Design Of Laser Velocimetry System With Active Modulating Light Source

With the development of photoelectric detection technology,the measurement of projectile velocity using photoelectric detection technology is one of the most common ways.Nowadays,in the projectile velocimetry experiments in the shooting range,the non-active velocimetry scheme with cooperative targets is mainly adopted.Through the way of original reflection,the detector receives the variation of reflected light flux and then converts it into electrical signals for later processing.However,for the non-active speed measurement is not suitable for installation and debugging,and the signal is easy to be interfered by messy signals.Therefore,the improvement of the traditional photoelectric velocimetry system is proposed.The speed measurement method adopted by the project is active(the cooperative goal is cancelled).The whole system is divided into three parts: the first part is the modulation of weak over-target signal;The second part is the demodulation of weak signal.The third part is the acquisition and processing of data.The whole process is briefly described as follows: First,a3 MHz high-power modulated laser is extended to a divergence Angle of 30.By irradiating on the surface of the projectile,a scattering modulation signal is generated,and then a large-area array photodetector is used to receive AM signal.Finally,the AM weak optical signal is transferred to the pre-I-V circuit and converted into electrical signal,so as to facilitate the demodulation of signal processing circuit.According to the signal demodulation receiver model,the demodulation methods include envelopment detector and coherent demodulation method.As it is aimed at the detection and extraction of weak signals,envelopment detector has the limitation of threshold effect on the signals with small SNR,which is not conducive to the extraction of weak signals.The subject adopts the receiver model of coherent demodulation.After the weak modulation over-target signal is filtered by BPF frequency band,then the signal is mixed with the coherent carrier,and finally the baseband over-target signal is obtained by low-pass filtering.Through simulation and experiment,the expected waveform is obtained.The paper mainly completes the design of the transmitting and receiving modules of the system,and the principle analysis and design of the signal processing circuit of each part.Finally,the upper computer software written on the Lab VIEW platform is used to store and process data.Through the projectile simulation experiment,the feasibility of the system is verified.In the active velocity measurement,the weak passing target signal can be detected and extracted effectively.After the demodulated signal is collected by high speed AD and processed by upper computer software,the baseband waveform is obtained and the velocity is calculated.According to the analysis of velocity measurement error,the theoretical relative error is 0.21%when the standard 7.62 mm projectile passes through the system with a target distance of 300 mm at a speed of 740m/s.