Optical fibre sensors

Abstract: A sensor comprises an optical fibre 24 and a casing 22 formed from first and second elongate, flexible strip portions which sandwich the fibre therebetween. At least one of the strip portions is substantially inextendable and formations are provided on it to cause microbending of the fibre in response to an applied force.

Description: This invention relates to elongate and flexible sensors, primarily for sensing an applied force or pressure at points distributed along their length, which employ an optical fibre which is bent in operation of the sensor so that the light transmittance of the fibre changes and can be detected. Such a use of an optical fibre has been known for several years. In a known example described in US 4572950 and US 4618764, in order to achieve the required bending of the fibre, known as "microbending", in response to an applied force, a filament is wound helically around the fibre and the applied force causes the fibre to be bent into the spaces between the turns of the filament. Winding of the filament complicates manufacture of the sensor, as too does the need for an outer sheath for mechanical protection if the sensor is to be of any practical use. The application of the helical filament and sheath to only a portion of the fibre, if required, is inconvenient. JP-A-60-185626 and JP-A-59-128009 describe a sensor fitted to a window frame of a car door having an electrically-operated window. The sensor has first and second strips or members apparently of resilient material which sandwich the optical fibre between them. One of the strips has a series of transverse projections which engage the fibre, and when pressure is applied, the fibre bends locally. Another type of sensor is described in EP-A-0240100, and comprises a pair of opposed jaws gripping the fibre between them. Each jaw has projections opposing flat areas on the other jaw, and microbending of the fibre around these projections permits the sensing of a pressure which tends to close the jaws. As mentioned in the United States patents cited above, however, such an arrangement is not readily adapted to provide an elongated flexible sensor of the type presently desired. In accordance with the present invention, the fibre is encased between two strip portions, with at least one of these strip portion being of a flexible but substantially inextendable material, which may for example be an extruded and pre-stretched plastics material similar to that used for packaging strapping. This first strip portion, at least, is provided with a series of tranverse ridges, formed for example by embossing, which cause microbending of the fibre under pressure. Thus, a sensor is produced which can be made strong enough in itself to resist mechanical damage without needing to be secured to a rigid structure for support, and which is thus of a general purpose nature. The strip portion or portions can easily be provided over a vast range of lengths to provide a sensing length as small as 10mm or less, or as long as many kilometres. In one arrangement, the fibre is sandwiched between two such inextendable strip portions, but in other arrangements, only one such inextendable strip portion is provided, with the first strip portion and fibre encased in a sheath, part of which may thus form the second strip portion, or alternatively with just the inextendable strip portion and a further strip encasing the fibre. The two strip portions may be formed by a single strip which is folded longitudinally to encase the fibre. In this case, the edges of the strip portions opposite the fold may be secured together. This provides for easy manufacture and assembly of the sensor. In embodiments of the invention, each of a pair of strip portions encasing the fibre may be formed with a series of transverse ridges, while means are provided to locate the strip portions relative to each other so that the ridges of the two series are staggered, ensuring that microbending can occur in the most effective manner. The locating means may be provided by the fold between the strip portions in the case of a folded sensor as mentioned above, or by inter-engaging formations on the strip portions such as inter-engaging projections and holes, or inter-engaging series of ridges and grooves. It will be appreciated that a sensor embodying the invention may comprise the above and other features as described below or as set out in the accompanying claims. Specific embodiments of the invention will now be described by way of example, with reference to the accompanying drawings in which: Figure 1 is an overall view of an optical fibre sensor according to the invention; Figure 2 is a cross-sectional view of a strip used in forming one embodiment of the sensor; Figure 3 is a sectional view taken on the line 3-3 shown in Figure 2; Figure 4 is a cross-sectional view of the sensor; Figures 5 to 8 are cross-sectional views of alternative forms of sensor; Figures 9a and 9b are sectional views taken along the lines 9-9 shown in each of Figures 4 to 7, with Figure 9a showing the sensor without an applied pressure, and with Figure 9b showing the sensor with pressure applied; Figure 10 is a sectional view taken along the line 10-10 in Figure 5; Figure 11 is a sectional view taken along the line 11-11 in Figure 7; Figure 12 is a cross-sectional view of a further sensor according to the invention; Figures 13a and 13b are sectional views taken along the line 13-13 in Figure 12, with Figure 13a showing the sensor without an applied pressure, and