Note: Descriptions are shown in the official language in which they were submitted.
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STRAIN GAUGE
This invention relates to strain gauges for ~ngineering structures e.g. bridges,b~ ilding~7 pipes, plant and the like whether made from steel or concrete, and in particular
to strain gauges that incorporate optical fibres as the strain sensing elements.
Strain gauges formed from optical fibres and having dim~.n~ions, in the order ofs 0.1 metre to 10 metres and especially in the order of 0.1 to 1 metre would be particularly
useful in detecting and monitoring strain in large engineering structures. However, one
signific~mt problem in the use of optical fibres for such purposes is the issue of
supporting the fibres on the structure so that the fibres are subjected to strains in the
structw-e but without d~m~ging the fibres or requiring costly and time-con~umingo method.s of mounting the fibres on the surface of the structure.
According to the present invention, there is provided an optical fibre strain gauge
for an engineering structure, which comprises:
a) a plurality of supports for the optical fibre that are, or can be, located on a
surface of the structure and are spaced apart from one another over a part of
the surface; and
b) at least one optical fibre that is looped around the supports so that it extends
between the supports, the optical fibre being fixed to the supports so that the
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length of the part of the fibre e~en~ling between the supports will vary in
accordance with strain of the surface of the structure.
I'he strain gauge according to the invention has the advantage that the optical
fibres can be looped around the supports a number of times. This enables the optical
s fibre or fibres to be held by the supports more easily so that the fibre or fibres are subject
to the strain of the surface of the structure without the necessity of complex ~tt~chment
procedures such as met~ ing and welding. In addition, the strain gauge according to
the invention will incorporate a length of optical fibre that is significantly greater than the
dimension of the area of the structure that is being monitored. This increases the
o flexibility of the design and enables, for example, areas of structures to be monitored
which have dimensions smaller than the resolution of the equipment employed to
monitor l:hem.
The optical fibre or fibres will normally contain one or more reflectors so that-- light will be caused to pass in both directions along that part of the optical fibre
5 exten~ling between the supports. Thus, for example, the increase in length may be
measurecl by a reflectometry method in which light is sent along the fibre and reflected
back to a detector and changes in the length of the fibre alter the time taken before the
light is detected at the detector. Such a detector may be formed by a mirror, a Bragg
grating formed in the fibre, or even, in the broadest aspect of the invention, simply a
20 cleaved end of the fibre. Such arrangements have the advantage that the reflector, and
any additional elements that may be present, can be located at a position remote from the
supports, so that if the structure to be monitored is subjected to very high tempel ~L~Ires
or is otherwise located in a hostile environment, only that part of the or each optical fibre
that is looped around the supports need be located in that environment. Alternatively the
2~ optical fibre may contain a strain-sensitive reflector such as a Bragg grating in that part
of the fibre that extends between the supports. For example, in the case of a Bragg
grating, the spacing of the grating will therefore vary in accordance with strain of the
surface. Thus light of a broad wavelength spectrum could be l~lln~.hed into the optical
fibre and the wavelength of the re~lected light would vary in accordance with the strain
30 ofthe surface. Instead, it may be appropliate to employ a Bragg grating whose grating
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spacing varies along its length and to launch monocl~oll.alic light into the optical fibre.
In this case the position along the optical fibre at which the grating spacing m~tçh~s the
light wavelength will vary with the strain on the surface and the path length of the light
will change accordingly.
s The optical fibre or optical fibres may simply be looped around the supports as a
whole or they may additionally be wound around individual supports in a plurality of
turns. This may enable the optical fibre or fibres to be held to the supports at least
principally by friction, although it may be appropriate to provide some additional form
of adhesion.
o The or each optical fibre should be looped around the supports so that it is taut.
However, in many cases it is pl ere- ~ ed for the fibre to be under tension so that it is in a
stretched state even when the structure surface is not strained. In this way the strain
gauge will be able to record a degree of col~lpl essive strain in the structure surface as the
-- separation between the supports decreases. Typically the optical fibre or fibres would be
15 stretched to an elongation of 0.2 to 0.5% at zero structural strain.
