Optical In-Process Measurement Systems

A special issue of Applied Sciences (ISSN 2076-3417). This special issue belongs to the section "Optics and Lasers".

Deadline for manuscript submissions: closed (1 January 2022) | Viewed by 23515

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Special Issue Editor

Bremen Institute for Metrology, Automation and Quality Science (BIMAQ), University of Bremen, 28359 Bremen, Germany
Interests: measurement techniques; measurement uncertainty; high-speed imaging; flow field measurement; physical limits

Special Issue Information

Dear Colleagues,

Optical principles enable precise measurements down to the quantum mechanical limits and provide the fastest possible measurement speed, which is the speed of light. Driven by the ongoing advances in powerful light sources, accurate light modulation possibilities, and efficient light detectors, the capabilities of optical measurement systems are increasing. However, a current challenge is to make use of the benefits of optical principles for in-process measurements on real-world objects. Examples are flow processes on wind turbines, on airplanes, and in combustors, thermal and mechanical processes on a workpiece during manufacturing, and the exploration of natural processes on Earth and in space. Studying non-idealized, non-scaled objects during their actual operation is an important task for the measurement science, since it allows us to gain new insights from the actual process behavior to be engineered. Furthermore, in-process measurements are required to create in-process controls. In-process measurement conditions are often challenging and can mean a limited optical access, an uncooperative measurement environment, a large measurement distance, a large measurement object, or just a low signal-to-noise ratio. In order to break new ground for the transition “from the lab to the app” (from the laboratory to the application), optical in-process measurements have to be realized at the limits of measurability and beyond.

We are therefore interested in articles that investigate in-process measurements by means of optical principles (VIS, IR, UV). Potential topics include, but are not limited to, the following:

  • innovative measurement principles using visible, infrared, or ultraviolet light;
  • model-based measurement approaches and/or data fusion that enable new measurands;
  • scaling of the measurement range from the lab to the app;
  • knowledge extraction from less-reproducible in-process measurement conditions;
  • fast measurements using scanning and multiplexing techniques;
  • measurement systems with adaptive components;
  • investigation and correction of cross-sensitivities;
  • limits of measurability by studying the minimal achievable measurement uncertainty;
  • novel findings about the investigated process due to in-process measurements;
  • application case studies;
  • applicability of a single measurement principle for different in-process applications and vice versa.

Original work highlighting the latest research and technical development is encouraged, but review papers and comparative studies are also welcome.

Prof. Andreas Fischer
Guest Editor

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Keywords

  • in-process measurement
  • optical measurement principle (visible, infrared, ultraviolet)
  • real-world object
  • measurement system
  • measurement uncertainty
  • measurability

Published Papers (12 papers)

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Editorial

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2 pages, 171 KiB  
Editorial
Special Issue on Optical In-Process Measurement Systems
Appl. Sci. 2022, 12(5), 2664; https://doi.org/10.3390/app12052664 - 04 Mar 2022
Viewed by 939
Abstract
Optical principles enable precise measurements, down to the quantum mechanical limits, and provide the fastest possible measurement speed: the speed of light [...] Full article
(This article belongs to the Special Issue Optical In-Process Measurement Systems)

