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Research On The Scanning Image Motion Compensation Technology Based On Fast Steering Mirrors With High Precision And Wide Frequency Range

Posted on:2017-03-20Degree:DoctorType:Dissertation
Country:ChinaCandidate:C S SunFull Text:PDF
GTID:1108330482991319Subject:Optical Engineering
Abstract/Summary:PDF Full Text Request
Since the battlefield situation changes rapidly and constantly in modern wars, acquiring the accurate real-time location information of the target becomes the primary task of the intelligence, surveillance and reconnaissance systems. Besides the capacity of wide area reconnaissance, the next generation of aerial camera should also be endowed with the ability of real-time monitoring of hot targets. With the advantage of accomplishing wide area coverage and high resolution imaging as well as surveillance on potential targets with high frame rate staring by means of using only one imaging system, dynamic scan and stare imaging has become one of the key technologies of airborne imaging systems. However, limited by the precision and bandwidth of the existing image motion compensation system, it!s hard to compensate the scanning image motion in dynamic scan and stare imaging systems. Therefore, scanning image motion compensation technology with high precision and wide frequency range must be developed to solve this problem.Compared with conventional mass-stabilized and mirror-stabilized systems, fast steering mirrors(FSMs) have been provided with a higher bandwidth and higher precision with the merits of a smaller angular range and a smaller size. FSMs have been widely applied in many optical systems to improve beam control performance, ranging from free space communication to pointing and tracking systems and line of sight(LOS) stabilization systems. In this thesis, the FSM is employed to compensate the scanning image motion to improve the compensation precision and banwidth. The objective of this research is to develop a scanning image motion compensation technology based on the FSM, and the major work has been conducted as follows:The dynamic scan and stare imaging system is proposed, in which the FSM is used to cancel the scanning image motion caused by the outer gimbal. The backscan operation of the FSM allows for efficient wide area coverage by continuously scanning the gimbals, thus reducing frame-to-frame settling times and extending gimbal hardware life. The working principle and the control system design of the imaging system are analyzed. The reference coordinate system is set up and scan command generation is given, then simulation results are presented to support the design.The mathematical model of the FSM control system is established. A pseudo random binary sequence(PBRS) is used as the input signal, and the model of the FSM control system is identified by the least square method. The effect of the factors on the performance of the FSM is analyzed, including the mechanical resonance and the digital control system. In order to eliminate the mechanical resonance, rate feedback is introduced to increase the damping coefficient of the system.The control method based on disturbance observer(DOB) and zero phase error tracking controller(ZPETC) is applied in the FSM control system. The inner-loop DOB can cancel the external disturbance and improve the robustness of the control system, and ZPETC is used as the feedforward controller of the closed loop system to improve the tracking performance. The performance and the robust stability of the DOB are analyzed, and the design of the low pass filter is studied. Base on the analysis, the performance and the robustness stability of the two-degrees-of-freedom(2DOF) control based on the DOB are investigated, and the design method is addressed. The principle of the ZPETC in digital control system is presented, and how to identify the model of the closed loop control system is discussed. By using the proposed method, the bandwidth of the FSM closed loop control system is more than 100 Hz, and the positioning precision is 5.7 rad. Simulation and experimental results show that, compared with the traditional PID controller, the proposed method can effectively improve the positioning precision and the control bandwidth of the FSM.The modulation transfer function(MTF) of the dynamic scan and stare imaging system is measured by the slanted edge method, and a quantitative analysis is made to evaluate the image motion compensation performance of the FSM. Firstly, the modeling analysis of the scanning image motion is made, and the MTF is introduced to assess the control performance directly. The effect of the control error of the FSM on the imaging quality is analyzed. Then the slanted edge method is used to measure the MTF of the imaging system for the sake of providing a quantitative evaluation standard of the image motion compensation control system. And experiments are designed and conducted to prove the effectiveness of this method. Finally, a proof-of-concept dynamic scan and stare imaging system is established, the exposure time is 5ms and the frame rate is set up to 50 Hz. The measured MTF of the system when the scanning image motion is compensated by the FSM is nearly equal to the MTF when the whole system is still. The results prove that the backscan operation of the FSM can compensate the scanning image motion effectively, thus meeting the staring requirements during exposure.The research of this thesis confirms that the control method based on DOB and ZPETC can improve the control performance of the FSM, and the FSM can compensate the scanning image motion with high precision and wide frequency range. Compared with traditional step and stare imaging system, both the frame rate and the image quality of the dynamic scan and stare imaging system are improved significantly.
Keywords/Search Tags:Scanning image motion compensation, Fast sreering mirrors, Dynamic scan and stare imaging system, Modulation transfer function, Disturbance observer, Zero phase error tracking controller, Slanted edge method
PDF Full Text Request
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