Calibration Setup Research Articles

Efficient vision ray calibration of multi-camera systems.

Vision ray calibration provides imaging properties of cameras for application in optical metrology by identifying an independent vision ray for each sensor pixel. Due to this generic description of imaging properties, setups of multiple cameras can be considered as one imaging device. This enables holistic calibration of such setups with the same algorithm that is used for the calibration of a single camera. Obtaining reference points for the calculation of independent vision rays requires knowledge of the parameters of the calibration setup. This is achieved by numerical optimization which comes with high computational effort due to the large amount of calibration data. Using the collinearity of reference points corresponding to individual sensor pixels as the measure of accuracy of system parameters, we derived a cost function that does not require explicit calculation of vision rays. We analytically derived formulae for gradient and Hessian matrix of this cost function to improve computational efficiency of vision ray calibration. Fringe projection measurements using a holistically vision ray calibrated system of two cameras demonstrate the effectiveness of our approach. To the best of our knowledge, neither any explicit description of vision ray calibration calculations nor the application of vision ray calibration in holistic camera system calibration can be found in literature.

Gas Expansion Process in the Dynamic Vacuum Calibration

By analyzing the gas expansion process from the upstream chamber via an orifice and a very rapid-opening ultra-high vacuum (UHV) gate valve to downstream one, the standard pressure analytical model based on the dynamic vacuum calibration apparatus in millisecond range developed by Lanzhou Institute of Physics (LIP) is deduced theoretically and corrected by real gas characteristics and temperature changes. According to the Knudsen criterion, there is no free molecular flow regime in the area in front of the orifice during the gas expansion, so the chocked flow approximation is adopted to reduce the difficulty of numerical computation. Under this approximation and the full opening of the rapid valve, the standard pressure expression is calculated theoretically, and the upstream chamber pressure and temperature changes are obtained by numerical simulation during the gas expansion from 100 kPa to 10 kPa. Also, experiments are performed using capacitance diaphragm gauges (model CDG045Dhs). The uncertainties between the measured pressure and the simulated and theoretical ones are 10% and 4.65%, respectively, which indicates that the apparatus can generate predictable pressure changes in the millisecond range and the conceptual model can better approximate the calibration results. Finally, the orifice conductance, and the correction factors of real gas characteristics and temperature change to the standard pressure are calculated according to the simulation results; the corrected standard pressure expression is obtained.

Design of helix based calibration setup for low-[formula omitted] beam position monitors

Responses of beam position monitors (BPMs) in low-energy beamlines are sensitive to beam velocity and bunching frequency due to non-relativistic effects. However, traditional BPM calibration methods based on the coaxial wire or wire antenna can only simulate electromagnetic fields of high-energy beams with β close to 1. A helix with an inner conductor was adopted to simulate low-β beams. The dispersion relation and characteristic impedance of the helix are analyzed based on the sheath-helix model. The phase velocity and characteristic impedance of the helix have a low dependency on the frequency and helix position inside a BPM, which is crucial for calibrating low-β BPMs. A helix with a phase velocity of 0.12c and characteristic impedance of 50 Ω is designed to calibrate the button BPMs at the Xi’an Proton Application Facility. Responses of the BPMs to the helix and a beam with β = 0.12 are simulated with CST Studio Suite. The maximum relative calibration error of the helix compared to the beam is 0.47 mm in the transverse positions within three-quarters of the BPM aperture, which shows the remarkable capability of the helix to calibrate low-β BPMs offline.

