METHOD OF MANUFACTURING A FIBRE-OPTICAL STRAIN-GAUGE AND STRAIN-GAUGE MANUFACTURED BY SAID METHOD

Abstract: In a fibre-optical strain-gauge of the kind comprising a mechanical construction providing a linear, strain-dependent relative movement of oppositely positioned spaced-apart first and second optical fibre-holding elements in a direction substantially perpendicular to the axial direction of the optical fibres, and in which the varying transmission of light between the two fibres is used as an indication of the strain, the main new feature is that the strain-gauge is manufactured using the following steps: a) prestressing an optical fibre by applying a pulling force, or prestressing the mechanical construction by applying a compressing force; b) fixing said optical fibre to the fibre-holding elements in the mechanical construction; and c) cleaving the optical fibre at the interspace between the fibre-holding elements. With this arrangement the optical fibre ends are aligned in an exact position in a simple manner without using any expensive micro-manipulation tools, and a high precision of the gap between the two fibre ends can be provided in a simple manner by controlling the prestressing of the optical fibre, or the mechanical construction.

Description: METHOD OF MANUFACTURING A FIBRE-OPTICAL STRAIN-GAUGE AND STRAIN-GAUGE MANUFACTURED BY SAID METHOD. TECHNICAL FIELD The present invention relates to a method of manufacturing a fibre optic strain- gauge of the kind set forth in the preamble of claim 1 and a fibre-optical strain- gauge manufactured in accordance with said method. BACKGROUND ART In strain-gauges of this kind it is known to position two optical fibres oppositely in order to transmit light from one fibre to the other and to provide a strain-dependent relative movement of the fibres, whereby the transmission of light is dependent on the strain. In such systems it is most common to use focusing lenses in order to have a relatively high transmission of light and the use of fibres without any optical elements is setting a high demand on the precision of the positioning of the fibres which is at least expensive and difficult to achieve. In this connection both alignment and mutual distance between the fibres have to be controlled in an exact manner, in order to achieve a stable correlation between strain and transmission of light. One example of a fibre-optical strain-gauge of this kind is described in US- 4,611 ,378. This document describes a method for positioning optical fibres on fibre-holding elements in alignment about a mechanical quiescent point in the absence of strain. Due to the use of relatively thick fibres, e.g. 0,3 mm 0,6 mm and 1 ,0 mm in diameter, the demand on the precision of the positioning of the fibres is not very high and this document does not address any of the problems in connection with precise alignment and positioning of the fibres. Other types of fibre-optical strain-gauges use bending, stretching and compression of the light transmission passage between the optical fibre ends, and is e.g. described in US-5, 812,251. Due to the necessary relatively long distance between the optical fibre ends in the constructions described in this document, however, these types of strain-gauges require the use of lenses in order to achieve an acceptable output signal. DISCLOSURE OF THE INVENTION It is the object of the present invention to provide a method of manufacturing a fibre-optical strain-gauge of the kind referred to above and a strain-gauge manufactured by said method, with which it is possible to achieve a very precise alignment and positioning of the two oppositely positioned optical fibres in the strain-gauge construction in a simple manner, and this object is achieved with a method of manufacturing a fibre-optical strain-gauge of said kind, which according to the present invention comprises the steps set forth in the characterising clause of claim 1. With this arrangement an exact alignment of the two optical fibres is automatically achieved in a simple manner and the distance between the two fibre ends can be controlled by controlling the prestressing of the optical fibre, or the mechanical construction, i.e. controlling the applied force. Preferred embodiments of the method are revealed in the subordinate claims 2-8 and a fibre-optical strain-gauge manufactured in accordance with the method and preferred embodiments thereof are revealed in the claims 9-12. BRIEF DESCRIPTION OF THE DRAWINGS In the following detailed part of the present description, the invention will be explained in more detail with reference to the exemplary embodiment of a strain- gauge manufactured in accordance with the invention shown in the drawings, in which Figure 1 shows a stripped optical fibre ready to be fixed in the fibre-holding elements in the mechanical construction of the strain-gauge, Figure 2 shows the fibre in Figure 1 after cleaving, Figure 3 shows a mechanical construction providing a linear strain dependent relative movement of the two fibre ends, Figure 4 shows an alternative mechanical construction for providing the linear strain dependent movement of the fibre ends, Figure 5 shows a schematic representation of the mechanical constructions in Figures 3 and 4 for explanation of the conversion of the axial movement of the fixation parts into oppositely directed movements of the holding elements, and Figure 6 shows another alternative mechanical construction for providing the strain dependent relative movement of the fibre ends. DESCRIPTION OF THE PREFERRED EMBODIMENT The optical fibre shown in Figure 1 has been prepared for use in the method in accordance