Research

Jump to: Editorial

11 pages, 1922 KiB  
Article
The Choice of Optical Flame Detectors for Automatic Explosion Containment Systems Based on the Results of Explosion Radiation Analysis of Methane- and Dust-Air Mixtures
Appl. Sci. 2022, 12(3), 1515; https://doi.org/10.3390/app12031515 - 30 Jan 2022
Cited by 6 | Viewed by 2374
Abstract
A review of the existing optoelectron monitoring devices revealed that the design of optoelectron detectors of the mine atmosphere does not sufficiently take into account the factor of external optical interference. This includes any extraneous source of thermal emission: a source of artificial [...] Read more.
A review of the existing optoelectron monitoring devices revealed that the design of optoelectron detectors of the mine atmosphere does not sufficiently take into account the factor of external optical interference. This includes any extraneous source of thermal emission: a source of artificial lighting or enterprises. As a consequence, the optoelectron detectors -based safety systems currently installed at mining sites are not able to ensure properly the detection of the ignition source in the presence of optical interference. Thus, it is necessary to determine the working spectral wavelength ranges from methane and coal dust explosions. The article presents the results of experimental research devoted to the methane-air mixture and coal dust explosion spectral analysis by means of the photoelectric method. The ignition of a methane-air mixture of stoichiometric concentration (9.5%) and coal dust of size characterized by the dispersion of 63–94 microns and concentration of 200 g/m3 was carried out in a 20 L spherical chamber with an initial temperature in the setup of 18–22 °C at atmospheric pressure. Then, photometry of the explosion light flux was conducted on a photoelectric unit. Operating spectral wavelength ranges from methane and coal dust explosions were determined. For the methane-air mixture, it is advisable to use the spectral regions at the maximum emission of 390 and 900 nm. The spectrum section at the maximum emission of 620 nm was sufficient for dust-air mixture. It enabled us to select the wavelength ranges for automatic explosion suppression systems’ launching references. This will exclude false triggering of the explosion suppression system from other radiation sources. The research results will help to improve the decision-making credibility of the device in its direct design. The results will be used in further research to design noise-resistant optical flame detection sensors with a high response rate. Full article
(This article belongs to the Special Issue Optical In-Process Measurement Systems)
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20 pages, 3497 KiB  
Article
Diffusional Behavior of New Insulating Gas Mixtures as Alternatives to the SF6-Use in Medium Voltage Switchgear
Appl. Sci. 2022, 12(3), 1436; https://doi.org/10.3390/app12031436 - 28 Jan 2022
Cited by 2 | Viewed by 2130
Abstract
Regarding the use of SF6 in medium voltage switchgear (MVS), a review of alternatives was encouraged by the European Parliament in Regulation No 517/2014. This is aimed at a new regulatory change, that is expected soon, which will include its prohibition, similar [...] Read more.
Regarding the use of SF6 in medium voltage switchgear (MVS), a review of alternatives was encouraged by the European Parliament in Regulation No 517/2014. This is aimed at a new regulatory change, that is expected soon, which will include its prohibition, similar to what has happened with other fluorinated greenhouse gases in other fields, like refrigeration. Therefore, there is an urgent need to study the physical and chemical properties of alternative gas mixtures to determine if they are suitable to replace SF6. In this context, this work addresses the difusional analysis of new gases. Binary and ternary mixtures made of 1,3,3,3-tetrafluoropropene (C3F4H2) and heptafluoroisopropyl trifluoromethyl ketone (C5F10O), using dry air as a carrier gas, were studied. The mixtures were analyzed using original equipment, composed of UV-Vis spectroscopy technology in a sealed gas chamber, which is similar to MVS. Consequently, an experimental equipment that monitors the concentration of a gas mixture online and a model that predicts the mixing process were designed and tested. The concentration profiles were obtained concerning both the time and position in the gas chamber, and the diffusional and convectional parameters were numerically calculated and optimized in an algorithm created in Scilab. Full article
(This article belongs to the Special Issue Optical In-Process Measurement Systems)
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24 pages, 9290 KiB  
Article
Optical In-Process Measurement: Concepts for Precise, Fast and Robust Optical Metrology for Complex Measurement Situations
Appl. Sci. 2021, 11(22), 10533; https://doi.org/10.3390/app112210533 - 09 Nov 2021
Cited by 8 | Viewed by 2295
Abstract
Optical metrology is a key element for many areas of modern production. Preferably, measurements should take place within the production line (in-process) and keep pace with production speed, even if the parts have a complex geometry or are difficult to access. The challenge [...] Read more.