Comparability of calibration strategies for measuring mercury concentrations in gas emission sources and the atmosphere

Abstract. A primary mercury gas standard was developed at Van Swinden Laboratory (VSL) to establish an International System of Units (SI)-traceable reference point for mercury concentrations at emission and background levels in the atmosphere. The majority of mercury concentration measurements are currently made traceable to the empirically determined vapour pressure of mercury. The primary mercury gas standard can be used for the accurate and precise calibration of analytical systems used for measuring mercury concentrations in air. It has been especially developed to support measurements related to ambient air monitoring (1–2 ng m−3), indoor and workplace-related mercury concentration levels according to health standards (from 50 ng m−3 upwards) as well as stationary source emissions (from 1 µg m−3 upwards). The primary mercury gas standard is based on diffusion according to ISO 6154-8. Calibration gas mixtures are obtained by combining calibrated mass flows of nitrogen and air through a generator holding diffusion cells containing elemental mercury. In this paper, we present the results of comparisons between the primary gas standard and mercury calibration methods maintained by NPL (National Physical Laboratory in the United Kingdom), a National Metrology Institute (NMI), and the Jozef Stefan Institute (JSI), a Designated Institute (DI). The calibration methods currently used at NPL and JSI are based on the bell-jar calibration apparatus in combination with the Dumarey equation or a NIST (National Institute of Standards and Technology in the United States) reference material. For the comparisons, mercury was sampled on sorbent traps to obtain transfer standards with levels between 2 and 1000 ng with an expanded uncertainty not exceeding 3 % (k=2). The comparisons performed show that the results for the primary gas standard and the NIST reference material are comparable, whereas a difference of −8 % exists between results traceable to the primary gas standard and the Dumarey equation.

Artificial neural networks for positioning of gamma interactions in monolithic PET detectors

To detect gamma rays with good spatial, timing and energy resolution while maintaining high sensitivity we need accurate and efficient algorithms to estimate the first gamma interaction position from the measured light distribution. Furthermore, monolithic detectors are investigated as an alternative to pixelated detectors due to increased sensitivity, resolution and intrinsic DOI encoding. Monolithic detectors, however, are challenging because of complicated calibration setups and edge effects. In this work, we evaluate the use of neural networks to estimate the 3D first (Compton or photoelectric) interaction position. Using optical simulation data of a 50 × 50 × 16 mm3 LYSO crystal, performance is evaluated as a function of network complexity (two to five hidden layers with 64 to 1024 neurons) and amount of training data (1000−8000 training events per calibration position). We identify and address the potential pitfall of overfitting on the training grid through evaluation on intermediate positions that are not in the training set. Additionally, the performance of neural networks is directly compared with nearest neighbour positioning. Optimal performance was achieved with a network containing three hidden layers of 256 neurons trained on 1000 events/position. For more complex networks, the performance degrades at intermediate positions and overfitting starts to occur. A median 3D positioning error of 0.77 mm and a 2D FWHM of 0.46 mm is obtained. This is a 17% improvement in terms of FWHM compared to the nearest neighbour algorithm. Evaluation only on events that are not Compton scattered results in a 3D positioning error of 0.40 mm and 2D FWHM of 0.42 mm. This reveals that Compton scatter results in a considerable increase of 93% in positioning error. This study demonstrates that very good spatial resolutions can be achieved with neural networks, superior to nearest neighbour positioning. However, potential overfitting on the training grid should be carefully evaluated.

On the contribution of remote sensing-based calibration to model hydrological and hydraulic processes in tropical regions

The accuracy of hydrological model predictions is limited by uncertainties in model structure and parameterization, and observations used for calibration, validation and model forcing. While calibration is usually performed with discharge estimates, the internal model processes might be misrepresented, and the model might be getting the “right results for the wrong reasons”, thus compromising model reliability. An alternative is to calibrate model parameters with remote sensing (RS) observations of the water cycle. Previous studies highlighted the potential of RS-based calibration to improve discharge estimates, focusing less on other variables of the water cycle. In this study, we analyzed in detail the contribution of five RS-based variables (water level (h), flood extent (A), terrestrial water storage (TWS), evapotranspiration (ET) and soil moisture (W)) to calibrate a coupled hydrologic-hydrodynamic model for a large Amazon sub-basin with extensive floodplains. Single-variable calibration experiments with all variables were able to improve discharge KGE from around 6.1% to 52.9% when compared to a priori parameter sets. Water cycle representation was improved with multi-variable calibration: KGE for all variables were improved in the evaluation period. By analyzing different calibration setups, a consistent selection of complementary variables for model calibration resulted in a better performance than incorporating all RS variables into the calibration. By looking at multiple RS observations of the water cycle, inconsistencies in model structure and parameterization were found, which would remain unknown if only discharge observations were considered.

