Papers by Author: Ki Bok Kim

Abstract: In this study, microwave free-space transmission technique was applied to measure the moisture content of powdered food (wheat flour, milk powder, and coffee powder). In frequency range from 1 to 15 GHz, the microwave attenuation and phase shift due to moisture content of food samples were measured and analyzed using vector network analyzer, double rigid horn antennas, and sample holder filled with moist food samples. To estimate the relationship between moisture density of powdered food and the attenuation and phase shift, correlation analysis was performed. The correlation coefficients at each food sample were greater than 0.91. The calibration equation for moisture content measurement having attenuation and phase shift as independent variables at 15 GHz was developed and evaluated. The coefficient of determination and root mean square for all food samples were 0.974 and 0.328 % respectively.

Abstract: This study was conducted to analyze the ultrasonic transmitted signal for apple using wavelet transform. Fruit consists of non-linear visco-elastic properties such as flesh, an ovary and rind and hence most ultrasonic wave is attenuated and its frequency is shifted during passing the fruit. Thus it was not easy to evaluate the internal quality of the fruit using typical ultrasonic parameters such as wave velocity, attenuation, and frequency spectrum. The discrete wavelet transform was applied to the ultrasonic transmitted signal for apple. The magnitude of the first peak frequency of the wavelet basis from the ultrasonic transmitted signal showed a close correlation to the storage time of apple.

Abstract: Conventional eddy current bobbin probes, multi-pancake and/or rotating pancake probes, and transmit-receive eddy current probes are currently utilized in testing metal tubing. Each method has respective strengths and weaknesses. This paper proposes another eddy current probe with new features. The structure is designed to be sensitive to circumferential cracks, which are not easily detected with ordinary bobbin coil probes. The directions of the magnetic field and the eddy current around the coil were considered in design of the probe structure. Signals of these probes from the artificial defects were acquired and analyzed. Experimental results show that the developed probes are more sensitive to circumferential defects than comparable ordinary bobbin probes. In addition, the new probes are insensitive to axial defects. By employing both the new probes and ordinary bobbin probes, more reliable ECT can be performed.

Abstract: As cMUTs (capacitive Micromachined Ultrasonic Transducer) offer numerous advantages over traditional transducers in terms of efficiency, bandwidth, and cost, they are expected to replace piezoelectric transducers in many applications. In particular, 2D-array cMUTs have aroused great interest in the medical engineering society because of their ability to materialize a true volumetric ultrasonic image. In this study, single element cMUTs with 32 x 32 and 64 x 64 cells were successfully fabricated. The diameter and thickness of the membrane are 35
and 1000 nm, respectively, with a sacrificial layer thickness of 600 nm. The electric characteristics of the fabricated cMUT were measured. Tests on the efficiencies of the cMUT in terms of wave generation and in terms of detection according to the bias and pulse voltage were performed in an air atmosphere.

Abstract: This study was performed to classify the acoustic emission (AE) signal due to surface check and water movement of the flat-sawn boards of oak (Quercus Variablilis) during drying using the principle component analysis (PCA) and artificial neural network (ANN). To reduce the multicollinearity among AE parameters such as peak amplitude, ring-down count, event duration, ring-down count divided by event duration, energy, rise time, and peak amplitude divided by rise time and to extract the significant AE parameters, correlation analysis was performed. Over 96 % of the variance of AE parameters could be accounted for by the first and second principal components. An ANN was successfully used to classify the AE signals into two patterns. The ANN classifier based on PCA appeared to be a promising tool to classify the AE signals from wood drying.