The fiber positioner of LAMOST is used to realize the accurate positioning of optical fiber in the process of astronomical observation. However, the existence of various error sources can affect the position accuracy of the fiber, and cause the actual results cannot satisfy the requirement of precision. In this paper, a fiber positioner is taken as research object, and the sources of positioning error, including mechanism error and joint clearance, are taken into consideration. The corresponding error model is established based on Monte Carlo numerical simulation method and virtual prototyping technology respectively, and the impact of different error sources on positioning accuracy of fiber positioner is analyzed. The results show that when only considering mechanism error, the mean positioning error of the fiber positioner is small, which can satisfy the requirement of the observation accuracy. Furthermore, when the clearance between journal and bearing is considered, there is a deviation between fiber’s actual trajectory and ideal trajectory. With the increase of clearance, the deviation is becoming larger and larger making positioning accuracy of the fiber positioner worse even cannot meet the requirements of precision. The above analysis provides a theoretical basis for the design of a new generation of high precision fiber positioner in the future.
LAMOST requires fast and accurate alignment of 4000 optical fiber units on a 1.75m-diameter convex focal plane to simultaneously observe 4000 targets, and the positioning accuracy of the unit is very demanding. But in fact there are a variety of factors such as motor motion error, machining error, assembly error, etc. which may lead to great positioning error. Moreover gear meshing transmission at the eccentric shaft and the anti-backlash spring may also cause greater positioning accuracy error. Besides,the inevitable gap between the center shaft and the bearing leads to motion error of the rotation mechanism, which also affects the precise positioning of the optical fiber. Therefore, in this paper, model and simulate the eccentric shaft basing on virtual prototype ADAMS set parameters and change the axial restraint of the gear, then obtain the axial movement curve. Design 9 groups of comparative tests through orthogonal experimental design.The results in case that: (1) The maximal movement of the gear at the eccentric shaft is within the allowable range, and has little effect on the unit positioning accuracy; (2) The overall error of the unit shaft clearance is in the range of -0.06deg/s -0.2deg/s, which leads to low positioning accuracy that may not meet the observation requirement. This research provides a theoretical basis for the design of a new generation high-precision positioning unit in the future.
This paper gives a scheme of optical fiber positioner structure of a miniature, by use of the DC servo motor with the diameter of 3mm driver, the distance can designed to 8.5mm, and can arrange more than 12000 fibers in the focal plane with the diameter of 1 meters, it is especially suitable for telescope with small dimension focal plane and has high density fiber positioning requirements. Based on the principle of double rotary fiber positioning principle, It consists of a hollow shaft revolving mechanism, and eccentric axis revolving mechanism relative to hollow shaft. The hollow shaft turns round at the range of -180 degrees to +180 degrees and the eccentric axis turns round at the range of -90 degrees to +90 degrees at the half of radius driving by each control motor. When positioning, the optical fiber end moves on the focal plate throughout, and can never deviate from focal plane. optical fiber is fixed in the mounting hole of fiber support which installed on the eccentric rotary shaft (fiber support’s hole axis is parallel to the axis of the hollow shaft), and fiber will lead to pass through the inner hole of the hollow shaft and focal plate then connected to the spectrometer. positioner center shaft adopts planetary gear driving principle, with small module motor’s gear and the fixed ring gear can driving motor and positioner planetary rotate, the eccentric shaft by DC servo motor with the diameter of 3mm drived coaxial optical fiber on the eccentric shaft, the center and the eccentric shafts adopts micro rolling bearing support; in order to prevent the positioner’s center and eccentric shaft to rotate out of bounds, both limiting devices have designed to ensure the safety of fiber positioning; both center and eccentric shaft are designed with a spring structure to eliminate the influence of gear clearance; because positioner size is very small, the positioner driving wire is embedded in the slot of the hollow shaft sleeve wall. This will not affect the fiber go through the center shaft’s holes and pass through the focal plane; positioner sample test results show that the closed-loop positioning can reached accuracy of 0.01mm unit, and can meet with the demand of optical fiber positioning.