Characterization of MEMS Acoustic Sensors and Amplifiers in Cryogenic Fluids for Quench Detection Applications in HTS CICC

An acoustic quench detection method utilizing MEMS (Micro Electro-Mechanical System) acoustic sensors is proposed. To investigate this method, a commercially available MEMS piezoelectric microphone, the Vesper VM1000, and two types of second stage amplifiers, using either an OPA344 or a LMH6629 based amplifier circuit, were characterized at cryogenic temperatures in helium gas. The MEMS microphones were in their original package with an integrated preamplifier. The tests were performed inside a two-stage Gifford-McMahon cryocooler from room temperature down to 60 K, at static pressures between 1.2 and 1.4 bar in gaseous helium, over the frequency band from 100 Hz to 10 kHz. Second stage amplifiers were needed to achieve signal to noise ratios approaching the manufacturer specified operating levels. The OPA344 based amplifier reduced in gain by >55 dB below 230 K, while the LMH6629 based amplifier performed well down to 60 K. The MEMS microphones appear to perform acoustic measurements down to 165 K but with some reduction in sensitivity down to 60 K. An acoustic model of the cryocooler plane wave tube calibration setup is developed and used to calibrate the microphone despite the presence of a significant thermal gradients down the plane wave tube.

Absolute calibration of the Lyman-α measurement apparatus at DIII-D.

The LLAMA (Lyman-Alpha Measurement Apparatus) diagnostic was recently installed on the DIII-D tokamak [Rosenthal et al., Rev. Sci. Instrum. (submitted) (2020)]. LLAMA is a pinhole camera system with a narrow band Bragg mirror, a bandpass interference filter, and an absolute extreme ultraviolet photodiode detector array, which measures the Ly-α brightness in the toroidal direction on the inboard, high field side (HFS) and outboard, low field side (LFS). This contribution presents a setup and a procedure for an absolute calibration near the Ly-α line at 121.6nm. The LLAMA in-vacuum components are designed as a compact, transferable setup that can be mounted in an ex situ vacuum enclosure that is equipped with an absolutely calibrated Ly-α source. The spectral purity and stability of the Ly-α source are characterized using a vacuum ultraviolet spectrometer, while the Ly-α source brightness is measured by a NIST-calibrated photodiode. The non-uniform nature of the Ly-α source emission was overcome by performing a calibration procedure that scans the Ly-α source position and employs a numerical optimization to determine the emission pattern. Nominal and measured calibration factors are determined and compared, showing agreement within their uncertainties. A first conversion of the measured signal obtained from DIII-D indicates that the Ly-α brightness on the HFS and LFS is on the order of 1020 Ph sr-1m-2s-1. The established calibration setup and procedure will be regularly used to re-calibrate the LLAMA during DIII-D vents to monitor possible degradation of optical components and detectors.

A new calibration setup for lock-in amplifiers in the low frequency range and its validation in a bilateral comparison

This paper addresses the calibration of lock-in amplifiers in the low frequency range. A simple but effective calibration setup implemented at INRIM is described. This includes a stable low-voltage source, composed of a digital sine wave generator and a cascade of two voltage dividers, one inductive and one resistive. The magnitude error is the lock-in magnitude error, which can be determined with an uncertainty of the order of 10−4 or better, that is, two orders of magnitude lower than the typical lock-in accuracy specification. The system can operate with an input voltage from 0.1 µV to 10 µV and a frequency from a few hertz to about 1 kHz. The performances of the calibration setup were evaluated at 1 µV and 10 μV at 20 Hz and 103 Hz and compared with those of another existing setup, implemented at METAS, through a direct comparison in the framework of an EURAMET research project.

