QMT Features: March 2012
SoftGauges for areal surface texture parameters
SoftGauges to test the numerical correctness of software for evaluating surface texture parameters have been extended to cover areal surface texture parameters. By Prof. Richard Leach and Dr Peter Harris, National Physical Laboratory.



Knowledge of the topography of a machined surface is necessary in order to understand the functional performance of the surface, and is consequently essential to the manufacturing process. As with the instruments used in many other areas of metrology, those that measure surface texture generally interface to a computer that collects and analyses numerical data. As part of the data analysis, surface texture parameters are evaluated. Whereas it is commonplace to use physical artefacts to calibrate surface measuring instruments, such artefacts do not allow validation and verification in isolation of the software components of the instrument.

ISO 5436–2 introduced into international standardisation the concept of the software measurement standard or SoftGauge in the context of the measurement of surface texture. The standard defines software measurement standards in the forms of reference data and reference software for testing the numerical correctness of software used in surface texture measurement.

The National Measurement Office of the UK Department of Business, Innovation and Skills recently supported, as part of its Engineering Measurement Programme, a project to develop reference software for the measurement of a number of areal surface texture parameters. The project, carried out by the National Physical Laboratory in partnership with the Centre for Precision Technologies at the University of Huddersfield, builds on previous work to develop SoftGauges for the evaluation of profile surface texture parameters.

Standards for areal surface texture parameters
International standards in the series ISO 21578 and ISO 16610 are under development by ISO/TC 213 Dimensional and geometrical product specifications and verification, and a number of parts in these series are published. The standards define terms relating to the measurement of surface texture, provide information about filtering of profiles and areal surfaces, and give definitions of areal surface texture parameters. The standards are the basis of ensuring traceability and comparability of the measurement results provided by surface texture measuring instruments.

Reference software for areal surface texture parameters
It is accepted that there can be differences in how areal parameters defined in international standards may be implemented in practice. One source of such differences is the way that standards define the parameters in terms of a surface that is assumed to be known everywhere whereas, in practice, the surface is known (approximately by measurement) only at discretely-sampled points. Another difference relates to the relationships assumed to hold between the properties of the measured surface data, such as sampling distances, and values of parameters for the various operators and filters applied to the data, including nesting indices and cut-off wavelengths.

Consequently, it is expected that instrument manufacturers will make different choices, albeit choices that are consistent with the definitions given in standards, when developing their own algorithms and software.
Reference software is used as a benchmark against which software in a measuring instrument can be compared. A data set is used as input to both the software under test and the reference software, and the results delivered by the software under test are compared with those provided by the reference software (figure 1). The most important consideration in the design and development of reference software is its numerical correctness. This is in contrast to the considerations for production software, for which the requirements on numerical correctness are generally more modest, but issues of efficiency, such as computing time and memory, and usability, are of concern.

The reference software takes the form of a single application program with a simple user-interface (figure 2) and is available from the web-site of the National Physical Laboratory. The main functions of the reference software are as follow:

•    Read data defining an S?F surface, i.e., a surface that has been S?filtered (to remove small scale components) and from which form has been removed (using an F?operator);
•    Apply a Gaussian areal filter to the data defining an S?F surface to obtain data defining the corresponding S?L surface, i.e., a surface derived from the S?F surface by removing large scale components using an L?filter;
•    Apply surface interpolation to the data defining the S?F and S?L surfaces to obtain continuous representations of those surfaces;
•    Evaluate areal surface texture parameters in terms of these continuous representations of the S?F and S?L surfaces;
•    Write the values of the areal surface texture parameters to an output file.l

For further information and to obtain the reference software, visit
http://www.npl.co.uk/engineering-measurements/dimensional/nano-dimensional/products-and-services/softgauges-for-surface-topography
  
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