with the present invention for manufacturing a fibre-optical strain- gauge by stripping of the protective coating 3 in a zone, in which the fibre is to be cleaved after fixing in the mechanical construction as shown in Figure 3. The optical fibre consists of a core 1 surrounded by a cladding 2 and a protective coating 3. The optical fibre shown in Figure 1 is prestressed by applying a pulling force, or the mechanical construction is prestressed by applying a compressing force, and the optical fibre is fixed in the holding elements 9, 10 in the mechanical construction shown in Figure 3, e.g. by gluing. After hardening of the glue the optical fibre is cleaved by scoring the cladding 2 using a hard-metal knife, whereby the optical fibre is cleaved as shown in Figure 2 and a certain gap is established between the two parts of the cleaved optical fibre. The gap, as shown in Figure 2, will be dependent on the prestressing of the optical fibre or mechanical construction before fixing, and is preferably approximately 5-50 μim, more preferred approximately 10-30 μm and further more preferred 10-20 μm. The transmission of light from one part of the fibre to the other across the established gap is dependent on the mutual position of the two fibre ends and in the non-strained condition the fibre ends are, due to the method of manufacturing, positioned exactly aligned, as shown in Figure 2. The mechanical construction shown in Figure 3 provides a strain dependent, relative movement of the oppositely positioned optical fibres in a direction substantially perpendicular to the axial direction of the optical fibres whereby the transmission of light between the two fibres can be used as an indication of the strain. The mechanical construction shown in Figure 3 comprises two fixation zones 5, 6 for fixing the mechanical construction to the surface, the strain of which is to be measured, two double-hinge zones 7, 8 for converting the axial relative movement of the fixation zones 5, 6 into oppositely directed vertical movements of the holding elements 9, 10 positioned in the fibre-holding zone 4. The mechanical construction further comprises grooves 11 , 12 in order to be able to position the fibre in the mechanical construction running from one end to the other through the grooves 11 , 9, 10 and 12. The fixing of the optical fibre to the holding elements 9, 10 is preferably provided by gluing the optical fibre in the two grooves 9, 10. The mechanical construction shown in Figure 4 corresponds essentially to the mechanical construction shown in Figure 3 but the relative movement of the holding elements 9, 10 is in Figure 4 in a horizontal direction. Figure 5 is a sketch showing the principle of providing the oppositely directed strain-dependent, relative movements of the fibre-holding elements 9, 10. Movement of the fixation zone 5 towards the fixation zones 6 will by means of the double hinges 7 and 8 be converted into a movement of the holding elements 9, 10 in the direction of the arrows in the fibre-holding zone 4. The special shape, T-profile mechanical construction shown in Figure 6 comprises two parts 13, 14 of composite material. The lower part 13 is of smaller width than the upper part 14 and is intended to be mounted on the surface of the construction to be measured. A groove 16 for mounting the optical fibre is provided in the upper part 14 running traversely from one side of the construction to the other offset towards one end of the construction. A hole 15 in line with the groove 16 is provided through the upper part 14 close to but outside the edge of the lower part 13. The cleaving of the optical fibre is performed in said hole 15 and strain in the construction will stretch the lower part 13 and the central part of the upper part 14, whereas the edge part of the upper part 14 will be substantially unaltered by such strain, whereby the desired relative movement of the optical fibre ends is provided by the warping of the upper part 14 in the edge area. Different sensitivities can be obtained by changing the relative position of the groove 16 in the upper part 14. This composite material mechanical construction is especially advantageous for measuring strain on composite material constructions such as wind turbine blades, due to the common thermal expansion coefficient of the construction to be measured on and the mechanical construction of the strain-gauge. If it is desired to produce a strain-gauge with an offset in the unstrained condition, this can be provided by prestressing the mechanical construction during fixation of the optical fibre, where this prestressing naturally is performed in a direction perpendicular to the mentioned prestressing for providing the air gap between the fibre ends. In the foregoing, the fibre-optical strain-gauge has been described with reference to the preferred embodiments shown in the figures and it will be obvious to a man skilled in the art, that many modifications can be made within the scope of the following claims without departing from the basic principle of the invention. Among such modifications it should be mentioned that the mechanical construction could naturally be any construction in which there is provided a strain-dependent, relative movement of the oppositely positioned fibre-holding elements, as long as the fibre can be positioned in the fibre-holding elements and fixed in a prestressed condition before cleaving, and where the mutual position of the fibre ends can be maintained after cleaving, in such a way that the exact alignment of the fibre ends is maintained until the strain-gauge is mounted on the surface, on which to measure.

