Abstract:
A tension sensor including an optical fibre having a Bragg grating is disclosed, the sensor including a housing and a support member shorter that the housing coaxially disposed therein and connected to the housing at one end. The fiber runs along the longitudinal axis of the housing and is connected to one end of the housing and to at least one location on the support member. A second Bragg grating may be provided within the support member to allow for additional measurements for temperature compensation purposes.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a device for measuring tensile forces as defined in the introductory part of claim  1 . 
     BACKGROUND OF THE INVENTION 
     The present invention is based on the principle of utilizing a fibreoptical Bragg grating. A Bragg grating is single modus fibre with permanent periodic variation of the refractive index over a fibre length of, for example 0.1 to 10 cm. The variation in the refractive index is established by illuminating the fibre with a UV laser. A Bragg grating reflects light with a wavelength that depends upon the refractive index and the space related period of the variation of the refractive index (the grating period), while light beyond this wavelength will pass through the grating more or less unhindered. The light reflected by the Bragg grating will exhibit a wavelength that varies as a function of a measurable quantity that changes the refractive index of the fibre material grating and/or the fibre length in the gratina zone (grating period). Tension in the fibre or temperature variations will therefore lead to a change of the wavelength of the light reflected by the Bragg grating. 
     For practical purposes one can, for example measure the temperature in the region −100° C to +250° C with (in the order of) 20 different points along the fibre for fibres with a length of up to 50-100 km. Using various multiplexing techniques, the number of measurement points can be increased. Examples of areas of application are temperature surveillance of power cables, pipelines, electrical transformers, engines and temperature monitoring of industrial processes. 
     A number of devices for measurement of tension in mechanical constructions exist. For special purposes where there is little space available, high temperature. high tension and so forth, all known devices for measurement of tensile forces have functional disadvantages. For example present measurement of tension under water is made with tensile sensitive sensors based on electrical elements, which in such environments exhibit low reliability. For other areas of application there may be little space available for installing extra components, such as tension sensors based on electrical induction or capacity (typical diameter 10-20 mm). Another example is the surveillance of darn with sensors based on electrical strain gauges. In such connections lightening strikes have sometimes rendered the sensor elements or the electronic circuits passive, and thus disabled the tension surveillance. 
     Accordingly there is a need for a tension sensor with mainly passive components that can be utilized in difficult environments and narrow spaces. 
     The objective of the present invention is to provide a device of this type for tension measurement in and on mechanical constructions. 
     SUMMERY OF THE INVENTION 
     This objective is achieved with a device according to the characterizing part of claim  1 . Beneficial features are disclosed by the dependent claims. 
     The invention relates to a device for measuring tension in mechanical constructions, the device comprising: 
     an optical fibre provided with a first Bragg grating, 
     an elongated housing arranged to encompass the optical fibre and to be attached to the construction to be measured, whereby the housing includes a first end and a second end and includes a first attachment site at the first end of the housing in order to establish a solid attachment between the housing and the optical fibre, 
     an elongated support member with a mechanical strength greater than the strength of the optical fibre and with a length shorter than the length of the housing. whereby one end or section of the support member is solidly attached to the housing at the second end of the same, and a second end extending freely along a part of the length of the housing. the support member exhibiting a second attachment site in order to establish a solid attachment between the support member and the optical fibre, 
     thus establishing a segment of an optical fibre comprising said Braog grating strapped between said first and said second attachment site of the housing and the elongated support member respectively . 
     This principal design of a tension sensor renders it possible to produce tension sensors with very small dimensions and with a measurement range from low tensions to tensions of several thousand microstrain in distant positions. The device also has the possibility of measuring tension in different positions along the same optical fibre. 
     Examples of mechanical constructions is meant constructions which can benefit from the invention are bridges, dams, platforms. cables, flexible pipes and the like. 
