| MEMS accelerometers possess the advantages of small volume,light weight,low power consumption and reliable performance.As the core element of inertial navigation system,MEMS accelerometers are widly applicated in consumer electronics,medical devices,aerospace,national defense and military industry.In the recent years,MEMS accelerometers have occupied the middle and low-end market,and been capturing the high-end market of strategic accuracy.As the development of our country's society and economy,especially the emerging industries of internet of things(IoT),artificial intelligence(AI)and big data,huge demand of high-performance MEMS accelerometers of our country will happen in the near future.Therefore,the research and development of high-performance MEMS accelerometers are strategically significant.Fabricated by MEMS fabrication process,MEMS resonant acceleromters are a kind of inertial sensors and based on resonant sensing principle.Generally,the resonant accelerometers consist of proof mass,micro leverage mechanisms and resonators.The inertial force of the proof mass,caused by the acceleration,is amplified by the leverages and applied to the resonators.Consequently,the resonant frequencies of the resonators are shift.After the calculation by the peripheral circuits,the acceleration can be detected.Compared with the analog MEMS acceleromters,such as piezoresistive,piezoelectric,capacitive and tunnel current accelerometers,resonant accelerometers have the advantages of quasi-digital output,digital circuit compatible,anti-interference,high stability,and great potential of accuracy improvement.In addition,the resonant accelerometers are immune to the pull-in effect,so they have large dynamic range and anti-shock function.However,the embargo policy of foreign resonant accelerometers products to our country is implementing,and the developments of resonant accelerometers in our country are still in the lab.By comprehensively analyzing the related research at home and abroad,it can be found that there are still many unsolved issues of resonant accelerometers to be explored,for example:the sensitivity enhancement by microleverage mechanisms can be further optimized;the sensitivity enhancement is limited if it's only based on microleverage optimization,the range self-adaption cannot be realized if the structure parameters are determined;and the relative sensitivities based on the mechanism of axial force sensibility are generally low(<1%/g).To counter the issues above,this thesis is working on the amplification mechanisms,sensing elements and sensing mechanisms,and the feasibilities are verified.The devices were fabricated and the testing platforms were constructed.The preliminary experiment results were obtained.The related researches are:The sensitivity enhancement mechanism based on microleverage mechanisms,including the modified model of micro leverage mechanisms(guiding the design of resonant accelerometers),the biaxial resonant accelerometer with two-stage microleverage mechanisms and its application of duplex mode tilt measurements(realizing biaxial acceleration measurements and duplex mode tilt measurements with high level integration and high sensitivity);The sensitivity enhancement mechanism based on h igh order mode resonators and the range adaption mechanisms,including the fish-bone shaped resonators and the resonators with multi-sectionalized electrodes(as the sensing elements of resonant accelerometers to improve the sensitivity and adjust the range);The sensitivity enhancement mechanism and voltage-controlled range adaption based on the perturbing force sensibility,which was realized by electrostatic coupled perturbing force.The contents can be summarized as:1.The MEMS accelerometers are classified and the working principles of each kind of accelerometers are introduced.The development history and research status of resonant accelerometers are reviewed and summarized.The research objects,technological means,research contents and innovation points are proposed on the existing issues and application requirements.2.Aiming at the issue of big error of present microleverage mechanism mechanical model,a modified model is proposed and verified by FEM simulation.The modified model and amplification factor calculation of two-stage microleverage mechanisms are deduced based on the modified model of single-stage micro leverage mechanisms.Compared with the present model,the modified model is more accurate and more capable for the design of resonant accelerometers,and it offers the theoretical basis to the subsequent chapters.3.A biaxial resonant accelerometer with two-stage microleverage mechanisms is firstly proposed.The working principle,performance parameters and equivalent circuit model of electrostatic DETF resonator are gone into details.The single proof mass biaxial acceleration measurements are realized by the difference between the axial and lateral stiffness of decoupling beams.The sensitivity of the device is calculated by the modified model of two-stage micro leverage mechanisms and optimized by the systematic FEM simulation.The electromechanical characteristics and the temperature characteristics of the resonators and the calibration of the accelerometer were tested by the vacuum open-loop frequency response testing platform.Benefit by the closed-loop self-oscillation circuit,the real time tracking of series resonant frequency and the resolution measurements were achieved.The two-stage microleverage maximizes the optimization of microleverage mechanisms and gives high sensitivity to the accelerometer.The adoption of decoupling beams makes the accelerometer compact,highly integrated,low cross-sensitive,so the proposed accelerometer has the potential for highly sensitive biaxial acceleration measurements.4.The duplex mode and highly sensitive tilt measurements,including biaxial tilt measurement with small angle range and uniaxial tilt measurement with large angle range,are firstly realized based on the biaxial resonant accelerometer.So different application requirements can be met.For the biaxial tilt measurement with small angle range,the decoupling beams isolate the crosstalk of two quadrature axes so the single proof mass biaxial tilt measurements are realized.But limited by the nonlinearity between the weight component and tilt angle,the angle range is small(±40°).For the uniaxial tilt measurement with large angle range,based on the frequency shift phase difference,the ±90° angle range tilt measurement can be realized by segment detection,which greatly expand the tilt measurement angle range.5.Aiming at the issues that the sensitivity improvement is limited if it's only based on microleverage optimization and the range self-adaption cannot be realized if the structure parameters are determined,based on the theoretical basis that the resonators have higher force sensibility at higher vibration modes,the fishbone-shaped resonators and the resonators with multi sectionalized electrodes are proposed,both of them are able to vibrate at mode 1 to 3 and the vibration mode can be switched freely.The higher mode resonators are firstly adopted as the sensing elements of resonant accelerometers.The sensitivity improvement is realized by higher vibration modes and the range adaption is achieved by mode adjustment6.Aiming at the issue that the relative sensitivities based on the mechanism of axial force sensibility are generally low(<1%/g),the highly-sensitive resonant accelerometer based on the mechanism of perturbing force sensibility is firstly proposed.Different from traditional resonant accelerometers based on the mechanism of axial force sensibility,in the new type of device,the proof mass is electrostaticly coupled to the resonator and then changes the resonant frequency of the resonator to detect the acceleration.In addition,the system nature frequency is optimized to meet the bandwidth of consumer applications,and the dimensions of resonator are also optimized for the in-atmosphere work.Finally,the major researches and the achievements are summarized,and the future work based on the research progress and unresolved issues are planned and outlooked. |
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