Papers by Author: Young Sup Lee

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Authors: Young Sup Lee, Hyoung Jin Im, Jaehwa Kwon, Dong Jin Yoon
Abstract: This paper presents concept, analysis and experiment of a novel sensor which is based on biologically inspired approach for acoustic emission (AE) detection. It is known that a conventional AE sensor consists of a matching layer, piezoelectric transducer, backing layer, supporting electric circuit and casing. The conventional AE sensors have been widely used to detect defects in various structures and they have designed as either broadband or resonant type. However, the novel sensor described in this paper utilizes the concept of hearing organs in animals with the help of micro electro-mechanical systems (MEMS) technology. The basic design with theoretical investigation including finite element analysis showed the core hearing element such as a hair cell could be implemented with the piezoeletric material. Also it is found that the dimensional variety and proper distribution of such elements inside the sensor are critical parameters to the detectability of AE signals from structures. Both the broadband and resonant type AE sensors with relevant electric circuits could be implemented with this novel sensor concept.
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Authors: Ki Hong Shin, Young Sup Lee
Abstract: In-plane characteristics of a single degree of freedom frictional system with a compressed spring are considered. The compressed spring is vertically installed to the mass moving horizontally along the friction interface. The compressed spring can introduce a nonlinear negative stiffness into the in-plane motion. The resulting system has a multiple equilibrium points; an unstable point at the center and two stable points on either side. It is shown that two stable equilibrium points can be separated far apart by increasing the compression ratio and the stiffness of the spring. The friction system is often characterized by stick and slip motions that cause unfavorable effects such as wear, noise, and chatter etc. It is demonstrated that increasing the compression ratio and the stiffness of the spring results in decreasing the size of the stick regions.
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Authors: Young Sup Lee, Ki Hong Shin
Abstract: This paper presents a theoretical and experimental study of a non-collocated pair of piezopolymer PVDF sensor and piezoceramic PZT actuator, which are bonded on a cantilever beam, in order to suppress unwanted vibration at the tip of the beam. The PZT actuator patch was bonded near the clamped part and the PVDF sensor, which was triangularly shaped, was bonded on the other part of the beam. This is because the triangular PVDF sensor is known that it can detect the tip velocity of a cantilever beam. Because the arrangement of the sensor and actuator pair is not collocated and overlapped each other, the pair can avoid so called "in-plane coupling", which can be found at a matched piezoelectric sensor and actuator pair and restricts the stability and performance of direct velocity feedback control. The test beam is made of aluminum with the dimension of 200 × 20 × 2 mm. Before control, the sensor-actuator frequency response function is confirmed to have a nice phase response without accumulation in a reasonable frequency range of up to 5000 Hz. The feedback control attenuates the magnitude of the first two resonances in the error spectrum of about 6 -7 dB.
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