The supports for the fibre may take any approp. iate form, although it is prefe. 1 ed
for them to comprise protuberances that extend from the surface of the structure and
around which the or each optical fibre is looped. The supports preferably have no
corners or edges that contact the optical fibre and which could cause light loss from the
20 optical fibre by microbending. In addition the supports prefe, ably have a curvature of
radius of at least 30mm so that no light is lost from the fibre by macrobending. The
supports may, for example, be forrned as cylindrical protuberances of circular cross-
section. However, in some circl-m~t~nces it may be preferable for the protuberances to
have lateral dimensions that ~imini~h in a direction (normal to the surface) that extends
25 away from the surface, for in~t~n~e they may be frusto-conical in shape. Such forms of
support can facilitate location of the optical fibre on them and removal of the optical
fibre therefrom if the fibre is arranged in a capping element as explained below. In
addition, such supports provide an easy method of stretching the optical fibre since, if
the optical fibre is held in a loop, the loop of fibre will be stretched as it is pushed over
30 the supports toward the surface of the structure.
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In. its simplest form of construction the strain gauge may have supports for theoptical fibre that are formed integrally with the structure surface. However, it is
pl ~re. l ed for the gauge to be formed separately from the structure so that it can be
attached to the structure at any convenient time. For example, the supports may be
5 located o:n a base plate that can be attflched to the structure, for instAnce by welding or
bolting.
Tihe optical fibres may be looped around the supports by hand in situ, but it ispie~lred to package the optical fibres in a more rugged arrangement that will withe1~ncl
normal abuse to be expected on a construction site and in use. For this reason the
0 optical fibres may be provided in a capping element that is located on the supports. If
the supports are frusto-conical or otherwise taper, the degree to which the capping
element is pushed on to the supports will determine the degree to which the optical fibre
is stretched as it is installed.
~~ The strain gauge may include any applop.iate number of supports. If it has two
supports, the optical fibre will extend between the supports in one direction and will
therefore detect strain in a single direction only. The strain gauge may alternatively
include three or more supports arranged on the surface so that the optical fibre or fibres
will be subject to strain occurring on the surface in two directions. For example they
may be arranged to form the vertices of a triangle, preferably a right-angled triangle so
20 that optical fibres extend over part of the surface in mutually perpendicular directions. It
is possible for the strain gauge to have, for example, four supports arranged at corners of
a rectangle, and for the optical fibre or optical fibres to extend between adjflcent
supports along the edges of the rectangle In this case, if one of the supports is
decoupled from the surface ofthe structure it will ~ Ain a constant separation from
25 the adjacent supports. Optical fibres extending along the edges of the rectangle that
meet at tlhat support will not be subject to strain of the surface of the structure and can
be used for temperature compensation.
Several forms of strain gauge in accordance with the present invention will now
be described by way of example, with reference to the accompanying drawings, in which:
Figure 1 is a partially cut-away perspective view of one form of strain gauge;
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Figure 2.is a schematic top view of a second form of strain gauge, and
Figure 3 is a sr.h~m~tic top view of a third form of strain gauge.
Referring to the accol,lpa"ying drawings, figure I shows a strain gauge 1 which
comprises a rect~ng~ r base plate 2 that can be firmly att~hed to a metal structure, for
example, it can be welded to the structure along its edges 4 and 6 so that points on the
base plate 2 follow strains on the underlying structure. A pair of protuberances 8 and 10
stand up from the base plate 2 and act as supports for an optical fibre strain sensing
elemçnt of the strain gauge 1. The protuberances 8 and 10 are each frusto-conical in
shape having a circular cross-section that is of minim~lm diameter of 60mm to prevent
o any light loss in the optical fibre by macrobending, and are each located at one end
region of the base plate 2.
The strain gauge includes an optical fibre 12 that forms a strain sensing el~m~nt~
and is looped around the protuberances three times before being led away from the base
plate 2 in a steel tube 14. Although only three loops of the optical fibre are shown for
the sake of clarity, in practice the fibre may be looped around the protuberances many
more times if desired, for example up to fifty or one hundred times. In addition the
thickness of the optical fibre will be much less than as shown. The optical fibre may
have a polymeric jacket formed, for example, from an acrylic polymer, and will typically
meter ~in--lu~ling jacket) of about 1 2511m. Alternatively the optical fibre may have a
20 carbon coating or a metallic coating e.g. formed from ~lllminillm or gold which will
exhibit less creep, will give the fibre a higher degree of protection and will load to a
reduced fibre diameter, thereby enabling a larger number of optical fibre loops if desired.
