Convenient Calibration Research Articles

Traceability of network time service to UTC(NIM): online calibration

Network time service provided by Network Time Protocol (NTP) servers is widely used to synchronize the clocks of computers and other networked equipment. With the rapid development of smart devices and their applications, more NTP servers as time sources need to be calibrated to a legal time reference. This work proposes using a low-cost and convenient calibration method to investigate the time service capability of ten NTP servers located at different cities in China. The uncertainties (k = 2) of the time offsets between Coordinated Universal Time (National Institute of Metrology) and network time from NTP servers for a single measurement are generally less than 150 ms, and the uncertainties (k = 2) of the mean time offsets for 24 h are less than 85 ms. Calibration results were found at the same level when applying the two time references, which were maintained a time offset less than 10 ns and located more than 1000 km apart.

A Convenient and High-Accuracy Multicamera Calibration Method Based on Imperfect Spherical Objects

Multicamera systems have important applications in industrial online measurement, attracting wide interest due to their encouraging performance. However, how to develop a convenient and high-accuracy multicamera calibration model is a big challenge. Traditional calibration methods based on 2-D objects require the preparation of high-precision and large-scale calibration objects and have poor adaptability to the spatial distribution of cameras. To address these issues, a convenient and accurate multicamera calibration method is proposed based on an imperfect spherical object. Specifically, a special calibration object, i.e., an imperfect sphere with many coded targets is designed for calibrating the multicamera system, where the optical axes of the cameras are different in orientations and converge into the measurement field. Then, a Euclidean reconstruction method is employed to calculate the camera poses and the spatial coordinates of feature points in the local coordinate system after the adaptive grouping of cameras is completed. Moreover, a graph theory-based optimal path transformation algorithm is developed to obtain the camera poses and spatial point coordinates in the global coordinate system (GCS), and a spherical projection optimization algorithm is designed to spheroidize the spatial coordinates of feature points. In the end, the camera poses and the spatial coordinates of feature points in the GCS are optimized by a bundle adjustment algorithm. We build a multicamera system and conduct extensive experiments to demonstrate the superiority of the proposed method.

A Calibration and 3-D Measurement Method for an Active Vision System With Symmetric Yawing Cameras

In this study, we propose a 3-D measurement method for an active stereo vision system with two symmetric yawing cameras. First, we establish a measurement model with five constant systematic parameters and five variables. Second, we present a design for a relatively simple and convenient calibration method to determine the constant systematic parameters for the active vision system. In our proposed method, the initial yaw angles of two cameras are calibrated with a minimum of three equal interval points in a common field of view (FOV). The initial pitch angles are calibrated with at least one point in this common FOV. The initial yaw angles are used in the following calibration processes of the baseline length and the optical offset. The baseline length is calibrated with respect to two such points with known horizontal distance. An offset used to compensate for the difference between the actual optical centers of the two cameras along their optical axes is also computed with at least one feature point with the cameras yawed at different angles. Third, an associated measurement method for objects in the common FOV is presented. The proposed method computes 3-D positions of feature points in the common FOV using these five constant systematic parameters and five variables. This active stereo vision system design does not require repeated calibration for varying yaw of the two cameras to conduct measurements once the five constant systematic parameters have been calibrated offline. We conduct comparative experiments, and the results verify the effectiveness of the proposed method.

Flexible multicamera calibration method with a rotating calibration plate.

Camera calibration is necessary for accurate image measurements, particularly in multicamera systems. The calibration process involves corresponding the coordinates of 3D calibration points with a 2D image and requires the establishment of a reliable 3D world coordinate system. This paper presents a convenient multicamera calibration method that uses a rotating calibration plate and multi-view stereo vision to calculate 3D points and their relationship with the image coordinates. Despite simple implementation, the rotation of the calibration plate presents numerous calibration points from various planes, increasing the stability of the solution and the noise reduction. The relocation accuracy and reprojection error are experimentally verified.

