Research And Implementation Of Laser Doppler Signal Processing In Non-ideal Environment

Laser Doppler vibration measurement technology is a non-contact vibration measurement technology.It has been widely used in various high-precision measurement since its proposal,such as medical fields,aerospace,and detection of military targets.Compared with the traditional contact sensor vibration measurement technology,the laser Doppler vibration measurement technology has higher spatial resolution and sensitivity.However,the vibration measurement system works in a non-ideal environment generally.The non-ideality of the optical system will cause carrier leakage,and the dynamic the speckle pattern of the rough target surface will cause speckle noise.In this thesis "Research and Implementation of Laser Doppler Signal Processing in Non-ideal Environment",the carrier cancellation structure and impulse noise cancellation algorithm based on adaptive filter are studied and finally the real-time processing of laser Doppler signal is implemented on FPGA,including real-time demodulation,carrier leakage suppression and impulse noise elimination.All modules are portable and have been verified by simulation and FPGA experiment.The influence of different measuring distances on the demodulation system is tested through the theoretical analysis and the experimental verification.The results show that the carrier leakage is only related to the optical system.The carrier leakage power of the laser Doppler vibration measurement system in this article is-12 d Bm(±3d Bm),the carrier leakage is equivalent to the amplitude of the laser Doppler echo signal when the vibration measurement distance is 15m(±5m),and the impulse noise in the demodulation system increases significantly and when the phase difference is 180。.The carrier partial cancellation structure proposed in this paper can achieve a carrier cancellation ratio of up to 38 d B,achieve a signal-to-noise ratio improvement of about2.5 times,and reduce the frequency of impulse noise.Compared with the adaptive carrier cancellation algorithm implemented by analog circuits,the FPGA-based adaptive carrier cancellation algorithm achieves about 50% performance improvement.In order to eliminate the impulse noise generated in the demodulation system,the kurtosis detection algorithm based on the fourth order cumulant is studied in this thesis,and the additive non-Gaussian characteristic of impulse noise is used to implement the detection and separation of impulse noise and normal frequency signal.The iterative formula of kurtosis coefficient is deduced,and the calculation is simplified by the method of approximate substitution of parameters.The performance analysis and simulation of the improved kurtosis coefficient iterative algorithm was carried out by MATLAB.The simulation results show that the improved kurtosis coefficient iterative formula has more sensitive detection performance to impulse noise.In order to realize the real-time detection and elimination of impulse noise on FPGA,a dynamic threshold decision method is proposed.This method can avoid the division operation in the algorithm structure,and the reasonable design reduces the use of multipliers to two.The final experimental results show that the algorithm has a good ability to detect impulse noise while reducing the hardware resources by 25% and improves the vibration measurement performance of the vibration measurement system at a long distance(50m-100m).