Wenjuan Sun

Laser tracker error determination using a network measurement

We report on a fast, easily implemented method to determine all the geometrical alignment errors of a laser tracker, to high precision. The technique requires no specialist equipment and can be performed in less than an hour. The technique is based on the determination of parameters of a geometric model of the laser tracker, using measurements of a set of fixed target locations, from multiple locations of the tracker. After fitting of the model parameters to the observed data, the model can be used to perform error correction of the raw laser tracker data or to derive correction parameters in the format of the tracker manufacturers internal error map. In addition to determination of the model parameters, the method also determines the uncertainties and correlations associated with the parameters. We have tested the technique on a commercial laser tracker in the following way. We disabled the trackers internal error compensation, and used a five-position, fifteen-target network to estimate all the geometric errors of the instrument. Using the error map generated from this network test, the tracker was able to pass a full performance validation test, conducted according to a recognized specification standard (ASME B89.4.19-2006). We conclude that the error correction determined from the network test is as effective as the manufacturers own error correction methodologies.

Advances in engineering nanometrology at the National Physical Laboratory

The National Physical Laboratory, UK, has been active in the field of engineering nanometrology for a number of years. A summary of progress over the last five years is presented in this paper and the following research projects discussed in detail. (1) Development of an infrastructure for the calibration of instruments for measuring areal surface topography, along with the development of areal software measurement standards. This work comprises the use of the optical transfer function and a technique for the simultaneous measurement of topography and the phase change on reflection, allowing composite materials to be measured. (2) Development of a vibrating micro-CMM probe with isotropic probing reaction and the ability to operate in a non-contact mode. (3) A review of x-ray computed tomography and its use in dimensional metrology. (4) The further development of a metrology infrastructure for atomic force microscopy and the development of an instrument for the measurement of the effect of the probe?surface interaction. (5) Traceable measurement of displacement using optical and x-ray interferometry to picometre accuracy. (6) Development of an infrastructure for low-force metrology, including the development of appropriate transfer artefacts.

A reference sample for investigating the stability of the imaging system of x-ray computed tomography

The use of x-ray computed tomography for dimensional measurements associated with engineering applications has flourished in recent years. However, error sources associated with the technology are not well understood. In this paper, a novel two-sphere reference sample has been developed and used to investigate the stability of the imaging system that consists of an x-ray tube and a detector. In contrast with other research work reported, this work considered relative positional variation along the x-, y- and z-axes. This sample is a significant improvement over the one sphere sample proposed previously, which can only be used to observe the stability of the imaging system along x- and y-axes. Temperature variations of different parts of the system have been monitored and the relationship between temperature variations and x-ray image stability has been studied. Other effects that may also influence the stability of the imaging system have been discussed. The proposed reference sample and testing method are transferable to other types of x-ray computed tomography systems, for example, systems with transmission targets and systems with sub-micrometre focal spots.

Advanced optical techniques for the measurement of the internal geometry of MEMS structures

Many modern MEMS devices incorporate multi-layered structures. However, the metrology of such structures is struggling to keep pace with their manufacture. In this paper the measurement of the internal geometry of MEMS devices using optical coherence tomography and infra-red confocal microscopy is discussed. Both measurement techniques provide non-contact, non-invasive measurements of internal geometry. However, both techniques use relatively new technologies for measuring internal geometry and the understanding of their capabilities is limited. The study reported in this paper has mainly focused on the thickness measurement of layers. The performance of both instruments has been investigated by measuring a 50 μm thick plate artefact that had been calibrated using a traceable stylus instrument. The standard uncertainties associated with the measurements of the artefact are 0.391 μm for optical coherence tomography and 1.085 μm for infra-red confocal microscopy. Finally, the capabilities of the two instruments have been highlighted by measuring a pressure sensor containing multi-layered structures. These measurements demonstrated that both instruments have the ability to measure the thickness of layers and to image internal geometrical structures.