Light-Sheet Microscopy for Surface Topography Measurements and Quantitative Analysis.

A novel light-sheet microscopy (LSM) system that uses the laser triangulation method to quantitatively calculate and analyze the surface topography of opaque samples is discussed. A spatial resolution of at least 10 μm in z-direction, 10 μm in x-direction and 25 μm in y-direction with a large field-of-view (FOV) is achieved. A set of sample measurements that verify the system′s functionality in various applications are presented. The system has a simple mechanical structure, such that the spatial resolution is easily improved by replacement of the objective, and a linear calibration formula, which enables convenient system calibration. As implemented, the system has strong potential for, e.g., industrial sample line inspections, however, since the method utilizes reflected/scattered light, it also has the potential for three-dimensional analysis of translucent and layered structures.

Calibration of large neutron detection arrays using cosmic rays

Cosmic muons are used to calibrate the position, light output and the relative timing offset of the Large Area Neutron Array (LANA). Each 2×2m2 LANA wall consists of twenty-five horizontal neutron detection bars. Each bar is 2 m long with a cross-section of 6.35×7.62cm2 Pyrex container filled with NE-213 organic scintillation liquid. The average position and time resolution of a bar is found to be about 8 cm and 500 ps FWHM, respectively. Our method provides an accurate, fast and convenient calibration of LANA that can be applied to general scintillation arrays without the use of radioactive sources or beams.

Titta: A toolbox for creating PsychToolbox and Psychopy experiments with Tobii eye trackers

We present Titta, an open-source toolbox for controlling eye trackers manufactured by Tobii AB from MATLAB and Python. The toolbox provides a wrapper around the Tobii Pro SDK, providing a convenient graphical participant setup, calibration and validation interface implemented using the PsychToolbox and PsychoPy toolboxes. The toolbox furthermore enables MATLAB and Python experiments to communicate with Tobii Pro Lab through the TalkToProLab tool. This enables experiments to be created and run using the freedom of MATLAB and Python, while the recording can be visualized and analyzed in Tobii Pro Lab. All screen-mounted Tobii eye trackers that are supported by the Tobii Pro SDK are also supported by Titta. At the time of writing, these are the Spectrum, Nano, TX300, T60XL, X3-120, X2-60, X2-30, X60, X120, T60 and T120 from Tobii Pro, and the 4C from Tobii Tech.

A convenient protein library for spectroscopic calibrations

While several Raman, CD or FTIR spectral libraries are available for well-characterized proteins of known structure, proteins themselves are usually very difficult to acquire, preventing a convenient calibration of new instruments and new recording methods. The problem is particularly critical in the field of FTIR spectroscopy where numerous new methods are becoming available on the market.The present papers reports the construction of a protein library (cSP92) including commercially available products, that are well characterized experimentally for their purity and solubility in conditions compatible with the recording of FTIR spectra and whose high-resolution structure is available.Overall, 92 proteins were selected. These proteins cover well the CATH space at the level of classes and architectures. In terms of secondary structure content, an analysis of their high-resolution structure by DSSP shows that the mean content in the different secondary structures present in cSP92 is very similar to the mean content found in the PDB.The 92-protein set is analyzed in details for the distribution of helix length, number of strands in β- sheets, length of β-strands and amino acid content, all features that may be important for the interpretation of FTIR spectra.

Improved Method for Acoustic Identification of Free Conductive Particle Defects in GIL

The presence of free conductive particles in gas-insulated transmission lines (GIL) is the most significant threat to their insulation. The non-invasive identification of free conductive particle defects in GILs is extremely challenging. Existing identification methods which use the acoustic amplitude-flight time (AAFT) pattern are not often adopted on site due to their faulty theory, non-robust extraction algorithm, and generally inaccurate calibration. In this study, the theory established by L. E. Lundgaard is expanded by deriving estimation equations related to the particle's moving state. A new method for extracting the state information from the raw data of acoustic signals is established and the robustness of the acoustic identification method is enhanced. A convenient calibration method is also developed by building a standard collision signal generator. The proposed methods are validated in a scale GIL model and a real GIL model. The results of this study may represent a workable reference for GIL condition monitoring.

