• 한국재료학회지
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• 제22권3호
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• pp.136-139
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• 2012

Magnetic nanoparticles for ferromagnetic fluids and magnetorheological fluids were prepared by chemical coprecipitation and mechanical milling, respectively. The surface-treated particles were dispersed at various weight ratios into a medium of polyethylene glycol. In order to evaluate the elastic modulus of the fluids, ultrasonic pulse velocities were measured with an ultrasonic test using transducers of 5MHz and 2.25MHz. The ultrasonic signals were only available with a transducer of 2.25 MHz at fluid concentrations of 5 mg/ml and lower. In the case of applying transducers over 2.25 MHz and concentrations over 5 mg/ml to the fluids, it was impossible to observe effective ultrasonic signals due to an excessive scattering of the pulses by the dispersed particles. Elastic moduli of the magnetorheological fluids were 5.44 GPa and 6.13 GPa with concentrations of 25 mg/ml and 50 mg/ml, respectively; these values were higher by 40% than the values of 4.04 GPa and 4.28 GPa of ferromagnetic fluids at the same concentrations. As for the effect of an external magnetic field on these dilute fluids, the ultrasonic signals were positioned in a very similar way, which was probably due to insufficient arrangement of the particles even though the reflection energy of the ultrasonic waves apparently increased.

• 비파괴검사학회지
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• 제22권4호
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• pp.422-429
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• 2002

용접부 초음파 사각탐상은 용접 루트부나 counter bore와 같은 결함과 관련 없는 신호를 발생하는 기하학적 반사체로 인하여 신호의 식별이 매우 어렵다. 본 논문에서는 이와 같은 상황에서 결함 신호를 식별하는 새로운 기법을 제안하였다. 디컨볼루션(deconvolution)을 사용한 유사성함수의 개념을 도입한 새로운 기법에서는 먼저 결합과 기하학적 반사체에 대하여 기준신호와 대상신호를 획득하였으며 이들을 정규화 하였다. 대상신호를 기준신호로 디컨볼루션하여 얻은 유사성함수의 패턴으로부터 기하학적 반사 신호와 결함신호를 구분할 수 있었다. 그 결과로, 제안하고 있는 기법이 기하학적 반사체와 노치 모서리부에서 반사되는 신호를 분리하는데에 유용함을 알 수 있었다.

• 전기학회논문지
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• 제65권11호
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• pp.1887-1893
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• 2016

The VSA(Video Signal Analysis) method is the time-domain approach for estimating ultrasonic attenuation which utilizes the envelop signals from backscattered rf signals. The echogenicity of backscattered ultrasonic signals, however, from deeper depths are distorted when the broadband transmit pulse is used and it degrades the estimation accuracy of attenuation coefficients. We propose the modified VSA method using adaptive bandpass filters according to the centroid shift of echo signals as a pulse propagates. The technique of dual-reference diffraction compensation is also proposed to minimize the estimation errors because the difference of attenuation properties between the reference and sample aggravates the estimation accuracy when the differences are accumulated in deeper depth. The proposed techniques minimize the distortion of relative echogenicity and maximize the signal-to-noise ratio at the given depth. Simulation results for numerical tissue-mimicking phantoms show that the Rectangular-shaped filter with the appropriate center frequency exhibits the best estimation performance and the technique of the dual-reference diffraction compensation dramatically improves accuracy for the region after the beam focus.

• 대한전기학회논문지:전기물성ㆍ응용부문C
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• 제49권1호
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• pp.44-51
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• 2000

In this paper, using a wide-band AE sensor with the frequency range from 100[kHz] to 1.5[kHz], the frequency spectra of AE signals generated from the corona discharges of the needle-plane gap and from the partial discharges of an epoxy void were analyzed to determine the proper ultrasonic sensor with optimum frequency range according to the patterns of corona discharges. We also examined the propagation characteristics of AE signals in oil and the relationship between the magnitude of corona discharge and the magnitude of AE signals in peak-to-peak value under the application of 60[Hz] ac high-voltage. From these results, the main frequency spectra of AE signals emitted from the corona discharges of the needle-plane gap were about 130[kHz] by the fast fourier transform, but the main frequency spectra appeared to be 230[kHz] in the partial discharges of an epoxy void. The magnitude of AE signals was proportional to the magnitude of corona discharge and discharge current pulse with increasing the applied voltages.

• 제어로봇시스템학회논문지
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• 제14권6호
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• pp.608-614
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• 2008

This paper describes a location tracking system to guide landing process of an Unmanned Helicopter(UMH) exploiting MIT Cricket nodes. For automatic landing of a UMH, a precise positioning system is indispensable. However, GPS(Global Positioning System) is inadequate for tracking the three dimensional position of a UMH because of large positioning errors. The Cricket systems use Time-Difference-of-Arrival(TDoA) method with ultrasonic and RF(Radio Frequency) signals to measure distances. They operate in passive mode in that a listener attached to a moving device receives distance signals from several beacons located at fixed points on ground. Inevitably, this passive type of implementation causes large disturbances in measuring distances between beacons and the listener due to wind blow from propeller and turbulence of UMH body. To cope with this problem, we proposed active type of implementation for positioning a UMH. In this implementation, a beacon is set up at UMH body and four listeners are located at ground area at least where the UMH will land. A pair of Ultrasonic and RF signals from the beacon arrives at several listeners to calculate the position of the UMH. The distance signals among listeners are synchronized with a counter value appended to each distance signals from the beacon.