Claims: CLAIMS 1. Method of manufacturing a fibre-optical strain-gauge of the kind comprising a mechanical construction providing a strain dependent, relative movement of oppositely positioned spaced apart first and second optical fibre- holding elements in a direction substantially perpendicular to the axial direction of the optical fibres and using the varying transmission of light between the two fibres as an indication of the strain, characterized by said method of manufacturing comprising the following steps: a) prestressing an optical fibre by applying a pulling force, or prestressing the mechanical construction by applying a compressing force, b) fixing said optical fibre to the fibre-holding elements in the mechanical construction, and c) cleaving the optical fibre at the interspace between the fibre-holding elements. 2. Method in accordance with claim 1, characterized by further comprising before step a) performing a stripping of the optical fibre adjacent the part of the fibre which is to be cleaved in step c. 3. Method in accordance with claim 1 or2, characterized by further comprising performing the fixing in step b) by positioning the fibre in grooves formed coaxially in the fibre-holding elements, gluing the fibre in said grooves, and hardening the glue before performing step c). 4. Method in accordance with any of the preceding claims, characterized by further comprising that the cleaving of the fibre in step c) is performed by scoring the fibre. 5. Method in accordance with claim 4, characterized by the scoring being performed by means of a hard-metal knife. 6. Method in accordance with any of the claims 1-5, characterized by the prestressing of the optical fibre or the mechanical construction in step a) being preformed to an extent, which after cleaving in step c) provides an exact gap within the range 5-50 μm. 7. Method in accordance with claim 6, c h a r a c t e r i z e d by the prestressing of the optical fibre or the mechanical construction in step a) is performed to an extent which after cleaving in step c) provides an exact gap within the range 10-30 μm. 8. Method in accordance with claim 7, c h a r a c t e r i z e d by the prestressing of the optical fibre or the mechanical construction in step a) is performed to an extent which after cleaving in step c) provides an exact gap within the range 10-20 μm. 9. Fibre-optical strain-gauge characterized by being manufactured in accordance with any of the preceding claims. 10. Fibre-optical strain-gauge in accordance with claim 9, c h a r a c t eri z e d by the mechanical construction comprising two symmetrically disposed fibre-holding elements (9, 10), each connected to two mounting elements (5, 6) for mounting the strain-gauge to the surface of the construction to be measured, by means of double hinges (7, 8), the double hinges for the first fibre-holding element (10) having the hinge close to the fibre-holding elements positioned close to a first surface of the mechanical construction and the hinge distant from the fibre-holding element positioned at a distance from said first surface, whereas the hinges for the oppositely positioned fibre-holding element (9) have the hinges close to the fibre- holding element positioned close to a second surface opposite said first surface of the mechanical construction and the hinges distant from the fibre-holding element positioned at a distance from said second surface, whereby the fibre-holding elements (9, 10) will move in opposite directions when moving the mounting elements (5, 6) towards each other and vice versa. 11. Fibre-optical strain-gauge in accordance with claim 9, c h a r a c t e r i z e d by the mechanical construction comprising a special shape, T-profile of composite material with a first part (13) of smaller width, intended to be connected to the construction to be measured and a second part (14) of greater width positioned symmetrically above the first part (13) and comprising a traversing groove (16) for mounting the optical fibre and a hole (15) positioned close to the edge of the first part (13) and centrally relative to the groove (16), the cleaving of the optical fibre being performed in said hole (15). 12. Fibre-optical strain-gauge in accordance with claim 9, 10 or 11, c h a r a c- t e r i z e d by the optical fibre being of the type usually used for telecommunication purposes, with a fibre diameter of approximately 125 μm. 13. Fibre-optical strain-gauge in accordance with any of the claims 9 - 12, c h a r- a c t e r i z e d by the material of the mechanical construction being selected to have the same thermal expansion coefficient as the material of the surface on which to measure.

Inventors: THORSEN PER, THORSEN, PER
Applicants: THORSEN PER, VESTAS WIND SYS AS, THORSEN, PER, VESTAS WIND SYSTEMS A/S
Application year: 1999
Main classification: G01L1
IPC codes: G01D5, G01L1, G02B6