Digital image correlation method is widely used in macro- and mesoscopic mechanical tests because of the advantages of non-contact, high precision, full field measurement, and simple experimental equipment. But it still has many shortcomings in theory and application. For example, the traditional DIC method has difficulty obtaining the desired result when the surface of the test object is rotated substantially or the deformation involves large rotation (i.e. measuring helicopter blade deformation dynamically). Some scholars argue that a rotation angle greater than 7° will not work with the DIC algorithm, and this phenomenon is called decorrelation [1-3]. The application of various nonlinear optimization algorithms greatly reduces the computation time of the DIC iteration process.
The positional accuracy of a near-infrared (NIR) dynamic navigator is remarkably affected by two factors. One is the calibration accuracy of the navigator’s two NIR cameras, and the other is the accuracy of feature point extraction. The current lack of accurate calibration devices for NIR cameras limits further application. Therefore, in this study, an NIR camera calibration device was designed by placing the NIR light source and heat dissipation system at the back of a Halcon transparent glass template. Usage of camera calibration in a specific band and the gray centroid method based on elliptic boundary to extract feature points can further improve the accuracy of vision system calibration and measurement. Repeated tests and verifications showed that the reconstruction accuracy (<0.1 pixels) of the binocular vision system calibrated by the NIR calibration device in a specific band was better than that calibrated by traditional methods.
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.
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).