction of the accelerometer.

[0009] Preferably, the MEMS resonator is obtained by processing through a deep silicon etching process.

[0010] 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.

[0011] Preferably, the metal atom species used in the self-traceable grating is one of chromium, aluminum, iron or barium.

[0012] Preferably, the grating line direction of the self-traceable grating is perpendicular to the motion sensitive direction of the MEMS resonator.

[0013] Preferably, the grating period of the self-traceable grating can be directly traced back to a constant in nature.

[0014] Preferably, the interferometric measurement reading head adopts a homodyne structure or a heterodyne structure.

[0015] 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.

[0016] 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.

[0017] Therefore, the present invention adopts the above-mentioned ultra-high linearity MOEMS accelerometer based on self-traceable grating, which has the following beneficial effects:

[0018] (1) The present invention adopts a self-traceable grating as a displacement measurement reference element, so that the accelerometer displacement sensor can be directly traced to the pitch value of the self-traceable grating, and indirectly traced to the atomic transition energy level constant. This not only greatly shortens the measurement value traceability chain, but also strictly inherits the high period uniformity of the self-traceable grating in displacement measurement, solves the high nonlinear measurement error of traditional accelerometers, and realizes ultra-high linear acceleration measurement.

[0019] (2) The self-traceable grating is an atomic deposition grating prepared by one-dimensional laser focusing atomic deposition technology. It is a direct materialization of the laser wavelength and can be directly traced back to a constant in nature without the need for additional calibration. Therefore, the grating has a 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 the picometer level.

[0020] The technical solution of the present invention is further described in detail below through the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 Schematic diagram of the structure of an ultra-high linearity MOEMS accelerometer based on a self-traceable grating according to an embodiment of the present invention;

[0022] Figure 2 A schematic structural diagram of a MEMS resonator of an ultra-high linearity MOEMS accelerometer based on a self-traceable grating according to an embodiment of the present invention;

[0023] Figure 3 The present invention provides a process for preparing a self-traceable grating of an ultra-high linearity MOEMS accelerometer based on a self-traceable grating according to an embodiment of the present invention;

[0024] Figure 4 A linearity diagram of an ultra-high linearity MOEMS accelerometer based on a self-traceable grating according to an embodiment of the present invention;

[0025] Reference numerals

[0026] 1. Folding beam; 2. Fixed outer frame; 3. Mass block; 4. Self-traceable grating; 5. Interference measurement reading head; 6. Data processing module; 7. Cr atoms; 8. Laser cooling; 9. Laser standing wave field; 10. Substrate;

[0027] 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

[0028] 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.

[0029] 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.

[0030] Example

[0031] like Figure 1 As shown, the present invention provides an ultra-high linearity MOEMS accelerometer 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 acceleration signal to be measured into a corresponding displacement, and the self-traceable grating interferometer measures the displacement to obtain an interference light signal, which is then transmitted to the data processing module 6 and the magnitude of the acceleration to be measured is obtained through the data processing module 6.

[0032] like Figure 2 As shown, the MEMS resonator includes a folding beam 1, a fixed outer frame 2 and a mass block 3. The outer wall of the folding beam 1 is connected to the inner wall of the fixed outer frame 2, the mass block 3 is arranged at the bottom of the folding beam 1, the fixed outer frame 2 is used to fix the mass block 3 and the object to be measured, and a displacement movement space greater than the range is reserved on the fixed outer frame 2 in the sensitive direction. The fixing process should adopt a rigid connection to reduce the measurement error caused by the deformation of the outer frame. When the MEMS resonator is working, the fixed outer frame 2 is fixed to the carrier, so the acceleration corresponds to the inertial force acting on the mass block 3, which is deformed with the folding beam 1 to produce a stress balance, thereby causing the mass block 3 to produce a linear response relative displacement, through which the corresponding acceleration can be inverted.

[0033] The MEMS resonator 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.

[0034] 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.

[0035] 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.

[0036] 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, and the photoelectric signal reference of the grating displacement is d multiples of the grating period. The displacement of the self-traceable grating 4 is measured by means of interference structures such as homodyne structure, heterodyne and self-mixing structure.

[0037] Since the laser standing wave field is locked to the atomic transition frequency, the atoms are subjected to the laser dipole force and are periodically deposited as a grating structure on the MEMS resonator mass block. The grating line direction of the self-tracing grating 4 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.