     To compensate for temperature related variations in measurements detected by the first Bragg grating, the support member preferentially includes a third attachment site for an optical fibre, localized in the region between the second point of attachment and the holding member of the section of the support member that extends freely along the housing, whereas the optical fibre exhibits a second Bragg-grating (reference grating) localized between the second and the third attachment sites. Since the reference grating is arranged in the part of the support member which is free in relation to the housing, there will only be minor strain on it from mechanical strain that is exerted to the housing, so that variations in measurements conducted by the reference grating mainly relate to temperature variations. 
     In a preferred embodiment of the invention the housing has a generally cylindrical shape and a generally cylindrical bore, the support member is a generally cylindrical shaped tubing with an external diameter less than the internal diameter of the housing. 
     The support member is preferably constructed from a material with the same thermal expansion coefficient as the surrounding construction to be measured, thus compensating tension loads which are merely temperature related. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The invention is described in further detail by reference to a preferred embodiment illustrated by the accompanying drawings, where 
     FIG. 1 shows an axial crossection schematic of an example of the device according to the invention for monitoring tension in mechanical structures, and 
     FIG. 2 shows an example of the practical utilization of the principle of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows a first embodiment of a tension sensor according to the invention. The tension sensor includes an elongated generally cylindrical housing  101  with an inner cylindrical bore  102  and with a first end  101   a  and a second end  101   b , arranged to encompass an optical fibre  120  extending through the bore  102  of the housing. 
     Inside the bore  102 , at the second end  101   b  of the housing is arranged a support member in the form of a generally cylindrical tube  103  with a diameter less than the internal diameter of the housing  101  and a length shorter than the length of the housing  101 . The internal tube  103  extends generally coaxially with the housing  101  and has an internal opening with dimension sufficient to encompass optical fibre  120 . The tube  103  is in its one end solidly attached to the second end  101   b  of the cylindrical housing  101  by means of a holding member  110 , such as a glue joint, and extends freely into the cylindrical bore  102  of the housing  101  thus establishing a ring shaped room  109  between the external surface of the tube  103  and the internal surface of the housing  101 . The free part of the tube  103  is thus not exposed to tension forces that is exerted to the housing  101 . 
     At the first end of the housing  101   a  is arranged a first point of attachment  111  for the optical fibre  120  to the housing  101 . In this embodiment the first point of attachment  111  also constitutes a sealing or a barrier against intrusion of pressure or fluids from the surroundings, and may be established, for example, in the form of a glued joint arranged sealingly in the space between the fibre  120  and the internal surface of the housing  101 . 
     The fibre includes a first Bragg grating  121  freely strapped between the first point of attachment  111  and a second point of attachment  112  localized at the free end of the internal tube  103 , thus providing a strapped fibre segment between the housing  101  by the point of attachment  111  and the internal tube  103  by the second point of attachment  112 . 
     A second Bragg grating  122  is strapped between the second attachment site  112  and a third attachment site localized to the part of the tubing  103  which is not exerted to tension forces that effect the housing  101 , i.e. in a distance from the holding member  110 . 
     The housing  101  to be exposed to tension is externally attached to the surrounding mechanical construction to be monitored along the entire length of the housing ( 101 ) (e.g. by gluing) or at both ends of the housing. 
     When the housing  101  is stretched the first Bragg grating  121  will undergo the same total elongation as the housing when the tension is transferred by the tubing  103 . The second Bragg grating  122  and the tubing  103  will experience an elongation that is substantially less as the mechanical strength (crossection and elasticity modulus) of the tube  103  is substantially greater than that of the fibre. 
     The optical fibre  120  is made of glass with a small diameter and must be protected against the surroundings in most of the practical applications in the embodiment shown in FIG. 2, which shows an alternative embodiment in the form of an elongated tube, separate tubes  130  and  131  extend partially into the bore  102  of the housing  101   a  nd are solidly attached to the housing  101 . This embodiment provides extra protection and may be utilized on both sides of the housing  101 . The tubes  130  and  131  are attached to the housing  101  so that each end may constitute or be a part of the attachment site  111  and the holding member  110  respectively, as shown in FIG.  2 . The Bragg gratings are for the sake of clarity not shown here. 
