Chan Yik Park

Localizations and force reconstruction of low-velocity impact in a composite panel using optical fiber sensors

A new approach of monitoring low-velocity impact events on composite structures was presented and experimentally evaluated. In this approach, impact strain histories of a composite plate were measured with optical fiber sensors. The recorded signals were used to estimate locations and forces of low-velocity impact events. For experimental validation, four-sensor heads were surface mounted at the four corners of a composite panel. The optical sensors caught dynamic strain due to impacts on the panel, and the optical cables transmitted wavelength changes of the sensors. The multiplexed wavelength changes were measured using a high-speed fiber Bragg grating interrogator for the determination of impact location. The locations of impact events were estimated using a neural network program and measured signals. The estimated locations and converted strain signals were taken as the inputs to the process of reconstructing impact forces. Even though the current study is an initial investigation on a simple impact problem, the result shows the possibility that the proposed technique can be applied to low-velocity impact monitoring of general composite structures.

Flight Test Measurement and Assessment of a Flapping Micro Air Vehicle

Flight test of flapping micro air vehicles (FMAVs) is carried out using an instrumented measurement system to obtain various engineering parameters and hence to assess the flight performance of the vehicles through the data investigation. An indoor flight test facility equipped with a motion capture system and tracking cameras is used for the work presented in this paper. Maneuvers including straight-level flight, ground flapping, takeoff and landing are tested. Spatial position and orientation data are obtained from the retro-reflective tracking markers attached to the vehicles. Subsequent test analysis is carried out by generating performance parameters from raw data and then assessing the flight performance by comparison of the vehicles. The main findings of this work confirm that the test method and procedures presented here enable the systematic numerical data measurement and assessment of the flying performances of these vehicles, and show the applicability for the test and evaluation of general flapping MAVs.

Detection of Impact Location for Composite Stiffened Panel Using FBG Sensors

We carried out experiments to detect impact locations on a composite plate using two types of composite plates, a composite flat plate with a constant thickness of 5 mm and a composite stiffened panel with stringers. Four multiplexed FBG sensors were attached to the bottom surface of the composite plates to acquire impact signals. The FBG sensor wavelength shift data were collected at a sampling frequency of 40 kHz using a high-speed FBG interrogator (SFI-710, Fiberpro Inc., Korea). The arrival times of the impact signals at each FBG sensor were obtained using a signal processing procedure. The arrival times were affected by noise level and signal-to-noise ratio. In order to overcome this weakness, signal processing techniques such as wavelet decomposition, normalization using each noise level and filtering with a moving average were adopted. To calculate the impact locations of the composite plate, a neural network algorithm was applied.

Repeat scanning technology for laser ultrasonic propagation imaging

Laser ultrasonic scanning in combination with contact or non-contact sensors provides new paradigms in structural health management (SHM) and non-destructive in-process quality control (IPQC) for large composite structures. Wave propagation imaging technology based on laser ultrasonic scanning and fixed-point sensing shows remarkable advantages, such as minimal need for embedded sensors in SHM, minimum invasive defect visualization in IPQC and general capabilities of curved and complex target inspection, and temporal reference-free inspection. However, as with other SHM methods and non-destructive evaluation based on ultrasound, the signal-to-noise ratio (SNR) is a prevalent issue in real structural applications, especially with non-contact thin-composite sensing or with thick and heterogeneous composites. This study proposes a high-speed repeat scanning technique for laser ultrasonic propagation imaging (UPI) technology, which is realized with the scanning speed of 1 kHz of a Q-switched continuous wave laser, and precise control of the laser beam pulses for identical point scanning. As a result, the technique enables the achievement of significant improvement in the SNR to inspect real-world composite structures. The proposed technique provides enhanced results for impact damage detection in a 2 mm thick wing box made of carbon-fiber-reinforced plastic, despite the low sensitivity of non-contact laser ultrasonic sensing. A field-applicable pure laser UPI system has been developed using a laser Doppler vibrometer as the non-contact ultrasonic sensor. The proposed technique enables the visualization of the disbond defect in a 15 mm thick wind blade specimen made of glass-fiber-reinforced plastic, despite the high dissipation of ultrasound in the thick composite.

High-Speed Wavelength Interrogator of Fiber Bragg Gratings for Capturing Impulsive Strain Waveforms

It is highly desirable to increase the sampling rate of a fiber Bragg grating (FBG) interrogator in other to sense dynamic strains caused by impulsive acoustic wave. We have developed a wavelength interrogator featuring 100k samplings per second that consists of a solid-state spectrometer, a photodiode array and fully parallel read-out circuits. Central wavelengths on the reflected partial spectra corresponding to FBGs are calculated by the centroid method with the selected groups of the consecutive photodiodes at which each FBG spectrum is imaged. The centroid calculation is simple to be implemented in a field-programmable gate array (FPGA) and fast enough to capture impulsive strain waveforms in real time. Short-term noise on the interrogated wavelengths is estimated to be around 0.5 in terms of stain within the sampling bandwidth.