Calibration of an LDR-Lux meter using a smartphone

In this study, it is aimed to calibrate an LDR-Lux meter with a calibration setup which includes a light bulb whose brightness can be changed by using a dimmer circuit, a data acquisition card, a smartphone and a lux meter application. In order to make the measurements more accurate and to prevent errors in the measurements, a smartphone and a laptop were connected to each other via internet browser and the data obtained with physical phone experiments was displayed on the computer screen instantly. In the experimental setup, the brightness of the light was changed with the dimmer circuit and height adjustment. The data were obtained as a result of 32 experiments for different illumination values. As a result of this study, it is seen that the calibrated experimental setup works with high accuracy. With this experimental setup, it is thought that students can also easily test the inverse square law of light. The main factors that make this experimental setup effective are that, unlike other studies in the literature, a professional lux meter is not used and the cost of the developed LDR-Lux meter is very low.

Calibration methodology for contact heat flux sensors with enhanced accuracy

This paper presents a method for the calibration of contact heat flow sensors and the measures taken to improve the overall standard uncertainty of the measurements. This may be particularly relevant for applications such as the thermal characterization of Li-ion battery cells, where small amounts of transferred heat must be measured. The designed calibration set-up is based on the measurement of temperature gradients within a reference object, which is a common and well known approach. However, certain aspects must be considered in order to obtain accurate results. In this paper all steps taken to improve the overall measurement uncertainty are described. One of the most important characteristics of the presented methodology is the experimental determination of an equivalent thermal conductivity of the used reference object. To do so, a single-sided guarded hot plate apparatus is integrated into the calibration set-up to generate a precisely known heat flow. This approach is comparable to the determination of the ‘energy equivalent’ in calorimeters. The measured equivalent thermal conductivity agrees well to literature values, but the achieved standard uncertainty is significantly lower. By way of example, the calibration method is applied on a thermoelectric generator, where the temperature dependent sensitivity and the Seebeck coefficient are determined and compared to literature values. The average standard uncertainty of the sensor sensitivity is ±1.86%, which is lower than most results published in the literature. At 20 °C the absolute value of the determined Seebeck coefficient of the examined thermoelectric element is only 2.7% higher than the value expected for the used material ().

Measurement Setup and Procedure for Precise Step Gauge Calibration

The rapid increase in precision of tactile coordinate measurement machines (CMMs) in recent years has caused an increased demand for high precision calibration artifacts, by which the traceability of coordinate measurements can be assured. This article presents research done in the development and verification of a high precision calibration system for step gauges, which are among the most frequently used CMM calibration artifacts. The main goals for the calibration system are measurement uncertainty of ${U}= ({{{(0.1}\,\,{{\mu m)}}}^{{2}}+{{(0.5\cdot }{{10}}^{{-6}} {\cdot L)}}^{{2}}})^{1/2} $ and full automatization of the measurement process, with human intervention only needed during the initial setup of the step gauge onto the system. Final verification has confirmed that both goals were achieved. The automation allows for autonomous measurement of the step gauge after the operator has performed the step gauge setup and entered nominal values of the measurement points, as well as the number of repetitions.

