Optical fiber strain sensor for measuring maximum strain

An optical fiber strain sensor is provided which remembers the maximum stn a structure has experienced in a given timeframe. A pair of multi-mode optical fibers with flat ends meet end-to-end within a microbore capillary tube with which the fibers have frictional contact. The fibers are fastened to the structure at two points a known distance apart on either side of the capillary tube. Increasing strain in the structure pulls the fiber ends apart in proportion to the amount of strain; however, when strain is decreasing, the fibers buckle rather than move closer together. Therefore, the maximum strain is reflected as a maximum distance between the fiber ends. When the maximum strain experienced is to be measured, the free end of one fiber is connected to a light source and the free end of the other fiber is connected to a detector. The intensity of light transmitted to the detector will vary inversely with the distance apart the fiber ends are; therefore, this distance can be measured and correlated with strain to a high degree of accuracy.

BACKGROUND OF THE INVENTION 
The present invention relates generally to a sensor for measuring 
displacement, and more particularly to an optical fiber strain sensor. 
It is often desirable to monitor strain in a structure, such as a bridge or 
highway overpass, a building, or a component in a vehicle, in order to get 
advance warning of fracture or other failure of the structure. However, it 
is not always convenient (or even necessary) to monitor the strain as it 
is occurring, and it is sufficient to know merely the maximum strain the 
structure has experienced in a given timeframe. For example, for vehicles 
such as aircraft, weight, space and other limitations would preclude 
monitoring strain of components therein in real time, but for purposes of 
evaluating the likelihood of future failure, it would be sufficient to 
know the maximum strain the component in the vehicle experienced during a 
particular trip. Therefore, it would be convenient to have a means of 
sensing and maintaining for later measurement the maximum strain the 
component experienced. 
Optical fiber sensors for measuring strain are known in the art, and have 
many advantages over other types of sensors. They are economical, durable, 
light-weight, and can be used in electromagnetically noisy environments, 
and are therefore ideal for use in aircraft. Currently-used optical fiber 
sensors either require active and continuous recording of the strain in 
real time or are of the on-off type wherein the optical fiber fails at a 
predetermined strain level. None of them provides a maximum strain 
measurement without continuous monitoring. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide an optical 
fiber strain sensor which can monitor the maximum strain a structure has 
experienced in a given timeframe and maintain it for later measurement. 
It is another object of the present invention to provide a simple, durable, 
inexpensive, light-weight, and highly accurate strain sensor. 
These and other objects are accomplished by apparatus for sensing and 
retaining for later measurement the maximum displacement in a given 
direction which has occurred between a first location and a second 
location. The apparatus comprises a first optical waveguide capable of 
transmitting light therethrough and having a flat end, attachable on a 
portion of the outer surface thereof to the first location, and a second 
optical waveguide capable of transmitting light therethrough and having a 
flat end, attachable on a portion of the outer surface thereof to the 
second location and positioned so that the flat ends of the first and 
second waveguides are in contact with each other across their entire 
surfaces when both waveguides are taut. A tube slidably encloses the flat 
ends of the first and second optical waveguides therein, and retains the 
waveguides in position. The displacement pulls the flat ends apart and 
friction between the waveguides and the tube retains the maximum distance 
the flat ends separate. This maximum distance correlates with the maximum 
displacement and can be measured by launching light of a known intensity 
into the first optical waveguide and measuring the intensity of the light 
emerging from the second optical waveguide. 
Other objects, advantages, and novel features of the invention will become 
apparent from the following detailed description of the invention when 
considered in conjunction with the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to the drawings, wherein like characters represent like or 
corresponding parts throughout the several views, one sees in FIG. 1 one 
embodiment of the optical fiber strain sensor 10 of the present invention 
being used to sense the strain in a structure 12. The strain to be 
measured is that which occurs in the known distance between points A and B 
in the direction shown by arrows S. A first optical waveguide or fiber 14 
having a flat end 16 is fixed, such as by an adhesive 17 like epoxy, to 
structure 12 at point A on the structure, and is positioned so that flat 
end 16 extends in the direction of point B. A second optical waveguide or 
fiber 18 having a flat end 20 is fixed, such as by an adhesive 17 like 
epoxy, to structure 12 at point B on the structure, and is positioned so 
that flat end 20 extends in the direction of point A until it meets flat 
end 16 of optical fiber 14 without any slack in fiber 18. Optical fibers 
14 and 18 should preferably be multi-mode as opposed to single-mode, since 
multi-mode fibers have a larger core and are therefore less sensitive to 
transverse displacement. Their associated components are less expensive as 
well. 
