Research On Closed Loop Control Of Optical Fiber Positioning Based On Vision Measurement Technology

At present,the fiber positioning of Large Sky Area Multi-Object Fiber Spectroscopy Telescope adopts the open-loop control mode,and the accuracy of fiber positioning is completely dependent on the accuracy of the pre-calibration.The pre-calibration error affects the accuracy of fiber positioning for a long time and the calibration process is complicated.Therefore,the accuracy of fiber positioning will directly affect the observed efficiency of the telescope and the quality of spectral data obtained by the spectrometer.In this paper,the fiber position is measured in real time and fed back to the upper control system through the establishment of a vision measurement system.According to the positioning error,the fiber unit is controlled to approach the target position for many times to ensure the accuracy of fiber positioning.A vision measurement system consisting of a high-resolution CMOS camera and a telephoto lens is installed around the MB primary mirror about 20 meters away from the focal plane in order to avoid blocking the light and enable real-time measurement of the fiber position.The vision measurement system receives the measurement instructions of the upper control system through the local area network,collects the images in real time and extracts the coordinates of the spot center position as the current position of the fiber through the light center of gravity method.In order to improve the stability of fiber position detection in the vision measurement system,this paper reduces the fixed reference fiber to remove the system drift and reduces the random error by averaging multiple images,improving the stability of fiber position detection from 50 microns to within 10 microns in the LAMOST field.In the laboratory,the operation mode of the fiber unit in the closed-loop control process was optimized by taking advantage of the ultra-low distortion of the telecentric measurement system with eliminating the measurement error of the fiber positioning.The expansion angle of the unit was normalized to the circumference of 0?2? starting from the initial angle,which solved the problem that the forward and reverse of the unit exceed the maximum angle due to the existence of mechanical zero.At the same time,the backlash of unit reversal caused by gear gap was solved.The closed-loop control experiment of 84 target points was carried out on three fiber units through the telecentric measurement system.After two times of positioning,the positioning accuracy of the three fiber units at 84 target points all met the positioning requirements of 40 microns.On this basis,93 fixed reference fibers and 140 moving fibers were selected to carry out the closed-loop control experiment on 84 target points in the LAMOST field.The results show that the positioning accuracy of 95%fiber units can reach 40 microns after 3?4 times of positioning.The experimental results of fiber positioning closed-loop control in the laboratory and in the LAMOST field show that it is one of the effective means to improve the precision and reliability of fiber positioning by introducing the vision measurement system which measures the fiber position in real time and form the closed-loop control.