[0038] The interferometric reading head 5 obtains a photoelectric signal carrying displacement information by incident and emitting laser light from the traceable grating 4. The data processing module 6 collects the photoelectric signal carrying displacement, and uses a corresponding algorithm to solve the displacement of the mass block according to its interference structure, thereby obtaining the corresponding acceleration.

[0039] The following takes the MOEMS accelerometer using a homodyne four-way Cr atom 7 self-traceable grating 4 as an example to explain its working process in detail:

[0040] The sensitive axis direction of the folded beam 1 of the MEMS resonator is the sensitive measurement direction of the accelerometer. When the accelerometer carrier generates acceleration, the mass block 3 of the resonator will generate a corresponding linear displacement Δx, and accordingly, the self-traceable grating 4 deposited on the mass block 3 will also generate 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:

[0041]

[0042] 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.

[0043] 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:

[0044]

[0045]

[0046] 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 amplitude of the sinusoidal light intensity voltage detected by the photodetector, is the initial phase of the interference signal.

[0047] The data processing module 6 collects the signal for processing, and two adjacent orthogonal interference signals with phase difference are randomly selected, and the two can demodulate the displacement information containing the motion direction. The phase of the interference signal is calculated by the inverse tangent phase unwrapping algorithm:

[0048]

[0049] 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.

[0050] The corresponding displacement signal can be obtained:

[0051]

[0052] According to the designed sensitivity of the resonator, the acceleration value measured by the inverse accelerometer is:

[0053] a=kΔx

[0054] Where k is the sensitivity of the accelerometer, Δx is the demodulated relative displacement, and a is the acceleration value measured by the MOEMS accelerometer.

[0055] like Figure 3 As shown, the self-traceable grating 4 adopts laser convergence atomic deposition technology to heat the grating material to gas in a vacuum environment, and uses a metal atomic 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.

[0056] Compared with the grating used in general MOEMS accelerometers, the self-traceable grating 4 used in this solution 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 acceleration displacement sensing.

[0057] On the basis of the above-mentioned atomic lithography grating preparation, in order to meet the measurement requirements of the large stroke of the MOEMS accelerometer resonator, the scanning and splicing atomic lithography method can be used to prepare a large-area atomic lithography grating without splicing errors, thereby realizing acceleration measurement with a large dynamic range.

[0058] 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.

[0059] like Figure 4 As shown in Figure 2, the displacement output of the MOEMS accelerometer has ultra-high linearity when measured, and its fitting R 2 The measurement linearity is 0.9997, which is better than the traditional measurement linearity of more than 1%, thus improving the measurement level.

[0060] Therefore, the ultra-high linearity MOEMS accelerometer based on self-traceable grating disclosed in the present invention has extremely high measurement linearity, and the displacement measurement can be traced in real time. Compared with the traditional MOMES accelerometer, it can improve the accuracy of its displacement measurement and reduce the nonlinear effect of acceleration measurement.

[0061] 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. An ultra-high linearity MOEMS accelerometer based on a self-traceable grating, characterized in that: It includes a MEMS resonator, a self-traceable grating interferometer and a data processing module. The MEMS resonator includes a folded beam, a fixed outer frame and a mass block. The outer wall of the folded beam is connected to the inner wall of the fixed outer frame. The mass block is arranged at the bottom of the folded beam. 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. The ultra-high linearity MOEMS accelerometer based on self-traceable grating according to claim 1, characterized in that: The sensitive axis direction of the MEMS resonator coincides with the measurement sensitive direction of the accelerometer.

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

4. The ultra-high linearity MOEMS accelerometer 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 ultra-high linearity MOEMS accelerometer 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 ultra-high linearity MOEMS accelerometer based on self-traceable grating according to claim 4, characterized in that: The grating line direction of the self-traceable grating is perpendicular to the motion sensitive direction of the MEMS resonator.

7. The ultra-high linearity MOEMS accelerometer based on self-traceable grating according to claim 4, characterized in that: The grating period of the self-traceable grating can be directly traced back to a constant in nature.

8. The ultra-high linearity MOEMS accelerometer 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.

9. The high-precision MOEMS accelerometer 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.

10. The high-precision MOEMS accelerometer 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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