| In recent years,with the rapid development of ultra-precision manufacturing,preci-sion metrology and other fields,the demand for accuracy of displacement measurement has gradually increased from nanometers to sub-nanometers and even picometers.Since restraining the periodic nonlinear error which is a few nanometers to more than ten nanometers in traditional heterodyne interferometer with coaxial beams,heterodyne in-terferometer with separated beams(HISB)has become the development direction of the next generation of interferometer.However,the optical configuration of the heterodyne interferometer with separated beams is more complicated and optical path is longer,which make it more susceptible to temperature than the traditional heterodyne interferometer with coaxial beams.In the meanwhile,the thermal drift of optics in the heterodyne inter-ferometer is highlighted and becomes a key factor restricting the further improvement of the measurement accuracy of HISB.In order to solve the problem that the measurement accuracy of the HISB is restricted by the thermal drift caused by temperature change,this paper has established the mathe-matical model of thermal drift of optics in the HISB,proposed corresponding suppression methods,and a method that can accurately and quickly test the thermal drift of optics the interferometer is also proposed.The specific research content is as follows:As to the thermal drift of optics in HISB under the condition of uniform temperature field,this paper proposed a the mathematical model of the thermal drift of optics in HISB under the condition of uniform temperature field.This model quantifies the relationship between thermal drift of optics,optical material,optical configuration and temperature of the interferometer.Meanwhile,the model also points out that the optical configuration is the key of affecting the error.Based on the above,a passive suppression method based on balanced optical configuration is proposed.By making the optical path length of the reference arm and the measurement arm meet condition of balanced optical configuration during designing the interferometer,which elimates the additional optical path caused by temperature changes and achieves the purpose of eliminating the thermal drift of optics under the condition of uniform temperature field.Theoretical and simulation results show that the method of making optical configuration balanced can effectively suppress the thermal drift of optics in HISB under conditions of uniform temperature field.As to the thermal drift of optics in HIBS under the condition of gradient temperature field,this paper proposed a the mathematical model of the thermal drift of optics in HISB under the condition of gradient temperature field.The model indicates that under the condition of gradient thermal field the HISB which with balanced optical configuration still has the thermal drift of optics,and indicates that the gradient thermal field can destroy the optical balance between measurement arm and reference arm is the main cause of error.Based on the above,this paper proposed an active compensation method based on RBF neural network,which establishes a corresponding RBF neural network by learning the behavior of interferometer' s output with temperature under certain boundary conditions,and realizes the active suppression of thermal drift of optics in the interferometer.Simulation results show that when the impact surface temperature change rate is 1?/h under the x-axis double-sided thermal shock condition,the thermal drift of optics in the interferometer reaches 15nm,and the peak-to-peak of error can be reduced to 0.8 nm compensated by the RBF neural network.As to the lack of a accurate and specifical method and system for measuring thermal drift of optics in the interferometer,this paper proposed a thermal control unit for measur-ing thermal drift of HISB was proposed.Based on the characteristics of the thermal drift of optics in the HISB under the condition of uniform and gradient thermal field,a test system which operating in a vacuum environment for measuring the thermal drift of optics is proposed,and a thermal unit composed of multi-layer composite wall based on thermal radiation exchange is also designed.The thermal unit mainly includes two characteristics:First,by actively controlling the distribution of heat sources in the thermal radiation unit,the above-mentioned thermal unit can be capable of specificially measuring the thermal drift of optics in HISB under the condition of both the uniform temperature field and the gradient thermal field;Second,the thermal unit composed of multi-layer composite wall can effectively shield and homogenize the external radiation disturbance.The simulation results show that when the temperature of the upper surface of the vacuum chamber changes linearly from 20? to 21? within 2 hours due to the external environment temperature change,the temperature of the interferometer increases with the temperature of the cavity nonlinearly and the maximum temperature rise of the upper surface of the interferometer is suppressed from 0.44? to 0.042? by the thermal unit.In this dissertation a system for measuring thermal drift of optics in a HISB is builded to verify the research mentioned above.First of all,performance of the thermal drift test system was tested.The multi-layer composite wall structure reduce temperature change of the interferometer caused by the vacuum cavity to about 1/7 of that without the the structure,which is close to the theoretical analysis and proves that the structure effectively suppresses the temperature change of the interferometer caused by the disturbance of the external environment.Secondly,the formation mechanism of the thermal drift of optics and the suppression methods under the uniform thermal field are verified by experiment.The results show that when the interferometer with balanced optical configuration in the uniform thermal field,no matter in 1 h short-term(temperature change rate 0.1?/h)or 10 h long-term(temperature change rate 0.01?/h)test conditions,the gradient of the interferometer remains unchange and the output of the interferometer shows no drift.However,the output of the interferometer with unbalanced optical configuration shows obvious drift under the same conditions,and the thermal drift coefficient of the interfer-ometer measured in the experiment is about 28.6 nm/?,which is basically consistent with the theoretical calculation value of 26.4 nm/?.Finally,the formation mechanism and compensation method based on RBF neural network of the thermal drift of optics un-der the condition of gradient thermal field are experimentally verified.The experimental results show that the temperature distribution of the interferometer changes with time and the overall temperature of fully symmetric HISB has changed by about 0.07? during the 0.7 h test time.And the temperature change rate of each test point on the interferometer is different,which causes the output value of the interferometer drifting about 2.5 nm.An RBF neural network is builded by learning the first 2/3 of this group of data to compensate the last 1/3,the result shows that the output value of the interferometer is maintained at the noise level whose peak-to-peak value is less than 1 nm without obvious drift;then the same network is used to compensate the thermal drift of optics measured under different temperautre changing rates,after compensating the result shows that the drift of in output value of the interferometer is effectively suppressed. |
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