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Manual and automatice large-scale dimensional metrology

By P. H. Sydenham


Current techniques for manual and automated determination\ud of decametre-range are reviewed from information gained by extensive\ud literature search and from visits made to a wide cross section of\ud European establishments concerned with large-scale dimensional\ud measurements. The reviews, which contain nearly 200 references, provide\ud background information needed by systems designers.\ud A 12m steel measuring base is described which was length\ud stabilised to within 3 parts in 10 for periods in excess of 500 hours.\ud Temperature of the internal water flow is monitored by a contact\ud thermometer which controls the heat input.\ud The theory and practice of tensioned-wire dimensional\ud transducers are given which enables this new-class of dimensional\ud measuring instrument to be designed. Two distinct groups of instrument\ud described are those for transducing continuous length changes into\ud electrical signals with better than 5 parts in 10^6 error and those for\ud detecting changes of 1 part in 10^10 or smaller, of a nominally fixed\ud length.\ud Continuous subdivision transducers with 12m and 1m range are\ud reported which incorporate mechanical mechanisms for obtaining adjustable\ud absolute length, reduction of in-scale accuracy, errors, linear rotary\ud output, rapid following response and simple temperature compensation.\ud A method and practical apparatus are described which uses this type of\ud transducer for automatic control of the position of a workhea moved\ud on a cartesian-frame manipulator. Actual two-dimensional position\ud is measured on a trilateral basis, a technique which eliminates the need\ud for a massive framework when machining or inspecting large workpieces.\ud In-situ numerically-controlled machining is possible with this system.\ud A second frameless technique is described for automatically\ud recording roundness variations of large rings or spigots. Repeatability\ud of 5 parts in 10^6 or better has been proven to be possible with\ud inexpensive equipment. This method illustrates the use of deformation\ud wire-transducers for dynamic dimensional measurement. A similar type\ud of instrument was used for creep determination of invariwires and\ud carbon-fibres supported on the 12m base. Results are given which have\ud shown that 1 part in 10^8 length stability is attainable over considerable\ud periods.\ud It is shown that measurement of earth strains is possible with\ud an invar wire tensioned by a simple beam-balance. Details are given of\ud an experiment performed in a tunnel in which solid-earth tidal strains\ud are recorded within hours of installation.\ud Several other possible applications are discussed for which\ud tensioned-wire transducers seem suited.\ud The use of position-sensitive photocells in dimensional\ud metrology is outlined with particular reference to wire transducers\ud where linear and rotary movements may need monitoring. It is shown\ud that they are a simple and economic way to measure small displacements of\ud millimetre range

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