Measuring the position of the end of 4000 optical fibers on the spherical focal plate for the LAMOST (Large Sky Area Multi-Object Fiber Spectroscopy Telescope) optical fibers positioning system is one of the key problems for LAMOST. The accuracy of optical fibers positioning system is guaranteed by feedback from measuring the position of the end of optical fiber. The position of the end of optical fiber is measured by photogrammetry with precision calibration. However, given the complexities in the optical fiber focal plane and the fiber positioner, the accurate standard point is considerably difficult to obtain, which results in insufficient calibration accuracy. To solve this problem, a convenient calibration method based on the Flexible Planar Target (FPT) is proposed. In this method, each fiber positioning unit positions the fiber to 16 designed locations, which are relatively accurate. These points form a high-precision 2D point array that can be used as the planar target. In this manner, each fiber positioning unit can be regarded as a small high-precision planar target. All small high-precision planar targets are assembled to form the Flexible Planar Target (FPT), which is used for calibration. Experimental results indicate that this improved method can reach a higher precision than that of previous method.
Metrology Camera System (MCS) was designed to make a closed-loop control of the optical fiber position in Fiber Positioning System (FPS) on the focal plate of the LAMOST. The stability of the metrology platform is the key factor to the quality of camera shooting. A precise adjustable mechanism was designed in this paper to achieve the platform’s pitching and horizontal rotation adjustment. And also a vibration isolation system using Magnetic Negative Stiffness (MNS) and positive spring in parallel was designed to decrease the effect of vibration, which was caused by the multiple complex vibration loads existing in the working environment, on the platform. Furthermore, an air conditioning system using the semiconductor refrigerator and resistance heater was designed to ensure working temperature of the camera and lens in extreme temperature environments. The simulation results showed that these designs were effective to improve the stability of the metrology system
The LAMOST telescope has been officially observed for the past seven years since 2009, and many parts of the telescope are currently being upgraded. The fiber positioning unit of the focal plane instrument is also planned to be upgraded again. In order to ensure a higher positioning accuracy of the fiber positioning unit, the newly developed fiber positioning system adopts a closed-loop camera to photograph the unit fiber position in real time, and feeds back to the control system to implement multiple positioning to improve the positioning accuracy. This article focuses on an improved optical center of gravity algorithm for optical fiber location based on the optical center of gravity algorithm. The factors affecting the position measurement of the optical fiber spot are optimized, and the recognition accuracy of the spot position under different conditions is improved.
Since the large scale use of paralleled controllable fiber positioner in LAMOST, the newly designed spectral survey telescope project generally uses the fiber position unit which similar to LAMOST to obtain the target spectrum. The positioning accuracy of the fiber positioner is directly related to the performance of the telescope. In order to further improve the positioning accuracy of positioners system, it is an important way to improve the accuracy by measuring the position of the optical fiber end on the positioners by using the visual metrology system. This paper mainly introduces the research design of LAMOST closed-loop metrology system, and the closed-loop system was established in different positions within the telescope to acquire best results. The metrology system will improve the fiber positioner system operation accuracy and reliability after the completion of the entire system in the future.
With the rapid development of multi-objective astronomical survey telescope technology, the heat of focal plate which high-density optical fiber positioners were mounted in has become the key factor of system precision. The new integrated cooling system designed multi curved composite grooves on the surface of focal plate for forced convection was proposed. Meanwhile, the manufacturing process, sealing structure and heat dissipation performance of the system were analyzed and tested with detail in the paper. The experimental results suggested that the new integrated cooling system of focal plate has a fast response speed and good heat dissipation performance.
In this paper, a compact optical fiber positioner is proposed, which is especially suitable for small scale and high density optical fiber positioning. Based on the positioning principle of double rotation, positioner’s center shaft depends on planetary gear drive principle, meshing with the fixed annular gear central motor gear driving device to rotate, and the eccentric shaft rotated driving by a coaxial eccentric motor, both center and the eccentric shaft are supported by a rolling bearings; center and eccentric shaft are both designed with electrical zero as a reference point, and both of them have position-limiting capability to ensure the safety of fiber positioning; both eccentric and center shaft are designed to eliminating clearance with spring structure, and can eliminate the influence of gear gap; both eccentric and center motor and their driving circuit can be installed in the positioner’s body, and a favorable heat sink have designed, the heat bring by positioning operation can be effectively transmit to design a focal plane unit through the aluminum component, on sleeve cooling spiral airway have designed, when positioning, the cooling air flow is inlet into install hole on the focal plate, the cooling air flow can effectively take away the positioning’s heat, to eliminate the impact of the focus seeing. By measuring position device’s sample results show that: the unit accuracy reached 0.01mm, can meet the needs of fiber positioning.
