An Acceleration Sensing Mechanism Based On Time Measurement

High-accuracy microelectromechanical systems(MEMS)accelerometer is the key element in inertial navigation,and the accuracy of the MEMS accelerometer is crucial to that of the navigation.This dissertation studies on a sensing mechanism based on time measurement.The measurement model based on the mechanism are initially established,and the theoretical formulas of the main performance indexes are deduced and many new characteristics are found.An accelerometer array using one device based on the mechanism is first proposed.Then the research results are experimentally demonstrated,which provides a theoretical foundation for developing high-accuracy MEMS accelerometer.The major works covered in this dissertation are listed as follows:(1)The measurement model of the acceleration based on time measurement is studied.By judging the times when the proof mass passes one set of two known displacement reference points(DRPs)during arbitrary vibration period,three time intervals are measured.The applied inertial acceleration is a function of the measured three time intervals.A mechanical structure of the MEMS accelerometer based on time measurement is developed and the acceleration measurement technique based on signal post-processing is proposed.The developed MEMS accelerometer system is tested and analyzed.The acceleration sensing mechanism based on time measurement is first experimentally verified.(2)The formulas of the main performance indexes of the accelerometer based on time measurement,including sensitivity,mechanical bandwidth,measurement range,equivalent acceleration caused by mechanical-thermal noise,and limit resolution of the acceleration detection circuit are defined and analyzed.The sensitivity is defined by the increment of the measured time interval per one meter per second squared.And it is in inverse proportion to the first power of vibration amplitude and the third power of resonant frequency.The mechanical bandwidth is defined by one percent of the resonant frequency.The measurement range is defined by the critical input acceleration amax that the maximum shifted displacement of center position of vibration trajectory of the proof mass corresponds to under the condition that the proof mass passes the DRPs.And it is equal to resonant frequency ω0 times the difference between vibration amplitude A and trigger gap d0,i.e.,amax=ω02(A-d0).The equivalent acceleration noise caused by the mechanical-thermal noise is the root square mean(RMS)acceleration that the displacement noise of center position of vibration trajectory of the proof mass due to thermal noise driving force corresponds to.The limit resolution of the acceleration detection circuit Rl,elec is decided by the time clock resolution Δt.The magnitude of Rl,elec is proportional to the first power of vibration amplitude times,the third power of resonant frequency times,and the first power of time clock resolution,i.e.,Rl,elec=Aω03Δt/2。The influence of time clock resolution,vibration amplitude,and trigger gap on the limit resolution of the acceleration detection circuit is consistent with the theoretical analysis.The measured step response time of the developed sensor is about 0.013 second.(3)Two novel characteristics of the acceleration sensing mechanism are found.First,it is found that the acceleration measurement is insensitive to the change of vibration amplitude and resonant frequency of the proof mass.With this characteristic,the acceleration can be measured during the process of attenuation vibration.The analytic expression of acceleration measurement during the process is deduced.Consequently,even if the signal used for driving the proof mass in the resonant state is disturbed and shielded,the sensor operates normally during the process of attenuation vibration.The measured acceleration during the process of attenuation vibration is experimentally verified.The measured time is accordance with the calculated time.Second,it is found that the acceleration measurement is insensitive to the change of DRPs.Based on this characteristic,an accelerometer array using one device based on time measurement is first proposed.N measurements can be simultaneously solved by predefining N sets of DRPs,which overcomes the drawback that individual differences in conventional array using multiple sensors degrade the expected improvement.A more accurate measurement can be obtained via combining the N measurements,which is called a method of virtual accelerometer based on one device.The 1σ noise,velocity random walk,and noise floor of the virtual accelerometer built in this dissertation are reduced by factors of 4.45,1.36,and 1.29 respectively,compared with individual component in the array.In conclusion,the acceleration sensing mechanism converts measurement of displacement caused by inertial acceleration to that of time.And time of the seven basic units of the international unit system is a physical quantity with the highest measurement accuracy.Thereby it provides a new method for measuring acceleration accurately.