Nikola Stan

Instrumentation of integrally stiffened composite panel with fiber Bragg grating sensors for vibration measurements

We evaluate the performance of fiber Bragg grating (FBG) sensors for the measurement of dynamic strains in complex composite structures. The particular structure used in this study is an integrally stiffened composite panel for which the stiffeners and skin are fabricated in a single layup and cure process. Surface-mounted FBG sensors are bonded to the panels after curing, whereas embedded FBG sensors are successfully incorporated during the fabrication process. A finite element model was also constructed of the stiffened panel. The panels were subjected to repeated impacts and the post-impact vibration response of the panel was measured through the FBG sensor responses. Little change to the global response of the panel was observed after the repeated impacts, through the dynamic response of the surface-mounted FBGs. Pulsed phase thermography and micro-computer-tomography imaging of the panel confirmed that the damage was localized near the impact locations, producing negligible changes to the global response of the panel. All of the embedded FBG sensors survived the fabrication and multiple impacts; however, as these were embedded close to the neutral axis of the panel, they were not very sensitive to the vibration modes. Excitation of the panel near the first natural frequency did produce a measurable response in the FBG sensors, confirming their functionality.

Increasing dynamic range of a fibre Bragg grating edge-filtering interrogator with a proportional control loop

We present a fibre Bragg grating (FBG) interrogator that uses a microcontroller board and a tunable optical filter in a proportional control loop to increase dynamic range and achieve high strain sensitivity. It is an edge-filtering interrogator with added proportional control loop that locks the operating wavelength to the mid-reflection point on the FBG spectrum. The interrogator separates low-frequency (LF) components of strain and measures them with extended dynamic range, while at the same time measuring high-frequency (HF) strain without loss in strain sensitivity. In this paper, we describe the implementation of the interrogator and analyse the characteristics of individual components, such as the speed and voltage resolution of the microcontroller and the tunable optical filter. We measure the performance of the proportional control loop at frequencies up to 1?kHz and characterize the system using control theory. We illustrate the limitation of the conventional interrogator to measure strains greater than 40 ?? and demonstrate successful application of the proposed interrogator for simultaneous measurement of 450 ?? LF strain at 50?Hz superimposed with 32?kHz HF strain.

Characterization of fatigue damage in adhesively bonded lap joints through dynamic, full-spectral interrogation of fiber Bragg grating sensors: 2. Simulations

In this study we measure the in situ response of a fiber Bragg grating (FBG) sensor embedded in the adhesive layer of a single composite lap joint, subjected to harmonic excitation after fatigue loading. After a fully reversed cyclic fatigue loading is applied to the composite lap joint, the full-spectral response of the sensor is interrogated at 100 kHz during two loading conditions: with and without an added harmonic excitation. The full-spectral information avoided dynamic measurement errors often experienced using conventional peak wavelength and edge filtering techniques. The short-time Fourier transform (STFT) is computed for the extracted peak wavelength information to reveal time-dependent frequencies and amplitudes of the dynamic FBG sensor response. The dynamic response of the FBG sensor indicated a transition to strong nonlinear dynamic behavior as fatigue-induced damage progressed. The ability to measure the dynamic response of the lap joint through sensors embedded in the adhesive layer can provide in situ monitoring of the lap joint condition.

High voltage measurements using slab coupled optical sensors (SCOS)

One of the most common methods used to measure high voltage is using a voltage divider. While this method is fairly reliable for most low frequency high voltage measurements, the voltage divider encounters difficulty when measuring higher frequency voltage signals. In these instances, the signal measured by the voltage divider becomes susceptible to distortion and inaccuracy. One solution to measuring voltages where a voltage divider would not suffice is using an electrode structure in conjunction with a fiber-based electric field sensor. A high voltage generator was constructed utilizing automotive ignition coils and was used in a capacitor-charging circuit. The voltage on the capacitor was measured using a common resistive voltage divider as well as with a fiber-based electric field sensor. Where a resistor divider was not reliable in characterizing the system, the optical sensor was successful in measuring the charge and discharge voltages of the capacitor circuit.