In the telescope observation, the position of fiber will highly influence the spectra efficient input in the fiber to the spectrograph. When the fibers were back illuminated on the spectra end, they would export light on the positioner end, so the CCD cameras could capture the photo of fiber tip position covered the focal plane, calculates the precise position information by light centroid method and feeds back to control system. A set of fiber back illuminated system was developed which combined to the low revolution spectro instruments in LAMOST. It could provide uniform light output to the fibers, meet the requirements for the CCD camera measurement. The paper was introduced the back illuminated system design and different test for the light resource. After optimization, the effect illuminated system could compare with the integrating sphere, meet the conditions of fiber position measurement.Using parallel controlled fiber positioner as the spectroscopic receiver is an efficiency observation system for spectra survey, has been used in LAMOST recently, and will be proposed in CFHT and rebuilt telescope Mayall. In the telescope observation, the position of fiber will highly influence the spectra efficient input in the fiber to the spectrograph. When the fibers were back illuminated on the spectra end, they would export light on the positioner end, so the CCD cameras could capture the photo of fiber tip position covered the focal plane, calculates the precise position information by light centroid method and feeds back to control system. After many years on these research, the back illuminated fiber measurement was the best method to acquire the precision position of fibers. In LAMOST, a set of fiber back illuminated system was developed which combined to the low revolution spectro instruments in LAMOST. It could provide uniform light output to the fibers, meet the requirements for the CCD camera measurement and was controlled by high-level observation system which could shut down during the telescope observation. The paper was introduced the back illuminated system design and different test for the light resource. After optimization, the effect illuminated system could compare the integrating sphere, meet the conditions of fiber position measurement.
Parallel controlled fiber positioner as an efficiency observation system, has been used in LAMOST for four years, and
will be proposed in ngCFHT and rebuilt telescope Mayall. The fiber positioner research group in USTC have designed a
new generation prototype by a close-packed module robotic positioner mechanisms. The prototype includes about 150
groups fiber positioning module plugged in 1 meter diameter honeycombed focal plane. Each module has 37 12mm
diameter fiber positioners. Furthermore the new system promotes the accuracy from 40 um in LAMOST to 10um in MSDESI.
That’s a new challenge for measurement. Close-loop control system are to be used in new system. The CCD camera
captures the photo of fiber tip position covered the focal plane, calculates the precise position information and feeds back
to control system. After the positioner rotated several loops, the accuracy of all positioners will be confined to less than
10um. We report our component development and performance measurement program of new measuring system by using
multi CCD cameras. With the stereo vision and image processing method, we precisely measure the 3-demension position
of fiber tip carried by fiber positioner. Finally we present baseline parameters for the fiber positioner measurement as a
reference of next generation survey telescope design.
Multi-objects survey system because of its high efficiency have been planned to build in many telescope such as
Mayall 4m telescope and have been working well on LAMOST. The telescope could control massively robotic fiber-positioners
carried with fibers on the top, received thousand galaxies and quasi-stellar objects at one time observation.
How to measure every fiber's position accurately is the key techniques for the telescope to improve its performance.
There is a good way to measure the fiber’s position by photogrammetry with no touches measurement. The camera
could capture the position of backside illuminated fibers. In this paper we described the trial measurement for multi
positioners system in different measuring parameters, and compared these conditions which influenced the measuring
accuracy. Finally the test results were presented the baseline parameters for the measurement system to provide a site
measurement option for the positioner location.