Figure 13b showing the sensor with an applied pressure; Figures 14,16,17 and 19 are cross-sectional views of four further sensors according to the invention; and Figures 15 and 18 are sectional views taken along the lines 15-15 and 18-18 in Figures 14 and 17, respectively. Referring to Figure 1 of the drawings, an optical fibre sensor 20 is overall of strip form and has an outer casing 22, through which extends an optical fibre 24. A light signal may be applied to one end of the optical fibre, and the light which is transmitted to the other end of the optical fibre may be detected. The sensor 20 is constructed so that normally there is a substantial transmission of light from one end to the other of the optical fibre, but when the casing 22 is pressed part way along its length, as shown by the arrows 25 in Figure 1, microbending occurs in the fibre, for the reasons described below, and the amount of transmitted light is reduced. There are many applications for the sensor 20. For example, it may be used in a similar arrangement to that of the above-mentioned Japanese specifications as a "safe edge". It may be arranged around industrial machines to define a safe area, it may be distributed around a property for use in a burglar alarm system, or it may be distributed freely around open ground for military use. Referring to Figures 2 and 3, the casing 22 is formed as an extruded strip of plastics material such as polyethylene, polypropylene, or polyester which may be stretched and reduced in cross-sectional size to give a favourable molecular structure. Similar techniques are used in the manufacture of plastic strapping for packaging uses. The extruded and stretched strip is then acted upon by a roller with various surface formations so as to produce in the strip two series 26a, 26b of transverse projections 28a, 28b and grooves 30a, 30b having a generally sinusoidal form as shown in Figure 3. A longitudinal groove 32 is also formed along the middle of the strip, with raised ridges 34a, 34b immediately to either side of the longitudinal groove 32. This longitudinal groove 32 serves as a fold line. Referring to Figures 4 and 9, the casing 22 is folded longitudinally to sandwich an optical fibre 35 between the series 26a, 26b of grooves and projections on the strip portions 36a, 36b to either side of the fold. The free edges 38a, 38b of the strip portions are held together, for example by tape over the outside of the strip or by glue, welding or clipping. The longitudinal ridges 34a, 34b engage each other to produce bowing of the strip portions 36a, 36b to leave a natural gap for the optical fibre 35 and to ensure that the fibre is only compressed when an external force is applied. Referring particularly to Figures 9a and 9b it will be noted that the transverse projections 28a on the strip portion 36a are aligned with the transverse grooves 30b of the other strip portion 36b, and that the transverse grooves 38a on the strip portion 36a are aligned with the transverse projections 28b on the strip portion 36b. That is to say, the projections of the two strip portions are staggered. As shown in Figure 9a, when no external force is applied, the optical fibre 36 is not deformed by the casing. However, as shown in Figure 9b, when an external force is applied, the strip portions move together, and the transverse projections 28a, 28b on the strip portions bend the optical fibre 35 into the transverse grooves 30a, 30b of the opposite strip portion, thus producing microbending of the optical fibre 35, which can be detected by detecting the amount of transmitted light through the optical fibre. Typically, the optical fibre 35 may have an overall diameter of 0.25mm and may be of the monomode or multimode type. Also, typically, the depth of the transverse grooves 30a, 30b may be 0.1mm, and the pitch of the transverse grooves may be between 0.9mm and 5mm depending on the fibre. Preferably, when the strip is folded to encase the optical fibre, the fibre is not held taught, but undulates slightly to allow for differential expansion and contraction between the optical fibre and the casing as a result of changing temperatures or stretching. If desired, rather than the optical fibre starting at one end of the casing and terminating at the other end, at the other end the optical fibre may be bent back on itself and extend a second time through the casing, so that the light source and light sensor for the optical fibre can both be located at the same end of the sensor strip. As shown in Figure 4, two elongate filaments 39 may be disposed within the casing to either side of the optical fibre 35, preferably of the same or slightly larger diameter than that of the optical fibre 35. These additional filaments act so as to limit over-travel of the strip portions 36a, 36b and over-stressing of the optical fibre 35 when an external force is applied. The additional filaments 39 are of such a flexibility so as to allow microbending of the fibre, but prevent it from being crushed. There now follows a description of a number of modifications to the sensor described above, in which like features are designated by like reference numerals. These modified sensors may include all of the features of the sensor described above, except where otherwise stated. Referring to Figures 5, 9 and 10, the casing of the sensor is formed, in this case, from two separate extruded strip portions 36a, 36b. Along the edge of each strip portion, there is a series of triangular shaped projections 40a, 40b and grooves which are