On-site Calibration Method Research Articles

A flexible camera calibration method for pose vision measurement system of roadheader

Cameras used for visual measurement in underground roadway construction sites are difficult to calibrate. Therefore, this study proposed an on-site camera calibration method that does not require manual intervention. First, the virtual point and line features on the plane target are constructed using pole-polar constraints. Second, a homography model based on point-and-line features is established. Finally, a combined objective function based on the dynamic weight coefficient is proposed to optimise the calibration results further. Relevant simulations and experiments showed that the maximum internal parameter error obtained using the proposed method was 0.463 %, and the maximum position estimation error of the roadheader was 30 mm. In addition, the results show that this method is superior to other methods in terms of calibration accuracy and robustness, solving the difficulty of camera calibration in underground roadway construction sites and guaranteeing high-precision pose vision measurement.

On-site calibration method for an underwater stereoscopic camera system based on Scheimpflug principle

On-site calibration method for an underwater stereoscopic camera system based on Scheimpflug principle

Active on-site calibration technology of current transformer based on non-rated frequency current injection.

This paper proposes an active on-site calibration method through background current cancellation and non-rated current injection. It can measure the error of the current transformer in service from 1% to 120% rated current percentage without power supply interruption. In order to establish the error relationship between rated frequency and arbitrary frequency, a theoretical analysis of current transformer calibration at the arbitrary frequency has been developed by means of the equivalent circuit. It describes a method to determine the phase angle and ratio errors of the measuring transformers at arbitrary frequencies on the basis of the calibrated error values at rated frequency. To prove the theoretical analysis, an experimental validation was carried out. The experimental results demonstrate that this active onsite calibration is a valid tool for the evaluation of current transformer performances. The calibration results showed that, for both cases (non-rated frequency calibration and mixing frequency calibration), the difference between mean ratio error and rated frequency ratio error was lower than 0.01%, and the difference between mean phase error and rated frequency phase error was lower than 1', which meets the requirement of the 0.2 accuracy class calibration.

On-site calibration method for a binocular vision sensor based on the phase-shift algorithm.

When a binocular vision sensor (BVS) is installed in a narrow space, traditional calibration methods are limited as all targets should be placed in more than three different positions. To solve this problem, an on-site calibration method based on the phase-shift algorithm is proposed in our paper. Intrinsic parameters of these two cameras should be first calibrated offline. Series of phase-shift patterns are projected onto any one target with known three-dimensional information to determine the relationship between two cameras. The target utilized in our proposed method can be selected arbitrarily, which is suitable to achieve the on-site calibration of BVS, especially in industrial vibration environments. Experiments are conducted to validate the effectiveness and robustness of our proposed method.

On-site calibration method of the AACMM based on a high-precision CNC machine tool

An articulated arm coordinate measuring machine (AACMM) is a kind of instrument used to test workpieces in the industrial field, and its measurement uncertainty depends on the on-site environmental factors to a certain extent. If there is a problem with the AACMM during on-site testing, it needs to be re-calibrated to ensure the measurement uncertainty. In this work, a fast on-site calibration method for the AACMM is proposed using a high-precision computer numerical control (CNC) machine tool. The AACMM is fixed on the machine tool table. The calibration artifact is clamped on the machine tool spindle. This calibration artifact can move to any point in the global space of the AACMM along with the operation of the spindle. Then, a virtual calibration artifact with calibration points is achieved using the CNC machine tool. The positions of these points are calculated using the improved Hammersley sequence. The calibration method using a high-precision CNC machine tool is experimentally verified through the single-point articulation performance test of ASME B89.4.22-2004 and the length measurement test of ISO 10360-12. The calibration method proposed in this work only uses an on-site high-precision CNC machine tool, one of the most common machines in many industrial fields, as the calibration instrument for the AACMM calibration. No extra equipment is required, which is convenient and beneficial to calibrate the AACMM. In addition, the CNC machine tool can construct different kinds of virtual structures for the AACMM calibration, such as virtual rods, virtual circles, virtual balls and even non-uniform 3D artifacts with complex curved surfaces. This may give rise to many novel calibration methods and further improve the calibration accuracy.