The surface accuracy of astronomical telescope focal plate is a key indicator to precision stellar observation. To conduct
accurate deformation measurement for focal plate in different status, a 6-DOF hexapod platform was used for attitude
adjustment. For the small adjustment range of a classic 6-DOF hexapod platform, an improved structural arrangement
method was proposed in the paper to achieve ultimate adjustment of the focal plate in horizontal and vertical direction.
To validate the feasibility of this method, an angle change model which used ball hinge was set up for the movement and
base plate. Simulation results in MATLAB suggested that the ball hinge angle change of movement and base plate is
within the range of the limiting angle in the process of the platform plate adjusting to ultimate attitude. The proposed
method has some guiding significance for accurate surface measurement of focal plate.
Modern multi-spectral sky survey requires the use of greater quantity and smaller size of the fiber positioner. This paper
presents a high-density integrated optical focal plane positioning system, which includes 150 groups fiber positioning
module and a 1 meter diameter honeycomb-shaped focal plane framework in that have about 150 hexagonal hole. Each
module has a pedestal includes 37 holes and 37 fiber positioner of 11.8 mm diameter. 37 fiber positioner integrated can
greatly reduce the difficulty of the design and installation. The modular structure also facilitates maintenance and
replacement in the field of telescope, and greatly reduce the difficulty of the drive system design. Numerical simulation
results show that: the honeycomb-shaped focal plane framework whose thickness is 100mm and who is in a variety of
working positions and load conditions, its maximum deformation is about 0.02mm. This meet the needs of the
general astronomical telescopes. The positioning accuracy of test 12mm diameter fiber positioner is about 0.04 mm,
and it is expected to reach 0.01mm if have the closed-loop control.
Large sky area multi-object fiber spectroscopy telescope (LAMOST) is an innovative reflecting Schmidt telescope. One
of its key technology is 4000 dual rotational fiber robot located in the focal plane. This article analyzes the calibration
requirements of the 4000 fiber robot. And then, proposes a fast calibration method in the complex field environment, and
discribes the specific process how to obtain positioning parameters of the fiber robot rapidly.
The focal plate is one of the most important components of the LAMOST, whose shape precision to be centripetal and
spherical structure of multi-hole. The hole drilling distortion duing to residual stress becomes one of the striking
problems. Studying on the distortion prediction, this paper adopts the finite element simulation based on the metal
cutting principles. The distribution to the surface residual stress is achieved by building the FEM model using
ANSYS .The influence of cutting depths on the distortion of the focal plate was investigated. With the confirmation of
the final CMM test result, the deviation which compared the measuring point with the theoretical sphere is less than
0.066mm. The result showed that the FEM analysis is an effective method which predicts the machining distortion of the
LAMOST is National Ninth Five Great Scientific Project. In the fiber positioning system, geometrical coordinates of fibers need to be measured in order to verify the precision of fiber positioning. The small focal plane system for LAMOST includes more than 200 fiber positioning units and its diameter is 500mm, so it's difficult to cover it using only one area CCD. For measuring wide field of view, the measurement system based on one CCD rotating is designed. The CCD camera is placed on a mechanism liked a pan head and can rotate around two vertical axes. When the CCD camera rotates in a certain way, the measuring scope becomes a ring. When the initial angle of CCD is changed the size of the ring changed too, so the wide field of view is measured. In this plan different measuring has overlapped regions and one fiber point may be measured for several times. After the camera's calibration the different imaging points will be transformed to the same coordinate system using photogrammetry method and the average value of them is the final value in order to eliminate the imaging error and transformation error. The realization of the measurement system based on CCD rotation is described.
The aims of LAMOST(Large Area Multi-Object Fiber Spectroscope Telescope) optical fibers positioning system is carrying out 4000 fibers minutely position quickly on the focal plane plate. Base on the dividing domain, we are putting forward parallel controllable optical fiber positioning system, this system consists of several parts as follows: In the focal plate of LAMOST, A aluminous alloy plate with plate diameter 1.75 m, globe radius is 20m. Over 4000 holes are bored on the focal plate; one optical fiber positioning unit of double revolving freedom device is inserted in each holes of focal plate, it is drived by two micro-stepping motor and positioning one fiber-end, focal plate is sustained by 8 steel tubes on the focal mechanical framework; for driving 8000 stepping motors, a control system is needed; and a measuring system with 4K surface CCD is used to calibrate the fiber's position, besides a few accessorial devices for example 4000 wire and fiber setting up need to plan elaborately, According to plan, parallel controllable fiber positioning system will be made in the next three years.
