Research On Laser Doppler Vibrometry System Based On Pulse Interleaved Discrete Frequency Shift Keying Method

In recent years,there has been a growing interest in the optical non-contact measurement technique centered around laser-based technologies,particularly Laser Doppler Velocimetry(LDV).LDV stands out among various non-contact measurement techniques due to its non-invasive nature,high spatial resolution,wide measurement speed range,high accuracy,and broad application areas.However,conventional LDV systems commonly employ Fast Fourier Transform(FFT)for frequency spectrum analysis of the acquired Doppler vibration signals.Despite its ability to provide frequency spectra,FFT has drawbacks such as the requirement of high signal stability and non-real-time data processing.In this study,we address these issues associated with conventional laser vibrometers and propose a fast and real-time demodulation method for Doppler vibration signals.We present a laser Doppler vibrometry scheme based on the pulse interleaved phase cancellation frequency discrimination technique.The principle of this method is explored,and several research aspects are investigated.First,through the study of laser Doppler vibrometry theory and considering the optical path schemes of other LDV devices,we design an optical path scheme suitable for the pulse interleaved phase cancellation frequency discrimination technique.This model exhibits the characteristics of simplicity in structure,ease of operation,and the ability to accurately discriminate the vibration state of objects based on the final collected current signals.Furthermore,we analyze and study existing laser Doppler vibrometry methods,replacing the analog-to-digital converter chips commonly used in most LDV systems with high-speed comparators.We disregard the amplitude information of the vibration voltage signals and convert the original sinusoidal signals into square wave signals directly recognizable by FPGA.By analyzing the pulse pair frequency discrimination method,we establish mathematical relationships for the frequency difference between any two square wave signals.We further improve the pulse interleaved phase cancellation frequency discrimination method and propose a method based on the analysis of the period and phase relationship of two different frequency square wave signals.Finally,we extract the edge pulses of the square wave signals of different frequencies to obtain the frequency difference of the signal.Utilizing the characteristics of pulse interleaved phase cancellation frequency discrimination,we establish a connection between the frequency difference information output by the algorithm and the displacement of the vibrating object.Combined with the designed optical path scheme,we can obtain the displacement curve of the vibrating object.Compared to FFT,this method offers lower computational complexity and better real-time performance.Moreover,we design a laser Doppler vibrometer experimental setup based on the pulse interleaved phase cancellation frequency discrimination method.We design and fabricate circuit boards,perform circuit soldering and debugging,successfully incorporate the pulse interleaved phase cancellation frequency discrimination method into the designed system circuit board,and develop a graphical user interface that displays the displacement curve and calculates the vibration frequency of the object.Through the debugging of each part of the experimental setup and the analysis of the hardware circuit waveform during the experiment,we ultimately display the displacement curve of a standard vibration source using the graphical user interface and achieve the measurement of the frequency of the standard vibration source.By measuring the vibration state of a 159.2 Hz standard vibration source,we obtain a vibration frequency of 160 Hz with an average velocity of 10.843 mm/s,differing by only 0.843mm/s from the actual results.