     If the housing  101 , which is exposed to tension, is attached at each end of the housing  101 , e.g. at the holding members  111  and  110 , and is exerted to a force F (not illustrated), the elongation ε H  may be expressed by the following equation:          ε   H     =         Δ                 H     H     =     F       E   H     +     A   H                                  
     here delta ΔH is the absolute change in length of the housing  101 , H is the starting length of the housing  101  (alternatively between the attachment sites (not shown) on the housing to the construction to be measured), A H , is the crossectional area of the walls of the housing  101   a  nd E 11  is the elasticity modulus for the material of the housing. As made evident from the equation above. the specific extension compression of the housing will increase with increasing force and with decreasing strength (crossectional area and elasticity modulus) of the housing. 
     If the housing  101  is solidly attached to the surrounding construction along its entire length H. e.g. by gluing or welding, the relative extension ε H  of the housing will be the same as for the surrounding construction. 
     Since the real extension ΔH of the housing  101  is the same as the extension of the fibre with the Bragg grating  121  between the attachment sites  111  and  112 , the specific extension ε 1  of the grating  121  may be expressed by the following equation:          ε   1     =         H     L   1       ·     ε   H       =       H     H   -   R       ·     ε   H                                
     where H is the length of the housing  101 , L 1  is the length of the fibre with the Bragg grating  121  (between attachment sites  111  and  112 ) and ε H  is the specific extension of the housing  101   a  s defined by the previous equation. R is the length of the internal tube  103  between the attachment site  112  and the second holding member  110 . From this equation one can recognise that using a longer tube  103  as compared to the grating length L 1  (R+L 1 =H),will lead to an enhancement of the grating extension ε 1  compared to the extension ε H  of the housing  101 . 
     In one embodiment the entire device, with the obvious exception of the fibre, is constructed of metal. Metal materials are generally preferred as they combine high mechanical strength with weldability and a certain degree of elasticity, which is required for applications for the measurement tensile forces. For applications where small diameters of the sensors are required, both housing  101  , internal tube  103  and protecting tube  130 ,  131  can be made from cannula tubes. The tension sensors may thus be produced with diameters of the housing and tubes in the order of 1 mm in diameter. For measuring forces the crossection of the housing exposed to strain and the mechanical properties of the material of the housing, is adjusted according to the force range to be measured. 
     To compensate temperature dependent wavelength displacement with relation to light reflected from the Bragg grating, which primarily is caused by a change in the refractive index of the fibre material as a direct function of the temperature change, a reference grating  122  (Bragg grating) is established. As indicated above the reference grating is arranged between the attachment sited  112  and  113  in the internal tube  103 . Thus this grating, is only to a small extent exposed to mechanical forces caused by tensions on the housing  101  or forces that propagate along the fibre  120 . The device is calibrated at different temperatures to achieve a measurement of wavelength displacement as a function of tension which is as unaffected by temperature as possible. 
     If the internal tube  103  is manufactured from the same material as the surrounding construction, the sensor may be calibrated to eliminate extension caused by thermal expansion of the surrounding structure. The sensor will thus only respond to extensions caused by mechanical tension. 
     From the foregoing description it is evident for a person skilled in the art that the different components do not necessarily have the geometry showed by the drawings. Consequently, the member to be exposed to tension may for example, have a crossection that deviates from a circular shape. it may be oval, square etc. The same applies to the other components. The central issue with the invention is, however, that the member to be exposed to tension shall be able to transmit extension to the connective member and further to the connected Bragg, grating. 
     The invention thus provides a device for measuring tension in mechanical constructions. which enables measurement over a broad range of tensions and with high precision and which compensates for deviations caused by temperature fluctuations. In addition the device according to the invention can be designed to be very small and can therefore be installed in places where measurement usually has not been possible. Another advantage with the device according to the invention is that the fibre is not exposed to external hydrostatic pressure, and will therefore exhibit a high reliability. Finally, this design does not require pressure tight connections for the fibre.