This paper constructed two protecting methods of diminishing the collision during the opposite movement of the
adjoining fiber unit in the LAMOST Positioning System. Auto-positioning mode is applied to every fiber positioning unit
of LAMOST Positioning System. The observing region is a circular region with the diameter of 33 mm. To ensure the
whole focal plane is covered by the observing region of 4000 fiber units, there must be superposition of observing region
of each adjoining fiber units, which induced the collision of adjoining fiber holder in the movement process and resulted
in the failing of orientation and mangling of structure. The mode of avoiding the collision comprises two methods. One is
hard protected mode, according to this method sensors are installed at each fiber positioning unit, then the motion of the
fiber units will be stopped immediately when the adjoining fiber units close to a dangerous distance. The other is soft
protected mode, which deliberates every situation of software from the observation programming to the motion path
designing for avoiding the collision. This paper expounds the designing and achievement of these two methods
One essential technology of the entire project of LAMOST(Large-sky-area multiobjec fiber spectroscopic telescope) is the focal plane plate on which is installed over 4000 fiber units. The telescope receives the light passes through the reflector of MA (the reflecting Schmidt plate) and MB (the spherical mirror), forms focusing image in the focal plane. And through optical fiber the light is introduced to the spectroscope by the fiber units. So the focal plane is the basic of the plane installed 4000 fiber units, and influence the imaging quality of the whole system. In the project of LAMOST, the focal plane plate is a part of sphere which radius is 19880mm and the diameter of it is about 1750mm, but the surface area of the focal plane plate approximate 1/8000 of the whole global. Thereby the measurement of the focal plane plate which profile tolerance low than 40μm to obtain the reliable and accurate results is a difficult problem to deal with. All the installation of the fiber units and mending the focal plane are based on the measurement. We try to use CMM(Coordinate Measurement Machine) to survey the processed Small Focal Plane which diameter is 445mm. And we will carry on the analysis for several kinds of test and the data processing method to compare and optimize, finally determines the actual application plan.
An investigation on measuring precision of the measurement system is carried on, which is applied to optical fiber
positioning system for LAMOST. In the fiber positioning system, geometrical coordinates of fibers need to be measured
in order to verify the precision of fiber positioning and it is one of the most pivotal problems. The measurement system
consists of an area CCD sensor, an image acquisition card, a lens and a computer. Temperature, vibration, lens aberration
and CCD itself will probably cause measuring error. As fiber positioning is a dynamic process and fibers are reversing,
this will make additional error. The paper focuses on analyzing the influence to measuring precision which is made by
different status of fibers. The fibers are stuck to keep the relative positions steady which can rotate around the same point.
The distances between fibers are measured under different experimental conditions. Then the influence of fibers' status
can be obtained from the change of distances. Influence to position error made by different factors is analyzed according
to the theory and experiments. Position error would be decreased by changing a lens aperture setting and polishing fibers.
This paper proposes research of a structure of LAMOST fiber positioning medium-term system; its construction and its tests are briefly introduced. This medium-term system includes several parts as follow: a main control computer, a unit controller, a set of drive circuits, 19 optical positioning units that positioning 19 optical fiber ends on the small simulate focal plane with diameter of 180 mm, a CCD camera, a frame grabber, and control programs. Tests on the system have indicated that positioning precision of 19 units is less than 0.04 mm on the whole focal plane with diameter of 180 mm. On medium-term system, some important problem for LAMOST building has test and research, for example: Fiber positioning precision, mechanism interference among the units, anti-jamming of drive circuits, unit's work life-span and reliability, temperature raising, etc. Test results have established stability foundation for LAMOST construction.
