Research On Fundamental Theory And Key Techniques Of Millimeter Wave Imaging Security Inspection System

At present,terrorist attacks occur frequently all over the world.Facing such an increasingly severe security situation,security inspection is now commonplace and a great variety of imaging and detection devices are deployed at airports,major transportation hubs,places of entertainment,government centers and crowded public places.As people pay more and more attention to safety issues,higher requirements are put forward for the safety,efficiency,and intelligence of security inspection equipments,especially equipments used for personnel security inspection.Traditional security inspection methods such as metal detectors,infrared imagers,and X-ray imagers have their own limitations for human body screening.Millimeter wave(MMW)imging technology is a truly safe technology without ionizing radiation and expanding rapidly into security screening for dangerous items concealed under clothing.It has become a hot research topic.However,the research on low-cost,high-reliability,and high-resolution fast MMW imaging security inspection systems still faces huge challenges.This dissertation mainly focuses on the fundamental theory and key techniques of the MMW imaging security inspection system.Both passive and active security systems and key technologies are studied.The main innovative research efforts are summarized as follows:The first work is about passive millimeter wave(PMMW)imaging technology.This part focuses on the basic theory of passive imaging of human body and concealed objects in natural state,that is,the principle of blackbody radiation.Then the differences of effective radiation performance of objects of different materials and human skin at different ambient temperatures are analyzed through modeling.Based on this,an economical and efficient single-channel PMMW imaging system based on a double turntable spiral scanning mechanism and a radiometer is developed,which distinguishes itself with traditional ones by the innovative scanning mechanism.Scanning of the target scene is achieved through the uniform rotation of the double turntable,without acceleration and deceleration during the effective scanning process.The field of view(FOV)is adjustable according to the size of actual target scene,which solves the problem of the limited imaging FOV of the previous mechanical scanning systems.The measured imaging quality has reached the advanced level of the same type of security inspection systems.The angular resolution is about 0.7°,and the single-frame imaging speed can be as fast as 3s.The imaging spatial resolution is verified to be less than 3cm at 2m.The results of testing on various hidden dangerous goods such as metal,plastics and ceramics also verify the excellent performance of the system when used in human security inspections.In addition,two opposite windows can be used to realize the simultaneous measurement of the persons to be inspected in the two security inspection channels on both sides of the system,which doubles the efficiency of the security inspection.The rest are the innovative researchs of active millimeter wave(AMMW)imaging technology:Firstly,a general theory of three-dimensional(3-D)millimeter-wave holographic reconstruction based on LFM signals is proposed,namely GHI-LFM algorithm.For MMW security inspection application,GHI-LFM algorithm provides the theoretical basis for phase compensation and approximation when using LFM signals for holographic reconstruction,as well as the constraints that the corresponding parameters of the system need to meet.So,the GHI-LFM provides the necessary theoretical basis for the design of MMW security inspection system based on LFM radar in engineering,and has important guiding significance for the selection of system parameters.The efficiency and advantages of GHI-LFM algorithm are verified through the comparison with the classical algorithms,and the excellent robustness against to additive noise and frequency error is also verified.Then,based on the GHI-LFM theory,a Ka-band 3-D holographic imaging security inspection system based on LFM radar was designed and built,using two-dimensional plane scanning combined with frequency sweeping to achieve broadband MMW 3-D holographic imaging.Further,a simple but effective calibration approach is utilized based on a measurement with a metallic plate,which ensures that system uncertainties and channel fabrication tolerances are effectively de-embedded from the measured signals.Experimental tests have verified the effectiveness of the imaging system and the GHI-LFM algorithm.In addition,aiming at the time-consuming process of the traditional holographic reconstruction algorithm using the Stolt interpolation to resample the 3-D data from the(k_x,k_y,k)domain to(k_x,k_y,k_z)domain,an accelerated interpolation-free 3-D holographic reconstruction algorithm based on the range stacking(RS)method is proposed,namely AHI-LFM algorithm.Compared with GHI-LFM algorithm,AHI-LFM algorithm has obvious efficiency advantage in reconstructing large-scale targets on the premise of ensuring image quality.Reconstruction of the target through the measured human security data also confirms the superiority of the algorithm.According to the characteristic of AHI-LFM algorithm that every slice is reconstructed separately in range direction,the prior knowledge in the range direction can be used to reconstruct only the area covering the position of the target,which can further reduce the imaging time and realize real-time imaging.Finally,for the case of sparse array,combined with the error compensation of equivalent phase center approximation,a modified 3-D MMW holographic imaging algorithm is proposed.Meanwhile,aiming at the problem of missing equivalent sampling points in the existing sparse linear array scheme,a new linear sparse array is designed,which ensures the full coverage of all equivalent sampling positions and the same image quality,but higher sparsity.Finally,the advantages of the new sparse array method are verified by simulation in a variety of examples.