Multiple Natural Features Fusion for On-Site Calibration of LiDAR Boresight Angle Misalignment

Boresight angle misalignment is a major error source in a mobile LiDAR system (MLS), which directly affects the overall accuracy and quality of MLS scanned point clouds data. However, the current calibration of the boresight angle misalignment mainly relies on artificial target features or a manual adjustment, and the intensive labors dramatically limit the calibration flexibility. To solve these problems, this paper develops a novel on-site calibration method for boresight angle misalignment based on multiple natural features constraints, which can automatically incorporate multiple natural features extracted from surrounding environments to generate more accurate calibration results for MLS boresight angle without used any artificial targets or specific facilities. First of all, an improved 4-points congruent sets (I-4PCS) algorithm is proposed for registering the MLS point clouds in forward and backward scanned overlapping areas and realizing smooth global registration for point clouds data. Secondly, a weight principal component analysis (WPCA) approach is presented to automatically extract the appropriate multiple natural features from the well registered point clouds and establish the appropriate features representation. Thirdly, according to the extracted multiple features, the certain geometric constrains equations for spherical, linear/cylindrical, planar features are established based on a model adjustment strategy. Lastly, the boresight angle misalignment calibration can be achieved through fitting the corresponding geometric constrains equations and minimizing the weighted through a least-squares adjustment process. The experimental results demonstrate that the proposed method can effectively on-site calibrate the boresight angle misalignment error, and the overall performance of MLS is significantly improved after the calibration based on multiple natural features constraints.

On-Site Calibration Method for Line-Structured Light Sensor-Based Railway Wheel Size Measurement System.

Line-structured light has been widely used in the field of railway measurement, owing to its high capability of anti-interference, fast scanning speed and high accuracy. Traditional calibration methods of line-structured light sensors have the disadvantages of long calibration time and complicated calibration process, which is not suitable for railway field application. In this paper, a fast calibration method based on a self-developed calibration device was proposed. Compared with traditional methods, the calibration process is simplified and the calibration time is greatly shortened. This method does not need to extract light strips; thus, the influence of ambient light on the measurement is reduced. In addition, the calibration error resulting from the misalignment was corrected by epipolar constraint, and the calibration accuracy was improved. Calibration experiments in laboratory and field tests were conducted to verify the effectiveness of this method, and the results showed that the proposed method can achieve a better calibration accuracy compared to a traditional calibration method based on Zhang’s method.

A Novel Multifeature Based On-Site Calibration Method for LiDAR-IMU System

Calibration is an essential prerequisite for the combined application of light detection and ranging (LiDAR) and inertial measurement unit (IMU). However, current LiDAR-IMU calibration usually relies on particular artificial targets or facilities and the intensive labor greatly limits the calibration flexibility. For these reasons, this article presents a novel multifeature based on-site calibration method for LiDAR-IMU system without any artificial targets or specific facilities. This new on-site calibration combines the point/sphere, line/cylinder, and plane features from LiDAR scanned data to reduce the labor intensity. The main contribution is that a new method is developed for LiDAR extrinsic parameters on-site calibration and this method could incorporate two or more calibration models to generate more accurate calibration results. First of all, the calibration of LiDAR extrinsic parameters is performed through estimating the geometric features and solving the multifeature geometric constrained optimization problem. Then, the relationships between LiDAR and IMU intrinsic calibration parameters are determined by the coordinate transformation. Lastly, the full information maximum likelihood estimation (FIMLE) method is applied to solve the optimization of the IMU intrinsic parameters calibration. A series of experiments are conducted to evaluate the proposed method. The analysis results demonstrate that the proposed on-site calibration method can improve the performance of the LiDAR-IMU.

