| This thesis describes the research on the development of double-frequency laser interferometer and automatic measuring calibration system of displacement sensor made for Chengdu Aircraft Industrial (Group). Attention has been focused on the following six parts:1. The principles of the operation of the double-frequency laser interferometer were analysedtheoretically. The emphasis was put on the main factors affecting its performance, such as: drift of the average frequency of interferometer; drift of frequency difference of interferometer; the changes of the interference add-on term of the Gaussian beam during the measurement and its effects on the phase changes, as well as the relationship between the changes of the interference add-on term of the Gaussian beam during the measurement with the movement of the measurement mirror. The double-frequency laser interferometer belongs to the heterodyne interferometer. It has been proved that this kind of interferometer possesses the advantage of having a higher transform gain, better wave-filter capacity and high spatial resolution. Furthermore, this kind of interferometer can filter out background light interference and obtain a measuring signal with high signal/noise ratio. During adjusting beam paths, the co-beam, a combination of the measuring beam and the reference beam, is not restricted to being perpendicular to the light sensitive surface of the object, but keeping the measuring beam and the reference beam co-axial proved to be crucial. Therefore, the three fundamentals required to obtain high precise measurements are: 1) frequency stability and frequency difference stability of the double-frequency laser source as well as the stable equivalent length of bore; 2) the measuring beam and the reference beam are placed very co-axially; 3) Capability of recording the sub-division of the period of phase change and a reversible counter system.2. The definition of stability and repeatability of the laser frequency and factors affecting the stability of laser frequency were analysed. The normal methods used to stabilise frequency and produce double-frequency were explained. After analysing the working conditions and related parameters of the laser device, it was concluded that the laser tube should be put inside an insulating cover and an electric discharge tube with a bigger diameter and length, smaller wall thickness and a higher thermal conductivity coefficient should be chosen. For example, when using PID control method of controlling the electrical current of the discharge, with the discharge tube exposed to the outside with very good thermal dispersion conditions, unrestrained thermal expansion back-pack style He-Ne laser tube gave good results. However, unsatisfactory results were obtained under the following conditions: discharge tube covered with glass cover, thermal dispersion conditions not as effective, thermal expansion of the discharge tube restricted by the co-axis glass cover (longer preheat time, lower anti-interference). Therefore, the frequency-stabilised double longitudinal modes with thermoregulation were adopted. This method was realised by winding wire with high electrical resistance on to the neck of laser tube. Heating was controlled by self-organised fuzzy control method. A normal co-axis He-Ne laser tube with bore length L = 190 mm was used. With PID and self-organised fuzzy control, the frequency error could be confined to a 10~9 level. In addition, preheating time and anti-interference were within requirements. Experiments have demonstrated that, under the condition of constant temperature (20), the frequency error of double-frequency laser interferometer with two longitudinal modes could be controlled within 10"9 Even with conditions such as with cool or hot air blowing, it can still be kept within 10"7. Self-organised fuzzy control methods can be realised in a single-chip processor which contains fuzzy optimisation and self-organisation functions. The above-mentioned system possesses the following advantages: simple... |
PDF Full Text Request