Optical waveguides – Optical waveguide sensor – Including physical deformation or movement of waveguide
Reexamination Certificate
2000-04-21
2002-05-14
Spyrou, Cassandra (Department: 2872)
Optical waveguides
Optical waveguide sensor
Including physical deformation or movement of waveguide
Reexamination Certificate
active
06389187
ABSTRACT:
This invention relates to an optical fibre bend sensor.
A deformation of an optical fibre results in strain developing within the fibre. Strains can be categorised depending on the nature of the distortion which produces them. A longitudinal strain: a one-dimensional expansion or compression of the fibre along its length, is categorised as a scalar strain. A scalar strain by its nature requires only one parameter, the magnitude of strain along the axis of the stretch or compression, to characterise it. In terms of the strain tensor (&egr;) this longitudinal strain is the tensile component &egr;
zz
.
If however a fibre is also free to deform in a transverse plane i.e. that perpendicular to the length of the fibre, further components of the general strain tensor will have effect. If the fibre is restrained in its position, tensile strain (&egr;
xx
, &egr;
yy
) arises within a compressed or stretched fibre in perpendicular directions to the longitudinal strain described above. This is known as transverse strain.
More generally, if the fibre is not fixed rigidly in location, one end of the fibre may then be displaced with respect to the other. Such a displacement occurs in three dimensions and results in a “bending” of the fibre. Both this bend and three-dimensional tensile strain require characterisation by both magnitude and direction. In the simple case of a bend with negligible fibre elongation, the induced strain is conveniently described in terms of the bend magnitude and plane of curvature. This curvature (&kgr;) has magnitude equal to 1/R, where R is the radius of curvature, and direction defined by the normal vector pointing towards the centre of curvature. Assuming a linear strain gradient across the bend then, in mathematical terms, the deformation developed in bending is formally equivalent to the transverse strain gradient: &Dgr;
t
&egr;
zz
, where &Dgr;
t
is the transverse gradient operator.
Optical fibre sensors for strain measurement are known in the prior art. An optical fibre is embedded in or surface-bonded to the structure to be monitored and its optical properties observed. The monitored structure is not limited to engineering applications e.g. aeroplane structures, building walls; optical strain sensors have been found useful in the medical field. A number of external influences may cause strain to develop within a structure: applied stress (elasticity) and electric field (piezoelectricity) to name two. An optical fibre within such a strained structure will in turn experience the effects of such strains. Transverse strain components will affect the refractive index and longitudinal strain components will also stretch (or compress) the fibre. In either case the optical path length of radiation propagating within the fibre is changed. Thus information pertaining to strain within the monitored structure is manifest in the phase of radiation propagating within the fibre and is therefore extractable using interferometric techniques. Optical strain sensing is particularly attractive because interferometry offers an accurate detection tool and the sensitivity of optical properties to physical influences such as strain and temperature is high.
Temperature has a similar effect to strain on an optical fibre. Thermal expansion will change the length and refractive index of the fibre and additional strains may also be induced by the differential expansion of fibre and host material. Any optical technique purporting to measure strain must make allowance for this cross-sensitivity between temperature and strain.
A problem with prior art interferometric strain sensing techniques such as that described in patent application GB9606785.5 is that a single probe fibre measures only a scalar component of strain—the elongation of the fibre length. Transverse strain components are not measured and a general three dimensional contortion of a fibre is detectable only as a change in fibre length. Fibres have been multiplexed both in parallel and series in order to provide scalar strain measurements across a range of positions. From data gathered from an array of such single probes a map of strain gradient may be built up. However each probe intrusion inevitably weakens the structure being monitored. The interface region will be subject to increased strain and there is a clear benefit to be had in limiting the number of such interfaces.
Strain gradient measurements have been performed from sea-going vessels using a number of magnetic bearing sensors interspersed with depth sensors on a towed sonar array. However such arrays are very bulky, occupying considerable storage space when not in use and such considerations limit the practical length of the array.
Furthermore magnetic bearing sensors as currently used are affected by perturbation of the local magnetic field. Measurements are therefore influenced by metallic structures on the sea bed as the array passes above.
An optical fibre sensing device sensitive to the degree of fibre bending is disclosed in U.S. Pat. No. 4,443,698. The interference pattern developed between light propagating in two different cores of a multicore fibre. is used to monitor changes in the bending of the fibre. If two cores are used then a helical 90°-twist about each other is incorporated over the sensing region. This allows the fibre to be sensitive to bending regardless of bend plane, but removes any capability of measuring bend direction. With three or more non-coplanar cores the need for such a twist can be avoided. The invention employs phase tracking techniques to follow varying bend parameters in order to allow deduction of unambiguous measurements. However, tracking requires access to one of the interferometer optical paths, a clear disadvantage for applications that require remote addressing of passive sensing lengths. In this case. bend direction information will be lost. Tracking will also be lost if power is not continuously maintained.
There is a perceived need for non-intrusive shape sensing by means of bend measurement. Such a sensor would have many applications in diverse fields. In robotics, knowledge of the absolute position of moving parts is essential. This can be deduced if directional bending of an integrated fibre can be measured. In medical applications, any internal monitoring is safest with minimal intrusion from a foreign probe and, additionally negligible generation of external electromagnetic fields, Lightweight position monitoring is essential to promote mobility in an artificial limb. Prior art optical strain sensors do not measure strain gradients and prior art shape sensors are overly bulky and generally rely on magnetic effects which have neither the accuracy nor immunity from environmental perturbation afforded by optical measurement.
It is an object of this invention to provide an alternative form of bend sensor.
The present invention provides a bend sensor incorporating a multicore fibre assembly having first and second component cores and being arranged to convey radiation to and from a sensor length comprising a bend sensing region of the fibre assembly and thereafter to analysing means arranged to analyse radiation output from the fibre assembly, characterised in that the fibre assembly incorporates coupling means for coupling radiation propagating from the sensor length in the first core to the second core for return to the sensor length, and reflecting means arranged to define the sensor length by at least partial reflection of radiation thereat in both cores, wherein the reflecting means and coupling means are arranged to define first and second optical paths each of which traverses the sensor length in a respective one of the cores but not the other, and the analysing means is arranged to disperse interferograms formed between radiation contributions which have traversed the optical paths.
This invention provides the advantages of accuracy and relatively non-intrusive bend measurement. Optical fibre assemblies can be produced with very small diameter and embedding within a structure will thus result in minimal di
Blanchard Paul M
Burnett James G
Greenaway Alan H
Harvey Andrew R
Lloyd Peter A
Amari Alessandro V.
Nixon & Vanderhye P.C.
Qinetiq Limited
Spyrou Cassandra
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