WO2005083379A1

WO2005083379A1 - Multi-arm fiber optic sensor

Info

Publication number: WO2005083379A1
Application number: PCT/SG2004/000045
Authority: WO
Inventors: Swee Chuan Tjin
Original assignee: Sif Universal Private Limited
Priority date: 2004-02-26
Filing date: 2004-02-26
Publication date: 2005-09-09

Abstract

A fiber optic sensor comprising a fiber optic (203) having two operatively connected and parallel arms (205,206), each arm having a Bragg grating (204). The fiber optic is embedded between a plurality of embedding layers. The embedding layers comprise an upper portion (210) with at least one upper layer and a lower portion with at least one lower layer. The arms (205,206) are operatively connected by an arcuate portion (207). There is at least one embedding layer (201) between the first arm (205) and the second arm (206). The sensor is not affected by temperature and may be able to read temperature, or expansion/contraction due to temperature.

Description

Multi-Arm Fiber Optic Sensor Field of the Invention

This invention relates to a multi-arm fiber optic sensor and refers particularly, though not exclusively, to a multi-arm fiber optic sensor where each arm has a Bragg grating. The invention also relates to such a sensor used as a force sensor able to compensate for temperature variation, and such a sensor for obtaining temperature variations.

Reference to Related Application

In international patent application number PCT/SG01/00239 filed 27 November 2001 there is disclosed a fiber optic force sensor using a Bragg grating (the "earlier application"). The contents of the earlier application are incorporated herein by reference.

Background to the Invention

The earlier application discloses an optic force sensor that uses a single arm of a fiber optic with a Bragg grating. A fiber optic force sensor using a Bragg grating that is able to compensate for temperature variation would be of great advantage. Such a sensor able to obtain temperature variations would also be of considerable advantage.

Summary of the Invention

In accordance with a preferred aspect of the present invention, there is provided a fiber optic sensor comprising: (a) a fiber optic having a plurality of operatively connected arms, each arm having a Bragg grating; (b) the fiber optic being embedded between a plurality of embedding layers; (c) the embedding layers comprising an upper portion with at least one upper layer and a lower portion with at least one lower layer.

The fiber optic sensor may be used as a force sensor able to compensate for temperature variation; and preferably is able to read temperature, or expansion/ contraction due to temperature.

In another aspect, there is provided a fiber optic force sensor comprising:

(a) a fiber optic having a plurality of operatively connected arms, each arm having a Bragg grating;

(b) the fiber optic being embedded between a plurality of embedding layers; (c) the embedding layers comprising an upper portion with at least one upper layer and a lower portion with at least one lower layer;

(d) the fiber optic force sensor being able to compensate for temperature variation.

For all aspects, each of the plurality of arms is parallel. The fiber optic may comprise a first arm operatively connected to a second arm by an arcuate portion, the first arm having a first Bragg grating and the second arm having a second Bragg grating. The first Bragg grating may be axially aligned with the second Bragg grating. Preferably, the first Bragg grating and the second Bragg grating are in different planes, with there being at least one embedding layer between them. The embedding material may be one or more of: metal, polymer, ceramic, carbon fibre composite, and so forth. Preferably, the embedded layers in contact with each Bragg grating have fibers with their axis aligned with an optical axis of the arm of the fiber optic on which the Bragg grating is written. More preferably, all other layers have fibers oriented either cross-ply or parallel-ply.

The upper portion may comprise a plurality of upper layers, each of the plurality of upper layers being identical and parallel. The plurality of upper layers may be oriented in a cross-ply arrangement and/or parallel-ply arrangement.

The lower portion may comprise a plurality of lower layers, each of the plurality of lower layers being identical and parallel. The plurality of lower layers may be oriented in a cross-ply arrangement and/or parallel-ply arrangement.

The number of upper layers may be the same as the number of lower layers. Alternatively, the number of upper layers may be different to the number of lower layers.

Brief Description of the Drawings

In order that the invention may be clearly understood and readily put into practical effect there shall now be described by way of non-limitative example only preferred embodiments of the present invention, the description being with reference to the accompanying illustrative drawings in which:

Figure 1 is an exploded perspective view of a first, preferred embodiment; Figure 2 is a perspective view of a second, preferred embodiment;

Figure 3 is a perspective view of the embodiment of Figure 1 ;

Figure 4 is a graph illustrating the temperature spectrum for the embodiments of

Figures 1 , 2 and 3; Figure 5 is two graphs illustrating the temperature effect for the embodiment of Figures 1 , 2 and 3; Figure 6 is a schematic illustration of a first force situation; Figure 7 is a graph of the result of Figure 6; Figure 8 is a schematic illustration of a second force situation; and Figure 9 is a graph of the result of Figure 8.

