Lamb wave is a good method to detect some imperfections in a thin plate. In order to use this method, a sensor as well as an actuator is needed. Usually, a piezoceramic transducer is a good sensor and also a good actuator. Nowadays, fiber optic sensors are good alternative transducers for detecting Lamb wave as well as other ultrasonic waves. However, in the case of the fiber optic sensor, its sensitivity has directivity; that is, the sensitivity is variable according to the alignment direction of the sensor because the sensor dominantly measures the displacement induced by the change of gage length along the parallel direction to the sensor. Thus, considering the change of the sensitivity with respect to the alignment direction of the sensor to an ultrasonic source is essential in order to detect the ultrasonic wave using a fiber optic sensor and to determine the absolute amount of the measured value correctly. In this paper, the directivity of the fiber optic sensor was investigated through both a theoretical analysis and an experimental one. The theoretical analysis showed that the sensitivity was related to the alignment angle of the sensor and to the ratio (L/λ)of the gage length (L) of the sensor and the wavelength (λ) of the Lamb wave. In the experimental analysis, an extrinsic Fabry-Perot interferometric sensor was used for detecting the Lamb waves which were excited by a lot of piezoceramic transducers. One fiber optic sensor was attached on the center of the aluminum plate; otherwise these piezoceramic transducers were attached around the fiber optic sensor according to the alignment direction of the fiber optic sensor. Finally, the theoretical results were verified in the experimental analysis.
This paper presents the use of a novel fiber optic accelerometer system to monitor civil engineering structures in real time. This sensor system integrates the Moire fringe phenomenon with fiber optics to achieve accurate and reliable measurements. A low-cost signal processing unit implements unique algorithms to further enhance the resolution and increase the dynamic bandwidth of the sensors. There are two major benefits in using this fiber optic accelerometer system for monitoring civil engineering structures. One is its immunity to electromagnetic (EM) interference making it suitable for difficult applications in such environments involving strong EM fields, electrical spark-induced explosion risks, and cabling problems, prohibiting the use of conventional electromagnetic accelerometers. The other benefit is its ability to measure both low- and high-amplitude vibrations with a constantly high resolution without pre-setting a gain level, as usually required in a conventional accelerometer. This benefit makes the sensor system particularly useful for real-time measurement of both ambient vibration (that is often used for structural health monitoring) and strong motion. This paper presents the prototype hardware and software development, characterization tests, and applications to real-time damage assessment, demonstrating the uniquely high performance of the Moire fringe fiber optic sensor system.
For the translation stage of nanometer scale, fiber optic EFPI sensor is suggested for the feedback control system on account of its high sensitivity, small size, simple system and relatively low cost. The novel signal processing algorithm for the real-time demodulation of EFPI output signal was developed and verified. The local linearity in the adjacent fringe values was shown, and used for the sinusoidal approximation of the nonlinear output signal. The real-time signal processing program was designed and the intensity signal of the EFPI sensor was demodulated to the phase shift with this program. The theoretical resolution of 0.36~8.6 nm in the displacement range of 0~200 μm was obtained. The sensor system was applied to the 1-D nano-positioner with a Piezo-electric actuator. The positioner successfully reached to the desired destination within 1 nm accuracy.
We propose a new algorithm that generates intermediate scenes from the near and the far views. Assuming that two cameras can be approximated with the affine model, we align the optical axes of the near and the far views on the transition baseline using the Fourier method. Then we determine the scale of the desired novel scene, zoom in the far view, and zoom out the near view. The zoom-in far view is theoretically the same as the zoom-out near view, but disparities exist that cause the synthesized image to present the double-blurring effect. Therefore, we remove the disparities of the zoom-out near view and then synthesize it with the zoom-in far view. For the panorama-based navigation system, the proposed algorithm can also generate images having smooth and realistic transitions as the viewer walks from the current panoramic image to the next panoramic image.
Impact location monitoring is one of the major concerns of the smart health monitoring. For this application, multipoint ultrasonic sensors are to be employed. In this study, a multiplexed FBG sensor system with wide dynamic range was proposed and stabilization controlling system was also developed for the maintenance of maximum sensitivity of sensors. For the intensity demodulation system of FBG sensors, Fabry-Perot tunable filter (FP-TF) with 23.8 nm FSR (free spectral range) was used, which behaves as two separate filters between 1530 ~ 1560 nm range. Two FBG sensors were attached on the bottom side of the graphite/epoxy composite beam specimen, and low velocity impact tests were performed to detect the one-dimensional impact locations. Impact locations were calculated by the arrival time differences of the impact longitudinal waves acquired by the two FBGs. As a result, multiplexed in-line FBG sensors could detect the moment of impact precisely and found the impact locations with the average error of 1.32 mm.
To perform the real-time health monitoring of the smart composite structures, two fiber optic sensor systems are proposed, that can measure the strain and detect the moment of fracture simultaneously. The types of the coherent sources used for fracture signal detection classify the systems--EDFA with FBG and EDFA with Fabry-Perot filter, and these systems were applied to extrinsic Fabry-Perot interferometer sensors imbedded in composite specimens to monitor the tensile tests. To understand the characteristics of matrix cracking signals, at first, we performed tensile tests using surface attached PZT sensors. This paper describes the implementation of time-frequency analysis such as short time Fourier-transform for the quantitative evaluation of the fracture signals like matrix cracking. From the test of tensile load monitoring using optical fiber sensor systems, measured strain agreed with the value of electric strain gage and the fracture detection system could detect the moment of damage with high sensitivity to recognize the onset of micro-crack fracture signals.
This paper presents a new method of 3D surface feature extraction using not conventional surface normal vectors and curvatures but center, corner and subsidiary points. We assume that each region of object have a uniform surface curvature distribution. With a range image, we get an edge map through the scan line technique. Using this edge map, we label a 3D object and extract center and corner point from each segmented region. Then we determined whether the segmented region is a planar surface or a curved. From the quadric surface equation, we calculate the coefficients of the planar surface or the curved surface. In this article, we use synthetic and real (Odetics) range images including polyhedral and curved objects.
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