Measuring and testing – Specimen stress or strain – or testing by stress or strain... – By loading of specimen
Reexamination Certificate
2000-06-05
2001-08-21
Noori, Max (Department: 2856)
Measuring and testing
Specimen stress or strain, or testing by stress or strain...
By loading of specimen
C250S227140, C385S013000
Reexamination Certificate
active
06276215
ABSTRACT:
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.
REFERENCES:
patent: 5182779 (1993-01-01), D'Agostino et al.
patent: 5564832 (1996-10-01), Ball et al.
patent: 5682445 (1997-10-01), Smith
patent: 5745615 (1998-04-01), Atkins et al.
patent: 5889901 (1999-03-01), Anderson et al.
patent: 5940556 (1999-08-01), Moslehi et al.
patent: 5973317 (1999-10-01), Hay
Dennison, Scheiner Schultz & Wakeman
Noori Max
Optoplan AS
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