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An Active Geophone Sensor with Optimized State Variable Filter for Measuring Low-Band Frequencies
Jinsoo Choi,Honggi Yoo,Eunjong Choi,Kihyun Kim,Hyo-Young Kim 한국정밀공학회 2024 International Journal of Precision Engineering and Vol.25 No.5
An active vibration-isolation system (AVIS) utilizes a geophone sensor, a type of velocity sensor, to control microvibration. The structure of the sensor is modeled by mass, damper, and spring. The mathematical model of the geophone sensor is a second-order model with a resonant frequency. However, at low-band frequencies, the response characteristic is nonlinear and phase delay occurs. Compared with the ideal velocity signals of the system, the velocity signals measured from the geophone sensor were distorted in low-band frequencies. Consequently, this measurement issue in feedback control loops can affect the stability and performance of the AVIS. This paper proposes design rules for a state-variable filter (SVF) that can compensate for the nonlinearity of the geophone sensors in low-band frequencies and evaluates vibration attenuation performance of the AVIS by applying the proposed SVF. To evaluate the effectiveness of the filter in compensating for the nonlinear response of the geophone sensor, we compared Bode plots generated through simulation and experimental results obtained using a dynamic signal analyzer. The experimental results demonstrated that the proposed SVF effectively reduces the resonance peak of the geophone sensor and expands the frequency bands that maintain a constant magnitude in range of 0.8–10 Hz. By applying the geophone sensor with SVF to AVIS, the microvibration attenuation improved to − 18.4 dB near 4.5 Hz.
Multi-Functional Brain Computer Interface Using Convolutional Neural Networks
Woosung Choi,Honggi Yeom,Nakyong Ko 제어로봇시스템학회 2022 제어로봇시스템학회 국제학술대회 논문집 Vol.2022 No.11
Brain-computer interface (BCI) is a promising technology that controls computers or machines using brain signals. With this technology, people with various disabilities, such as neural paralysis, and spinal cord injury can control electric devices or express their intention by thinking. However, previous BCI studies have a limitation that they can predict only one type of intention. To use the BCI system in daily life, the BCI user should be able to achieve various tasks such as moving, text typing, and arm movements. In this paper, we propose a multi-functional BCI method that can predict various intentions simultaneously. To classify multiple intentions, we proposed two prediction models using Neural Networks (NN) and Convolutional Neural Networks (CNN) models. To evaluate the proposed BCI system, the classification accuracy of the model was measured and compared using steady state visually evoked potential (SSVEP), sensory motor rhythm (SMR), and both of them (Multiple Intention). The average prediction accuracies were 22.46% in NN, 55.86% in CNN. These results indicate that the proposed multi-functional BCI can predict multiple intentions. It also means that users of the proposed BCI system can control various electric devices simultaneously.
Branched Segments in Polymer Gate Dielectric as Intrinsic Charge Trap Sites in Organic Transistors
Lee, Junghwi,Choi, Hyun Ho,Park, Namwoo,Min, Honggi,Han, Singu,Jeong, Heejeong,Song, Inseok,Kim, Se Hyun,Lee, Hwa Sung American Chemical Society 2015 The Journal of Physical Chemistry Part C Vol.119 No.14
<P>Charge traps in polymer gate dielectrics determine the electrical stability of organic field-effect transistors (OFETs), and polar alkoxy groups are well-known extrinsic charge traps. However, the actual location of intrinsic charge traps in nonpolar polymer gate dielectrics has been poorly understood yet. Here, we demonstrate that the skeletal structure of polymer chain plays an important role in determining the electrical stability. To verify it, we prepared linear and branched polystyrene (<I>l</I>-PS and <I>b</I>-PS) and blended them, in which branched segments provide much larger free volume than the other segments. The current-insulating performance and field-effect mobility increased with decease of <I>b</I>-PS portion. In particular, the bias-stress stability was remarkably varied according to the change of <I>b</I>-PS portion even though all measurements excluded reactive components such as oxygen and water; the increase of <I>b</I>-PS resulted in time-dependent decay of mobility and threshold voltage under bias stress. This indicates that the branched segments in <I>b</I>-PS provide intrinsic and metastable charge trap sites. Our result suggests that the skeletal structure of polymeric chains in gate dielectric is one of the important factors affecting intrinsic long-term operational stability of OFET devices.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/jpccck/2015/jpccck.2015.119.issue-14/acs.jpcc.5b01522/production/images/medium/jp-2015-015226_0009.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/jp5b01522'>ACS Electronic Supporting Info</A></P>
최광일(Kwang-Il Choi),Pham Quang Vu,오종택(Jong-Taek Oh),조홍기(Honggi Cho),김태현(Taehun Kim),김중호(Jungho Kim),이창선(Changseon Lee) 대한설비공학회 2013 대한설비공학회 학술발표대회논문집 Vol.2013 No.11
An experimental investigation of condensation heat transfer of R410A was carried out for horizontal micro-fin tubes of 6.61 mm and 7.45 mm inner diameter in this study. The heat transfer coefficient and pressure drop were measured under the following conditions: the saturation temperature was at 48˚C, the mass flux ranged from 200 to 320 kg/m²s, the heat flux ranged from 9 to 12 kg/m²s, the quality of vapor reached to an unit and the length of test section was 1.2m. The effects of mass flux, heat flux and inner diameter on heat transfer coefficient were evaluated. The heat transfer coefficient and pressure drop were also compared using some existing correlations.
Vu, Pham-Quang,Choi, Kwang-Il,Oh, Jong-Taek,Cho, Honggi World Scientific Publishing Co. Pte. Ltd. 2017 International Journal of Air-Conditioning and Refr Vol.25 No.2
<P>The condensation heat transfer coefficients and pressure drops of R410A and R22 flowing inside a horizontal aluminum multiport mini-channel tube having 18 channels are investigated. Experimental data are presented for the range of vapor quality from 0.1 to 0.9, mass flux from 50 to 500<TEX>$ \,$</TEX>kg/m<SUP>2</SUP>s, heat flux from 3 to 15<TEX>$ \,$</TEX>kW/m<SUP>2</SUP> and the saturation temperature at 48<TEX>$ ^{\circ }$</TEX>C. The pressure drop across the test section was directly measured by a differential pressure transducer. At a small scale, the noncircular cross-sections can enhance the effect of the surface tension. The average heat transfer coefficient increased with the increase of vapor quality, mass flux and heat flux. Under the same test conditions, the heat transfer coefficients of R22 are higher than those for R410A, the pressure drops for R410A are 7-19% lower than those of R22. The lower pressure drop of R410A has an important advantage as an alternative working fluid for R22 in air-conditioning and heat pump systems.</P>
Single-phase heat transfer and pressure drop characteristics in multiport mini-channel tubes
Pham Quang Vu,Kwang-Il Choi,Jong-Taek Oh,Honggi Cho 대한설비공학회 2017 대한설비공학회 학술발표대회논문집 Vol.2017 No.6
The single-phase heat transfer and pressure drop in multiport mini-channel tubes were measured using water as the working fluid. Two different multiport tubes were used to obtain experiment heat transfer coefficient and frictional pressure drop with varying of Reynold number from 50 to 10.000. The inlet and outlet temperatures, the wall temperatures along the test section, the pressure drop and the mass flow rate were measured. The relationships between the mass flow rate, the heat flux on the pressure drop and heat transfer coefficient were analyzed. Also, the empirical correlation for heat transfer coefficient and pressure drop adjusted from the experimental data are proposed.