Detailed Description of Preferred Embodiments

To first refer to Figures 1 and 3, Figure 1 shows an exploded perspective view of a first embodiment of a sensor according to the present invention, and Figure 3 shows that same embodiment as assembled. Here, there is an optical fiber 203 with a fiber Bragg grating generally referred as 204, the optical fiber being embedded between embedding layers generally shown as 202.

As is explained in the earlier application at pages 5 and 6, the Bragg wavelength depends on the temperature of the Bragg grating 204. To be sensitive to the temperature, the optical fiber 203 is formed as a loop, with there being a first arm 205 and a second arm 206, the two arms 205, 206 being joined by an arcuate portion 207. First arm 205 has a first longitudinal axis 208 and second arm has a second longitudinal axis 209. Preferably, axes 208, 209 are parallel, and spaced apart. In that way the arms 205, 206 are parallel to each other.

Each arm 205, 206 has a Bragg grating 204. Preferably, the Bragg gratings 204 are axially aligned - they are at a location on each arm 205, 206 the same distance from arcuate portion 207. Each arm 205, 206 as well as their Bragg gratings 204, are in different planes with there being at least one embedding layer 201 between them. There may be more than one layer 201 between the arms 205, 206, and thus Bragg gratings 204. The embedding layer(s) 201 may be of any suitable material such as for example, metal, ceramic, polymer, carbon fiber composite, carbon fiber prepregs, and so forth. If a carbon fiber composite or carbon fiber prepregs, the fibers are preferably all aligned. For the layers of fiber composite immediately adjacent and/or in contact with the Bragg gratings 204, the axes of the fibers are preferably parallel to the optical axis of the arm 205, 206 on which the Bragg grating 204 is written. All other layers may be cross-ply or parallel-ply.

The layers 202 have an upper portion 210 and lower portion 211. The upper portion 210 has at least one layer (as illustrated) but preferably has a plurality of layers. The lower portion 211 preferably has at least one layer (as illustrated) but preferably has a plurality of layers. The layers of the upper portion 210 and lower portion 211 may be cross-ply or parallel-ply.

The number of layers in lower portion 211 may be the same as the number of layers in the upper portion 210. Alternatively, the number of layers in the lower portion 211 may be different to the number of layers in the upper portion 210. It is preferred for the layers in upper portion 210 to be the same as the layers in the lower portion 211. The orientation of the layers in the upper portion 210 is preferably the same as the orientation of the layers in the lower portion 211. If the layers in one of the portions 210, 211 are in a cross-ply arrangement, the layers in the other portion are preferably also in a cross-ply arrangement. Alternatively, the layers in one of the portions 210, 211 may be cross-ply, and the layers in the other portion may be parallel-ply. The arms 205 and 206 are aligned such that their axes 208, 209 are in planes generally parallel with the planes of upper portion 210 and lower portion 211 and are vertically aligned. Figure 2 shows a variation. Here, the axes 208, 209 are aligned at an angle to the horizontal and the vertical. The angle may be in the range 1° to 179°. For all forms of Figures 1 to 3, the optical fiber 203 may be located in or adjacent to the neutral layer between portions 210, 211, the location being as required or desired. Alternatively, the optical fiber may be remote from the neutral layer.

Due to the sensitivity to temperature, when there is a temperature differential between arms 205, 206 such as, for example, a temperature T| at arm 205 and a temperature T₂ at arm 206 (Figure 4), the temperature differential ΔT can be detected.

A fiber Bragg grating will reflect light that has a wavelength λt corresponding to twice its period λ, multiplied by the effective refractive index of the fiber n_θff that the propagating mode sees. λ^ = 2λn_Θff

This is called the Bragg condition, and λ^ is called the Bragg wavelength. Light at other wavelengths will be transmitted without significant attenuation. In other words, the grating operates as a narrow-band wavelength notch filter.

Any force or pressure or strain or stress or thermal energy applied to the fiber Bragg grating results in a shift in the Bragg wavelength _t of the sensor, which can be detected in either the reflected signal or spectrum l_ror in the transmitted signal or spectrum l . Since the measured information is encoded directly into wavelength, which is in absolute power, the resultant data acquired does not depend directly on the total light intensity, on losses in connectors or on the source power level. For this reason, a thermal and/or force sensor using a fiber Bragg grating is particularly insensitive to exterior deteriorations such as variations in the light intensity or light losses. Therefore, the thermal and/or force sensor according to the present invention is particularly reliable and robust.

