Xiaozheng Xing

New type optical fiber positioning unit device for LAMOST

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.

Study on the optical fiber positioning medium-term system for LAMOST

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, units work life-span and reliability, temperature raising, etc. Test results have established stability foundation for LAMOST construction.

Calibration method with separation patterns of a single-camera

Such as in LAMOST (Large Sky Area Multi-Object Fiber Spectroscopy Telescope), many photometric measurement systems need to reach sub-pixel accuracy with area scan CCD camera. The separation patterns are used to calibrate a single-camera with high precision. Several separation calibration patterns with small size are put on the position of object plane of the camera. Each pattern has some spot array with high precision. The position of each reference point on the image plane of the camera is calculated. The coordinates of the reference points on the calibration patterns are used to calibrate the camera. The curved-surface fitting method is applied to fit the perspective relationship between the object plane and the image plane. The integer pattern with large dimension can be replaced by the several small differential patterns in the situation of large field. The difficulty to manufacture the large pattern is avoided. The experimental results show that the mean value of residual error is less than 0.002mm with the separation calibration method.

Preliminary study on the measurement system for LAMOST small focal plane fiber positioning system

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 its 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 cameras 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.

Design of control system for fiber positioning system of LAMOST

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.

The measurement system for fiber positioning unit of LAMOST

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.

Implementary scheme of parallel controllable optical fiber positioning system for LAMOST

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 fibers 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.

Observation planning of LAMOST fiber positioning subsystem and its simulation study

The observation region of fiber positioning unit of LAMSOT is designed as a round area and overlapped each other in order to eliminate the un-observation region and increase the efficiency of the observation. But in such structure, the fiber holders have probability to touch each other during moving to the target images. This paper introduces a method of an observation planning for LAMOST sub-system including 19 fiber positioning units which can diminish the probability of mechanical interference by proper assignment ways and the preparatory processing in which the interference would be detected and eliminated through the retreat algorithm, and the strategy of allocating images and the moving routes of all units are obtained. The computer simulation indicates that this method successfully avoids the mechanical collisions during observations of Sub-system, at the same time, the efficiency of the observation is hardly decreased. This method is definitely valuable to the parallel controllable optical fiber position system of LAMOST which has 4000 fiber positioning units.

A new measurement method of profile tolerance for the LAMOST focal plane

There were a few methods taken in the profile tolerance measurement of the LAMOST Focal Plane Plate. One of the methods was to use CMM (Coordinate Measurement Machine) to measure the points on the small Focal Plane Plate and calculate the points whether or not in the tolerance zone. In this process there are some small shortcomings. The measuring point positions on the Focal Plane Plate are not the actual installation location of the optical fiber positioning system. In order to eliminate these principle errors, a measuring mandrel is inserted into the unit-holes, and the precision for the mandrel with the hole is controlled in the high level. Then measure the center of the precise target ball which is placed on the measuring mandrel by CMM. At last, fit a sphere surface with the measuring center points of the target ball and analyze the profile tolerance of the Focal Plane Plate. This process will be more in line with the actual installation location of the optical fiber positioning system. When use this method to judge the profile tolerance can provide the reference date for maintaining the ultra error unit-holes on the Focal Plane Plate. But when insert the measuring mandrel into the unit hole, there are manufacturing errors in the measuring mandrel, target ball and assembly errors. All these errors will bring the influence in the measurement. In the paper, an impact evaluation assesses the intermediate process with all these errors through experiments. And the experiment results show that there are little influence when use the target ball and the measuring mandrel in the measurement of the profile tolerance. Instead, there are more advantages than many past use of measuring methods.

The testing scheme for the LAMOST focal plane plate

At present, the LAMOST project is in a crucial period. The machining progressing of LAMOST Focal Plane Plate has completed. The inspection of the machining quality for the Focal Plane Plate in the machining process is a pivotal work. In all of the design requirements, the most crucial standards of accuracy are the profile tolerance and the unit-holes dimensional angle. Theirs precision will influence the observation efficiency of the LAMOST. But there are more than 4000 unit-holes on the 1.75m diameter Focal Plane Plate, it is impossible to measure all unit-holes and the whole area of the Focal Plane Plate. How to measure the minimal unit-hole and get the most accurate results about the machining process, judge whether the final machining Focal Plane Plate satisfy the design requirements. The measurement scheme optimization is discussed in the paper. There are two different ways to measure the Focal Plane Plate, one is the traditional way whish use specially designed implements for the every individual parameter, the other way used the CMM to measure the pivotal design requirements such as unit-hole dimensional angle and the profile tolerance of the Focal Plane Plate. The advantage of this is saving the time and cost on the CMM, improving the efficiency for the whole measurement work, and acquires the direct vision results before measuring the Focal Plane Plate on CMM. Whereas the implement which used in the measurement need to design and machine precisely for the credible measurement results. And all the measuring work is calibrated by the CMM sampling detection. The sampling detection based on the processing technology and some implements are mentioned in the paper.