The paper proposes the measurement system for the fiber positioning unit of LAMOST(Large Sky Area of Multi-Object Fiber Spectroscope Telescope). It consists of an area CCD sensor, an image acquisition card, and a lens. The fiber is illuminated by light source from one end. The end of the fiber on the focal plate is imaged on the area CCD sensor by the lens. The image of the fiber end is acquired by the area CCD sensor, and transferred into a computer by the image grabber. Some pro-processed methods are used to process the digital image of the fiber. According to further digital image processing, the position of the fiber is obtained. The paper focuses on the calibration method of the digital area CCD camera. The measurement system calibrates the camera with the calibration board. The calibration board has some holes illuminated by an area LED. The positions of those holes are pre-measured precisely. Then, the systematic error of the measurement is figured out through the calibration procession. The optical aberration is fitted by the quartic surface. The measurement system can measure the position of the fiber on the positioning unit precisely. The precision of the measurement system is 0.010mm.
The architecture of the software which controls the LAMOST fiber positioning sub-system is described. The software is composed of two parts as follows: a main control program in a computer and a unit controller program in a MCS51 single chip microcomputer ROM. And the function of the software includes: Client/Server model establishment, observation planning, collision handling, data transmission, pulse generation, CCD control, image capture and processing, and data analysis etc. Particular attention is paid to the ways in which different parts of the software can communicate. Also software techniques for multi threads, SOCKET programming, Microsoft Windows message response, and serial communications are discussed.
This paper proposes an optical fiber positioning unit device for LAMOST(Large Area Multi-Object Fiber Spectroscope Telescope), It consists of a central shaft revolving mechanism, and eccentric shaft revolving mechanism relative to central shaft. The central shaft turns round at the range of -180° to +180° and the eccentric shaft turns round at the range of -90° to +90° driving by each control motor. When positioning, the optical fiber end moves on the focal plate throughout, and can never deviate from focal plane. It has simple structure, could be machined and assembled and taken down easily and could be ensured machining practices easily, so could be reduced manufacture costs. The unit sets mechanical electrical zero position detecting device to reduce the accumulate error of multi-positioning. Testing result have demonstrated this new double revolving optical fiber positioning unit device can accord with the demand of LAMOST entirely.
This paper describes the design of control system of fiber positioning system. The fiber positioning system has more than 4000 fiber units with 2 stopping motor and 2 start position sensor in each unit, and whole units will be assembled at 1.75 meter diameter focal surface of LAMOST, the mechanism and control system have demanding requirements for high precision position control. Detail design, testing and performance evaluation is described in this paper, a special control unit which can control and monitor more than 20 fiber with 1000Hz driving frequency of stepping motor and response start position sensor with one pulse of stepping motor in each fiber unit is set as a CAN bus node, 200 control units combine a can bus real time control system which can control the whole fibers move to new position in 3 minutes. In order to get high precision position in this open loop control unit, a very simple and small sensor is used to eliminate the accumulate errors of mechanism with resetting the start position, and compensation data is measured and set in control software to diminish the mechanical transmission errors. For testing the mechanism and control system, a small fiber positioning system with 19 units have been made.
In present paper we have proposed an optical fiber positioning system for LAMOST (Large Sky Area Multi-Object Fiber Spectroscopy Telescope). In this system, the convex focal plate of the telescope (Its diameter is 1.75 m) is divided into about 4000 individual domains. Each domain contains a controllable unit which is named X-unit. Each unit is specified with the position of its domain with polar coordinates (the angle and the radius distance from the center) by using its center point as reference. Each X-unit can be moved smoothly by verifying the two parameters of polar coordinates. An optical fiber is hold with X-unit and introduced to spectroscope, thus the optical fiber can be moved anywhere within its designated circular domain at the convex focal surface. In addition, the circular domain are overlapped in a `honeycomb-like' arrangement to ensure that there are no blind spots on the convex focal surface, such as that the radius distance of the units is 25 mm, and the positioning range of the optical fibers is 30 mm, thus there will be no `blind areas'. The structural parameters of the X-unit could also be optimized to limit the directional error of the axis in a certain range. Each unit is driven by two stepping motors and mobilized by computer in a way to ensure that the units do not collide with each other. It is possible that very high or very low star densities in areas of the field of view may reduce the observational efficiency of the telescope. This should not a significant problem by using optimization of the observation program.