As is shown in Figure 5, the effect of the temperature effect ΔT will depend on whether Ti is greater or less than T₂.

Another advantage of having two arms 205, 206 for optical fibre 203, and each arm having a Bragg grating 204, is that the sensor is able to respond to forces in two directions. This is shown in Figures 6 to 9. In Figures 6 and 7, the force is applied in one direction - either as a compressive force from above, or as a tensile force from below. The force differential ΔF between arms 205 and 206, can be detected. In Figures 8 and 9 the force is applied in the opposite direction - either as a compressive force from below, or as a tensile force from above. The force differential ΔF can be detected.

Therefore, the exerted force results in a variation of the predetermined optical transmittance and reflectance of the Bragg grating 204.

Therefore, a force sensor having the fiber Bragg grating 204 can be used in particular to measure a force applied perpendicularly to the fiber optical axis 208, 209 and perpendicular to the plane of the layer portions 210, 211 and is, therefore, capable of measuring a pressure exerted upon either one of the layer portions 210, 211. This pressure sensor can nevertheless be used also to measure a stress or strain exerted in the plane of the layer portions 210, 211 and, in particular in the longitudinal direction of the optical fiber 203. Accordingly, this sensor can also be used as strain sensor.

Generally speaking, a force sensor having a fiber Bragg grating embedded between the lower portion 211 and the upper portion 210 according to the invention represents an indirect sensing device.

In comparison to a direct sensing device and method using a bare fiber Bragg rating, the indirect sensing device and technique of the invention using an embedded fiber Bragg grating offers several advantages. First, in case of a pressure sensor according to the invention, when a force is applied to the pressure sensor by means of a plate or similar device, it is easier to determine a contact area of the plate with respect to the pressure sensor, i.e. to an upper or lower layer. This allows the pressure to be easily estimated. Second, the embedded fiber Bragg grating extends, both in case of pressure and of a strain sensor according to the invention, extends the range of forces that can be applied to the fiber Bragg grating with no permanent damage to the fiber Bragg grating. Furthermore, the response of the embedded fiber shows improved stability with respect to time as compared to that of a bare fiber Bragg grating used in direct sensing techniques. Such stability is observed when a static force is applied to both the bare and embedded fiber Bragg grating and the drift in the axial strain is observed over a period of time.

Naturally, the fiber Bragg grating sensor with two fiber optic arms 205, 206 can be used as a force sensor not affected by temperature variation. Preferably, the sensor is able to read temperature, or expansion/contraction due to temperature. Such a sensor is sometimes referred to as an "athermal" sensor. According to the invention, the optical fiber 203 may have a diameter that is comparable to a thickness of each layer, although this is not necessary.

The optical fiber 203 can be acrylate-coated, polyimide-coated or uncoated, which is selected according to which materials are contained in the layers. The coating, if such is provided preferably has a thickness of typically 10 microns.

In the embodiments of Figures 1 to 3, each layer is made of a composite material comprising a polymer material and elongated carbon fibers being arranged in parallel within each layer. Each layer may be thermally and/or electrically insulating.

In alternative embodiments, each layer can comprise glass fibers instead of or in addition to the carbon fibers.

In further alternative embodiments an epoxy resin material or a polyester resin material is used in the layers.

Preferably, if at least a part of the layers is made of a material comprising elongated fibers, the optical fiber 203 has a diameter that is considerably larger than a diameter of the elongated fibers. This provides a smoother bending of the optical fiber 203 embedded between the lower portion 211 and the upper portion 210.

The thickness and number of the layers of portions 210, 211 should be selected such that the sensor has a predetermined sensitivity to the external force, F. Although a fiber optic with two arms 205, 206 has been described the arms 205, 206 being joined by an arcuate portion 207, and each arm 205, 206 having a Bragg grating 204, it will be realized that three or more (e.g. three, four, five, six and so forth) arms could be provided, each with a Bragg grating 204.

Whilst there has been described in the foregoing description various embodiments of the present invention, it will be realized by those skilled in the technology concerned that many variations or modifications in details of design or construction may be made without departing from the present invention.