Energy dependence of a scintillating fiber detector for preclinical dosimetry with an image guided micro-irradiator

The dosimetry of preclinical micro-irradiators is challenging due to their millimetric beams and medium x-ray energy range. Plastic scintillator dosimeters (PSD) are good candidates for such a purpose as they provide a high spatial resolution although they show an energy dependence below 100 keV. The purpose of this study was to assess the energy dependence of a dedicated PSD (called DosiRat) for micro-irradiators dosimetry.The response of the PSD relative to air kerma was measured for different beam qualities (40–225 kV) with the X-RAD 225Cx irradiator. The corresponding energy spectra, determined by Monte Carlo simulations, allowed for correcting the differences in absorbed dose between the DosiRat material (polystyrene) and the air and therefore allowed to compare DosiRat intrinsic energy response to the Birks scintillation quenching model. The energy response of DosiRat was then assessed under preclinical conditions through percentage depth dose curves (PDD) and relative output factor (ROF) measurements in water for beam diameters ranging from 1 to 25 mm.DosiRat energy response showed a coefficient of variation of 23% from 40 to 225 kV, mainly explained by the mass energy-absorption coefficient variation between polystyrene and air. A remaining variation was shown to be caused by the quenching of the scintillation and was correctly reproduced by the Birks model (with kB = 10.27 mg MeV−1 cm−2). PDD and ROF measurements highlighted an energy response variation with depth and collimation up to 10%. A dose accuracy better than 1% was finally achieved with appropriate calibration and correction factors (CF), for beam collimations larger than the detector (2 mm diameter).DosiRat energy dependence was fully characterized in preclinical energy range and shown to be negligible with convenient calibration and corrections factors. It provided accurate dosimetry for medium energy (225 kV) and millimetric beams (down to 2.5 mm).

IMU Mounting Angle Calibration for Pipeline Surveying Apparatus

Odometer and nonholonomic constraints are crucial external aids for inertial navigation systems (INSs) to achieve desirable positioning accuracy for pipeline inspection gauges (PIGs). Exploiting the maximum potential of these constraints requires the accurate knowledge of the inevitably existing misalignment in the attitudes of the inertial measurement unit (IMU) with respect to the host vehicle (i.e., the IMU mounting angle relative to the PIG). In this paper, we address the mounting angle calibration of a PIG without using sophisticated equipment. Instead, the proposed method only requires the PIG to be placed on a simple leveled frame and rotated about its longitudinal axis. In this case, the existing IMU mounting angles will make the IMU’s pitch and heading angles vary with rotation. Closed-form analytic solutions for the IMU’s attitude angles are obtained under this specific condition and show that the magnitude and phase of the IMU’s pitch and heading angles have a linear relation with the corresponding pitch and heading misalignment angles. Then, a method for the misalignment angle estimation is presented based on this principle. Simulations are performed to validate the proposed method and investigate the related error sources that influence the estimation accuracy. The results of the real tests indicate that the mounting angles can be accurately estimated and compensated through this simple and convenient calibration.

A magnification-continuous calibration method for SEM-based nanorobotic manipulation systems.

Calibration for scanning electron microscope (SEM) based nanorobotic manipulation systems is important and difficult. Most current calibration methods are cumbersome because they require customized high precision calibration boards and repeated calibration procedures in different magnifications. This paper presents a convenient magnification-continuous calibration method with high precision for SEM-based nanorobotic manipulation systems. The projection matrix containing a continuous magnification factor is obtained by modifying the affine camera model. This facilitates the simplification of the parameter computing process. Movement features are used to align the moving axes of micropositioning stages and calibrate the system, which benefits for the realization of efficient automatic calibration. Three experiments are carried out, and the results demonstrate that the proposed method is effective and practical for calibrating SEM-based nanorobotic manipulation systems under a wide range of continuous magnifications. Experiments also confirm that high precision measurements can be conducted in different magnifications with only once calibration and the relative error is within 1%.

