Thermal strain measurements by fiber Bragg grating (FBG) sensors mounted onto different host materials are
demonstrated for low coefficients of thermal expansion (CTE). Such low CTEs are typically found in carbon
fiber reinforced plastics (CFRP). This work has application potential for FBG sensor networks in the highprecision
control of thermal deformations in structures or in curing monitoring. For this purpose, a thermal
error model of the FBG sensor, which accounts for the thermo-optic coefficient and the thermal expansion of
the FBG, was characterized experimentally. The error-model characterization method is based on reference
measurements of FBGs bonded to ZERODUR ceramics. Using this error model, thermal strain can be measured
by surface-mounted FBGs on any given host structure using an external temperature reference and the FBG's
wavelength shift. This method is demonstrated successfully for unidirectional layers of CFRP with a CTE of
-0.4 · 10<sup>-6</sup> 1/K in fiber direction and for steel (316 Ti), which is commonly used in cryogenic applications.
Measurements are performed for temperatures from 100K to 320K and the results are verified by high-precision
dilatometer measurements. Accuracy limits of the FBG-based thermal strain measurements are discussed, as
well as the minimization of errors induced by the FBG's structural interface. Further, the reduction of errors in the adhesive bonding is discussed. This work expands the understanding of the separation of thermal and mechanical effects in the signals obtained by FBGs.
This paper investigates the use of filtering techniques such as the Low-pass and Kalman filter in combination
with the quasi-static strain-displacement transformation in order to estimate dynamic structural displacement
based on noisy strain measurements. Numerical simulations and vibration experiments were performed on simple
beam structures under various dynamic loading cases to determine the sensitivity of estimation procedures against
model errors and noise disturbance. In the experimental setup the multiplexing ability of fiber Bragg grating
(FBG) sensors was used to obtain strain data with high accuracy and low noise level. Estimated displacements in
the experiment were verified against laser displacement readings. Depending on the load case the noise-sensitive,
quasi-static shape estimation results could be highly improved by applying recursive filtering methods which
allow an application of the simple approach even for complex, vibrating structures.
A possible approach to meet the increasing performance requirements of lightweight structures in various engineering
fields is the application of smart structures. One of the functions, which are required, is the observation of the structures'
shape. During operation, however, the monitoring of displacement fields is difficult. This paper discusses the
displacement field estimation of a dynamically excited plate using fiber Bragg grating strain sensors. Using a modal
approach, it is possible to derive a transformation matrix to estimate the displacement field using only a few strain
measurements. To reduce systematic estimation errors due to residual modes, a parameter study was performed and the
sensor location optimized using the condition number of the transformation matrix as an objective function. An
experiment with an optimized sensor configuration including 16 fiber Bragg grating strain sensors was performed to
verify the method and the simulation results.
Fiber optic Bragg grating (FBG) sensors show promising capabilities in the measurement of strain and temperatures in structures at many locations. In this work, the potential of FBG sensors for high-precision deformation control in opto-mechanical applications is investigated. This requires a strain resolution of < 1 um/m. A test rig with a simply supported steel beam was developed which should represent the geometry of a lightweight optical mirror with a ribbed support structure. The deformation of this beam is controlled by a piezo actuator. The reference deformation measurement is done
using six capacitive displacement sensors with a resolution < 0.5 nm. It is being investigated to what level of accuracy FBG sensors can be used to reconstruct the displacement information. Different
methods to increase the accuracy are discussed: decreasing the sensor noise by oversampling and increasing the number of sensors. Tests were performed using different diffraction-based interrogation techniques for the wavelength detection: a CCD-based FBG sensor system and a PSD (Position Sensitive Detector)-based high-speed FBG sensor system which - to our knowledge - has not been used for an application of this kind yet. A comparison of both systems discussing the weaknesses and strengths is given for the recording of mechanical strain < 1 um/m. The results showed that a resolution of < 0.3 um/m for the strain measurement using FBG sensors can be achieved. This study shows an interesting application potential for FBG sensors in structural deformation control for various fields such as optics or high-precision machine tools.