Multidimensional Displacement Detection Using Maximum Length Sequence

During observation,a telescope usually adopts a multi-dimensional adjustment mechanism to adjust the position and orientation of its secondary mirror in real time to ensure the final image quality.As a precision functional component,the multi-dimensional adjustment mechanism requires pose measurement before installation or after long-term operation,in order to verify and calibrate its accuracy.In this paper,according to the performance requirements and working characteristics of the multi-dimensional mechanism of astronomical telescope,a visual pose detection method using the an MLS(Maximum-Length-Sequence)based target was implemented to carry out the research and experimental verification of the multi-dimensional displacement detection and calibration.Firstly,combining the principle of MLS with the actual measurement requirements,a two-dimensional visual target was designed,and an experiment system was built to fulfill the detection of two orthogonal linear displacements and one angular displacement.The reference points and locating experiments of two orthogonal linear displacements were performed.The measurement errors of the linear displacement in the range of 5.000 mm were reduced by calibration.The both translational displacements were measured 0.001 mm RMS.Based on this,the positioning experiment in the rotational direction in the range of 600? and repeated the positioning of the whole system were tested successively.The experimental results showed that,the angular displacement measurement accuracy was 0.15? RMS,and the standard deviations of repeated positioning displacement in the three sets of experimental data of linear displacements was both 0.001 mm,and those of angular displacement were 0.12?,0.14? and 0.12?,respectively.Secondly,in view of the actual situation of the application environment and system installation,addressing the problem that the lens out-of-focus,uneven illumination,target deflection and lens tilt,etc.may cause detection errors,comprehensive experiments and analysis were carried out to explore the impact of environmental factors on the measurement,and improved methods were proposed.The actual data indicated that the lens out-of-focus and the target deflection with lens tilt in the range of 1° had little effect on the measurement and cloud be ignored.Besides,the uneven illumination would affect the identification of the sequence,and the impact on the measurement after the algorithm improving could be ignored.Finally,the pose measurement system with visual targets was applied to a 3SPS+3(SP-U)six-DOF micro-motion parallel mechanism.The two translational directions and one rotational direction of the moving platform were selected for calibration,and the precise displacement measurement was realized when it moved with the other two DOF at the same time.The experimental data showed that among the three DOF of calibration,the RMS-errors of the two translational directions were both 0.001 mm,and the root mean square error of the rotation direction was 0.21?.Similar accuracy was thereby reached in the other two DOF measurements.Considering the stability of the system,the positioning repeatability experiments were completed in three DOF directions by the platform.The experimental data showed that the standard deviation of the three sets of experimental data in two translational directions were both 0.001 mm,and those in the rotational direction were 0.17",0.19" and 0.17",respectively,which indicated that the measurement system with visual targets had good stability.