Input Mapping for Model Calibration with Application to Wing Aerodynamics

A wide range of model calibration methods and formulas have been introduced in the literature for calibrating low-fidelity (LF) computer models against high-fidelity (HF) computer models or data. Although most existing model calibration techniques assume that LF and HF models have identical inputs, in some engineering applications, for example, wing aerodynamics computations, inputs to LF and HF models are often defined differently due to different levels of abstraction in modeling or simulation. For these problems, this paper proposes a new model calibration method that calibrates a mapping from HF model inputs to LF inputs by matching HF and LF model outputs. The method incorporates regularization to prevent overfitting and to allow calibration parameter selection. In the application to calibrating aerodynamic simulation models, three advantages of the proposed method are demonstrated. First, it achieves higher calibration accuracy than the traditional bias correction method when HF data are scarce. Second, it provides convenient and effective calibration parameter selection in the calibration process. Finally, it enables physical insights to be drawn from the calibrated input mapping. Specifically, for the test cases examined, the calibration adjusts wing twist angles to compensate for the neglect of thickness in the vortex lattice representation of the wing geometry.

A flexible calibration method for the intrinsic and mechanical parameters of panoramic line-scan cameras

Panoramic line-scan cameras provide high definition images with a quasi uniform distribution of the sensing elements. Unfortunately, calibration of such devices with classic techniques is not possible making their use, in computer vision field, not straightforward. We propose a flexible and convenient calibration method for the computation of intrinsic and mechanism parameters of such systems by using images of a simple calibration pattern. Unlike existing approaches, the proposed method considers a full geometrical model and does not make any unattainable assumption such as concentricity of images, a priori knowledges about camera poses or coincidence of the rotation axis with the optical center. Using few images taken with free poses, the calibration is performed in two steps. First, the intrinsic parameters are computed using sets of 2D/3D point correspondences that are generated automatically by exploiting the structure of the calibration rig. Then, thanks to the parameterization of the projection of a 3D straight line, the mechanical parameters are computed using points at infinity. For each step, closed form solutions and iterative refinement techniques are presented. The proposed approach is validated with both synthetic and real data collected by our prototype.

Preliminary research on focal plane calibration method in LAMOST based on flexible planar target

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.

First Interlaboratory Comparison on Calibration of Temperature-Controlled Enclosures in Turkey

The number of accredited laboratories in the field of calibration of temperature-controlled enclosures has been increasing in Turkey. One of the main criteria demonstrating the competence of a calibration laboratory is successful participation in interlaboratory comparisons. Therefore, TUBITAK UME Temperature Laboratory organized the first interlaboratory comparison on “Calibration of Temperature-Controlled Enclosures” in Turkey as a pilot laboratory between January and November, 2013. Forty accredited laboratories which provide routine calibration services to the industry in this field participated in the comparison. The standards used during the comparison was a climatic chamber for the measurements at $$-40\, {^{\circ }}\hbox {C},\,-20\, {^{\circ }}\hbox {C}, 40\, {^{\circ }}\hbox {C}$$ and $$100\, {^{\circ }}\hbox {C}$$ and an oven for the measurements at $$200\, {^{\circ }}\hbox {C}$$ . The protocol of the comparison was prepared considering guide EURAMET cg-20 and BS EN/IEC standards 600068-3-5 and 600068-3-11. During the comparison measurements, each participant had the liberty to choose the most convenient calibration points in terms of their accreditation scope among the values mentioned above and carried out on-site measurements at UME. The details and the results of this comparison are given in the paper. Determination of the statistical consistency of the results with the uncertainties given by the participants can be assessed by the method of $$E_{n}$$ value assessment for each laboratory. $$E_{n}$$ values for all measurement results based on the results of pilot and participating laboratories were calculated.