Claims

The claims

1. A fiber optic sensor comprising: (a) a fiber optic having a plurality of operatively connected arms, each arm having a Bragg grating; (b) the fiber optic being embedded between a plurality of embedding layers; (c) the embedding layers comprising an upper portion with at least one upper layer and a lower portion with at least one lower layer.

2. A fiber optic sensor as claimed in claim 1 , wherein the fiber optic sensor is used as a force sensor.

3. A fiber optic sensor as claimed in claim 2, wherein the force sensor is not affected by temperature.

4. A fiber optic sensor as claimed in claim 3, wherein the sensor is able to read at least one of: temperature, and expansion/contraction due to temperature.

5. A fiber optic sensor as claimed in any one of claims 1 to 4, wherein each of the plurality of arms is parallel.

6. A fiber optic sensor as claimed in any one of claims 1 to 5, wherein the fiber optic comprises a first arm operatively connected to a second arm by an arcuate portion, the first arm having a first Bragg grating and the second arm having a second Bragg grating.

7. A fiber optic sensor as claimed in claim 6, wherein there is at least one embedding layer between the first arm and the second arm.

8. A fiber optic sensor as claimed in claim 6 or claim 7, wherein the first Bragg grating is axially aligned with the second Bragg grating.

9. A fiber optic sensor as claimed in any one of claims 1 to 8, wherein the upper portion comprises a plurality of upper layers, each of the plurality of upper layers being identical.

10. A fiber optic sensor as claimed in claim 8, wherein the plurality of upper layers are oriented in an arrangement selected from the group consisting of: cross-ply, and parallel-ply.

11. A fiber optic sensor as claimed in any one of claims 1 to 10, wherein the lower portion comprises a plurality of lower layers, each of the plurality of lower layers being identical.

12. A fiber optic sensor as claimed in claim 11, wherein the plurality of lower layers are oriented in an arrangement selected from the group consisting of: cross-ply, and parallel-ply.

13. A fiber optic sensor as claimed in claim 11 or claim 12 when appended to claim 9 or claim 10, wherein the number of upper layers and the number of lower layers are selected from the group consisting of: the same, and different.

14. A fiber optic sensor as claimed in any one of claims 1 to 13, wherein the embedded layers are of a material selected from the group consisting of: metal, polymer, ceramic, and carbon fiber composite.

15. A fiber optic sensor as claimed in any one of claims 1 to 14, wherein the embedded layers in contact with each Bragg grating has fibers with axis aligned with an optical axis of the arm of the fiber optic on which the Bragg gratings is written.

16. A fiber optic sensor as claimed in claim 15, wherein all other layers have fibers oriented in a manner selected from the group consisting of: cross- ply, and parallel-ply.

17. A fiber optic force sensor comprising: (a) a fiber optic having a plurality of operatively connected arms, each arm having a Bragg grating; (b) the fiber optic being embedded between a plurality of embedding layers; (c) the embedding layers comprising an upper portion with at least one upper layer and a lower portion with at least one lower layer; and (d) the fiber optic force sensor being able to compensate for temperature variation.

18. A sensor as claimed in claim 17, wherein the sensor is able to read at least one: temperature, and expansion/contraction due to temperature.

19. A sensor as claimed in claim 17 or claim 18, wherein each of the plurality of arms is parallel.

20. A sensor as claimed in claim 19, wherein the fiber optic comprises a first arm operatively connected to a second arm by an arcuate portion, the first arm having a first Bragg grating and the second arm having a second Bragg grating.

21. A sensor as claimed in claim 20, wherein there is at least one embedding layer between the first arm and the second arm.

22. A sensor as claimed in claim 20 or claim 21 , wherein the first Bragg grating is axially aligned with the second Bragg grating.

23. A sensor as claimed in any one of claims 17 to 22, wherein the upper portion comprises a plurality of upper layers, each of the plurality of upper layers being identical.

24. A sensor as claimed in claim 23, wherein the plurality of upper layers are oriented in an arrangement selected from the group consisting: cross-ply, and parallel-ply.

25. A sensor as claimed in any one of claims 17 to 24, wherein the lower portion comprises a plurality of lower layers, each of the plurality of lower layers being identical.

26. A sensor as claimed in claim 25, wherein the plurality of lower layers are oriented in an arrangement selected from the group consisting of: cross- ply, and parallel-ply.

27. A sensor as claimed in claim 25 or claim 26 when appended to claim 23 or claim 24, wherein the number of upper layers and the number of lower layers are selected from the group consisting of: the same, and different.