Research On Measurement Method Of Laser Doppler Acceleration

Comparing with electrical accelerometers,fiber-optic accelerometers have advantages such as small size,light weight,corrosion resistance,and anti-electromagnetic interference,etc.Fiber-optic accelerometers can be widely used in the acceleration measurement of moving object under various harsh conditions and can have higher measuring accuracy,therefore,they have found wide applications in recent years.However,fiber-optic accelerometers commonly used measure accelerations by fix the sensing unit on the surface of the moving object,this will unavoidably interfere the moving of the object,and is especially disadvantageous for small acceleration measurement of moving object.Based on the laser Doppler effect,this paper proposed a noval non-contact fiber-optic accelerometer,which calculates acceleration by striving the difference between the Doppler frequency shifts of moving object at two adjacent moments.Main contents of this dissertation include the following aspects:1.Firstly,the phenomenon of laser Dopple effect is introduced,theorectical analysis and formula deduction are given for the process of velocity measurement of moving object based on the laser Doppler effect,and the relationship between the velocity and Doppler frequency shift are given.Then the working principle of the heterodyne laser Doppler measuring system is described,based on which,a specific scheme for acceleration measurement of moving object using the laser Doppler effect is demonstrated.Finally,an all-fiber laser Doppler acceleration measurement system is designed and the acceleration calculating formula is derived.2.The corresponding hardware circuit is designed according to output signal characteristics of the all-fiber laser Doppler acceleration measurement system.Considering that the output signal shows relatively low optical power and poor signal-to-noise ratio,it can not be directly demodulated.Therefore,the photodiode in the signal conditioning circuit is first used to convert the optical signal into a current signal,which is then transimpedance amplified and a bias voltage is introduced,the obtained voltage signal is input into an analog-to-digital conversion circuit to be transformed into a digital signal.The digital signal enters into an FPGA control circuit that including a demodulation algorithm to extraction the acceleration.Finally,the demodulation results are transmitted to a host computer through the network port communication circuit to display.3.Typical algorithms used for Doppler signal demodulation are compared and analyzed,their advantages and disadvantages are summarized,based on which,a digital correlation phase detection algorithm to demodulate the acceleration is proposed and theoretically analyzed.By using the digital correlation phase detection algorithm,not only the poor signal-to-noise ratio of the Doppler signal can be overcome,but also,the phase different for different time caused by the moving object can be calculated directly,which avoides extra errors that may be introduced by calculating the phase separately.4.Simulation analysis and experiment verification are accomplished for the all-fiber laser Doppler acceleration measurement system.Firstly,a piezoelectric ceramic oscillator is used as the moving object,and the acceleration's demodulation is simulated and analyzed using the digital correlation phase detection algorithm.When the signal-to-noise is 20d B,the acceleration demodulation error is the largest,which is0.135m/s~2.With the signal-to-noise ratio increases,the demodulation error continues to decrease.Then the all-fiber laser Doppler acceleration measurement system is built for experimental verification.The ceramic oscillator is measured at different accelerations,the maximum measurement error is-3.7660%,and the measurement range can cover0.115 m/s~2to 6.41×10~6m/s~2.The proposed all-fiber laser Doppler acceleration measurement system can be used for non-contact acceleration measurement of moving object,which has a high measurement accuracy and measurement stability,and having certain application potentials in fields such as a high-precision acceleration measurement of moving object with low signal-to-noise ratio.