Photogrammetry and laser scanner technology applied to length measurements in car testing laboratories

The important reforms developed in vehicles require geometrical measurements in car testing laboratories. They are usually performed by using coordinate measuring machines in gantry configuration or laser trackers. The uncertainty values obtained from this instrumentation is very low in comparison with the high tolerances required by the standards applied to the measurements, UNE 26-192:1987 and ISO 612:1978. This fact, together with the requirements about economical investments, lead to the research of the suitability of different metrological systems and procedures.Photogrammetric and laser scanning systems require lower economical investment so they can be useful in car testing laboratories. In this work, two of these systems are tested to verify their capability to fulfil the parameters and tolerances required for the standards UNE 26-192:1987 and ISO 612:1978. The uncertainty evaluation of these techniques is performed using a standard artefact based of five delrin spheres which is previously calibrated using a coordinate measuring machine.Geometrical parameters height, length, width, front track and wheelbase are measured from a car using the techniques under study, and MATLAB software aided in data processing. The results obtained are compared with the geometrical data previously provided by the manufacturer and the tolerance and uncertainty intervals defined by the standards. Both techniques demonstrate their metrological capacities for the performed measurements and required tolerances, although photogrammetry appears as a more advisable technique. Photogrammetry requires lower economic investment and offers higher portability, although human operator requires more expertise for target positioning and illumination controlling.     

Volumetric error modelling of a stereo vision system for error correction in photogrammetric three-dimensional coordinate metrology

Optical three-dimensional coordinate measurement using stereo vision has systematic errors that affect measurement quality. This paper presents a scheme for measuring, modelling and correcting these errors. The position and orientation of a linear stage are measured with a laser interferometer while a stereo vision system tracks target points on the moving stage. With reference to the higher accuracy laser interferometer measurement, the displacement errors of the tracked points are evaluated. Regression using a neural network is used to generate a volumetric error model from the evaluated displacement errors. The regression model is shown to outperform other interpolation methods. The volumetric error model is validated by correcting the three-dimensional coordinates of the point cloud from a photogrammetry instrument that uses the stereo vision system. The corrected points from the measurement of a calibrated spherical artefact are shown to have size and form errors of less than 50 μm and 110 μm respectively. A reduction of up to 30% in the magnitude of the probing size error is observed after error correction is applied.     

An artefact for traceable freeform measurements on coordinate measuring machines

The present paper describes an artefact based approach to obtain traceability of freeform measurements on coordinate measuring machines. First, the requirements for the traceability of freeform measurements and a strategy for the development of a feasible solution are presented. A new concept of artefact, called the “Modular Freeform Gauge” (MFG) has been developed. It is based on physical modeling of a given freeform surface by a combination of items with regular geometry, well calibrated on their dimensions and form. The relative position is accounted for during the procedure; this information is used to generate a “calibrated” CAD model as reference for freeform measurements. The architecture of the artefact, its collocation in the traceability chain, and the calibration procedure are described.Finally, a procedure for the uncertainty assessment of actual freeform measurements is presented. The work here described has been focused on implementation of the uncertainty assessment procedure for freeform measurements on turbine blades. A task-specific Modular Freeform Gauge was developed for this application.     

New method to calibrate and validate a high-resolution 3D scanner,based on photogrammetry

In the last years close-range photogrammetric scanning systems, are acquiring a larger market share. This is due to low-cost hardware, components and to new user-friendly software. The ultimate, photogrammetric 3D scanning systems are very accurate and precise because, of the high-resolution cameras (over 10 Mpixels) they are equipped of and, the more precise algorithms of their software. The calibration phase is the primary step for the development of a, precise photogrammetric scanner. Through a good calibration it is indeed, possible to eliminate optical aberration issues and to obtain precise and, accurate three-dimensional measurements. In this study a powerful, calibration method, named full-field calibration, was implemented to, obtain high-precision values, using an original three-dimensional, calibrator, developed so as to increase the performance of this type of, calibration. Prior to using any measurement or 3D scanning system, precision and, accuracy have to be assessed. In this study a robust validation method, for photogrammetric scanning systems has been proposed. The validation, procedure consisted in: (1) operator error analysis, (2) reproducibility, error-analysis, (3) control-system error analysis, (4) scanning system, error analysis., The measurements taken using the “control system” (certified equipment in, terms of precision and accuracy) were considered as “gold standard”. The, photogrammetric measurements, subsequently obtained by the scanning, system, were aligned to the “gold standard” using Procrustes, registration. The system error was expressed as the displacement between, these two sets of measurements.     

