Manual and automatice large-scale dimensional metrology

Abstract

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