Research Of Vibration Isolation Technique For A Type Of Aerial Camera With Long Focal Length

Since the 20 th century,airborne optical imaging and measurement technology has been developed rapidly,which has been used in resource census,terrain mapping,military reconnaissance and many other areas widely.In order to obtain high resolution images and geographical information of the scene,the focal length of aerial camera is becoming longer and longer.Vibration has become an important factor which affects imaging quality and measurement accuracy.In this paper,the influence of vibration on imaging quality and measurement performance is analyzed and the vibration isolation technique is studied to improve the dynamic imaging performance of aerial camera.The major work includes the following aspects:According to the working principle of the aerial camera,the mapping relationship between the object point and the image point during scan collection is established.The image motion model under squint imaging is deduced and image motion caused by linear vibration and angular vibration is analyzed.It is pointed that angular vibration(especially angular vibration about the roll axis)is the main factor which affects image quality.In order to evaluate the influence of vibration on image quality objectively,the quantitative relationship between image motion and optical transfer function(OTF)is established.Dynamic modulation transfer function(MTF)under sinusoidal vibration and random vibration is analyzed to evaluate image blur.A formula based on Bessel function of the first kind is established to calculate dynamic MTF due to sinusoidal vibration.Dynamic MTF due to random vibration is calculated via numerical integration based on time-domain signal which is replicated from the referenced power spectral density.The geometrical deformation of the image is evaluated via using the first-order statistical moment of image motion during the exposure time.The geometrical deformation changes with the vibration frequency when the vibration amplitude is constant.It points out that high-frequency vibration causes blurred images mainly and low-frequency vibration causes geometric deformation.Based on the working principle of TDICCD,a simulation model is established to analyze image degradation caused by vibration intuitively.Vibration in the scanning direction causes the image to be stretched or compressed and the image is smeared and twisted due to vibration which is perpendicular to the scanning direction.According to aircraft position,attitude information and the depression angle,position angle from the encoder in aerial camera,a geo-location algorithm is proposed based on the theory of homogeneous coordinate transformation and the calculation theory of global ellipsoid.The influence of angular vibration error due to the shock absorber on t he target location accuracy is analyzed emphatically.It is pointed out that angular vibration error about the roll axis has a serious effect on the target location accuracy and the measure to improve the location accuracy is given.With the requirement of image quality evaluation under vibration,an analysis model of vibration isolation and imaging performance is proposed based on power spectrum which can reflect the combined effects of aircraft vibration,vibration isolation system,vibration damping efficiency and residual vibration on i mage quality.The vibration isolation performance and imaging performance is analyzed with the simulation analysis interface which is written with MATLAB software.In order to increase the damping efficiency,the vibration isolation system is analyzed,designed and tested.Firstly,the vibration isolation model is established and the quantitative relationship between the stiffness,damping and installation spacing of the shock absorber is obtained when the vibration isolation system is not coupled,which provides theoretic basis for the design and assembly of the vibration isolation system.For the realization of the vibration isolation system whose natural frequency is 6Hz,the metal coil spring is chosen as the flexible support element and the adjustable rotary liquid damper is used to provide additional damping and to achieve damping adjustable.The arrangement of the rotating liquid dampers and the method of selecting the damping values are given.The frequency characteristics and vibration coupling characteristics of the vibration isolation system are analyzed by ADAMS software.Through the sine sweep test,the natural frequency and transmission rate curve of the vibration isolation system are tested,which is consistent with the simulation results.The dynamic response of the vibration isolation system is tested by using the random vibration test and the damping efficiency is above 92.2%.Angular vibration coupled by the vibration isolation system is measured by the collimator and angular vibration amplitude can be reduced by adjusting the damping value of each rotary damper.Finally,the effectiveness of the vibration isolation system in improving image quality is verified through the imaging experiment.The analysis model of vibration isolation and imaging performance based on power spectrum proposed in this paper can be used to predict image quality of the camera under the vibration environment,which also has certain guiding significance for the design of the vibration isolation system.Through the analysis,design and experiment of the vibration isolation system,the vibration isolation performance is improved,which makes the aerial camera can obtain high resolution images under the vibration environment.