Radiant energy – Photocells; circuits and apparatus – Optical or pre-photocell system
Patent
1985-11-25
1987-07-07
LaRoche, Eugene R.
Radiant energy
Photocells; circuits and apparatus
Optical or pre-photocell system
73705, 356358, G01B 1102
Patent
active
046789090
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The present invention relates to optical pressure sensing apparatus and, more particularly, to such apparatus utilising the principles of an optical fibre interferometer for sensing small pressure induced displacements of a pressure responsive element.
The principles of interferometery are well known and their application to the sensitive measurement of displacements is also established. However, the necessity of having to use large and heavy optical components and light sources in precise alignment has limited their practical application.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a pressure sensing apparatus which employs the high sensitivity of an optical fibre interferometer to achieve both improved sensitivity to small pressure changes and measurement over a large dynamic range.
To this end, the invention consists in an optical pressure sensing apparatus characterised by an optical fibre interferometer device arranged to sense the movement of a resilient pressure responsive element, whereby an interference signal produced at an output of the interferometer device is responsive to change in the pressure applied to the pressure responsive element. The latter may be any type of resilient element employed for pressure sensing purposes, for example, a diaphragm, capsule or one of a selection of tubes of different geometries, and an element of appropriate mechanical properties is selected depending upon the particular application.
Preferably, the interferometer device comprises individual optical fibre signal and reference arms, the signal arm being mechanically or optically coupled to the pressure responsive element so that the length of the optical path defined by the signal arm changes relatively to that of the reference arm in response to movement of the pressure responsive element. The signal and reference arms have equal responses to physical pertubations, such as, temperature changes, other than the desired pressure measurand and these cancel one another by the process of common mode rejection. Such apparatus therefore shows negligible cross sensitivity. Moreover, once calibrated, such apparatus may be used to measure gauge pressure regardless of other environmental changes. In a preferred embodiment, the signal optical fibre is secured at or adjacent one end to a fixed location on the apparatus and has its opposite end, or a portion of its length, fastened to the pressure responsive element so that it is under tension, whilst the reference optical fibre is secured at or adjacent both its opposite ends to fixed locations on the apparatus. In order to compensate more fully for physical pertubations other than the measurand, the reference arm is disposed, relatively to the signal arm, in the optimum position for this purpose and to this end, may, for example, also have its distal end, or a part of the reference fibre, attached to a non-movable location of the pressure responsive element.
The interferometer may be of the Michelson or Mach Zehnder type and have an input and output coupled by coupling means to signal and reference arms which are conveniently constructed from single mode optical fibre. With the Michelson configuration, the distal ends of the signal and reference fibres are reflective so as to reflect light beams propagating in the fibres, and the distal end of the signal fibre is mechanically attached to the pressure responsive element. The coupling means is adapted to mix the reflected beams in the two fibres to produce an interference signal at the output of the inferferometer. With the Mach Zehnder configuration, the input and output are disposed at opposite ends of the signal and reference fibres and a portion of the length of the signal fibre is fastened to the pressure responsive element. The output coupling means mixes the beams independently propagated in the signal and reference fibres to produce the interference signal at the output.
In other embodiments of the invention, the interferometer device may comprise a Fabry
REFERENCES:
patent: 4464936 (1984-08-01), McIntire et al.
patent: 4547668 (1985-10-01), Tsikos
patent: 4588886 (1986-05-01), Snider
Hesse, Joachim, and Sohler, W., "Faser Optishe Sensoren", Elecktronik, 6-18-82, pp. 89-92.
Corke Michael
Jackson David A.
Jones Julian D. C.
Kersey Alan D.
Kent Scientific and Industrial Projects Limited
LaRoche Eugene R.
Mottola Steven J.
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