Research on Pulsed Laser Doppler Vibration Measurement Method

Laser Doppler vibration detection technology which is charactered with long measurement distance, high measurement accuracy and non-contact measurement has received considerable attentions in industry, agriculture, military and other fields. Previous research usually uses a continuous light source as the emitted signal and the FFT algorithm to calculate frequency of the received signal. However, the difficulty of extracting the effective vibration signal is greatly increased when the target is far away from sensor because the echo signal is drowned in noise. Meanwhile, the complexity of FFT algorithm is high and the accuracy depends on the number of FFT points. In this paper, the continuous laser source is replaced by the pulse laser source to detect the target. It can improve the signal-to-noise ratio of the received signal in condition with the same laser power. Benefits from the pulse transmission method, the transmitted signal and the received signal can be separated in time domain. Meanwhile, the phase difference between the transmitted signal and the received signal is used to extract the vibration signal without FFT algorithm. In addition, the laser Doppler vibration measurement system with an all-fiber structure is built using FPGA as the core signal processor. The single-frequency signal between 300Hz ∼ 3000Hz and speech signal is tested at a distance of 170meters. The extraction ability of vibration signals of different reflective media is compared and analyzed in this paper. The results show that this method can effectively complete the extraction and measurement of vibration signals at a long distance, and the frequency measurement range and accuracy can meet the requirements of practical applications.