Flat ends 16 and 20 of optical fibers 14 and 18 meet and are physically 
flush across the entire surfaces of the flat ends, thereby allowing 
maximum coupling of light from one fiber into the other fiber. To 
encourage this, flat ends 16 and 20 should be perpendicular to the axis of 
their respective fibers 14 and 18, and should be as smooth as possible. 
Carefully cleaving or cleaving and polishing flat ends 16 and 20 should be 
sufficient. 
Fibers 14 and 18 are initially retained in this flat end-to-flat end 
position by a microbore capillary tube 22 which encloses fibers 14 and 18 
therein in the vicinity of their flat ends 16 and 20. Flat ends 16 and 20 
should preferably meet near the middle of tube 22. Tube 22 is fixed, such 
as by epoxy (not shown), to structure 12. Tube 22 may be colinear with the 
line between points A and B, or it may be off-line by being angled with it 
but centered on it, or by being shifted to the side but parallel with it. 
Fibers 14 and 18 are positioned and sized as necessary so that flat ends 
16 and 20 meet at approximately the center of tube 22 without any slack in 
either fiber. The inner diameter of tube 22 nearly matches the outer 
diameter of fibers 14 and 18, which are slidable therein. The tighter the 
fit is between tube 22 and fibers 14 and 18, the more perfectly aligned 
the fibers will be with each other, maximizing the coupling of light from 
one to the other. Some clearance is required to allow air to flow between 
fibers 14 and 18 and tube 22. Tube 22 is shorter than the distance between 
points A and B, leaving some of each of the fibers unenclosed near points 
A and B. 
Each of fibers 14 and 18 is in frictional contact with tube 22 at some 
point along the distance over which the tube encloses the fiber, that 
point preferably being at a respective end 24 or 26 of tube 22, as shown 
in FIG. 1. This frictional contact initially retains the fibers in the 
end-to-end position. Frictional contact between tube 22 and fibers 14 and 
18 may be established by providing a non-elastic adhesive 27 such as wax 
between each of the fibers and the tube at its respective end 24 or 26, 
effectively reducing the inner diameter of the tube at its ends to the 
outer diameter of the fibers. The frictional contact formed should be such 
that the frictional force can be overcome by the strain in structure 12 
before either one of fibers 14 or 18 breaks and before either bond between 
the fibers and the structure breaks. The bonding force between fibers 14 
and 18 and structure 12 can be increased by applying adhesive 17 over more 
fiber surface area at the respective points A and B. Fibers 14 and 18 are 
pretensioned before being adhered to either tube 22 or structure 12 to 
make them taut. 
In operation, structure 12 will strain in direction S.sub.1, shown in FIG. 
2, pulling fibers 14 and 18 from points A and B. The frictional forces at 
ends 24 and 26 of tube 22 will be overcome and flat ends 16 and 20 will 
separate. FIG. 2 shows sensor 10 and structure 12 under such a first 
maximum strain condition. Flat ends 16 and 20 have separated by a distance 
D.sub.max. 
FIG. 3 shows sensor 10 and structure 12 after the strain has been reduced 
from that shown in FIG. 2, and so points A and B have moved in the 
direction S.sub.2. Fibers 14 and 18 have buckled (shown pictorially to 
exaggerate the effect) between their respective attachment points at A and 
B and their respective points of entry into tube 22 at its ends 24 and 26. 