Optical metrology is a key element for many areas of modern production. Preferably, measurements should take place within the production line (in-process) and keep pace with production speed, even if the parts have a complex geometry or are difficult to access. The challenge for modern optical in-process measurements is, therefore, how to simultaneously make optical metrology precise, fast, robust and capable of handling geometrical complexity. The potential of individual techniques to achieve these demands can be visualized by the tetrahedron of optical metrology. Depending on the application, techniques based on interferometry or geometrical optics may have to be preferred. The paper emphasizes complexity and robustness as prime areas of improvement. Concerning interferometric techniques, we report on fast acquisition as used in holography, tailoring of coherence properties and use of Multiple simultaneous Viewing direction holography (MultiView), self reference used in Computational Shear Interferometry (CoSI) and the simultaneous use of several light sources in Multiple Aperture Shear Interferometry (MArS) based on CoSI as these techniques have proven to be particularly effective. The use of advanced approaches based on CoSI requires a transition of the description of light from the use of the well-known wave field to the coherence function of light. Techniques based on geometric optics are generally comparatively robust against environmental disturbances, and Fringe Projection (FP) is shown to be especially useful in very demanding measurement conditions. Full article
(This article belongs to the Special Issue Optical In-Process Measurement Systems)
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14 pages, 7824 KiB  
Article
Camera-Based In-Process Quality Measurement of Hairpin Welding
Appl. Sci. 2021, 11(21), 10375; https://doi.org/10.3390/app112110375 - 04 Nov 2021
Cited by 15 | Viewed by 2209
Abstract
The technology of hairpin welding, which is frequently used in the automotive industry, entails high-quality requirements in the welding process. It can be difficult to trace the defect back to the affected weld if a non-functioning stator is detected during the final inspection. [...] Read more.
The technology of hairpin welding, which is frequently used in the automotive industry, entails high-quality requirements in the welding process. It can be difficult to trace the defect back to the affected weld if a non-functioning stator is detected during the final inspection. Often, a visual assessment of a cooled weld seam does not provide any information about its strength. However, based on the behavior during welding, especially about spattering, conclusions can be made about the quality of the weld. In addition, spatter on the component can have serious consequences. In this paper, we present in-process monitoring of laser-based hairpin welding. Using an in-process image analyzed by a neural network, we present a spatter detection method that allows conclusions to be drawn about the quality of the weld. In this way, faults caused by spattering can be detected at an early stage and the affected components sorted out. The implementation is based on a small data set and under consideration of a fast process time on hardware with limited computing power. With a network architecture that uses dilated convolutions, we obtain a large receptive field and can therefore consider feature interrelation in the image. As a result, we obtain a pixel-wise classifier, which allows us to infer the spatter areas directly on the production lines. Full article
(This article belongs to the Special Issue Optical In-Process Measurement Systems)
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14 pages, 9121 KiB  
Article
Mirau-Based CSI with Oscillating Reference Mirror for Vibration Compensation in In-Process Applications
Appl. Sci. 2021, 11(20), 9642; https://doi.org/10.3390/app11209642 - 15 Oct 2021
Cited by 9 | Viewed by 1787
Abstract
We present a Mirau-type coherence scanning interferometer (CSI) with an oscillating reference mirror and an integrated interferometric distance sensor (IDS) sharing the optical path with the CSI. The IDS works simultaneously with the CSI and measures the distance changes during the depth scanning [...] Read more.
We present a Mirau-type coherence scanning interferometer (CSI) with an oscillating reference mirror and an integrated interferometric distance sensor (IDS) sharing the optical path with the CSI. The IDS works simultaneously with the CSI and measures the distance changes during the depth scanning process with high temporal resolution. The additional information acquired by the IDS is used to correct the CSI data disturbed by unwanted distance changes due to environmental vibrations subsequent to the measurement. Due to the fixed reference mirror in commercial Mirau objectives, a Mirau attachment (MA) comprising an oscillating reference mirror is designed and built. Compared to our previous systems based on the Michelson and the Linnik interferometer, the MA represents a novel solution that completes the range of possible applications. Due to its advantages, the Mirau setup is the preferred and most frequently used interferometer type in industry. Therefore, the industrial use is ensured by this development. We investigate the functioning of the system and the capability of the vibration compensation by several measurements on various surface topographies. Full article
(This article belongs to the Special Issue Optical In-Process Measurement Systems)
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23 pages, 24413 KiB  
Article
Automated Detection of Premature Flow Transitions on Wind Turbine Blades Using Model-Based Algorithms
Appl. Sci. 2021, 11(18), 8700; https://doi.org/10.3390/app11188700 - 18 Sep 2021
Cited by 5 | Viewed by 1195
Abstract
Defects on rotor blade leading edges of wind turbines can lead to premature laminar–turbulent transitions, whereby the turbulent boundary layer flow forms turbulence wedges. The increased area of turbulent flow around the blade is of interest here, as it can have a negative [...] Read more.
