Strain sensor with optical fibre Bragg gratings

Details Strain sensor with optical fibre Bragg gratings Strain sensor with optical fibre Bragg gratings

2001-02-20

2003-07-01

Kim, Robert H. (Department: 2882)

Radiant energy

Photocells; circuits and apparatus

Optical or pre-photocell system

C385S037000

Reexamination Certificate

active

06586722

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to strain sensors and in particular to multi-element strain sensors incorporating fibre Bragg gratings as the strain sensing element.
2. Description of Related Art
Strain rosettes are well known multi-element strain sensors and are widely used in mechanical testing. Strain rosettes typically comprise two or three non co-linear strain gauges mounted on a common substrate. The strain gauges are typically arranged at 45° or 60° to one another, to form rectangular or delta rosettes respectively.
Strain rosettes can be surface mounted or embedded inside structures and used to provide a variety of information on strain fields. For example, strain rosettes can be used to measure components of strain along and perpendicular to a principal axis, or to determine the orientation of the principal axis if it is not already known.
In the past, strain rosettes have typically employed three electrical strain gauges (e.g. resistance strain gauges) as their strain sensing elements. A known rectangular strain rosette is shown schematically in
FIG. 1
, and includes three resistance strain gauges S
1
, S
2
, S
3
arranged at 45° to each other. The strain gauges are mounted on a substantially planar common substrate
99
for ease of handling and to maintain their mutual orientation. For optimum results, the individual strain gauges and placed as close together as possible. Separate electrical connections are required to each sensor.
Fibre Bragg gratings are well known and can be used as temperature sensors or strain gauges as alternatives to electrical sensors, providing numerous advantages. Fibre Bragg gratings (FBGs) and their use as sensing elements are described in “Optical Fibre Bragg Grating Sensors: A Candidate for Smart Structure Applications”, Dunphy et al, Chapter 10 of Fibre Optic Smart Structures, edited by Eric Udd, 1995 John Wiley & sons, Inc., ISBN 0-471-55448-0.
A typical FBG is shown schematically in FIG.
2
(
a
) the FBG is formed of a length of optical fibre having a core
21
surrounded by cladding material
22
having a lower refractive index than the core. The optical fibre is typically a single mode fibre (mono mode), the core diameter being sufficiently small such that for a chosen light source, light can propagate along the core in only a single mode. The single mode is guided substantially by the core/cladding boundary. The “lines”
11
of the grating are a series of regularly spaced perturbations of the refractive index N
C
of the core. The grating extends along a length L of the fibre, where L is typically in the range 1 mm to 20 mm, and the variation of core refractive index along the longitudinal axis Z of the FBG is shown in FIG.
2
(
b
). A variety of techniques can be used to produce FBGs. In one of these techniques, the refractive index perturbations are formed in the core by masking the fibre with a phase mask and exposing it to intense ultra-violet light. In another technique, the index perturbations are formed by exposing the fibre to the interference pattern produced from two intersecting halves of a UV laser beam. The spacing X of the index perturbations is determined by the angle at which the two halves of the beam intersect. The perturbations in core refractive index produced by these techniques are typically of the order of one part in one thousand or less.
The optical fibres used to produce FBGs generally have a Protective coating outside the cladding. Before the fibre is exposed to UV light to form the grating the protective coating is removed. After exposure, the stripped portion of fibre is re-coated to restore its durability.
When a broad spectrum of light is input to the FBG as an input signal, most wavelengths pass through the grating region and form a transmitted output signal
82
. However, the periodic index perturbations produce a strong Bragg reflection of components of the input signal having a wavelength &lgr;
b
, the Bragg wavelength, where:
&lgr;
b
=2
XN
C
Thus, a tunable detector can be used to look for a peak in the reflected signal, or a trough in the transmitted signal. The wavelength at which the peak or trough occurs therefore gives an indication of the line spacing X of the grating.
When the FBG is subjected to longitudinal strain, the spacing X changes and so results in a shift in the Bragg wavelength. To a good approximation, the Bragg wavelength is proportional to strain along the longitudinal axis. Advantageously, the grating sensor inherently tends to reject the effects of strain fields not aligned with the longitudinal axis.
Advantageously, as strain is determined by measuring the Bragg wavelength, the measurement is not affected by fluctuations in the intensity of the input light.
The fibre Bragg grating provides other advantages associated with fibre optic sensors. For example it is immune to electromagnetic interference, has light weight and small size, exhibits high temperature and radiation tolerance, and is durable even in harsh environments.
Fibre optic strain rosettes employing three separate FBGs as the strain sensing elements are known, in which each FBG has its own input and output fibres, separate from those of the other FBGS. Although the sensing region may be suitably compact (i.e. the FBGs may be arranged close together) the three sets of related fibres are inconvenient.
Rather than connecting separately to each one of an array of FBGS, it is known to instead connect them in series, provided that their nominal Bragg wavelengths are sufficiently different. One such arrangement is shown schematically in FIG.
3
. Here, a light source
70
outputs a signal, a portion of which
80
is input to a series string of fibre Bragg gratings
1
A,
1
B,
1
C via a bi-directional coupler C. The three FBGs have different nominal Bragg wavelengths, &lgr;
BA
, &lgr;
BB
, and &lgr;
BC
respectively and the reflected signal
81
returning to the coupler essentially consists of light at just these three wavelengths. A portion of the reflected signal
81
is input to a light detector
71
via the coupler C. In this example, the light source
70
is a broad band source and the light detector
71
is a tunable narrow band detector. Thus, as the detector scans across a range of wavelengths, intensity peaks will be detected corresponding to the three Bragg wavelengths, and so the strain experienced by each FBG can be determined. Thus, in
FIG. 3
the fibre Bragg gratings are multiplexed.
It will be appreciated that in alternative arrangements a tunable narrow band light source may be used in conjunction with a broader band light detector to measure the Bragg wavelengths.
Strain rosettes incorporating series connected fibre Bragg gratings are known and an example is shown schematically in FIG.
4
. Here, the optical fibre components of the strain rosette are formed from a single continuous fibre comprising an input portion
50
connected to a first fibre Bragg grating
1
A. The first fibre Bragg grating is connected by a connecting loop
6
to the second FBG
1
B which in turn is connected by a second loop
6
to a third FBG
1
C. The FBGs are arranged at 0°, 45°, and 90° with respect to a nominal axis and the rosette is encapsulated in a thin film of encapsulating material
9
. The thickness of the optical fibre is exaggerated in the figure for clarity.
The three FBGs are arranged close together, forming a compact sensing portion, but the overall size of the rosette is significantly larger as a result of the connecting lengths of fibre
6
formed into loops. Although it is desirable to make the loops as small as possible to minimise the overall size of the rosette, the minimum bend radius must be large enough to avoid significant bend loss. For typical optical fibres having a cladding diameter of up to 200 &mgr;m the minimum bend radius without loss is approximately 1 cm. Thus, this large minimum bend radius of the fibres results in a large and cumbersome device when the multiplexed FBG sensors are arranged in the necessary geometry.
Delta rosett

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