| With the development of micro-nano technology and chip integration technologies,the low altitude,slow speed and small(LSS)targets such as drones have shown explosive growth.Many levels of black flying and malicious monitoring have brought huge security hidden risks to the tremendous low-altitude detection.The results can hardly satisfy situation in the traditional detection methods,therefore we urgently need a new technique to solve it to make up for the loopholes in low-altitude defense.The lidar detection system can just make up for the above-mentioned defects.Its advantages like narrow beam,strong directivity,miniaturization and fast response speed have drawn more and more attention in this field.It will be the main technical solution to fill the loopholes of the traditional radar in the low-altitude field,and it is also one of many technical routes that have developed rapidly in recent years.This thesis focuses on the key scientific problems for the process of "low,slow and small" targets detection and recognition in the lidar system,such as laser scanning system,scanning control method,long-distance divergence angle compression,background noise suppression,high-precision time interval measurement method and small-angle recognition auxiliary method for lidar,etc.,the main research content is as follows:(1)The high-speed scanning and transmitting technology of high collimation laser is studied.By using resonant galvanometer,the design scheme of twodimensional scanning optical system based on resonant line scanning and vector frame scanning mirror is completed,the linear range of the two-dimensional galvanometer voltage and scanning angle was measured,the scanning track and methods was also tested.The Time-of-flight measurement is analyzed and determined,and the laser in low loss waveband is determined according to the solar radiation and atmospheric laser absorption spectrum.The relationship between the laser pulse refrequency and the farthest detection blur distance is taken as a constraint,furthermore,the key parameters,such as laser repetition frequency,single frame resolution and scanning angle,are determined by integrating the farthest target coverage relationship.According to the above,the optical transmitting antenna structure is designed,the core FPGA enabling circuit is also designed to make the pulse laser transmit stably for the meanwhile.The distortion of scanning track is analyzed,the causes of pincushion distortion are described,the distortion is corrected by changing the control input voltage model,and then the simulation and experimental analysis are carried out,the full field scanning design of lidar is completed by using electric turntable.(2)High isolation instantaneous field of view laser detection technology is studied.Based on the analysis of outdoor complex enviro nment and strong radiation background noise,by using the relationship between the noise power radiated from the sun to the system and the instantaneous field of view angle of the system,the simulated noise curve under the condition between the coaxial and off-axis transceiver are carried out,the coaxial receiving mode with low noise is selected.According to the selected APD,the corresponding aspheric optical receiving antenna is designed,the isolation degree of polarization isolation and central opening reflector device in the same system is studied.Finally,the superiority of the central opening transceiver isolation device is determined after the simulation and comparison,and the isolation device with 2 mm central opening is designed to achieve the optimal receiving effect.(3)High precision and high data rate laser ranging technology is studied,combined with the problems of timing and time measurement in the measurement of target flight time interval based on LSS-target detection lidar system,through the study of centroid and constant fraction discrimination(CFD)timing scheme,by analyzing the cost at the same timing accuracy,GW6042 high precision CFD module is selected as identification time unit.The TDC-GP22 high-speed time measurement chip is used to complete the design of time measurement module,the combination plan of two time measurement modes reach the kilometer level time measurement requirements,the FPGA module is also used as the high-speed core control module to design the high-precision time measurement and mode switching circuit,the self calibration in GP22 internal parameters help to finish the high time measurement accuracy on the order of 100 picoseconds while achieving the high-speed data transmission.(4)The host passive compound device of infrared camera and lidar was built to conduct related in-door and out-door experiments.The principle of lidar is analyzed,and lambert body is taken as the target analysis model of LSS-target,the semiphysical simulation is under the following laser parameters: 128 k Hz repetition rate,the 1ns pulse width and 1.1 W average power.By comparing the relation curve between the theoretical detection distance and the target reflectivity,compared with the actual reflectivity of the UAVs,the possibility of detecting LSS-targets in kilometer level lidar is confirmed,and then the overall design of lidar system is completed.Aiming at the infrared camera and matching lens,the linkage scheme between detection target and electric turntable is designed,and the frame difference method which is less affected by the external changing light is selected as the target detection scheme.In order to reduce the delay time of target detection and recognition in video sequence,convolutional neural network deep learning method is added to accelerate the process of target recognition,the Res Nets-50 residual neural network with good application effect is also used to improve its average accuracy,and finally the complex inside and outside enviro nment imaging experiment is carried out with the compound device. |
