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Production sequence of Si-spheres and interferometrical determination of the sphere volume

Precision Engineering

Division 5

In ICT, the usable dynamic range of the X-ray images, i.e. the useful contrast, limits the maximum material thickness and the accuracy of the surface determination. In the EMPIR project "AdvanCT", various influences on the imaging, such as parasitic source regions and light scattering in the scintillator of the detector, were examined and model-based corrections implemented. This made it possible, for example, to determine the surfaces with an accuracy of 1/10 of the voxel size even with an array of overlapping steel gauge blocks.

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In the WIPANO project CTSimU a procedure for the basic qualification of X-ray CT simulation software for dimensional measurements of macroscopic parts was developed. This basic qualification tests the correct implementation of basic physical effects and software functionalities. The outcomes of the project are currently being incorporated in a VDI/VDE guideline for the basic qualification of CT simulation software. The basic qualification represents the first step towards standardisation for the determination of measurement uncertainty in dimensional CT measurements by means of simulation.

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The published standard ISO 10360-13 describes for the first time acceptance and reverification tests for optical 3D coordinate measuring systems (3D-CMS) such as those used in the automotive industry for measurements on car bodies. German manufacturers and users of optical 3D CMS and PTB were involved in the preparation of the standard. PTB coordinated this work in Germany and contributed the results to the international standardization body ISO TC 213. The German version of the standard will be published at the beginning of 2022.

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The EMPIR project „Standards for the evaluation of the uncertainty of coordinate measurements in industry” (EUCoM) has been working on the development of new methods for determining the measurement uncertainty of tactile coordinate measurements. One method relies on specific measurement data, in order to estimate the uncertainty once the measurement is complete. Two other approaches utilise already available information for the estimation. The intention is to include these methods in the ISO 15530 series of standards once the validation has been concluded successfully.

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Crystalline standards based on the lattice parameter of the silicon crystal are a promising way to pass on the unit of length in the sub-nanometer range. To accelerate the complex sample preparation and to realize new sample geometries, a new preparation system was developed, and its process parameters were optimized.

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With the advantage of high dynamics, micro probes are promising candidates for high-speed roughness measurements. However, a very fast wear of the silicon tips is observed. A tip characteriser with rectangular structures developed at PTB was used to investigate the wear mechanisms. First results show that the main wear mechanism is due to tip breakage and not tip wear.

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PTB’s Virtual Coordinate Measuring Machine VCMM is a digital metrological twin of coordinate measuring machines that is capable of determining the task-specific measurement uncertainties of complex measurement task. It is used in productive industries as well as in accredited laboratories. PTB recently started a long-term collaboration with developers, manufacturers, and users of coordinate measuring machines to identify new fields of application in length metrology and enhance and improve the existing software.

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The EU EMPIR project TracOptic started in June 2021. The aim is to develop practical guidelines for traceable areal roughness measurements with optical 3D microscopy and for dimensional measurements with optical distance sensors. By closely linking experimental characterisation of devices and modelling of the devices, a better understanding of the influencing variables is also to be achieved together with research partners.

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For the traceability of the measurement results of large distances with low uncertainties, a field-compatible multi-wavelength interferometer was developed and manufactured at PTB. The optics were designed for distances of up to 5 km and the entire system concept was designed to achieve the best possible stability under outdoor conditions. In autumn 2021, the first measurement campaigns took place on reference baselines in Europe to verify the measurement properties of the system.

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The algorithm for compensating the group refractive index of electromagnetic waves in air, which has been officially recommended by the International Association of Geodesy (IAG) since 1999 contains a subtle sign error in a partial formula. The algorithm is essential for optical distance measurements, e.g. in total stations. Depending on the light wavelength used, this resulted in measurement errors of up to several µm/m. This error was discovered by PTB within the EMPIR GeoMetre project and a corrected version of the algorithm was published.

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As an alternative to the Monte-Carlo method, a GUM-compatible approach is developed and investigated, which simulates the uncertainty contributions of measurements in a as systematic and separate way as possible. The aim of this approach is to estimate measurement uncertainties simulatively in a significantly reduced time compared to the Monte-Carlo method and at the same time offering a higher flexibility for everyday work.

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For the determination of the volume of silicon spheres to realize the kilogram according to the 2019 redefinition, spheres have been used so far with very good optical quality and low deviation from roundness. For these spheres a negligible effect of wavefront aberration is assumed when measured by interferometry. New simulations and the availability of two 28Si-spheres with very different roundness deviation, but made from the same crystal, now fulfill for the first time the conditions to confirm experimentally the predicted effect of wavefront aberration.

