Research On Laser Doppler Velocimetry For The Projectile Velocity Measurement

With the rapid development of ordnance industry, modern engineering testing technique plays an important role in the estimation and examination of weapons performance parameters. The accurately measurement of projectile velocity can provide reliable data for ballistic research in the artillery development, which can also evaluate damage degree of attacking target simultaneously. The existing measuring systems, like start-end method and radar system, have the disadvantages of average error and easy loss of alignment, which can not meet requirement of high accurate weapons already. Laser Doppler technique has several advantages, such as noncontact nature, small measurement volume giving high spatial resolution, rapid dynamic response, and large measuring range, so it is becoming the developing direction of speed measurement.The dissertation analyzes the defects of the existing methods in projectile velocity measurement, and consider by using laser Doppler velocimeter(LDV) to detect the velocity of projectile. The laser Doppler technique of solid surface is studied from a view of practical application, and the feasibility is demonstrated.The basic theory of LDV is introduced, and the superiority of detecting Doppler signal by utilizing optical heterodyne technique is illustrated. The characteristics of four conventional optical structures are discussed and compared. The mechanism of laser Doppler signal, produced by the motion of scattering surface, is elaborated using Fraunhofer diffraction theory, and on this basis the relationship between the spot size on moving surface and the visibility of signal is explained. The mathematic model, based on fringe model and the principle of stochastic scattering, is established for analyzing the formation of Doppler signal. The relationship between the number of particles and the visibility of signal is calculated by numerical simulations, and the result has a great agreement with the former. On this basis, the beam expanded LDV and multipoint illumination LDV are designed respectively based on the dual-beam model, which can solve the problem of misalignment in differential LDV result from projectile deviation. The hardware circuit of preprocessing is designed for Doppler signal extraction.In order to enhance the signal to noise ratio(SNR) of Doppler signal, a finite impulse response(FIR) filter with dynamic frequency band is designed. An improved signal processing method, in which the spectrum of autocorrelation function is used in the frequency refinement and correction algorithms, is proposed to solve the problem of low accuracy in the real-time fast Fourier transform(FFT). Besides, the method can improve the reliability of correction algorithm simultaneously. Measurement error of the LDV is analyzed concisely and measures to reduce and control the error in different situations are put forward.Experimental setup of beam-expanded LDV is built to measure the tangential speed of rotating disk. Compared the measured value to the actual value, the mean relative accuracy is superior to 1.4%. The experimental systems of one-dimensional and two-dimensional multipoint LDV are built respectively to measure the velocity of rotary plate, and the mean relative errors between the measured value and reference value are 1.52% and 0.98%. Finally, the simulation experiment of projectile velocity is performed by using the two-dimensional multipoint LDV. The above results indicate that the laser Doppler systems designed in the dissertation are feasible for realizing the velocity measurement of projectile.