Beam Deflection Tracking Based Autocollimation Micro-angle Measurement Method With Long Working Distance And Large Range

Autocollimation technique is the main approach for high precision micro-angle measurement.In order to realize adjustment and measurement with high efficiency,the micro-angle measurement range is required to be large enough.However,large measurement range and high resolution are restricted with each other.Increasing the measurement range will decrease the resolution.With the rapid development in the fields of large-scale equipment assembling and testing,large-scale scientific engineering,long working distance is also urgently required on the basis of both large range and high resolution.Long working distance,large measurement range and high resolution cannot be obtained simultaneously in current autocollimation technique.Especially,the measurement range decreases rapidly when the working distance is larger than 7.5 m.There is measurement capability while the working distance is larger than 25 m because the measurement range decreases to 0.Thus,it is a very critical question to increase both working distance and measurement range on the prelimilary of unchanged resolution which needs to be urgently resolved in these fields.Aiming at the problems presented above,in this study of “Beam deflection tracking based autocollimation micro-angle measurement method with long working distance and large range”,a beam deflection tracking based autocollimation method is proposed to highly improve the contradiction of large measurement range and long working distance,thus to increase the measurement range under the condition of long working distance.The resolution remains unchanged in large measurement range while the working distance varies.First,aiming at the problem that the reason of limitation of working distance in the traditional autocollimation is not clear and cannot be effectively improved,the existing autocollimation techniques are summarized into four basic types,and the variation between the measurement range and working distance of the four basic types are analyzed in details and the expressions of variations are derived.The analysis results show that the measurement range of autocollimation system is mainly limited by the ratio of the size of photodetector and the focal length of collimating lense,and by the ratio of the pupil size of collimating lens and the working distance in the working distance of less and more than 2 m,respectively.Based on the above analysis,a beam deflection tracking based autocollimation method is proposed.A beam deflection unit is used to adjust the direction of the collimated beam in realtime during the measurement process according to the deflection angle of target mirror.A 2D micro-angle monitoring unit is used to monitor the deflection angle of the beam deflection unit,and the deflection angle of target mirror is calculated through the angle conversion relationship.The model of the proposed autocollimation method is established and the analysis of the model reveals that its measurement range is mainly limited by the ratio of the diameter of target mirror and working distance.The measurement range of the method is significantly larger than the existing autocollimation system,which largely improves the contradiction between large measurement range and long working distance.Second,aiming at the problem that the existing 2D micro-angle monitoring technique can hardly realize high resolution in large measurement range,the models of capacitive sensor-based and interfometer-based tangent micro-angle monitoring unit are established for analysis of the generation mechanism of principle errors and their impacts on micro-angle measurement results.Simulation results indicate that principle errors are the main impacts on the micro-angle measurement results in the range of ±900 arcsec,and the principle errors are further compensated according to the simulation results.On the basis of this,the Monte Carlo method is applied to analyze the remaining error sources while principle error is compensated,and the influence of each error source on the measurement results is obtained so as to determine the main error sources to provide guidance for further improving the micro-angle measurement accuracy of them.At the same time,the influence of air turbulence on the micro-angle measurement results in autocollimation with long working distance is analyzed.The models of air refractive index structure constant are summarized as well as the air turbulence power spectrum model and phase screen model.And according to the applicable conditions and applications of each model,the phase screen model is determined as the air turbulence model in autocollimation with long working distance.Furthermore,based on the actual air turbulence in autocollimation with long working distance,the parameters of phase screen is determined for simulation of the influence of air turbulence.The analysis and simulation provide theoretical basis for solving the problem of air turbulence in autocollimation with long working distance using adaptive optics and improving measurement accuracy.Finally,an experimental beam deflection tracking based autocollimation system is constructed for performance tests.Experimental results indicate that the effective resolution is better than 0.04 arcsec and the measurement range is from ±1050 arcsec to ±350 arcsec at long working distance from 7.5 m to 25 m.The experimental tests verified the characterization of long working distance,large measurement range and high resolution of the proposed autocollimation method.