Defects on rotor blade leading edges of wind turbines can lead to premature laminar–turbulent transitions, whereby the turbulent boundary layer flow forms turbulence wedges. The increased area of turbulent flow around the blade is of interest here, as it can have a negative effect on the energy production of the wind turbine. Infrared thermography is an established method to visualize the transition from laminar to turbulent flow, but the contrast-to-noise ratio (CNR) of the turbulence wedges is often too low to allow a reliable wedge detection with the existing image processing techniques. To facilitate a reliable detection, a model-based algorithm is presented that uses prior knowledge about the wedge-like shape of the premature flow transition. A verification of the algorithm with simulated thermograms and a validation with measured thermograms of a rotor blade from an operating wind turbine are performed. As a result, the proposed algorithm is able to detect turbulence wedges and to determine their area down to a CNR of 2. For turbulence wedges in a recorded thermogram on a wind turbine with CNR as low as 0.2, at least 80% of the area of the turbulence wedges is detected. Thus, the model-based algorithm is proven to be a powerful tool for the detection of turbulence wedges in thermograms of rotor blades of in-service wind turbines and for determining the resulting areas of the additional turbulent flow regions with a low measurement error. Full article
(This article belongs to the Special Issue Optical In-Process Measurement Systems)
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16 pages, 6072 KiB  
Article
In-Situ Evaluation of the Pitch of a Reflective-Type Scale Grating by Using a Mode-Locked Femtosecond Laser
Appl. Sci. 2021, 11(17), 8028; https://doi.org/10.3390/app11178028 - 30 Aug 2021
Cited by 5 | Viewed by 1831
Abstract
Major modifications are made to the setup and signal processing of the method of in-situ measurement of the pitch of a diffraction grating based on the angles of diffraction of the diffracted optical frequency comb laser emanated from the grating. In the method, [...] Read more.
Major modifications are made to the setup and signal processing of the method of in-situ measurement of the pitch of a diffraction grating based on the angles of diffraction of the diffracted optical frequency comb laser emanated from the grating. In the method, the improvement of the uncertainty of in-situ pitch measurement can be expected since every mode in the diffracted optical frequency comb laser can be utilized. Instead of employing a Fabry-Pérot etalon for the separation of the neighboring modes in the group of the diffracted laser beams, the weight-of-mass method is introduced in the method to detect the light wavelength in the Littrow configuration. An attempt is also made to reduce the influence of the non-uniform spectrum of the optical comb laser employed in the setup through normalization operation. In addition, an optical alignment technique with the employment of a retroreflector is introduced for the precise alignment of optical components in the setup. Furthermore, a mathematical model of the pitch measurement by the proposed method is established, and theoretical analysis on the uncertainty of pitch measurement is carried out based on the guide to the expression of uncertainty in measurement (GUM). Full article
(This article belongs to the Special Issue Optical In-Process Measurement Systems)
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17 pages, 2374 KiB  
Article
High Spatial and Temporal Resolution Bistatic Wind Lidar
Appl. Sci. 2021, 11(16), 7602; https://doi.org/10.3390/app11167602 - 19 Aug 2021
Cited by 4 | Viewed by 1915
Abstract
The high-resolution bistatic lidar developed at the Physikalisch-Technische Bundesanstalt (PTB) aims to overcome the limitations of conventional monostatic lidar technology, which is widely used for wind velocity measurements in wind energy and meteorology applications. Due to the large measurement volume of a combined [...] Read more.
The high-resolution bistatic lidar developed at the Physikalisch-Technische Bundesanstalt (PTB) aims to overcome the limitations of conventional monostatic lidar technology, which is widely used for wind velocity measurements in wind energy and meteorology applications. Due to the large measurement volume of a combined optical transmitter and receiver tilting in multiple directions, monostatic lidar generally has poor spatial and temporal resolution. It also exhibits large measurement uncertainty when operated in inhomogeneous flow; for instance, over complex terrain. In contrast, PTB’s bistatic lidar uses three dedicated receivers arranged around a central transmitter, resulting in an exceptionally small measurement volume. The coherent detection and modulation schemes used allow the detection of backscattered, Doppler shifted light down to the scale of single aerosols, realising the simultaneous measurement of all three wind velocity components. This paper outlines the design details and theory of operation of PTB’s bistatic lidar and provides an overview of selected comparative measurements. The results of these measurements show that the measurement uncertainty of PTB’s bistatic lidar is well within the measurement uncertainty of traditional cup anemometers while being fully independent of its site and traceable to the SI units. This allows its use as a transfer standard for the calibration of other remote sensing devices. Overall, PTB’s bistatic lidar shows great potential to improve the capability and accuracy of wind velocity measurements, such as for the investigation of highly dynamic flow processes upstream and in the wake of wind turbines. Full article
(This article belongs to the Special Issue Optical In-Process Measurement Systems)