Verification of a Capacitive Voltage Divider With 6-μrad Uncertainty Up To 100 kV

A reference setup for system calibration of industrial transformer loss measurement (TLM) systems consists of three main components, a voltage divider, a current transformer, and a power meter, and their phase displacements should each not be more than 10 $\mathrm {\mu }$ rad to achieve an overall system uncertainty of better than 20 $\mathrm {\mu }\text{W}$ /VA. We have extensively verified the uncertainty level of the current-comparator-based capacitive voltage divider (CVD) used in the TLM reference setup of the Van Swinden Laboratorium (VSL), both from the component level and the system as a whole. Different practical conditions relevant for on-site measurements are considered, e.g., measurement cable lengths, cable types, and grounding. The verification measurement results show an agreement of better than $(6 \pm 6) \times 10^{-6}$ in ratio error and $(4 \pm 6)~\mathrm {\mu }$ rad in phase displacement between the CVD component and system calibrations up to 100 kV. Requirements for achieving this agreement are adequate grounding of the CVD and the use of triax cable between the high-voltage (HV) capacitor and the CVD low-voltage electronics in case large distances have to be covered on-site. The $(4 \pm 6)~\mathrm {\mu }$ rad agreement in phase displacement is well within the required 10 $\mathrm {\mu }$ rad limit for voltage measurements as part of on-site TLM system calibrations with 20 $\mathrm {\mu }\text{W}$ /VA overall uncertainty at low power factors.

Industrial robot performance analysis using low cost set- up

Industrial robot calibration is technique used to increase industrial robot repeatability and accuracy. In our project, we have analyzed various calibration techniques of articulated industrial robots and proposed an original method to analyze Industrial robot performance analysis. The proposed calibration set-up technique is using a mechanical setup of Linear Variable differential Transformer (LVDT) probe and inductive proximity sensor. The Statistical process control (SPC) software is using to monitor the real time data of the process and extracting the data for statistical analysis. The proposed Industrial robot calibration technique is applying at ABB Six Degrees of Freedom industrial robot and analyzing the performance gets affected by various Robot parameter and Internal/External factors. The calibration method is proposed to evaluate the repeatability of an articulated industrial robot using component of a Statistical Analysis course.

Analysis of the measurement uncertainty of a new measurement flexure calibration set-up

A measurement flexure calibration set-up is presented in this paper. The measurement flexures under test are part of a new 5 MN · m standard torque machine measurement system at the PTB. The calibration set-up can create transversal forces up to 200 N and bending moments up to 100 N · m and respectively up to 150 N · m torque moments simultaneously. The measurement uncertainty budget of the set-up is investigated in a theoretical analysis.

Traceable calibration of pressure based water level data loggers

This paper investigates accuracy, influence of temperature variations and time drift of water level data loggers, operating on a principle of hydrostatic pressure measurement. The laboratory setup for calibration of such devices, equipped with self-designed pressure chamber and commercial climatic chamber is presented. Calibration procedure for logger’s built-in pressure transducer, with ensured traceability to the primary standards of pressure and temperature, is proposed. Influence of temperature on the pressure reading was examined by conducting calibration at 10 °C, 15 °C and 25 °C.

A Comparison of Different Calibration Techniques for Hydrophones Used in Medical Ultrasonic Field Measurement.

The acoustic output characterization of medical ultrasonic equipment requires regular calibration of the hydrophones used to ensure the reliability of measurements. Such hydrophone calibration is offered as a service by several institutions. Various calibration techniques using a variety of ultrasonic excitation pressure waveforms comprising different pressure amplitude ranges and frequency compositions as well as different reference measurement systems have been proposed and applied over the past decades. Currently, four different setups for hydrophone calibration are available at the Physikalisch-Technische Bundesanstalt (PTB). This internal comparison study addresses the consistency of all four methods, including direct primary calibration and substitution calibration using reference hydrophones. The methods apply single-frequency tonebursts and swept tonebursts in the kPa amplitude range of quasi-linear acoustics as well as impulse excitation including nonlinear propagation. In recent years, a new primary calibration setup using a high-frequency vibrometer has been implemented at PTB, enabling the characterization of hydrophone frequency responses in modulus and phase and extending the upper frequency limit to up to 100 MHz. For the comparison in the frequency range from 0.5 MHz to 60 MHz, two passive membrane hydrophones with well-known characteristics gained from many years of measurements were used. Another membrane hydrophone with a nominal diameter of 0.2 mm and an integrated preamplifier was applied to address the frequency range up to 100 MHz. The results obtained with the different setups showed good agreement with average root-mean-square (rms) deviations of 3% (primary calibrations, 1-60 MHz) and 4% (1-100 MHz). The consistency of the implementations was thus verified in this comparison.