The protuberances 8 and 10 and the optical fibre 12 are enclosed in a steel
capping element 16 that is also rect~n~ r in shape and of subst~nti~lly the same2~ dimensions as the base plate 2. Apart from the protuberances 8 and 10 and the optical
fibre 12, the interior ofthe capping element 16 is filled with a potting compound 18, for
example polyurethane, a cured acrylic polymer or the like
The strain sensor is rn~nllf~ct~1red and delivered to the in~t~ tion site in twoparts: the base plate with protuberances, and an assembly of the capping element 16
30 cont~ining the optical fibre 12 and the potting compound 18. The capping element part
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of the strain gauge is m~mlf~ct~lred by looping the optical fibre 12 the required number
of times around a former having the sarne shape and dimensions as the protuberances 8
and 10 or perhaps very slightly smaller diametersl placing the capping el~m~nt over the
former and optical fibre 12, filling the interior of the capping element 12 with the potting
5 compouncl 18 and curing the potting compound. After the potting compound 18 has
fully curecl the former is removed.
In order to install the strain gauge, the base plate 2 is attached to the surface of
the structure for example by we}ding, and the capping element assembly is pushed on to
the two protuberances 8 and 10 sufficiently to cause the taper of the protuberances to
o stretch the optical fibre 12 by a small amount e.g. 0.2 to 0.5%. The app,opliate degree
of stretch of the optical fibre 12 may, for example, be ensured by providing one of the
base plate or the capping element assembly with a stop and the capping element
assembly may be forced on to the protuberances, for example by hammering, until
further movement is prevented by the stop. The capping element 16 is retained on the
5 base plate by means of screws 20 that are received by tapped holes 22 in the
protuberances.
In use the length of the optical fibre will vary in accordance with çh~ngeS in the
separation of the protuberances 8 and 10, the total length of the fibre r~h~nging by 2n
times the change in separation of the protuberances, where n is the number of times the
20 optical fibre is looped around the protuberances. Appropriate choice of potting
compound 18 and jacket material for the optical fibre 12 will cause adhesion between the
two and will prevent or at least substantially reduce slippage of the fibre around the
protuberances. Strain in the structure may be monitored by any of the following
methods:
25 1 ) by providing a pair of reflectors such as Bragg gratings in the parts of the optical fibre
remote: from the strain gauge, and using reflectometry methods to monitor the change
in the overall length of the optical fibre;
2) by providing Bragg gratings in the parts ofthe optical fibre that are subject to
stretching and monitoring changes in wavelength of light reflected by the grating; or
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3) by monitoring the variation of an intrinsic property of the fibre such as the propogation constant.
Figure 2 shows scl-~m~ticAlly a second form of strain gauge according to the
invention which can monitor strain in two orthogonal directions A and B as shown in
5 the drawing. The gauge is of the same construction as that of figure 1, and comprises a
base plate 2 and three protuberances 8, 9 and 10, the protuberances subtending an angle
of 90~ about protuberance 9. Two separate optical fibres 12 and 12' are looped about
protuberances 8 and 9 and about protuberances 9 and 10 ~es~ec~ ely so that each
optical fibre lies predominantly parallel to one of the directions A or B and
0 predominantly perpendicular to the other of the directions.
Figure 3 shows sch~m~tically a third forrn of strain gauge according to the
invention which can monitor strain in two orthogonal directions and is temperature
compensaled. The strain gauge is also of the same general construction as that shown in
~ figure I but comprises a generally square base plate 2 having four protuberances 8, 9, 10
15 and 1 1, one protuberance in the region of each corner of the base plate 2. Four optical
fibres 12, 12', 13 and 13' are wound around Adj~cçnt pairs ofthe protuberances so that
each of the optical fibres extends generally along one edge of the base plate, optical
fibres 12 and 13 being disposed along opposite parallel edges as are optical fibres 12'
and 13'. The base plate is welded to the underlying structure surface by weld 20 which
20 extends along two adjacent edges 21 and 22 ofthe base plate 2 but not along the other
~dj~cçnt edges 23 and 24 (although the weld 20 could, if desired, be extended along part
of the edges 23 and 24 in the region of protuberances 9 and 1 1). In this way,
protuberances 8, 9 and 11 are fixed to the structure surface while protuberance 10 is
decoupled from the surface and will IllA;I~IA;l~ a constant separation from protuberances 9
25 and 11 other than due to variations in temperature. Optical fibres 12 and 12' will
therefore act as strain sensing elements in respect of directions A and B respectively,
while optical fibres 13 and 13' can be used to compensate for temperature effects
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