High-speed interrogation of multiplexed Fiber Bragg gratings enabling real-time visualization of dynamic events such as impact loading

Fiber Bragg gratings (FBGs) are widely recognized for their unobtrusive sensing character and multiplexing possibilities. Within this article, these advantages are fully exploited through the development of a high-speed full-spectral FBG interrogation system capable of simultaneously reading out a range of FBG sensors. For the first time, the full-spectra of multiplexed FBG sensors are dynamically interrogated up to 100 kHz with a spectral resolution down to 40 pm. The feasibility of this unique sensing system is demonstrated using carbon fiber integrally stiffened panels which are monitored for their structural health. Detailed analysis based on the full-spectral datasets is enabling the assessment of non-linear events involving non-uniform strain distributions, such as impact loading. Moreover, we show real-time visualization of these impact events based on the multiplexed FBG responses.

High-Speed Interrogation of Multiplexed Fiber Bragg Gratings With Spectral Distortion

Fiber Bragg grating (FBG) sensors can be multiplexed in large numbers to monitor the performance of large structures. This paper addresses the collection of FBG reflection spectra from wavelength division multiplexed sensors at fast acquisition rates. The spectral and temporal resolution is first derived as a function of the tunable filter and measurement system properties. The method is applied to impact loading investigations of a stiffened composite skin panel. The reflected spectrum of each FBG in an array, embedded in the panel, is collected at 100 kHz during the impact events with a spectral resolution down to 40 pm. Visualization of the FBG responses to these impact events, including the presence of spectral distortion in some FBG spectra, is presented. Future analyses based on the full-spectral data sets can enable the assessment of the localized progression of internal damage in such structures.

Optical sensing of electrical fields in harsh environments

This paper shows a method allowing electric field sensing in high vibration environments. The vibration sensitivity is reduced using a push-pull configuration and external phase modulation. This paper provides both theoretical description and experimental verification.

Slab-coupled optical sensor fabrication using side-polished Panda fibers

A new device structure used for slab-coupled optical sensor (SCOS) technology was developed to fabricate electric field sensors. This new device structure replaces the D-fiber used in traditional SCOS technology with a side-polished Panda fiber. Unlike the D-fiber SCOS, the Panda fiber SCOS is made from commercially available materials and is simpler to fabricate. The Panda SCOS interfaces easier with lab equipment and exhibits ∼3  dB less loss at link points than the D-fiber SCOS. The optical system for the D-fiber is bandwidth limited by a transimpedance amplifier (TIA) used to amplify to the electric signal. The Panda SCOS exhibits less loss than the D-fiber and, as a result, does not require as high a gain setting on the TIA, which results in an overall higher bandwidth range. Results show that the Panda sensor also achieves comparable sensitivity results to the D-fiber SCOS. Although the Panda SCOS is not as sensitive as other side-polished fiber electric field sensors, it can be fabricated much easier because the fabrication process does not require special alignment techniques, and it is made from commercially available materials.

Push–pull slab coupled optical sensor for measuring electric fields in a vibrational environment

Vibration-insensitive fiber optic electric field sensor is created by fabricating two sensing elements in close proximity onto the same optical fiber and subtracting the two signals. The device is used to measure an electric field of 10  kV/m, while the sensor is being bent and impacted.

High dynamic range high sensitivity FBG interrogation

We present a fiber Bragg grating (FBG) interrogation method using a micro-controller board and optical filter that achieves high strain sensitivity and high dynamic range. This interrogation method allows high sensitivity detection of ultrasonic waves superimposed on low-frequency (on the order of 100Hz) vibrations of arbitrary magnitude. One possible application is in-situ structural health monitoring of windmill blades exposed to strong winds by using FBG sensors for detection of ultrasonic waves. Interrogator operation is based on the edge filtering method using a broadband source, fiber Fabry-Perot filter and a micro-controller board which acts as a control feedback loop that locks the filter wavelength to the mid-reflection point on the FBG spectrum. Wavelength locking method allows high sensitivity for edge filtering of high-frequency waves, while the feedback signal is the measurement of low-frequency vibration with high dynamic range. The concept of the interrogator operation and different implementations are described and discussed with experimental results.