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Measurement of deformation: fiber optic sensors, the company HBM

Measurement of deformation: fiber optic sensors, the company HBM

Measurement of deformation: fiber optic sensors, the company HBM

Measurement of deformation: fiber optic sensors, the company HBM

T.Kleckers, B.Gunther

Today for measuring strain have been successfully used electrical deformation sensors (strain gages). Developed over 50 years ago, today they are technically upgraded and are available in different versions. Not so long ago, for measuring the strain began to use the optical (fiber) sensors. In many areas they have become a good alternative to strain gauges.

In the fiber deformation sensors [1] used a diffraction grating Bragg, which includes a large number of points of reflection located inside the fiber with a certain interval. When passing the laser radiation through the fiber portion of a certain wavelength reflected from the grating. This peak reflected radiation is recorded by measuring equipment. As a result, deformation changes the spacing between the nodes of the Bragg gratings and the refractive index of the fiber. Consequently, changing the wavelength of radiation reflected from the grating. By changing the wavelength you can determine the amount of deformation.

Let's see what are the characteristics of the optical deformation sensors in comparison with strain gauges, and where their application is most effective.

The resistance change of a lode cell is caused by two physical effects [2]. When a metal conductor is subjected to strain, changing its length and cross section. If Poisson's ratio for the material of the measuring grid strain gages is 0.3, then the resulting ratio of the stress sensitivity, determined by the change of the geometrical sizes of the sensor, taking the value of 1.6. The resistivity of the material of the measuring grid is also changed depending on the degree of deformation. Therefore, the sensitivity of the measuring gratings of different materials varies. The contribution of changes in specific resistance coefficient of the strain ranges from 0.4 to 0.6. Both effects can overlap each other. Therefore, the ratio of the stress sensitivity of strain gages is typically close to two.

The sensitivity of the optical sensors is determined by the change of the distance between the points of reflection in the Bragg grating and the refractive index of fiber due to the deformation. The stress sensitivity coefficient k, determined by the first factor equal to unity – in this case, the impact of changes in cross-sections of fibers for the strain gauges is taken into account through the Poisson's ratio can be neglected. As a result of changes of the refractive index k value is reduced to approximately 0.22. Thus, the ratio of the stress sensitivity of fiber Bragg gratings is about 0,78.

Full article in the attached pdf file.

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