Introduction
Chapter 4 Temperature Insensitive Fiber Bragg Grating Based Two
4.1 Introduction
M
easurement of structural vibrations induced by instantaneous impacts or natural environmental perturbations is a critical issue, particularly in civil engineering applications and rail transport network to name a few. Response of any given structure to such vibrational perturbations depends on the natural frequency of such structures, which is dictated by their physical properties. Often, a decrease in its value results in a damage proliferation [73]. Damage identification and prevention of its evolution in any civil engineering infrastructure, thus, becomes an integral part of structural-health-monitoring (SHM). Vibration measurement analysis is a vital approach to implement SHM strategies. Vibration sensors such as accelerometers are one of the key smart-diagnostic systems to study the impact of vibration perturbations in this strategy. Realizing low-cost and reliable vibration sensor system is quite challenging. A temperature insensitive, simple, compact, highly sensitive vibration sensor with a capability to resolve any arbitrarily directed vibration perturbation would be of extreme importance.Conventional vibration sensors are electronic in nature as they exploit either piezoelectric or capacitive effects. However, inherent limitations such as vulnerability towards electromagnetic/radio-frequency interference (EMI/RFI) and inability towards multiplexing and remote-sensing make such sensors impractical for real-field applications. On the other hand,
Chapter 4: Temperature Insensitive Fiber Bragg Grating Based Two Dimensional Vibration Sensor For Structural Health Monitoring
71 with these conventional sensors. Owing to their inherent merits several kinds of optical fiber sensors have been developed over the last two decades for many real-field applications and especially for SHM [127]. In particular, several research efforts have been made to develop optical fiber vibration sensors. These efforts mainly employed cantilever based mechanism where the vibration is encoded in terms of uniaxial misalignment between two similar or different kinds of optical fiber [76, 77, 128]. A more complicated sensing scheme that integrated Moiré fringe with fiber optics is reported in [78]. Recently, a variety of optical fiber vibration sensors exploiting interferometric techniques have been reported [79, 80]. However, performance of all these optical fiber vibration sensors which are based on the intensity modulated scheme suffers from the source power fluctuations. Also, these sensors are unfit for multiplexing and distributed sensing. Importantly, being one-dimensional, they are unable of resolve arbitrary vibrations. On the other hand, optical fiber Bragg gratings (FBGs) have attracted considerable amount of research interests over the past few years. A range of FBG sensors have been reported for monitoring various parameters of practical importance and interest for science/engineering applications such as strain, pressure, displacement, curvature, tilt etc. [127]. Some attempts have also been made to develop FBG based vibration sensors. For example, Berkoff et al. in [82] embedded FBG element into a thin layer of elastomer between a rigid base and a seismic mass. Sensor suffered seriously from the cross-axis response. Zhou et al.
in [84] employed vibration induced chirping mechanism of a surface loaded FBG. Such power- referenced sensing scheme suffers from light source power fluctuations and is unsuitable for distributed sensing. Weng et al. in [85] employed a very complicated sensing structure based on a U-shaped rigid cantilever beam, a diaphragm and a FBG fixed between the two legs of the cantilever beam. Researcher in [86] reported another diaphragm based sensor employing a
Chapter 4: Temperature Insensitive Fiber Bragg Grating Based Two Dimensional Vibration Sensor For Structural Health Monitoring
72 centrally attached inertial mass and the FBG. Another sensor design based on a tapered fiber and tilted FBG was employed in [87]. In another attempt, Stefani et al. employed microstructured polymer optical fiber Bragg grating to capture vibration/acceleration [89]. Temperature cross- talk, cross-axis sensitivity and complexity in designing remained some of the serious issues with all these sensors reported in the literature. Importantly, being uniaxial, they are incapable of resolving the impact to two-dimensional vibration perturbations. In order to address the later issue and to develop two-axial sensor, Fender et al. employed mass loaded multicore optical fiber carrying FBG in all the four cores of the fiber [90]. However, fabrication of such complicated structure required high level of sophistication and increased the experimental difficulty.
The objective of the research reported in this chapter is to design and construct a simple, compact, all-optical FBG based two-dimensional temperature-insensitive vibration sensor by employing chirp-free strain tuning mechanism. Sensor design employs four FBGs that are fixed onto the two sets of a pair of thin parallel plates, each carrying centrally fixed concentrated mass element. Design strategy insures a chirp-free Bragg grating response. Performance characteristics for the design strategy of the proposed sensor are first modeled using a 3-D CAD model and finite element analysis using ANSYS software. Afterwards, a rigorous experimental investigation is carried out to characterize the sensor. Sensor is excited with an electromechanical shaker over a frequency band ranging from 10Hz to 350Hz with varying accelerations from 0.65m/s2 to ~70m/s2. The response of the reported sensor is observed to be in good agreement with the applied signal with an excellent capability of successfully resolving the impact to two-dimensional vibration perturbations. Frequency response of the sensor is also analyzed to determine the natural frequency of the sensor. Further, in the proposed scheme,
Chapter 4: Temperature Insensitive Fiber Bragg Grating Based Two Dimensional Vibration Sensor For Structural Health Monitoring
73 vibration information from the peak wavelength separation between the two FBGs. This wavelength based optical diagnosing mechanism makes the proposed sensor temperature insensitive.