Development and testing of an error compensation algorithm for photogrammetry assisted robotic machining

Robotic machining of relatively small features on large components potentially offers an opportunity to reduce capital expenditure in various industries. A barrier to this is the inability of robotic machine tools to machine to the tolerances of conventional equipment. This paper proposes and tests a photogrammetry-based metrology assistance algorithm to compensate for robotic machining inaccuracy, as measured in the part, and investigates the associated measurement challenges. The algorithm is executed in a two stage process, whereby the closest point to nominal cutting coordinates on an aligned inspection surface is used for compensation, created a penultimate measured cut. Finally, the finishing program coordinates are compensated to correct under-cuts during the measured cut stage. Conceptual tests using simulated measurement data give confidence that the proposed approach works well. In experiments, a key area for further R&D effort is found to be uneven inspect point coverage, which results in alignment issues and a poor surface finish. Ultimately, direction is given to improve measurement system performance to enable the metrology assistance approach proposed to be implemented and therefore the benefits of “process-to-part” robotic machining to be realised.     

一种线阵声纳阵列群延时偏差标定方法

提出了一种新的线阵声纳阵列群延时偏差标定方法。从群延时偏差导致的阵列测向系统误差的角度出发，建立了群延时偏差的标定模型，利用多个频点的信息进行最小二乘拟合来精确测量群延时偏差。理论分析和实验表明，新的群延时标定方法是有效且技术可行的，标定精度可以优于1ns，高于传统基于单频的标定方法。     

Uncertainty evaluation procedure and intercomparison of bell provers as a calibration system for gas flow meters

Many NMIs (national measurement institutes) and calibration laboratories worldwide use a bell prover as a calibration system for gas flow meters. The basic definition and procedure to estimate the bell prover uncertainty have been given in previous studies. After the announcement of a mutual recognition agreement (MRA) in 1998 by the BIPM (Bureau International des Poids at Mesures), many NMIs have needed to have more details of uncertainty evaluation procedure of the bell prover. In this study, more details of the test procedure of bell prover uncertainty are presented with a modification of position of the temperature and the pressure sensor to measure more exact values. The other modification is to use three precise guide rods with bearings to make the pressure in the bell more stable. Furthermore, a laser interferometer is used to enhance the measurement accuracy of the testing time and the traveling length of the bell. The uncertainty of the bells estimated by the procedure given in this study is 0.13% at the confidence level of 95%, rather than 0.17% of the previous one. Recently, the results of CCM.FF-K6, which is an international comparison of NMIs organized by the CIPM (Comité International des Poids et Mesures), show that the uncertainty of the KRISS (Korea Research Institute of Standards and Science) bell prover estimated by this study is reasonable.     

Accurate Calibration of Mercury Vapour Indicators for Occupational Exposure Measurements using a Dynamic Mercury Vapour Generator

Abstract

An accurate and rapid calibration procedure for mercury vapour indicators used for occupational exposure measurements is described, using a dynamic mercury vapour generator traceable to the standard bell-jar calibration apparatus. The method provides greater flexibility, accuracy, and trace ability than the usual methods for the calibration of mercury vapour indicators used for occupational exposure measurements. The calibration procedure allows a correction to be applied to the mass concentration value displayed by the mercury vapour indicator. Measurement results from the mercury vapour indicator at the occupational exposure limit of 25 µg ·m^?3, following calibration, have been estimated to have an expanded uncertainty of 8%.     

Design,realization and uncertainty analysis of a laser-based primary calibration facility for solar cells at PTB

A completely newly designed multi-functional facility for the primary calibration of reference solar cells and the spectral characterization of all solar cell types has been developed and built at PTB. The new facility is based on the successfully applied Differential Spectral Responsivity (DSR) method that allows the determination of the absolute spectral responsivity and nonlinearity of solar cells with the lowest uncertainties. By using a tunable laser system, the new setup avoids the main problem of monochromator-based systems: the low optical power level of the monochromatic beam. Thus it enables a significant reduction of the uncertainty for the short circuit current under standard test conditions I_STC of solar cells. It enables the calibration of World Photovoltaic Scale (WPVS) reference solar cells with an uncertainty of 0.4% (k = 2), the lowest value stated by any WPVS laboratory.     

