| As a kind of optical imaging equipment for shooting ground targets,aerial camera is widely used in forestry,agriculture,resource exploration and other fields.With the increasing demand for the quality of aerial images,the development of a high-resolution aerial camera has become an urgent need.However,because the working environment of the aerial camera is very complex,the changes of atmospheric pressure,temperature and imaging distance will lead to the deviation between the photosensitive surface of the aerial camera detector and the square focal plane of the ideal image,resulting in the decrease of imaging clarity and resolution,that is,defocus.Therefore,the proposal of an automatic focusing scheme with high precision and strong real-time performance is a key topic in the aviation field.In this paper,the working principle and development status of automatic focusing technology of aerial camera are discussed.In order to solve the problem that ordinary image focusing method is not suitable for moving aerial camera focal plane detection,a heterogeneous differential filter image focusing method is proposed.Finally,the experimental platform of auto-focusing system is designed by using DSP+FPGA,which verifies the effectiveness of the method in this paper.The main contents of this paper are as follows:(1)The research background,significance and development status of aerial camera focusing technology at home and abroad are introduced,and the defocus principle of aerial camera optical system are analyzed.Five commonly used methods of focusing at this stage such as:photoelectric autocollimation method,image-based autocollimation method,ground calibration method,phase detection method,and image processing focus detection method are summarized.(2)An image focusing method based on spatial filtering effect is proposed aimed at the problem of the ordinary image focusing method.Spatial filtering velocity mea-surement and spatial filtering effect are introduced,and spectral analysis of spatial filter transmission function and the influence of transmission function on filter filtering characteristics and aerial camera focus detection accuracy are introduced in detail,the key parameters of the spatial filter that affects the focus detection effect are discussed and designed.In view of the fact that a large number of fundamental frequencies was contained in the output signal of the rectangular filter so that the image plane position cannot be distinguished and the focus detection accuracy is poor,an out-of-phase differ-ential filter is designed to remove the fundamental frequency in the output signal of the rectangular filter and the differential filter is analyzed on the spectrum.The reason that the accuracy of the differential filter focusing method is higher than that of the rectan-gular filter focusing method is theoretically explained,and the specific implementation process of the differential filter method focusing method is introduced.The simulation experiments to detect the focus of the same group of engineering outfield flying images using differential filtering,spatial domain,frequency domain,image entropy,and sta-tistical focus functions are designed,and the steepness,sensitivity,sharpness ratio are analyzed.It is showed that the focusing accuracy of the differential filter focusing method proposed is higher than that of the traditional image focusing operator.A static imaging experiment are designed to detect the same group of exterior scene images by using the traditional image focusing operators Robert and Brenner and the heterogeneous differential image focusing algorithm.It is proved that the heterogeneous differential image focusing algorithm has an accuracy of 19.87μm when taking pictures of the same scene,which is less than 76.8μm of the half focal depth of the optical system and meets engineering needs.(3)The commonly used method of focusing window selection is studied,and the optimal focusing area is selected based on an adaptive genetic algorithm.The commonly used focusing search algorithm is studied,and in order to solve the problem that the mountain climbing method is easy to fall into the local extreme point,the focusing model and the improved mountain climbing algorithm are adopted as the search strategy of this paper.The focus search algorithm is verified to meet the requirements of focusing accuracy and real-time performance.(4)An auto-focusing hardware circuit with DSP+FPGA as the core is designed,and the software development process of the auto-focusing system is introduced in detail.Finally,a dynamic imaging experimental device is designed to simulate the aerial camera taking photos of the ground during actual work.The automatic focusing system designed in this paper is used to conduct multiple sets of focusing experiments,and the system is carried out from the perspectives of focusing repeatability and real-time performance.It is showed that the focusing accuracy of the automatic focusing system based on the out-of-phase differential filtering effect designed is 31.05μm,which meets the engineering requirements,and the time required to complete one-step focusing is less than 125ms,and the real-time performance is strong.The automatic focusing system designed in this paper is not necessary to repeatedly image the same scene,which is feasible with automatic inspection and focusing of aerial cameras in complex aviation environments and high practical value. |
