Low Frequency Vibration Measurement Based On A Dual-frequency Fiber Grating Laser And The Doppler Effect

Nowadays,vibration measurement has wide range of applications in mechanical engineering,aerospace,civil engineering and so on.Generally,vibration can be measured through mechanical,electronic and optical methods.Each of these methods has its own unique advantages.Optical measurement methods are highly demanded in applications due to their wide measurement range,high precision,contactless measurement and other advantages.In this thesis,optic fiber vibration sensor is proposed based on the laser Doppler effect to measure the vibration frequency and amplitude of vibrating objects.When there is relative movement between a measured object and a light source,the reflected light from the surface of the moving object will be frequency shifted for Doppler effect,which is the Doppler shift.And the amount of frequency shift is dependent on the speed of the moving object.In this thesis,vibration measurement is realized through the measurement of the vibration speed of a vibration object.Conventional single-frequency laser has large DC drift in vibration measurement and is susceptible to external influences,which results low measurement accuracy.However,the proposed heterodyning measurement structure based on dual-frequency fiber laser can greatly reduce the impact of the DC drift and the measurement error caused by external environment disturbance.The thesis focuses on the vibration measurement at low frequency below 200 Hz by employing fiber lasers and the laser Doppler effect to realize high accuracy and high signal-tonoise ratio(SNR)real-time measurement of low frequency vibration.A measurement scheme for low frequency and weak vibration is proposed based on dual-frequency fiber grating lasers and the laser Doppler effect,which is easy for implementation due to its simple structure.The scheme is demonstrated and verified through experiments by using a weak vibration platform with the vibration frequency and amplitude tunable.For the demodulation of vibration signals,the thesis adopts a digital scheme by digitizing the received sensing signals and differentiating the phase of the signal in digital domain to recover the original vibration signal.The scheme demonstrates high SNR of about 50 dB and a linear response and high sensitivity at low frequency in which conventional accelerometers are quite inefficient.The scheme works very well in the band below 200 Hz and can works down to 1 Hz,and the frequency resolution is 1 Hz.