Traceability and uncertainty analysis in volume measurements

Laboratories must ensure that obtained results are reliable, for that it is necessary to calibrate glassware and proving tanks using the correct method and estimate of uncertainties. The aim of this work is to present the gravimetric method as a suitable and traceable method to calibrate glassware according to laboratory requirements. Moreover, the model for the calculation of the measurement uncertainty is described and it is proved to be completely suitable for the calibration by gravimetric method through a numerical example drawn from the INRIM uncertainty budget in the CCM.FF-K4 key comparison.     

Uncertainty budgeting for range calibration

The Guide to the Expression of Uncertainty in Measurement (GUM) established a general procedure to evaluate measurement uncertainty. The Guide covers only the evaluation of a single result or a set of individual results. Modern measuring instruments and procedures operate over a wide range of values. Therefore in practice a calibration procedure is needed that is valid for this range. The procedure should include an evaluation of uncertainty associated with the calibration results and for the subsequent measurements performed with the calibrated instrument. Traditionally regression analysis is used for this purpose. In this paper we will discuss some weaknesses of the regression approach and suggest an alternative. We show that for instruments with a linear response function the regression can be replaced by 2-point calibration. We introduce a limit of the deviation from linearity to address observed deviations from a linear response function of the instrument. To improve an existing instrument with a non-linear response function a combination of the instrument and a correction function can be treated as a virtual linear measuring device and a 2-point calibration can be applied. As an example we use the calibration of a pressure sensor to illustrate the procedure. The approach can be used for instruments and measurement procedures with a linear or non-linear response function.     

Systematic approach to realizing optimal moving order selection for multi-axis precise motion system

This paper presents a systematic approach for selecting the optimal moving order (MO) in a multi-axis precise motion system (MPMS). According to the proposed procedure, an optimal high-efficiency MO selection approach for high-efficiency and high-accuracy requirement is introduced. First, the characteristics of the giving MPMS are analysed, and the error model is established. The orthogonal test method is used to evaluate the influence on the MO caused by different MPMS configurations. Second, the number of possible MOs can be narrowed to limited and satisfied MO types. By calculating the deviations after each step, all directional movements can be arranged. Third, considering that both the accuracy and efficiency are important indexes, a series of systematic formulations are developed to select the optimal MO to balance accuracy and efficiency. A case for which a six-axis precise platform is adopted in an optoelectronic packaging system is implemented, and the methods of high-quality MO selection are verified by performing a series of experiments, and the methods are shown to be useful and effective. To balance the proportion of efficiency and accuracy, the formula and corresponding model are proposed to select the MO. The approach is not only beneficial to the accuracy improvement and trajectory planning of MPMS, but also helpful in terms of reducing the computational processing for the following algorithm. For the engineers using in precise industry area, the proposed approach can significantly improve the operative precision of MPMS with an optimal MO. This methodology of MO can also be the basis of references to error-related analyses on MPMS.     

Application of unscented R–T–S smoothing based on novel mathematical model in SINS/GPS integrated system post processing

The integrated system of Strapdown Inertial Navigation System (SINS) and Global Positioning System (GPS) has become widely-applied equipment in airborne remote sensing for getting the motion information. For the SINS/GPS integration used estimation theory, the mathematical model plays an important role in the optimal estimation of state. In this paper, the main factors that affect the precision of SINS/GPS integrated estimation in IMU calibration errors are considered and a novel nonlinear mathematical model is proposed. An unscented Rauch–Tung–Striebel smoother (URTSS) is implemented based on this model, and Monto Carlo simulation and flight test with a loaded SINS/GPS integrated system are conducted to verify the performance of this model. The comparison results indicate that the proposed nonlinear model can effectively improve the attitude estimate precision.     

Multiport vector network analyzer calibration using the generalized 3-term error model

A generalized 3-term error model, where only three error matrixes are defined, is proposed for the calibration of n-port vector network analyzer (VNA) with n + 1 measurement channels. In this model, the node is replaced by the complex vector, and correspondingly the branch gain is represented by the complex square matrix. According to this error model, which also obeys the fundamental rules of flow graph, the formula for actual scattering matrix of an n-port DUT can be deduced. Finally, the actual S-parameters of a four-port device can be corrected and they are compared with the measurement results by Agilent VNA E5071B. The good agreement attests the precision of the calibration algorithm.     