Damage Index Comparison for a Composite Stiffened Panel Using Lamb Wave

Various damage index (DI) algorithms of detecting changes such as a loosen bolt and a delamination development in a composite structure were examined using ultrasonic Lamb waves generated by embedded piezoelectric active sensors. The DI is a single value that is a function of response signal’s attenuation due to any damage or changes in a structure. Various DI algorithms such as active damage interrogation (ADI), time domain root men square (RMS), short time Fourier Transform (STFT) and time reversal (TR) were discussed. For experimental validation, a composite stiffened panel was used, and loosen bolt damage and low-velocity- impact damage were tested. In order to pitch and catch Lamb waves, surface mounted PZTs (lead zirconate titanate) were used. According to the DI algorithms, appropriate ultrasonic guided Lamb waves were selected for actuators. Each set of DI algorithm and drive signal showed different characteristics to detect the damage.

Bird-mimetic Wing System of Flapping-wing Micro Air Vehicle with Autonomous Flight Control Capability

A micro air vehicle with a bird-mimetic up-down and twisting wing drive system was developed in this study. The Flapping-wing Micro Air Vehicle (FMAV), with a 50 cm wingspan and a double-crank drive system, performed successful flights of up to 23 min. The performance and capabilities of the FMAV were enhanced by adapting a number of unique features, such as a bird-mimetic wing shape with a span-wise camber and an up-down and twisting wing drive mechanism with double-crank linkages. This lift-enhancing design by mimicking the flapping mechanism of a bird’s wing enabled the 210 g FMAV to fly autonomously in an outdoor field under wind speeds of less than 5 m·s−1. Autonomous flight was enabled by installing a flight control computer with a micro-electro-mechanical gyroscope and accelerometers, along with a micro video camera and an ultralight wireless communication system inside the fuselage. A comprehensive wind tunnel test shows that the FMAV with a high-camber wing and double-crank mechanism generates more lift and less net thrust than the FMAV with a flat wing and single-crank mechanism, which confirms the improved performance of the developed FMAV, as well as the superior slow flying or hovering capabilities of the FMAV with a high-camber wing and double-crank wing drive system.

Guided Wave Damage Detection in Composite Plates under Temperature Variations

One of the main challenges of guided wave structural damage detection is to find an effective way of compensating temperature changes and to apply it to existing damage detection methods. This article describes a simple method for applying guided waves to the problem of detecting damage in the presence of temperature changes. In order to examine the effectiveness of the presented method, delaminations due to a low-velocity impact on composite plate specimens are detected. The results show that the presented approach can be a potential candidate to detect damage in aircraft structures under the temperature variations.

Development of a wireless pilot arm-wearable haptic interface for unmanned aerial vehicle wing deflection sensing

When the flight of an unmanned aerial vehicle is controlled by a ground pilot, a wing deflection monitoring is required to avoid overload wing structural failures. Therefore, integrated structural health monitoring technologies are being developed to transfer such information to the pilot. In general, this information can be monitored visually by the ground pilot. In this study, a haptic interface enables human–machine communication through tactile sense and provides synchronized information exchange between a pilot and an unmanned aerial vehicle. In other words, we propose not a vision interface but a haptic interface to transfer the wing deflection information to the ground pilot; this interface is named “Fly-by-haptic,” which is beneficial because the vision of the ground pilot is already performing multiple tasks. For a proof of concept, four integrated fiber Bragg grating sensors were installed on a half wing specimen to measure dynamic strains. The wing deflection information was estimated by the displacement–strain transformation matrix. The wing deflection information was wirelessly transferred to actuate vibro-haptic motors installed in a pilot arm–wearable haptic interface. Finally, a human test was performed using the developed haptic interface; the test results determined that the 15 participants, who are novices, showed 100% accuracy for wing deflection.

Structural Damage Monitoring of a Composite Wing Using Multiple Types of Sensors

This paper outlines an ongoing SHM project called KASHMOS where the damage detection in composite wing structures using multiple sensors is investigated. The test bed is a composite UAV wing section and the ground structural tests are being performed to evaluate developed systems. Different kinds of damage modes in prearranged areas are intentionally induced. Optical fiber sensors, piezo-electric active sensors and Q-switched pulse lasers are utilized for detecting the damage and monitoring the integrity of the wing structure. Strain-based, impedance-based, and ultrasonic-wave-based damage monitoring methods are studied. Although this investigation is based on the ground tests, the operation concepts for the onboard system as well as the ground system are also considered.