<title>High-temperature optical sensor for displacement measurement</title>

Abstract

Two slit and bar grids and optical fibers for lighting and reading are specially conceived to resist at high temperature <some hundred oF °C) and constitue a fully optical moire set-up, allowing measurement of displacement with a sensitivity of O%25 jim. Automatic compensation of temperature is possible.1. LNTRODUCT ION Metrology at high temperature is more and more important for industrial applications in measure of strain, displacements. .. fields of structures. One of the important problems for users consists to separate the effect of temperature from loading forces on the relative displacement between two parts of the structure, as for instance the variation of the diameter of a tube. If the critical value before rupture depends on the amplitude of the mechanical displacement due to the forces, of course the addition of the thermal effect is a source of perturbation for the validity of the decision. Furthermore it is evident that the accuracy, requested for such measurements, is the same as at room temperature and it can appear that the desired sensitivity is of the order of some tenth of jim. We can summarize this kind of measurement by the sche of Figure Ia. A transducer, situated between the points A and B of the structure, must be able to measure the variation of this basis AB with the desired accuracy and without influence of parasitic effect. At high temperature the electronic transducers are prohibited and only full optical transducers are applicable, the automatic electronic treatment being far away from the high temperature envrronment.If laser systems are appreciated br their good coherence comportment, they are often unadapted to the agressivity of the ambiance, namely because of speckle degradation. The accurated interferometdc techniques are thus avoided in those cases and incoherent or pseudo-coherent techniques are selected preferably.We propose here a classical moire technique illustrated by Figure lb. Two gratings are jointed separatly to points A and B and give rise to a moire effect, The influence of air gap on fringe contrast in this kind of application limites the value of the grating pitch which must be greater than a critical value of the order of 50 jim for practical use. With this value of pitch it is possible to choice an air-gap equal to the Fresnel distance p/X, where p is the pitch of the gratings and X° a typical wavelength of the source. This typical distance is great enough to avoid any friction between the gratings due to parasitic vibrations, but any tranverse vibration must be without influence on the fringe contrast in view of interfringe measurement. 268 / SPIE Vol. 1504 Fiber-Optic Metro/ogy and Standards (1991) 0-8194-0613-9/91 /$4.0O Downloaded From: http://proceedings.spiedigitallibrary.org/ on 06/16/2016 Terms of Use: http://spiedigitallibrary.org/ss/TermsOfUse.aspx