| Laser Doppler velocimetry(LDV)has the advantages of high accuracy,good linearity,fast dynamic response,large measurement range,non-contact measurement,and is widely used in aviation,aerospace,machinery,energy and other fields.Digital signal processing has the advantages of high measurement accuracy,good sensitivity,and the ability to measure intermittent signals,which is the key to achieving accurate velocity measurement in LDV.This article conducts research on Doppler frequency calculation and signal denoising.Firstly,a dual beam dual scattering model optical path was constructed,and a 150 mm diameter metal disk driven by an electric motor was used to simulate the measured moving object.An avalanche diode photoelectric conversion module was used to receive optical signals,and a laser Doppler velocity measurement experimental platform was established.On this basis,frequency correction and signal denoising for laser Doppler signals were proposed.The signal information is obtained through FFT spectrum analysis.Aiming at the spectrum analysis error,the mixed convolution window of the four term Nuttall window function and the five term MSD window function is used to suppress the spectrum aliasing phenomenon.Then,the improved six spectral line interpolation algorithm is used to adjust the frequency,and the cubic B-spline is used to fit the interpolation coefficient to avoid solving the high-order equation,obtain the optimal spectral line,and obtain a better speed measurement signal.A joint denoising method is proposed to address the noise interference in the signal.Firstly,the Sparrow search algorithm(SSA)is used to determine the number of decomposition layers K,and then the noisy laser Doppler signal is decomposed using Variational Mode Decomposition(VMD)to preserve the relatively pure components and remove the noisy ones,Then,wavelet packet threshold denoising(WPTD)is performed on the components containing partial noise,and pure nodes and components are selected for reconstruction to form a denoised signal.Finally,frequency extraction is performed to obtain high-precision measurement signals.Verify the frequency correction and joint denoising methods proposed in this paper using simulated and measured signals.The sampling rates of 5MHz and 5GHz were selected for the low-frequency and high-frequency domains of the simulated signal,and the actual signal sampling frequency was 10 MHz.The disc speed was measured using a photoelectric encoder,and the linear velocity of the disc rotation was calculated based on the disc diameter.The Doppler frequency shift was calculated based on the frequency velocity formula of laser Doppler velocity measurement,and compared with the Doppler frequency shift calculated using the method described in this paper.Using the hybrid convolutional window six spectral line interpolation algorithm proposed in this article for Doppler frequency calculation,the relative error of simulated signal frequency is less than0.0005%,and the relative error of measured signal speed is between 0.00146% and0.00213%.By using SSA-VMD-WPTD joint denoising,the signal-to-noise ratio of the simulated signal and the measured signal has been significantly improved.The relative error of the measured signal speed is between 0.00081% and 0.00114%,and the measurement error is reduced by more than 50% compared to direct frequency calculation.This effectively reduces the noise component in the laser signal and improves the signal detection accuracy. |