Validation of Photon Collimation Techniques for Monolithic PET Detector Calibration

Monolithic-based positron emission tomography (PET) detectors have become an attractive alternative to traditional pixelated scintillator modules. One of the main limiting factors to further expand the use of the monolithic approach in commercial PET scanners is the requirement of more complex calibration setups than for their pixelated counterparts. These calibration procedures are usually based on hardware-collimation methods in which a physically collimated beam source is moved across the front face of the scintillator. These procedures require the use of manifold hardware setups and are time consuming, especially when dealing with systems composed of many detector modules. To reduce the mechanical complexity and long acquisition times required to obtain calibration datasets when using monolithic-based detectors, we have validated in this work the recently proposed method of using software-collimated calibration data for monolithic PET detectors. software-collimated methods require less hardware, are easier to implement, and have the potential to be used in an assembled PET scanner. We are evaluating a defined bias estimator and the overall detector spatial resolution using data obtained with the two different collimation techniques. Results showed bias behavior and spatial resolution discrepancies as small as 4.9% between the two datasets for a monolithic crystal with dimensions of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$50 \times 50 \times 8$ </tex-math></inline-formula> mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> . These results demonstrate that software-collimated data in combination with accurate positioning algorithms promises to be competitive against traditional hardware-collimation procedures. Finally, we have explored and confirmed the use of software-collimated data for the calibration step using two procedures, the PC3 method (calibrated data is shown) and Voronoi diagrams.

Effect of Contacting Surface on the Performance of Thin-Film Force and Pressure Sensors

Flexible force and pressure sensors are important for assessing the wear comfort of tightly fitting apparel. Their accuracy and repeatability depend on the sensor itself and the contacting surface. Measurements of the contact pressure on soft surfaces like human skin tend to be erroneous, which could be due to incorrect sensor calibrations. This study aims to examine the effects of human body parameters such as the hardness and temperature of the contacting surface by using a custom-made calibration setup and investigating the incorporation of rigid discs on the sensor surface. Two commercial force sensors, FlexiForce and SingleTact, and one pressure sensor, Pliance X, are used in the investigation. The findings reveal that adding rigid discs on both sides of the force sensors improves their sensitivity. Systematic calibration has been performed on the surfaces with different temperatures and hardness. The results show that FlexiForce and Pliance X tend to be affected by the changes in surface temperature and surface hardness. Prolonged testing time shows that the time dependence of SingleTact and Pliance X sensor is lower, which suggests that they are more suitable for lengthier evaluations in which interface pressure is exerted on the human body. In brief, sensor attachment and proper calibration should be thoroughly considered before using sensors for applications on soft surfaces, like the human body.

Machine learning-based method for linearization and error compensation of a novel absolute rotary encoder

The main objective of this work is to develop a miniaturized, high accuracy, single-turn absolute, rotary encoder called ASTRAS360. Its measurement principle is based on capturing an image that uniquely identifies the rotation angle. To evaluate this angle, the image first has to be classified into its sector based on its color, and only then can the angle be regressed. Inspired by machine learning, we built a calibration setup, able to generate labeled training data automatically. We used these training data to test, characterize, and compare several machine learning algorithms for the classification and the regression. In an additional experiment, we also characterized the tolerance of our rotary encoder to eccentric mounting. Our findings demonstrate that various algorithms can perform these tasks with high accuracy and reliability; furthermore, providing extra-inputs (e.g. rotation direction) allows the machine learning algorithms to compensate for the mechanical imperfections of the rotary encoder.