Line Structured-Light Vision Sensor Calibration Based on Multi-Tooth Free-Moving Target and Its Application in Railway Fields

In the railway detection systems, the existing online calibration of operational outdoor structured-light vision sensors usually encounters complex lighting, camera equipped with optical filters and large fields of view (FOVs) etc. These problems seriously limit calibration precision and efficiency. A novel onsite line structured-light vision sensor calibration method independent of target reference feature points and which utilizes a multi-tooth free-moving target is proposed. The intersection points are determined as feature points by using a laser irradiated on the target, and the feature points in the target coordinate system are established on the basis of cross-ratio invariant. Then, the 3D coordinates of feature points in the camera coordinate system are calculated by combining the intrinsic parameters. Meanwhile, the method based on the uncertainty model is used to optimize the location deviation of the feature points after compensation, and the equation of the laser plane is finally obtained. High-accuracy calibration of outdoor line structured-light vision sensor can be achieved when moving freely even without the removal of the optical filter and the use of any auxiliary equipment. The measurement accuracy of our method is equivalent to that of other methods using planar target in ideal imaging conditions. In addition, our method can be applied to online wheel-size measurement system and can meet the calibration and measurement requirements in complex onsite railway environment.

An Onsite Calibration Method for MEMS-IMU in Building Mapping Fields.

Light detection and ranging (LiDAR) is one of the popular technologies to acquire critical information for building information modelling. To allow an automatic acquirement of building information, the first and most important step of LiDAR technology is to accurately determine the important gesture information that micro electromechanical (MEMS) based inertial measurement unit (IMU) sensors can provide from the moving robot. However, during the practical building mapping, serious errors may happen due to the inappropriate installation of a MEMS-IMU. Through this study, we analyzed the different systematic errors, such as biases, scale errors, and axial installation deviation, that happened during the building mapping, based on a robot equipped with MEMS-IMU. Based on this, an error calibration model was developed. The problems of the deviation between the calibrated and horizontal planes were solved by a new sampling method. For this method, the calibrated plane was rotated twice; the gravity acceleration of the six sides of the MEMS-IMU was also calibrated by the practical values, and the whole calibration process was completed after solving developed model based on the least-squares method. Finally, the building mapping was then calibrated based on the error calibration model, and also the Gmapping algorithm. It was indicated from the experiments that the proposed model is useful for the error calibration, which can increase the prediction accuracy of yaw by 1–2° based on MEMS-IMU; the mapping results are more accurate when compared to the previous methods. The research outcomes can provide a practical basis for the construction of the building information modelling model.

Improving operational efficiency of power plants through on-site calibration of flow sensors

Growing industrial and legal need for precise heat and flow measurements in district heating facilities pushed the development and application of a technology for laser based on-site calibrations of large flow sensors. This calibration method works with direct flow velocity measurements along the pipe diameter through a custom made ball valve which is mountable during plant operations.To accredit this calibration method an extensive uncertainty budget was created. The largest contribution to the uncertainty budget, the flow distribution in the relevant cross section of the pipe, was researched and quantified in an extensive project. The result of this project is a profile class model, where the resulting uncertainty of the measurement can be derived directly from the measurement itself.The on-site calibration method is already deployed and executed in over 120 cases with a great variety in resulting flow sensor accuracies in within their operational conditions.

On-Site Calibration Method of Dosimeter Based on X-Ray Source

The real-time monitoring of environmental radiation dose for nuclear fa-cilities is an important part of safety, in order to guarantee the accuracy of the monitoring results regular calibration is necessary. Around nuclear facilities there are so many environmental dosimeters installed dispers-edly, because of its huge quantity, widely distributed, and in real-time monitoring state; it will cost lots of manpower and finance if it were tak-en to calibrate on standard laboratory; what’s more it will make the en-vironment out of control. To solve the problem of the measurement ac-curacy of the stationary gamma radiation dosimeter, an on-site calibra-tion method is proposed. The radioactive source is X-ray spectrum, and the dose reference instrument which has been calibrated by the national standard laboratory is a high pressure ionization. On-site calibration is divided into two parts; firstly the energy response experiment of dosim-eter for high and low energy is done in the laboratory, and the energy response curve is obtained combining with Monte Carlo simulation; sec-ondly experiment is carried out in the field of the measuring dosimeter, and the substitution method to calibrate the dosimeter is used; finally the calibration coefficient is gotten through energy curve correction. In order to verify the accuracy of on-site calibration method, the calibrated dosimeter is test in the standard laboratory and the error is 3.4%. The re-sult shows that the on-site calibration method using X-ray is feasible, and it can improves the accuracy of the measurement results of the stationary γ-ray instrument; what’s more important is that it has great reference value for the radiation safety management and radiation environment evaluation.