Reliability of parameters of associated base straight line in step height samples: Uncertainty evaluation in step height measurements using nanometrological AFM

Step height is widely used as one of the important nanometrological standards for the calibration of nanometrological instruments. In the calculation of step height, a method of determining a base straight line as a reference line is very important. In nanometrology, which is a field of dimensional metrology, an associated feature (Gaussian associated feature), such as a base straight line, is normally calculated from a measured dataset of a metrological instrument on a real feature using the least squares method. The reliability of a base straight line varies depending on the position and number of measured points for the line and the uncertainty in step height calibration also varies depending on the reliability of the base straight line. In this study, we carried the out step height measurement of micropatterned thin films (10, 7, 5, and 3 nm) using an atomic force microscope (AFM) equipped with a three-axis laser interferometer (nanometrological AFM) and evaluated the uncertainty in these measurements. From the uncertainty evaluation results, the uncertainty derived from the reliability of the parameters of the base straight line was one of the major sources of uncertainty when the measured points for the base straight line were varied. An expanded uncertainty (k = 2) of less than 0.4 nm was obtained. Furthermore, the reliable range of an associated base straight line in a single step height, such as that in an atomic step sample, was calculated and in importance of the calculation of the reliable range was shown in the uncertainty evaluation and in determining the measurement strategy.     

Uncertainty reducing method for the reference standards in gauge block comparator calibration

A simple method for reducing the uncertainty of reference standards has been proposed for gauge block comparator calibration. Errors due to the reference value of the length difference for mechanical comparator evaluation can be reduced by using the average of two reference values. Two reference values of length difference are obtained by the appropriate combination of three gauge blocks. Errors caused by reference gauges are canceled and averaged. An error reduction effect was successfully demonstrated using an actual mechanical comparator.     

Development and evaluation of the calibration facility for high-pressure hydrogen gas flow meters

The calibration facility with the multi-nozzle calibrator was developed for the calibration of flow meters to be used with high-pressure, high-flow-rate hydrogen gas. The critical nozzles installed in the multi-nozzle calibrator were calibrated with traceability to the national standard. The relative standard uncertainty of the mass flow rates produced from the calibration facility is 0.09% when the flow rate is between 150 g/min and 550 g/min. In this study, the Coriolis flow meter was calibrated for a pressure range of 15–35 MPa. The relative standard uncertainty of the flow rates obtained from the Coriolis flow meter was 0.44% for the case of the worst fluctuations in the output of the flow meter; based on the calibration curve, this is 0.91%. The present result shows that there is a maximum 3% difference between the output of the Coriolis flow meter and the mass flow rates of the multi-nozzle calibrator, even though the Coriolis flow meter was calibrated using water. Therefore, for the development of a calibration facility that can calibrate a flow meter under the same conditions as those encountered in actual use, it will be important to develop a new flow meter.     

Use of low-cost UAV photogrammetry to analyze the accuracy of a digital elevation model in a case study

For the geodetic survey of the terrain surface, the use of conventional surveying methods and instruments is common. New technologies, such as e.g. UAVs and their combination with a digital camera, bring new opportunities also in the documentation of Earth’s surface. This combination of technologies allows for the use of low-cost digital photogrammetry to document the Earth’s surface in relation to mining activities. The aim of the research presented in this paper is to analyze the accuracy of the digital elevation model (DEM) obtained using low-cost UAV photogrammetry. The surface mine Jastrabá (Slovakia) was chosen as the test area. The mine has morphologically dissected surface and is thus suitable for verifying the use of UAV photogrammetry to capture fairly intricate details on the surface. The research, which has been carried out, showed that the model created from photogrammetric data from the UAV achieves the accuracy required by current national legislation. From the selection of the test consisting of 237 points, only 3 points failed the conditions for the accuracy of the detailed points. These results indicate that the combination of low-cost UAV and a digital camera may be used as a viable alternative for the collection of data with the goal to document surface structures and to form 3D models formed from this data. The use of UAV in a photogrammetric survey of the mine brings new opportunities for the creation of documentation, because with this technology we can measure the entire surface in detail, create orthophoto maps of the entire area and document inaccessible parts of the area such as sludge dumps, and steep slopes.