| With the development of optical fiber positioning,the LAMOST(Large Sky Area Multi-Object Fiber Spectroscopy Telescope)system requires higher positioning accuracy.Despite the achieved remarkable results of open-loop control mode in previous techniques,it fails to meet the strict requirements of the next-generation optical fiber positioning techniques.In contrast to the open-loop system,in this paper,we focus on fiber positioning in the closed loop control mode.In the closed-loop control system,the detected optical fiber position is regarded as negative feedback,and the fiber unit is controlled to fretting multiple times to compensate the position deviation and improve the positioning accuracy of the system combined with the optimized element control algorithm.To this end,we add a photogrammetric system consisting of a CMOS camera,a telephoto lens and an image acquisition and processing host to the closed-loop control system.After receiving the command of the control system,the visual measurement subsystem controls the camera to take pictures and get the pictures containing the location information of the optical fiber.We design the image acquisition and processing program,and extract the image plane coordinates of the spot representing the optical fiber location unit are extracted by the method of light center of gravity.To convert the image plane coordinates into object plane coordinates,we use the least square method to solve the parameters in the camera calibration polynomial model.Finally,the minimum matching method is used to match the object plane coordinates with the optical fibers.The control system can obtain the position of the optical fiber unit as the starting point of each positioning.In the process of fiber positioning,the closed-loop control can be realized by combining with the visual measurement subsystem.In our proposed framework,by measuring the actual position of the optical fiber and feeding it back to the control system,the control unit positions and approaches the target position multiple times to improve the accuracy of optical fiber positioning.To this end,we add a photogrammetric system consisting of a CMOS camera and a telephoto lens to store and process the captured pictures.By virtue of the mathematical program processing,the fiber coordinates on the unit obtained by pictures are converted into the actual coordinates in space.The camera program sends the actual coordinates to the control program,then the computer calculates the pulses and controls the units according to these computed pulse parameters.In that case,the optical fiber gradually approaches the target position to realize the closed-loop control of the unit.Nevertheless,there still exist other challenges in our designed system such as the collision of the optical fibers and the deviation of positioning accuracy caused by element out-ofbounds.Firstly,according to the starting position and target position of each positioning process,we propose a collision prediction algorithm based on the area of interference region to predict the collisional optical fibers in the next operation process in time.Then,the target redistribution algorithm based on collision prediction is adopted to select the designated units for collision-free processing,and the collision avoidance processing between units is realized,which effectively reduces the collision probability between units.In addition,we normalize the central axis angle and eccentric axis angle on the current position and target position of the unit through normalization processing,so as to avoid the occurrence of the phenomenon that the unit cannot move successfully because it needs to cross the mechanical zero position.Furthermore,the angle correction algorithm based on the safety angle interval proposed in this work solves the problem of error in the calculation of the waiting angle of the two axes caused by the detection error of the visual measurement subsystem.Finally,we use pulse compensation algorithm to reduce the influence of gear cavitation and improve the positioning accuracy of the unit.The pilot test of LAMOST fiber positioning closed-loop feedback control system shows that the closed-loop control mode can effectively improve the positioning accuracy and positioning efficiency of fiber element.After three times of fiber positioning in closed-loop control mode,the positioning error of 97%fiber element is less than 40μm.After four steps of fiber positioning,the positioning error of 98.6%fiber element is less than 40μm,and the positioning time is less than 5 minutes,which are in line with the design goal of closed-loop feedback control system.The results provide theoretical support for the closed-loop feedback control system used in LAMOST field fiber positioning.Moreover,the intermediate test system has successfully passed the acceptance of the expert group,which not only verifies the reliability of the closed-loop control system proposed in this work,but also justifies the effectiveness of various algorithms proposed in this work. |
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