Improvements to the accuracy of prototype ship models measurement method using terrestrial laser scanner

Accuracy is a crucial requirement in the shipbuilding industry, especially during the design stage. The existence of errors in 3D prototype models can cause huge discrepancies when projected to full-scale objects. The ability to provide rapid and dense 3D data has made the terrestrial laser scanners (TLSs) a viable option in shipbuilding measurement. However, the numerous sources of systematic error that corrupt collected TLS data make a calibration procedure necessary. The requirement of laboratory with large number of targets to perform existing TLS calibration procedures make them unfeasible for most TLS users. Thus, this study has investigated the suitability of an on-site calibration method for application in high-accuracy shipbuilding. Two prototype boats were measured using a Faro Focus 3D scanner and close range photogrammetry was used for benchmarking purposes. By utilising four quantitative analyses, the results verify the ability of the developed calibration method to improve the accuracy of terrestrial laser scanner measurements in the field. With a relatively simple implementation, the developed calibration method is expected to be valuable for a wide variety of terrestrial laser scanner applications, especially the shipbuilding industry.

A fast on-site calibration method designed based on hierarchical coupled discrete mathematical model

For re-calibration all parameters if the sites layout changing, a fast on-site calibration algorithm for dual linear CCD intersection measuring system is designed, which can avoid invariant parameters re-calibration when the measuring system is moved. The measurement model of this system expresses the mathematical relation between output variables of the target coordinates and input variables of sites layout parameters as well as pixel coordinates. In accordance with the stability of interior parameters of the linear CCD camera, considering the stability of some external parameters, these stable parameters are coupled to a certain degree, which can reduce the calibration parameters number, and improve the speed of the calibration process. According to the coupling parameter, hierarchical mathematical model about the input variables, the coupling parameters and output variables is established, and target coordinates resolved mathematical model with changing site layout parameters is established, which can realize the fast on-site calibration for measuring system. The experiment results show that the calibration precision of proposed method meet the practical requirement of dual linear CCD intersection measuring system.

Extrinsic parameter calibration of stereo vision sensors using spot laser projector.

The on-site calibration of stereo vision sensors plays an important role in the measurement field. Image coordinate extraction of feature points of existing targets is difficult under complex light conditions in outdoor environments, such as strong light and backlight. This paper proposes an on-site calibration method for stereo vision sensors based on a spot laser projector for solving the above-mentioned problem. The proposed method is used to mediate the laser spots on the parallel planes for the purpose of calibrating the coordinate transformation matrix between two cameras. The optimal solution of a coordinate transformation matrix is then solved by nonlinear optimization. Simulation experiments and physical experiments are conducted to validate the performance of the proposed method. Under the condition that the field of view is approximately 400 mm×300 mm, the proposed method can reach a calibration accuracy of 0.02 mm. This accuracy value is comparable to that of the method using a planar target.

On-site calibration method for outdoor binocular stereo vision sensors

Using existing calibration methods for binocular stereo vision sensors (BSVS), it is very difficult to extract target characteristic points in outdoor environments under complex light conditions. To solve the problem, an online calibration method for BSVS based a double parallel cylindrical target and a line laser projector is proposed in this paper. The intrinsic parameters of two cameras are calibrated offline. Laser strips on the double parallel cylindrical target are mediated to calibrate the configuration parameters of BSVS. The proposed method only requires images of laser strips on the target and is suitable for the calibration of BSVS in outdoor environments. The effectiveness of the proposed method is validated through physical experiments.

On-Site Metrology Calibration Method for River-Piping Flowmeter

On-site metrology calibration is of practical significance and necessity to promote the rational development and utilization of water resources and to strengthen the management of water metering facilities specification. In this paper, on-site flowmeter calibration method and principle is detailed with standard instrument comparison test, deviation control range of on-site metrology calibration measurement is proposed with the application of error theory. Standard instrument reference method is proved to meet on-site metrology calibration requirements by examples analysis. Further research orientation is also put forward.

