Vibration Signal Processing Based On 2-MSMI

Self-mixing interference(SMI)technology is widely used in vibration measurement.Some measurement methods have problems such as large errors,lack of real-time performance,and only sinusoidal vibration can be reconstructed.Since the interference signal is modulated and demodulated,or two-multiple self-mixing interference(2-MSMI)technology can improve the accuracy of measurement.Therefore,this paper combines modulation and demodulation with 2-MSMI technology to measure the vibration by processing the vibration signal.Firstly,the three-mirror cavity theoretical model of the 2-MSMI system is obtained according to the three-mirror cavity theoretical model of SMI system.According to the energy conservation,the power equation and the phase equation are obtained in 2-MSMI system.The two equations are used as the theoretical basis to derive the vibration micro-displacement reconstruction equation.The principle of current modulation and phase modulation are briefly introduced.Secondly,2-MSMI signal is simulated with different parameters(feedback coefficient,object vibration amplitude and frequency).The effect of different external cavity lengths and reflection angles on the interference signal were studied.The relationship between each parameter and the interference signal waveform is obtained through simulation or experiment,which lays a foundation for the part of later experimental.Then,the principle of Fast Fourier Transform(FFT)spectrum analysis demodulation is introduced.The parameters of experiment are determined.Combining current modulation with 2-MSMI technology,the modulated vibration signal is processed with FFT spectrum analysis to reconstruct the micro-displacement of the vibrating object.Experimental results show that this method can effectively measure the vibration with vibration amplitude smaller than the wavelength,and the reconstruction error of 2-MSMI technology for measuring the micro-displacement of the vibration is smaller than the reconstruction error of SMI technology.Finally,the principle of five-step phase shift algorithm demodulation is introduced.Combining phase modulation with 2-MSMI technology,the modulated vibration signal is processed with five-step phase shift algorithm to reconstruct the micro-displacement of the vibrating object.The simulation and experimental results show that this method not only can reconstruct the waveform of the vibration amplitude within the wavelength,but also can reconstruct the waveform larger than the wavelength vibration amplitude.The measurement range becomes larger.And the 2-MSMI technology reconstructs the error of micro-displacement about half that of SMI technology.