inter- engaged to ensure the proper staggering of the series 26a, 26b of projections and grooves for microbending, and which are bonded in order to secure the strip portions 38a, 38b together. Referring to Figures 6 and 9, the strip portions 38a, 38b are, in this case, separately formed. The strip portion 38a has, along each edge, a series of upstanding headed pins 42 which snap-engage in corresponding holes in the strip portion 38b firstly to ensure correct staggering of the series 26a, 26b of projections and grooves for microbending, and secondly to secure the strip portions 38a, 38b together. Rather than as shown in Figure 6 where the strip portions 38a, 38b are substantially parallel, the headed pins 42 may be shorter in size to cause bowing of the strip portions 38a, 38b which will thus tend to provide an effect of centralising the optical fibre 35 and any additional filaments 38 within the casing. Referring to Figures 7, 9 and 11, the strip portions 38a, 38b are, in this case, integrally formed, and the free edge of the strip portion 38a has a hooked projection 44 therealong which is a snap fit with a formation along the edge of the strip portion 38b so as to hold the strip portions 38a, 38b in the folded arrrangement. Futhermore, to either side of the series 26a, 26b of projections and grooves for microbending, there are further series of triangular projections 46a, 46b, which as shown in Figure 11 are spaced apart when no external force is applied. When an external force is applied, the triangular projections 46a, 46b and grooves inter-engage in order to limit the degree to which the strip portions 38a, 38b can be pressed together, thus serving to protect the optical fibre 35. The modified sensor shown in Figure 8 is similar to the sensor of Figure 4, with the exception that the transverse projections are higher at their ends than in the middle, having a concave shaped upper surface. Therefore, in addition to causing microbending, the projections also serve to limit the degree of pressing together of the upper and lower strip portions 38a, 38b, thus protecting the optical fibre 35. Referring now to Figures 12 and 13, the series 26b of projections and grooves on the strip portion 36b is replaced by an elongate channel 48 into which is fitted an elongate resilient layer 50 of, for example, silicone rubber. As can be seen in Figure 13a, without any external force applied, the optical fibre 35 is not deformed by the casing, but as shown in Figure 13b, when an external force is applied, the resilient layer 50 presses the optical fibre into the grooves 30a between the projections 28a, in order to cause microbending. Referring now to Figures 14 and 15, an alternative form of sensor is shown. The sensor 52 comprises a strip 54 of flexible but inextendable plastics material having a series of transverse ridges 56 and grooves 58 embossed on one side of the strip. An optical fibre 60 lies along the ridged and grooved side of the strip 54, and a further strip 62 of resilient material such as a cork rubber composite is provided so that the strips 54,62 sandwich the optical fibre 60 between them. The assembly described so far has an outer protective sheath 64 which is of a flexible and possibly soft and extendable material such as a synthetic rubber. The sheath 64 may be extruded onto the layers 54,62, or may be formed by a strip which is folded and welded. The operation of the sensor shown in Figures 14 and 15 is similar to that described above with reference to Figures 12 and 13. Figure 16 shows a modification to the sensor of Figures 14 and 15. In order to facilitate manufacture and to ensure that the optical fibre is not stretched tight when the strips 54,62 and sheath 64 are applied, the fibre is mounted on the resilient strip 62 by adhesive tape 66 before being assembled with the inextendable strip 54 and the sheath 64. Thus, it can more easily be ensured that there are slight undulations in the optical fibre 60 when it is adhered to the resilient strip 62. Alternatively, the optical fibre 60 can be mounted on the inextendable strip 54 with adhesive tape prior to assembly with the resilient strip 62 and the sheath 64. Figures 17 and 18 show a further modification to the arrangement of Figures 14 and 15, in which the resilient strip 62 is omitted. In this case, the optical fibre 60 is sandwiched between the inextendable strip 54 and the upper portion of the sheath 64, and the sheath 64 is formed from a resilient material, such as synthetic rubber. Thus, when pressure is applied to the sensor 52, the upper portion of the sheath directly presses the optical fibre into the grooves 58 of the inextendable strip 54 to cause microbending, as shown in Figure 18. Figure 19 shows a modification to the arrangement of Figures 17 and 18. In this case, rather than providing a sheath 64 which encircles the inextendable strip 54 and the optical fibre 60, only the upper portion of the sheath is provided in the form of a strip 68 of resilient material, and this strip 68 is welded or adhered along its edges to the strip 54 of inextendable material 2. In the arrangements described above, the interior of the casing may be filed with a soft compressable material in order to prevent the ingress of dirt and liquids. In the arrangements described above, the series of ridges and grooves on the inextendable strip portion(s) need not be provided along the whole length of the sensor, but only in a particular portion or portions so that the sensor has sensitive and non-sensitive zones along its length.

