For decades three-dimensional (3D) measurements of engineering components have been made using fixed metrology-room based coordinate measuring machines (CMMs) fitted most commonly with single point or to a much lesser extent, scanning tactile probes. Over the past decade there has been a rapid uptake in development and subsequent use of portable optical-based 3D coordinate measuring systems. These optical-based systems capture vast quantities of point data in a very short time, often permitting freeform surfaces to be digitized. Documented standards, for example ISO 10360, for the verification of fixed CMMs fitted with tactile probes are now widely available, whereas verification procedures and more specifically verification artefacts for optical-based systems are still in their infancy. Furthermore, the industry is seeking traceability in 3D measurements of high precision components. A recent requirement is the demand for highly accurate measurements of large gears with diameters up to 1000 mm as used in gear boxes of wind turbines. Up until now it has been impossible to ensure traceability of 3D measurements of big gears, since no traceable standards were available. This paper describes three different types of artefacts that were developed during the project, namely tetrahedron artefacts for testing the basic measurement capability of optical 3D devices, freeform verification artefacts for testing the capability of measuring complex geometry, and a large gear artefact for task related calibration of different types of CMMs. In addition, artefact calibration data and associated measurement uncertainties and international intercomparisons are presented. These developments will be of considerable value to end users, calibration laboratories and producers of optical and tactile CMMs.
In contrast to measurements of the dimensions of machined parts realized by machine tools and characterized by CMMs, software results are not fully traceable and certified. Indeed, a computer is not a perfect machine and binary encoding of real numbers leads to rounding of successive intermediate calculations that may lead to globally false results. This is the case for poor implementations and poorly conditioned algorithms. Therefore, accurate geometric modelling and implementations will be detailed. Based on the works of National Metrology Institutes, the problem of software traceability will also be discussed. Some prospects for this complex task will finally be suggested.
A tuning fork-based atomic force microscope cantilever has been investigated for application as an encoding sensor for real-time displacement measurement. The algorithm used to encode the displacement is based on the direct count of the integer pitches of a known grating, and the calculation of the fractional parts of a pitch at the beginning and during displacement. A cross-correlation technique has been adopted and applied to the real-time signal filtering process for the determination of the pitch during scanning by using a half sinusoidal waveform template. For the first investigation, a 1D sinusoidal grating with the pitch of 300 nm is used. The repeatability of displacement measurements over a distance of 70 µm is better than 2.2 nm. As the first application, the real-time displacement of a scanning stage is measured by the new encoding principle as it is moved in an open-loop mode and closed-loop mode based on its built-in capacitance sensor.
Atomic force microscope cantilevers have been investigated for their use as the encoder for real-time high-resolution displacement measurements, when paired with a 1D sinusoidal grating of well-known pitch. For a known one-directional (forward or backward) displacement measurement, the decoding algorithm is based on directly counting the integer periods of the grating and calculating the fractional parts at the beginning of the displacement and at the actual position by using one cantilever. Using two cantilevers arranged in the quadrature phase shift positions on the grating makes the measurement of two-directional (forward and backward) displacements possible. The decoding algorithm directly unwraps the phase between two encoded signals. Cross-correlation filtering and the differentiation process of two encoded signals are found to be very successful to guarantee the implementation of real-time displacement measurements by suppressing noise and reducing the offset and tilt of the encoded signals.
<p class="Abstract">To achieve a new kilogram definition using the X-ray crystal density method, the Center for Measurement Standards, Industrial Technology Research Institute in Taiwan has established the combined XRF (X-ray fluorescence)/XPS (X-ray photoelectron spectroscopy) surface analysis system for the quantitative surface-layer analysis of Si spheres. The surface layer of a Si sphere is composed primarily of an oxide layer, carbonaceous contamination and physisorbed/chemisorbed water. This newly combined instrument has been implemented to measure the XRF for the direct determination of the mass deposition of oxygen (ng/cm<sup>2</sup>) with a calibrated silicon drift detector and the XPS for the ratio between the elements (O, Si, C) and composition identification. These two complementary methods of X-ray metrology allow an accurate determination of the surface-layer mass of the Si sphere. In this paper, the construction of a combined XRF/XPS surface-analysis system is reported, including the surface characterisation method, the assembly of parts of the load-lock chamber and ultra-high-vacuum analysis chamber, the vacuum-system design, hardware integration and the intended research on surface-layer measurement. It is anticipated that the measured surface-layer mass will be combined with the core mass of the Si sphere.</p>
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