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In order to calibrate the two orthogonal measuring axes of autocollimators simultaneously, including the characterisation of their crosstalk, PTB and VTT MIKES developed calibration systems which are based on different measuring principles and realise the task of metrological traceability differently. The comparison of calibrations of an autocollimator as a transfer standard enabled us to identify systematic measurement errors of the two systems and to validate their uncertainty budgets. The results confirm the corresponding CMC entries of PTB and VTT MIKES in the BIPM database.

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The Fizeau sphere interferometers available at PTB were built and optimised for the measurement of large spheres with a diameter of about 94 mm. For the measurement of smaller spheres, a concept was proposed which is based on the extension of the existing DEI that was originally developed for length measurements on prismatic bodies. The main advantage here is the possibility of using several light sources with significantly different wavelengths.

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During fast topography measurements, beam vibrations of the employed piezo-resistive microprobes can deteriorate the achievable measurement uncertainty considerably.
In-situ characterization of the dynamic microprobe response, acquired using high acquisition bandwidth and followed by spectral analysis reveals the dominant frequencies of these vibrations to be adjacent to the eigen-frequencies of the microprobe.
These investigations should help to determine promising strategies for active and passive damping of microprobes, improving their performance during precision high-speed surface scans.

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High measurement quality of capacitive displacement sensors can only be achieved if the electrodes of the sensor are aligned parallel to each other. A new non-contact tilting method was developed with which the inner surfaces of the electrodes of capacitive sensors can be automatically adjusted without the need for additional, cost-intensive measuring techniques.

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State of the art three-dimensional atomic force microscopes (3D-AFM) cannot selectively detect 3D displacements of the probes tip. The novel true 3D-AFM head detects these with a combination of interferometer and optical lever. It is tested for measurement tasks in 3D-Nanometrology using an optimized and structured cantilever-based probe, the 3D-Nanoprobe, with promising results.

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The uncertainty contribution of the alignment of the double-ended interferometer for length measurements of prismatic bodies without wringing was investigated and quantitatively estimated with the aid of virtual experiments. Previously unknown effects that do not occur in commonly used single-ended interferometers were identified and mathematical descriptions were validated by means of virtual experiments. Furthermore, the contribution has been reduced by revising the adjustment procedure.

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Within the framework of a TransMeT-project that has been accomplished in 2021, the range of PTB's TraCIM-service for the validation of dimensional evaluation methods has been expanded. So far, software test services for basic geometries according to Gauss and Chebyshev fit methods, the 3-Rosette method and algorithms for the evaluation of intercomparisons were covered. Since October 2021, a new expert is available. Users and developers of holistic 3D-algorithms for the evaluation of cylindrical and tapered screw thread gauges have the opportunity to certificate their algorithms by means of the TraCIM-service.

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The measurement of camshafts has not yet been standardised. In order to determine the state of the art in the measurement of camshafts and to create a basis for possible further pre-normative work, PTB has carried out an intercomparison with interested parties from the circle of measuring instrument and camshaft manufacturers. The results show that the field is largely mastered, although there is also potential for improvement.

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PTB’s young scientist, Mutaib Zackaria, member of the research staff of the working group of hardness metrology, has won the IХ International Competition "The Best Young Metrologist of COOMET” in the field of applied metrology. His work on an automated approach to perform Rockwell hardness test convinced the distinguished scientific committee of the competition which included the representatives of international (BIPM, BIML) and regional (COOMET, AFRIMETS, EURAMET, SIM) metrology organizations.

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In order to achieve the smallest possible measurement uncertainty in the traceability of gear measurement quantities, the gears on coordinate measuring machines were previously calibrated in single-point probing mode. In recent years, numerous test measurements have been carried out in scanning mode on gears of different dimensions and gear parameters. An optimized measurement strategy has made it possible to achieve the same measurement uncertainties as in single-point probing mode, but with a time gain of more than a factor of 10.

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During manufacturing of precision silicon spheres, their geometry must be controlled every day. A novel setup has been designed with a compact measurement loop, optical sensors and a handling strategy which avoids collision and contact with the very valuable, superpolished spheres. The design reduces the measurement time at each equator with sub-nanometre resolution to three minutes. 

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Anni Röse from PTB was featured as DPG Physicist of the week in April, 2021. She studied physics in Göttingen, Germany and received her M.S. degree from the Georg-August University in 2018. Since 2019 she has been pursuing a Ph.D. degree in mechanical engineering at the Ilmenau University of Technology and is a part of the working group 5.42 “Multiwavelength interferometry for geodetic lengths” at PTB.

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