Claims: An elongate, flexible pressure sensor (20) for detecting pressure applied at any point along the length of the sensor, or at least at any point within predetermined sensitive portions along said length, the sensor comprising an elongate optical fibre (24) and a casing (22) for the fibre, the casing including means for causing microbending of the fibre in response to external pressure being applied to the casing, at least within a sensitive portion of the sensor, characterised in that said microbending means comprises a first elongate, flexible but substantially inextendable strip portion and a second elongate, flexible strip portion arranged so that the fibre is sandwiched between the strip portions, in that at least the first strip portion has a surface provided along its length, at least within said sensitive portions, with a series (26A) of transverse ridges, and in that the second strip portion is adapted (26B) to bend the fibre over each ridge upon pressing together of the strip portions, so as to cause said microbending. A sensor as claimed in claim 1, wherein the second strip portion is substantially inextendable. A sensor as claimed in claim 2, wherein the first and second strip portions are formed from a single strip (22) which is folded longitudinally to encase the fibre. A sensor as claimed in Claim 3, wherein a formation (34A,34B) is provided along the strip adjacent to the fold to cause the strip portions to bow away from each other. A sensor as claimed in Claim 3 or 4, further comprising means (44) to hold together the edges of the strip portions opposite the fold. A sensor as claimed in any preceding claim, wherein said other strip portion is also provided with a series (26B) of transverse ridges for engaging the fibre, and means (40) being provided to locate the strip portions relative to each other so that the ridges of the two series are staggered. A sensor as claimed in Claim 6 when dependent on any of Claims 3 to 5, wherein the fold provides said locating means. A sensor as claimed in Claim 5, wherein the strip portions are separately formed (36A,36B) and secured together. A sensor as claimed in Claim 8, further comprising projections (42) formed on the first and/or second strip portion and engaging complementary holes in the other strip portion to secure the strip portions together and also serve as said locating means. A sensor as claimed in Claim 8, further comprising inter-engaging transverse ridges and grooves (40) on the strip portions, which are bonded together and also serve as said locating means. A sensor as claimed in any of Claims 1 to 5, wherein the second strip portion is formed with an insert layer (50) of resilient material for engaging the fibre, so that the fibre can be bent over the ridges by the resilient material. A sensor as claimed in claim 1, wherein the second strip portion (64,68) is of a resilient material. A sensor as claimed in claim 12, wherein the first and second strip portions (54, 62) are formed separately and held, together with the fibre, by a separately formed casing portion (64). A sensor as claimed in claim 12, wherein the second strip portion is formed by part of a sheath (64) around the first strip portion and the fibre. A sensor as claimed in claim 12, wherein the second strip portion (68) is secured along its edges to the first strip portion. A sensor as claimed in any preceding claim, wherein at least the first strip portion is formed from extruded and longitudinally pre-stretched plastics material. A sensor as claimed in any preceding claim, wherein the ridges (26a) are formed on at least the first strip portion by embossing. A sensor as claimed in any preceding claim, further comprising formations (46A,46B) on the first and second strip portion which come into engagment upon substantial pressing together of the strip portions to limit bending and stressing of the fibre. A sensor as claimed in any preceding claim, further comprising at least one elongate flexible filament (39) extending alongside the fibre to prevent crushing of the fibre.

Inventors: TRACEY PETER MARTIN, TRACEY, PETER MARTIN
Applicants: BESTQUINT LTD, BESTQUINT LIMITED
Application year: 1990
Main classification: G01L1
IPC codes: G01L1