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.
Fiber spectroscopic telescopes, such as LAMOST, require accurate alignment between the fiber ends and their corresponding celestial targets. In the measurement, the center of a light spot obtained through gray centroid method is regarded as the center of a fiber end. Nevertheless, we’ve observed that these two centers don’t coincide under wide visual angles when integrating sphere (Built in light source for bromine tungsten lamp) is used as light source. Basing on these phenomena we observed, this paper proposes a hypothesis that the maximum intensity of the light transmitted by the optical fiber to the end of the optical fiber is outside the end of the fiber. The intensity distribution at the output end of fibers under integrating sphere light source is simulated in this study. Keywords: accurate alignment
In the large sky area multiobject optical fiber spectroscopy telescope project, to capture the spectrum of a particular object, the optical fiber positioner must position the optical fiber end face to a specified location on the focal plane. The accuracy of the optical fiber positioner is guaranteed by feedback from photogrammetry. Photogrammetry accuracy is based on accurate calibration. However, given the complexities in the optical fiber focal plane and the optical 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 combination of small, planar targets is proposed. In this method, each optical fiber positioner positions the optical fiber to several designed locations, which are relatively accurate. These points form a high-precision, two-dimensional point array that can be used as the planar target. In this manner, each optical fiber positioner can be regarded as a small, high-precision planar target. All small, high-precision planar targets are assembled to form the flexible planar target, which is used for calibration. The experimental result indicates that this method is highly accurate and can be applied in focal plane calibration.
A new calibration technique for line-structured light scanning systems is proposed in this study. Compared with existing methods, this technique is more flexible and practical. Complicated operations, precision calibration target and positioning devices are all unnecessary. Only a blank planar board, which is placed at several(at least two) arbitrary orientations, and an additional camera that is calibrated under the global coordinate system are required. Control points are obtained through improved binocular intersection algorithm that avoids corresponding points matching and then used to calculate the light stripe plane through least square fitting. Experiment results indicate that the system calibrated by this technique is able to conduct surface measurement, offering an accuracy superior to 32μm(RMS).
In fiber spectroscopic telescopes, optical fiber positioning units are used to position thousands of fibers on the focal plane quickly and precisely. Stepper motors are used in existing units, however, it has some inherent deficiencies, such as serious heating and low efficiency. In this work, the universally adopted subdivision driving technology for stepper motors is transplanted to brushless DC motors. It keeps the advantages of stepper motors such as high positioning accuracy and resolution, while overcomes the disadvantages mentioned above. Thus, this research mainly focuses on develop a novel subdivision driving technology for brushless DC motor. By the proving of experiments of online debug and subdivision speed and position, the proposed brushless DC motor subdivision technology can achieve the expected functions.
In order to avoid the defects of contact measurement, such as limited range, complex constructing and disability of 3-D parameter acquisition, we built a binocular videogrammetric system for measuring 3-D geometry parameters of wind tunnel test models, for instance, displacement, rotation angle and vibration, in low-speed wind tunnel. The system is based on the principles of close-range digital photogrammetry. As a non-contact system, it acquires parameters without interference in the experiments, and it has adjustable range and simple structure. It is worth mentioning that this is a Realtime measurement system, so that it can greatly compress the experiment period, furthermore, it is also able to provide some specific experiments with parameters for online adjustment. In this system, images are acquired through two industrial digital cameras and a PCI-E image acquisition card, and they are processed in a PC. The two cameras are triggered by signals come from a function signal generator, so that images of different cameras will have good temporal synchronization to ensure the accuracy of 3-D reconstruction. A two-step stereo calibration technique using planar pattern developed by Zhengyou Zhang is used to calibrate these cameras. Results of wind tunnel test indicate that the system can provide displacement accuracy better than 0.1% and rotation angle accuracy better than 0.1 degree, besides, the vibration frequency accuracy is superior to 0.1Hz in the low-frequency range.
In this paper, combining optical measurement with conventional material testing machine, a real-time in-plane
displacement and strain measurement system is built, which is applied to the material testing machine. This system can
realize displacement and strain measurement of a large deformation sample moreover it can observe the sample crack on
line. The change of displacement field is obtained through the change of center coordinate of each point of a grid lattice
in the surface of the testing sample, according to two-dimensional sort coding for the grid in the traditional automated
grid method, in this paper, an improved one-dimensional code method is adopted which make calculating speed much
faster and the algorithm more adaptable. The measurement of the stability and precision of this system are made using
the calibration board whose position precision is about 1.5 micron. The results show that the short-time stability of this
system is about 0.5micron. At last, this system is used for strain measurement in a sample tension test, and the result
shows that the system can acquire in-plane displacement and strain measurement results accurately and real-time, the
velocity of image processing can reach 10 frame per second; or it can observe sample crack on line and storage the test
process, the max velocity of observation and storage is 100 frame per second.