• 대한전기학회:학술대회논문집
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• 대한전기학회 2000년도 하계학술대회 논문집 C
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• pp.1897-1899
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• 2000

This paper describes the frequency characteristics and the number of pulse of ultrasonic signals due to partial discharge occurred at each electrode. The defects which could occur in a transformer were simulated by using needle-plane electrode, IEC(b) electrode and void electrode. As a result, the dominant frequency of ultrasonic signals generated by corona in oil and partial discharge in void was hardly changed regardless with the applied voltage, but in case of surface discharge in oil, its dominant frequency moved to low frequency with the applied voltage. The increasing rate of pulse number per second was high in order of the surface discharge in oil, the partial discharge in void, the corona in oil.

• Journal of Electrical Engineering and Technology
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• 제13권1호
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• pp.430-437
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• 2018

Ultrasonic attenuation is an important parameter in Quantitative Ultrasound and many algorithms have been proposed to improve estimation accuracy and repeatability for multiple independent estimates. In this work, we propose an improved algorithm for estimating ultrasonic attenuation utilizing the optimal frequency compounding technique based on stochastic noise model. We formulate mathematical compounding equations in the AWGN channel model and solve optimization problems to maximize the signal-to-noise ratio for multiple frequency components. Individual estimates are calculated by the reference phantom method which provides very stable results in uniformly attenuating regions. We also propose the guideline to select frequency ranges of reflected RF signals. Simulation results using numerical phantoms show that the proposed optimal frequency compounding method provides improved accuracy while minimizing estimation bias. The estimation variance is reduced by only 16% for the un-compounding case, whereas it is reduced by 68% for the uniformly compounding case. The frequency range corresponding to the half-power for reflected signals also provides robust and efficient estimation performance.

• 대한기계학회:학술대회논문집
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• 대한기계학회 2004년도 추계학술대회
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• pp.1727-1732
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• 2004

For the purpose of monitoring by ultrasonic test of the ball bearing conditions in rotating machinery, a system for their diagnosis was developed. ultrasonic technique is used to detect abnormal conditions in the bearing system. And various data such as frequency spectrum, energy and amplitude of ultrasonic signals, and ultrasonic parameters were acquired during experiments with the simulated ball bearing system. Based on the above results and practical application for power plant, algorithms and judgement criteria for diagnosis system was established.

• 제어로봇시스템학회:학술대회논문집
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• 제어로봇시스템학회 2004년도 ICCAS
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• pp.582-587
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• 2004

Ultrasonic sensors are widely used in mobile robot applications to recognize external environments, because they are cheap, easy to use, and robust under varying lighting conditions. However, the recognition of objects using a ultrasonic sensor is not so easy due to its characteristics such as narrow beam width and no reflected signal from a inclined object. As one of the alternatives to resolve these problems, use of multiple sensors has been studied. A sequential driving system needs a long measurement time and does not take advantage of multiple sensors. Simultaneous and pulse coding driving system of ultrasonic sensor array cannot measure short distance as the length of the code becomes long. This problem can be resolved by multi-frequency driving of ultrasonic sensors, which allows multi-sensors to be fired simultaneously and adjacent objects to be distinguished. Accordingly, this paper presents a simultaneous and multi-frequency driving system for an ultrasonic sensor array for object recognition. The proposed system is designed and implemented using a DSP and FPGA. A micro-controller board is made using a DSP, Polaroid 6500 ranging modules are modified for firing the multi-frequency signals, and a 5-channel frequency modulated signal generating board is made using a FPGA. To verify the proposed method, experiments were conducted in an environment with overlapping signals, and the flight distances for each sensor were obtained from filtering of the received overlapping signals and calculation of the time-of-flights.

• 제어로봇시스템학회논문지
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• 제13권7호
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• pp.711-717
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• 2007

Location information will become increasingly important for future Pervasive Computing applications. Location tracking system of a moving device can be classified into two types of architectures: an active mobile architecture and a passive mobile architecture. In the former, a mobile device actively transmits signals for estimating distances to listeners. In the latter, a mobile device listens signals from beacons passively. Although the passive architecture such as Cricket location system is inexpensive, easy to set up, and safe, it is less precise than the active one. In this paper, we present a passive location system using Cricket Mote sensors which use RF and ultrasonic signals to estimate distances. In order to improve accuracy of the passive system, the transmission speed of ultrasound was compensated according to air temperature at the moment. Upper and lower bounds of a distance estimation were set up through measuring minimum and maximum distances that ultrasonic signal can reach to. Distance estimations beyond the upper and the lower bounds were filtered off as errors in our scheme. With collecting distance estimation data at various locations and comparing each distance estimation with real distance respectively, we proposed an equation to compensate the deviation at each point. Equations for proposed algorithm were derived to calculate relative coordinates of a moving device. At indoor and outdoor tests, average location error and average location tracking period were 3.5 cm and 0.5 second, respectively, which outperformed Cricket location system of MIT.