DE8915071U1 - Temperature-insensitive fiber optic strain sensor - Google Patents

Description

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Feiten & Guilleaume Energietechnik AG, D-5000 Cologne 80 Description: Temperature-insensitive

The invention relates to an optical waveguide sensor according to the preamble of claim 1.

DS 30 ZQ 966 describes an LVL sensor for strain gauges (an LVL strain sensor) in which at least one strand of a ketal or glass fiber reinforced plastic (GRP) is wound around the LVL and a wire-shaped, tensile-resistant sheath made of glass fiber reinforced plastic (GRP) is applied around it. Example: Primary coated LVT with 0.2 mm outer diameter, strand made of steel wire with a thickness of 0.03 mm, lay length in the mm range, GRP sheath with an outer diameter of around 2 mm. The interaction of the parameters LVL-strand-sheath is complex and difficult to reproduce, so that the manufacture of such an LVL sensor is complex.

An improvement is achieved with the LVL sensor for small tensile or compressive forces (LVL strain sensor) described in DE 89 00 090.9 Ul. It is essential that two coils made of a ketal wire or glass thread are wound in a cross lay around the primary coated LVL. Example: Kultimode LVL SO/125/175 (core-edge-primary coating diameter in μη», steel wire 0.09 mm thick, lay length 10 mm, GRP sheath as before. This LVL sensor is still temperature sensitive, so that some effort is required for it to function properly.

Therefore, the object of the invention is to design such an LVL sensor in such a way that it is temperature-insensitive.

The solution to this problem is specified by the characterizing features of claim 1. It essentially consists in

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that instead of two steel wires or glass threads, two additional optical fibres are laid in a cross lay around the central optical fibre.

g; The subclaims relate to advantageous developments of the optical fiber sensor according to the invention, thus claims 2 and 3 the optical fiber, , and 5 the casing, and 6 the sensor with measuring device.

, iiiThe invention is based on the knowledge that the temperature sensitivity of the previously known fiber optic sensors depends on the difference H in their construction-related material and stranding parameters. These % have a great influence on the sensor properties. In particular

In particular, the basic attenuation, which should be as small as possible, and the sensor sensitivity, which should be as high as possible, depend on the tension with which the stranding partners are wrapped around each other (and the sheath is wrapped around them). If, as seems appropriate due to the elasticity, one or two steel wires are wrapped around the central LVL, then changes in temperature will result in strong stress effects on the LVL due to the very different thermal expansion of quartz and steel. If, on the other hand, two further LVLs are wrapped around the central LVL, this is impossible.

The advantage of the invention is that you now have a temperature-insensitive LVL strain sensor. In addition, you now have two additional LVLs available, which significantly expands the measurement options.

An embodiment of the invention is shown in the drawing and is described in more detail below. They show the following as a schematic diagram:

- Fig. 1 an LVL strain sensor with a central LVL and ?.two LVL cross coils around it, and

- Fig. 2 the connection of the light transmitter and light receiver to this sensor.

As shown in Fig. 1, the two venules L2 and L3 are arranged around the central, primary coated optical waveguide (LVL) L1, which are also

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if primary coated optical fibers are used, they are stranded with a lay length S in a cross lay. As a protective cover U, a wire-shaped sheath made of glass fiber reinforced polyester resin is provided, which is directly adjacent to the stranded structure.

All three fiber optic cables are multimode/gradient fiber optic cables with the following external diameters: core 50 μm, cladding 125 μm and primary coating made of tJV acrylate 250 μm. However, other LVL types, such as single-mode fiber optic cables, can also be used. Likewise, different LVL types can be used for the different fiber optic cables L1, L2 and L3 of the sensor.

The pitch length S of the fiber optic coils L2 and L3 and thus the position of their crossing points K can be varied as desired. To increase the sensitivity of the sensor, as is known, the pitch length of the fiber optic coils is adapted to the pitch length (this is twice the focal length of the converging lens sequence simulated by a fiber optic cable) of the central fiber optic cable by selecting the ratio of both with η : 1 and η = 3, 4, 5 ... > 10.

