Study Of The Key Technology For Autocollimating Beam Shift Restraining Based On Target Feedback

With the development of the manufacturing technology in ultra-precision and micro-mechanical, higher requirements on the measurement stability of opto-electronic autocollimator become more and more stringent. In actual applications, the influences of laser beam shift on the measurement stability of the laser CCD autocollimator with higher accuracy are always fatal especially for the experimental conditions.The subject"study of the key technology for autocollimating beam shift restraining based on target feedback"aims at the measurement stability of laser CCD autocollimating system caused by the laser beam shift. The laser CCD autocollimating beam shift restraining technology to realize high measurement stability is discussed. The technical difficulty of the subject result from the laser beam shift virtually composed by three beam shifts, which are the thermal shift of the laser, the shift caused by the change of air refractive index, and the random shift caused by atmospheric disturbance. The characteristics of randomness and uncertainty of the laser beam shift synthetically composed by the three factors can not be simply mathematical modeled. That is also an important problem which still could not be well resolved in laser CCD autocollimation research.This paper presents a technology for autocollimating beam shift restraining based on target feedback. A laser CCD autocollimating measurement system based on target feedback is proposed, in which a beam-splitting target detector to separate and feedback the laser beam shift with the same characteristics as the measurement beam is developed. The beam shift coupled in the measurement beam could be restrained by real-time closed-loop feedback control, and then the higher stability for laser CCD autocollimating measurement is achieved.A mathematical model of the laser CCD autocollimating measurement system based on target feedback is researched using perspective projection transformation techniques. The relationships among every elements of the system are researched, the relationships among the beam spot location received by CCD and QPD, the rotating angle of the beam-splitting target detector, and the laser beam shift are analyzed. The establishment of the mathematical measuring system model could be used for the parameters adjusting and performance analysis of the laser CCD autocollimating measurement system based on target feedback.In order to improve the edge location accuracy, and then higher center location accuracy of beam spot in laser CCD autocollimating measurement system could be obtained, a new subpixel edge location method based on orthogonal Fourier-Meilin moments (OFMM's) and error compensation is proposed. The detailed information of the edge is fully extracted based on the rotation invariance and the lower radial orders of OFMM's in edge detection. Subpixel location of the edge is obtained according to the relationships among different orders of the OFMM's in vertical directions. Compensation of errors caused by templates with different sizes and the actual edge model are also provided to achieve higher edge location accuracies. Experimental results show that the peak-peak value of the repeated locations of the edge location method proposed in the paper is improved from 0.29 to 0.13 pixels compared to the traditional Zernike moments edge location method, the peak-peak value of the repeated locations is improved from 0.21 to 0.13 pixels and the peak-peak value of the edge location results is improved from 0.23 to 0.19 pixels compared to the gray-level weight center method.Finally, the experiment for the performance of the laser CCD autocollimating measurement system based on target feedback is performed. The performance of the laser beam shift, the flexible hinge, and the characteristics of the control system are tested. The measurement stability of the measuring system is also tested. The experimental results show that the peak-peak value of the stability of the laser CCD autocollimating measurement system is decreased from 0.46" to 0.15". At last, the main factors result in the laser beam shift is analyzed.