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13 pages, 3527 KiB  
Article
Dynamic, Adaptive Inline Process Monitoring for Laser Material Processing by Means of Low Coherence Interferometry
Appl. Sci. 2021, 11(16), 7556; https://doi.org/10.3390/app11167556 - 18 Aug 2021
Cited by 2 | Viewed by 1926
Abstract
Surface laser structuring of electrical steel sheets can be used to manipulate their magnetic properties, such as energy losses and contribute to a more efficient use. This requires a technology such as low coherence interferometry, which makes it possible to be coupled directly [...] Read more.
Surface laser structuring of electrical steel sheets can be used to manipulate their magnetic properties, such as energy losses and contribute to a more efficient use. This requires a technology such as low coherence interferometry, which makes it possible to be coupled directly into the existing beam path of the process laser and enables the possibility for an 100% inspection during the process. It opens the possibility of measuring directly in the machine, without removing the workpiece, as well as during the machining process. One of the biggest challenges in integrating an LCI measurement system into an existing machine is the need to use a different wavelength than the one for which the optical components were designed. This results in an offset between the measurement and processing spot. By integrating an additional scanning system exclusively for the measuring beam and developing a compensation model for the non-linear spot offset, this can be adaptively corrected by up to 98.9% so that the ablation point can be measured. The simulation model can also be easily applied to other systems with different components and at the same time allows further options for in-line quality assurance. Full article
(This article belongs to the Special Issue Optical In-Process Measurement Systems)
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10 pages, 7996 KiB  
Article
A Real-Time Automated System for Dual-Aperture Common-Path Interferometer Phase-Shifting
Appl. Sci. 2021, 11(16), 7438; https://doi.org/10.3390/app11167438 - 13 Aug 2021
Cited by 5 | Viewed by 1636
Abstract
We report a novel fully real-time automatized optomechatronic dual-aperture common-path interferometer system for obtaining the phase difference between two interferograms by using the technique of phase-shifting interferometry. A motorized system is used to shift an additional phase transversally to the optical axis by [...] Read more.
We report a novel fully real-time automatized optomechatronic dual-aperture common-path interferometer system for obtaining the phase difference between two interferograms by using the technique of phase-shifting interferometry. A motorized system is used to shift an additional phase transversally to the optical axis by ruling translation. For each high-resolution ruling displacement step of 0.793 μm, an interferogram is recorded by a CCD camera. The phase difference between the two successive recorded interferograms is then automatically calculated by computational self-calibrated algorithms. The proposed device provides more accurate measuring than typically used manual processes. Real-time phase differences are obtained from a robust low-cost optomechatronic system. Analytical calculation of the phase is performed automatically without the requirement of additional or external tools and processes, reducing the significant rework delay. A set of 47 interferograms were captured in real time then recorded and analyzed, obtaining an average phase shifting of 2.483 rad. Analytic explanation and experimental results are presented. Full article
(This article belongs to the Special Issue Optical In-Process Measurement Systems)
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11 pages, 4395 KiB  
Article
In-Process Measurement of Three-Dimensional Deformations Based on Speckle Photography
Appl. Sci. 2021, 11(11), 4981; https://doi.org/10.3390/app11114981 - 28 May 2021
Cited by 5 | Viewed by 1513
Abstract
In the concept of the process signature, the relationship between a material load and the modification remaining in the workpiece is used to better understand and optimize manufacturing processes. The basic prerequisite for this is to be able to measure the loads occurring [...] Read more.
In the concept of the process signature, the relationship between a material load and the modification remaining in the workpiece is used to better understand and optimize manufacturing processes. The basic prerequisite for this is to be able to measure the loads occurring during the machining process in the form of mechanical deformations. Speckle photography is suitable for this in-process measurement task and is already used in a variety of ways for in-plane deformation measurements. The shortcoming of this fast and robust measurement technique based on image correlation techniques is that out-of-plane deformations in the direction of the measurement system cannot be detected and increases the measurement error of in-plane deformations. In this paper, we investigate a method that infers local out-of-plane motions of the workpiece surface from the decorrelation of speckle patterns and is thus able to reconstruct three-dimensional deformation fields. The implementation of the evaluation method enables a fast reconstruction of 3D deformation fields, so that the in-process capability remains given. First measurements in a deep rolling process show that dynamic deformations underneath the die can be captured and demonstrate the suitability of the speckle method for manufacturing process analysis. Full article
(This article belongs to the Special Issue Optical In-Process Measurement Systems)
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