3D Deformation Measurement of Soft Material under Indentation Using Improved Diffraction-Assisted Image Correlation

In inverse finite element-based analysis, complete experimental data collection is critical for multi-parameter identification and physical modeling of all kinds of materials. In this paper, diffraction-assisted image correlation (DAIC) is improved and proposed for the deformation measurement of a soft material under indentation with no blind area. A simple and convenient image-based 3D calibration method was developed, and more accurate formulations for 3D displacement measurement based on a more rigorous imaging model were derived. Using the improved DAIC, a newly developed imaging device with indenter-fixed loading and no blind area is proposed that allows 3D displacements of the whole upper surface of a soft silica gel specimen to be retrieved. The experimental results demonstrate that the proposed method is an accurate, efficient and convenient tool with a simple structure for 3D indentation deformation measurement and illustrate its capabilities to capture deformation in indentation tests with tough testing requirements, such as in situ measurement with limited access (high integration level) and dynamic testing (capturing of synchronously stereo images).

Reflectance Intensity Assisted Automatic and Accurate Extrinsic Calibration of 3D LiDAR and Panoramic Camera Using a Printed Chessboard

This paper presents a novel method for fully automatic and convenient extrinsic calibration of a 3D LiDAR and a panoramic camera with a normally printed chessboard. The proposed method is based on the 3D corner estimation of the chessboard from the sparse point cloud generated by one frame scan of the LiDAR. To estimate the corners, we formulate a full-scale model of the chessboard and fit it to the segmented 3D points of the chessboard. The model is fitted by optimizing the cost function under constraints of correlation between the reflectance intensity of laser and the color of the chessboard’s patterns. Powell’s method is introduced for resolving the discontinuity problem in optimization. The corners of the fitted model are considered as the 3D corners of the chessboard. Once the corners of the chessboard in the 3D point cloud are estimated, the extrinsic calibration of the two sensors is converted to a 3D-2D matching problem. The corresponding 3D-2D points are used to calculate the absolute pose of the two sensors with Unified Perspective-n-Point (UPnP). Further, the calculated parameters are regarded as initial values and are refined using the Levenberg-Marquardt method. The performance of the proposed corner detection method from the 3D point cloud is evaluated using simulations. The results of experiments, conducted on a Velodyne HDL-32e LiDAR and a Ladybug3 camera under the proposed re-projection error metric, qualitatively and quantitatively demonstrate the accuracy and stability of the final extrinsic calibration parameters.

Development of Piezoelectric Actuator Calibration System

Piezoelectric actuator is used for the period of time will cause the fading of efficacy. Hence, it is an important issue about providing convenient, fast and accurate calibration mode. In the past, the values of calibration parameters are record by manual. Therefore, this method will cause the risk of personal errors. In this investigation, a self-developed automatic calibration system for piezoelectric actuator is proposed. The automatic calibration system integrated with the measurement software processing module, scheme of interferometer system, piezoelectric actuator and the voltage amplifier has been employed for the calibrating of piezoelectric actuator parameters, e.g. hysteresis and maximum stroke. Comparison experiments between the calibration parameters results of self-developed interferometer system and Renishaw RLE-10 system reveal that the maximum stroke error is 13.3% and the maximum hysteresis error is 14.6%.These results are all in the error range which is provided by the firm.

Calibration of excitation function measurement based on corona cage test results.

Corona cage approaches are crucial for research on the corona characteristics of conductors. Calibration is an indispensable task for determining excitation functions, which are used to predict corona performance of long transmission lines through extrapolation from measurements of short lines in corona cages. In this paper, the amplification factor G is calculated through a frequently adopted method, propagation analysis of high-frequency corona current along a short line. Another convenient calibration method, based on distributed parameter equivalent circuits, is established. The results for G obtained through propagation analysis and equivalent circuits are compared. To verify the rationality of calculation parameters in propagation analysis and equivalent circuits, a calibration experiment based on the excitation caused by a simulated monopulse current was performed. The results of the proposed calibration method and the calibration experiment are in good agreement.