Fibers 14 and 18 buckle instead of sliding back into tube 22 because the 
frictional force between the fibers and the tube exceeds the buckling 
load. The buckling load is low because the slenderness ratio of fibers 14 
and 18 is high. Buckling can be encouraged by positioning tube 22 out of 
alignment with the direction of strain S, as discussed above. The degree 
to which tube 22 is out of alignment can be extremely small. Since fibers 
14 and 18 do not slide back into tube 22, the distance D.sub.max between 
flat ends 16 and 20 is retained or "remembered". The frictional force and 
the small clearance between tube 22 and fibers 14 and 18 is sufficient to 
hold D.sub.max constant. If structure 12 were to again strain in the 
direction S.sub.1 shown in FIG. 2 to a greater extent than the previous 
strain S.sub.1, fibers 14 and 18 would further pull out of tube 22, 
separating the distance between flat ends 16 and 20. A new D.sub.max would 
thus be created. In this way, sensor 10 retains the maximum strain 
position for later measuring. 
The maximum strain experienced by structure 12 can be measured by measuring 
the distance D.sub.max between flat ends 16 and 20. Of course, 
trigonometry or calibration can be used to correlate the distance 
D.sub.max between flat ends 16 and 20 with the strain in structure 12 when 
tube 22 is not aligned with points A and B. The distance D.sub.max between 
the flat ends can be measured by measuring light transmission across the 
distance, as shown schematically in FIG. 4. Light of a known intensity 
from a light source 28 such as an IR laser diode or LED is launched into 
fiber 14 so that it propagates therethrough in the direction of fiber 18. 
The light then couples into fiber 18, propagates therethrough and is 
detected and measured by a detector 30 such as a silicon photodiode 32 
connected to a conventional detection circuit 34. As the distance between 
fibers 14 and 18 increases, proportionately less light is coupled from one 
fiber to the other. The intensity of transmitted light is very sensitive 
to the distance apart of flat ends 16 and 20, so strain can be measured to 
a high degree of accuracy. For example, a difference in strain of 1000 
microstrain between two points 2.54 cm apart can be measured as 
approximately a 20% difference in light output intensity using multi-mode 
optical fibers having a 68-micron core diameter and an overall diameter of 
75 microns and a one-cm long microbore quartz tube with a 90-micron inside 
diameter. Of course, sensor 10 must be calibrated in order to correlate 
strain with light intensity. 
FIG. 5 shows another embodiment of the invention which includes a substrate 
36, upon which fibers 14 and 18 and tube 22 are mounted, which makes 
sensor 10 more easily handled as a unit separate from structure 12. 
Substrate 36 has two holes 38 and 40 therethrough, which are positioned to 
align with points A and B, respectively, on structure 12 when the 
substrate is attached thereto. Two floating pads 42 and 44 are positioned 
in holes 38 and 40, respectively, and are sized to be smaller than the 
holes. Pads 42 and 44 are movably connected to subtrate 36 by an elastic 
adhesive 46 such as silicone rubber between their outer perimeters and the 
inner edges of holes 38 and 40. Substrate 36 is fixed to structure 12 via 
pads 42 and 44, which are rigidly fixed, such as by epoxy or cement, to 
the structure at respective points A and B. Pads 42 and 44 therefore move 
with points A and B when structure 12 experiences strain. Fibers 14 and 18 
and tube 22 are positioned on and fixed to substrate 36 in the same manner 
as they are fixed to structure 12 in the previously discussed embodiments, 
with fiber 14 being fixed to pad 42 with adhesive 17, and fiber 18 being 
fixed to pad 44 with adhesive 17. Holes 38 and 40 and pads 42 and 44 are 
sized such that there is sufficient space between the outer perimeters of 
the pads and the inner edges of the holes for the pads to move within 
substrate 36 when structure 12 is strained. Tube 22 may also be positioned 
on substrate 36 so that it is not aligned between holes 38 and 40, in 
which case fibers 14 and 18 are adjusted in length and position 
accordingly, as discussed with respect to the previous embodiment. 
Some of the many advantages of the present invention should now be readily 
apparent. For instance, a simple, durable, inexpensive, light-weight, and 
highly accurate strain sensor has been provided. Furthermore, an optical 
fiber strain sensor has been provided which can monitor the maximum strain 
a structure has experienced in a given timeframe and maintain it for later 
measurement without the need for continuous recording of the strain in 
real time. 
Those skilled in the art will appreciate without any further explanation 
that many modifications and variations are possible to the above-disclosed 
optical fiber strain sensor, within the concept of this invention. 
Consequently, it should be understood that all such modifications and 
variations fall within the scope of the following claims.