As is well known, a gradient fiber optic cable can be described as a lens cable. So any disturbance of the fiber optic cable caused by pressure applied at specific points at the spiral crossing points K must lead to internal imaging errors and thus cause a significant loss of light through attenuation, especially if the pitch length of the spirals is adapted to the pitch length of the central fiber optic cable. Accordingly, strandings with partners of unequal thermal length expansion are always in the compartment compared to strandings with the same partners,· because they cause strong pressure changes as a function of temperature and thus strong attenuation changes, but the latter do not.

With an inventive fiber optic sensor, which is made up of 3 quartz fiber optic cables 50/; 125/250, a high degree of temperature invariance of the attenuation can be achieved. Measurements showed 0.3 dB / 80 ⁰ C for a total length of 20 m.

A further improvement is indicated when the protective cover U

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around the stranded structure of the 3 LWL is a tube made of metal, plastic or GPK, which surrounds the stranded structure with a small distance (mm range) and in which it is fixed at regular larger distances (10 cra range>), for example by means of a construction adhesive.

Fig. 2 shows how a light emitting diode (LED) is connected in front of the three optical fibers of the sensor for the measuring device and each optical fiber o< np T 4 ^Y.+ **-wn-r\-fm-ntraA4 /-.H &OHgr; t &Bgr;/-» + &psgr;-tA &lgr; >->*4f»n &Pgr;&Tgr;11 Me &Tgr;&Ggr;&Ggr;&Pgr; % no nkiuusnl» T 4- &bgr;+·

In contrast to the previously known fiber optic strain sensors, where only one fiber optic cable is available to measure the light attenuation, the measurement reliability of the new sensor is tripled, since all three fiber optic cables are used to measure the light. Even if one fiber optic cable breaks, the other two fiber optic cables can still perform their measuring task. And accordingly, operational reliability has increased by querying more than one fiber optic cable per sensor.

The new fiber optic sensor is primarily used as a strain sensor for engineering structures. It enables reliable measurements even at higher and rapidly changing temperatures.

Claims (6)

Fl 45&THgr;7 «i- ·· ..* 21,12.69 Protection claims: 1. Optical waveguide (LVL) strain sensor constructed from a central, primary-coated optical waveguide (L1), around which at least one helix of a metal wire or glass thread is wound, preferably two helixes are wound in a cross lay, around which a protective sheath is then arranged, characterized in that the helixes (L2 and L3) are also primary-coated optical waveguides. 2. Fiber optic strain sensor according to claim 1, characterized in that the central optical fiber (Ll) and the two optical fiber helices (L2 and L3) laid around it in a cross-lay are preferably multi-wave/gradient optical fibers, for example optical fiber 50/125/250 (core-sheath-primary coating diameter in μπα). 3. Optical fiber strain sensor according to claim 2, characterized in that, as is known per se, the lay length (S) of the optical fiber coils (L2 and L3) corresponds to the pitch length (that is twice the lens focal length of the converging lens sequence simulated by the L\fL) of the central optical fiber (Ll) in the ratio η : 1 with η = 3, 4, 5 ... > 10. 4. Optical fiber strain sensor according to one of claims 1 to 3, characterized in that the protective sheath (U), as is known per se, is a wire-shaped sheath made of plastic, predominantly made of glass fiber reinforced plastic (GRP), which lies directly against the stranded structure (L1 to L3). 5. Fiber optic strain sensor according to one of claims 1 to 3, characterized in that the protective cover (U) is a tube made of metal, plastic or GRP, which surrounds the stranded structure at a small distance (mm range) and in which it is fixed at regular larger distances (10 cm range), for example by means of a construction adhesive. Fl 4&THgr;&THgr;7 ..'....'.. .'..Jg^ '..' .,* 21.12.89 6. LVL strain sensor according to one of claims 1 to 5, characterized in that for the measuring device, a light emitting diode (LBD) is connected upstream of the LVL (L1 to L3) and a light receiving diode (photodiodes FD1 to FD3) is connected downstream of each LWL.