Research On Key Technologies Of Laser Vibrometry For Micro-bead Retro-Reflection Target

The homodyne laser vibrometry is an important vibration detection method in industrial application and scientific research.In recent years,with the impetus of rapid development in the many fields,e.g.aerospace,ultra-precision manufacturing engineering,vibration measurement continues to move toward nanometer and subnanometer precision.Micro-bead retro-reflective material can effectively improve the signal-to-noise ratio during the measurement.However,intensity variation of the measurement light and speckle noise will introduce the displacement measurement error of several nanometers or even micrometers,which is a key issue that restricts the further improvement of the accuracy of homodyne laser vibrometer.With the in-depth study on the detection system of traditional homodyne laser vibrometer,this project establishes a vibrometric interference signal model for the micro-bead retro-reflection target.The influence mechanisms of measurement light intensity and speckle noise on vibration displacement measurement accuracy were analyzed.To increase the precision and stability of the vibrometer,a cyclic nonlinear error suppression method based on balanced spectroscopy strategy and a speckle noise suppression method based on micro-neighborhood scanning were proposed.The main works are as follows:In order to clarify the error mechanism of the micro-bead retro-reflective material reflection characteristics for homodyne laser vibrometry,combined with the laser interference theory and the principle of polarized light transmission,an interference signal model based on the reflection characteristics micro-bead retroreflective material was established.Model analysis shows that when measuring the change of light intensity,the unbalanced splitting light in detection system results in the variable cyclic nonlinear error will affect the accuracy of the demodulation displacement;the speckle noise will jointly modulate the measured light amplitude and phase,and thus results in distortion of the demodulation displacement.Therefore,it is a key factor that to improve the accuracy of vibration measurement by implementing the balanced light separation in the detection system and improving the stability of the amplitude and phase of the interference signal.In order to effectively compensate the variable cyclic nonlinear errors introduced by measurement light intensity variation,a cyclic nonlinear error suppression method based on balanced light separation strategy is proposed.According to the optical balance separation strategy,the balanced spectroscopic optical path is designed to suppress the DC offset error and overcome the interference of the measured light intensity variation,and improve the stability of subsequent cyclic nonlinear compensation;the feature parameters of interference signals are extracted on-line in the signal processing,multi-stage flow water treatment method is used to realize realtime compensation of residual cyclic nonlinear errors.This method has no complex iterative process and only contains fixed-point multiplication,thus improving the real-time performance of compensation.In addition,for nano-scale amplitude vibration measurement,the optical shutter is used to extract the characteristic parameters of the interference signal,thereby compensate the cyclic nonlinear error.Experiments show that the DC offset error is always suppressed below 5% of the full signal amplitude,when the normalized measurement light intensity changes from 1 to 0.1;the vibration harmonic introduced by the cyclic nonlinear error is attenuated from 9 dB to-26 dB.In the range of 1 m/s vibration speed,the peak-to-peak value of nonlinear error of the demodulation displacement is less than 2.0 nm.In order to effectively suppress the speckle noise,according to the statistical characteristics of laser speckle noise,a speckle noise suppression method based on micro-neighborhood scanning was proposed.The measurement light is scanned at a high speed in a micro area near the measured point,the speckle noise is periodically modulated;then low-pass filtering is used to achieve signal averaging and smoothing,filtering out periodic speckle noise.So that the amplitude and phase of the interference signal tend to be stable,reducing the distortion of the vibration measurement demodulation displacement.The experimental results show that demodulation displacement distortion is 58.3% at the defect point of micro-bead retro-reflection target,which can be reduced to 3.4% by micro-neighbor scanning.Finally,based on micro-bead retro-reflection tape,this project conducts experimental verification on the balanced light separation detection system,on-line non-linear error compensation method,and micro-neighborhood scanning speckle suppression method.The experimental results show that the cyclic nonlinear compensation method and the speckle noise suppression method proposed in this paper can effectively improve the anti-interference ability of the homodyne laser vibrometer for measurement light intensity and speckle noise,and improve the accuracy of the vibration measurement demodulation displacement.