An onsite structure parameters calibration of large FOV binocular stereovision based on small-size 2D target

An onsite structure-parameter calibration method for large field of view (FOV) binocular stereovision based on a small-size 2D target is proposed. Compared with the traditional large-size target, a small-size 2D target is much easier to be manufactured with high accuracy and can be used in onsite calibration. Compared with the 1D target, a 2D target can complete the calibration quickly with fewer calibration images. Additionally, operating and maintaining a small-size 2D target are considerably convenient and flexible. Calibration experiments show that only 5 placements of the calibration target can accomplish the accurate calibration of structure parameters and the average value of 10 measurements for a distance of 1071mm is 1071.057mm with a RMS less than 0.2mm. Moreover, the measurement accuracy of the stereovision calibrated with our method is a little more accurate than that of the one calibrated with traditional method.

Development of an on-site calibration method for a current transformer testing system and its application

The ratio error and phase displacement of a current transformer (CT) are measured by using a CT testing system, which consists of a current source, a standard CT, a CT under test, a CT comparator and a CT burden. Methods for on-site calibration of the components in CT testing systems in industry have been developed recently, which do not require any component to be detached from the systems. The method utilizes several travelling transfer standards: standard CTs, non-reactive standard resistors, a calculable wide-ratio error CT and non-reactive shunt resistors. The travelling standard CT is used for absolute evaluation of a current source and a standard CT of industry. The non-reactive standard resistors and the calculable wide-ratio error CT are used to evaluate the linearity of the errors in the CT comparator. The shunt resistors are used to evaluate the CT burden of industry. The on-site calibration method using these travelling transfer standards has been successfully applied to calibration of CT testing systems in industries.

Aerosol ultraviolet absorption experiment (2002 to 2004), part 1: ultraviolet multifilter rotating shadowband radiometer calibration and intercomparison with CIMEL sunphotometers

Radiative transfer calculations of UV irradiance from total ozone mapping spectrometer (TOMS) satellite data are frequently over- estimated compared to ground-based measurements because of the presence of undetected absorbing aerosols in the planetary boundary layer. To reduce these uncertainties, an aerosol UV absorption closure experiment has been conducted at the National Aeronautics and Space Administration/Goddard Space Flight Center (NASA/GSFC) site in Greenbelt, Maryland, using 17 months of data from a shadowband radi- ometer (UV-multifilter rotating shadowband radiometer (UV-MFRSR), U.S. Department of Agriculture (USDA) UV-B Monitoring and Research Network) colocated with a group of three sun-sky CIMEL radiometers (rotating reference instruments of the NASA Aerosol Robotic Network (AERONET)). We describe an improved UV-MFRSR on-site calibration method augmented by AERONET-CIMEL measurements of aerosol ex- tinction optical thickness (t a) interpolated or extrapolated to the UV- MFRSR wavelengths and measurement intervals. The estimated t a is used as input to a UV-MFRSR spectral-band model, along with indepen- dent column ozone and surface pressure measurements, to estimate zero air mass voltages V0 in three longer wavelength UV-MFRSR chan- nels (325, 332, 368 nm). Daily mean ^V0&, estimates and standard de- viations are obtained for cloud-free conditions and compared with the on-site UV-MFRSR Langley plot calibration method. By repeating the calibrations on clear days, relatively good stability (62% in ^V0& )i s found in summer, with larger relative changes in fall-winter seasons. The changes include systematic day-to-day ^V0& decline for extended peri- ods along with step jump changes after major precipitation periods (rain or snow) that affected the diffuser transmission. When daily ^V0& values are used to calculate t a for individual 3-min UV-MFRSR measurements on the same days, the results compare well with interpolated AERONET t a measurements (at 368 nm most daily 1s root mean square (rms) differences were within 0.01). When intercalibrated against an AERO- NET sunphotometer, the UV-MFRSR is proven reliable to retrieve t a , and hence can be used to retrieve aerosol column absorption in the UV. The advantage of the shadowband technique is that the calibration ob- tained for direct-sun voltage can then be applied to diffuse-radiance volt- age to obtain total and diffuse atmospheric transmittances. These trans- mittances, in combination with accurate t a data, provide the basis for estimating aerosol column absorption at many locations of the USDA UV-B Monitoring and Research network and for correction of satellite estimations of surface UV irradiance. © 2005 Society of Photo-Optical Instru-