• Progress in Superconductivity and Cryogenics
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• v.22 no.2
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• pp.38-43
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• 2020

This paper presents comparative research on characteristics of air-cored and iron-cored high-temperature superconductor (HTS) field winding synchronous motors. The 100 kW air-cored model is designed analytically by Spatial Harmonic Method, and based on this model, the iron-cored model having the same output power is designed for comparison. Due to the substantial difference of permeability property between air and iron-core, there is a difference of magnetic field magnitude and angle with respect to the HTS tape c-axis, resulting in a different critical current of the field winding considering the anisotropic property of HTS tape. For a detailed comparison between two models, the following key motor characteristics are calculated through the Finite Element Method (FEM) simulation: 1) critical current; 2) HTS wire length; and 3) torque characteristics. From the simulation results, it can be confirmed that the critical current value of the iron-cored model increases by 33 %. Also, in the case of the superconducting wire consumption, those of the iron-cored and air-cored models are 95.3 m and 815.6 m, respectively. So the wire usage can be reduced to about 88 % by using iron core. However, in terms of torque characteristics, the torque ripple of the iron-cored model is about twice as large as that of the air-cored model, which may be a disadvantage on vibration and acoustic noise.

• The KSFM Journal of Fluid Machinery
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• v.3 no.4 s.9
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• pp.38-43
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• 2000

Recently, KARI(Korea Aerospace Research Institute, Korea) and CIAM(Central Institute of Aviation Motors, Russia) have made an effort in developing a centrifugal compressor for a small gas turbine engine as part of a collaboration program. This compressor has been designed as a sub-component for an axial-centrifugal compression system for a small turbo-shaft engine aiming adiabatic efficiency higher than 0.81. The geometrical design requirement imposes restrictions to have high inlet hub-to-tip ratio and inlet swirl flow. In this study, the compressor has been designed using the generalized experimental data established from those compressors having pressure ratio of 3.7 to 5. From this generalized empirical correlation, desirable values of design parameters could be obtained. Subsequently, quasi-3D and 3D viscous flow analyses have been performed to ensure the adopted methodology. It is expected that the centrifugal compressor provides total pressure ratio of 4.89, corrected mass flow-rate of 1.64kg/sec, and adiabatic efficiency of 0.815 with inlet hub-to-tip ratio of 0.641. These relatively high total pressure ratio and inlet hub-to-tip ratio are the main distinctive features in this design. Besides, one of the main features of this centrifugal compressor is the adoption of a double-row bladed diffuser to effectively decelerate the transonic flow leaving the impeller. The compressor has been manufactured and will be tested in the near future.

• Journal of Communications and Networks
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• v.11 no.6
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• pp.574-588
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• 2009

Although much research has been conducted in the area of authentication in wireless networks, vehicular ad-hoc networks (VANETs) pose unique challenges, such as real-time constraints, processing limitations, memory constraints, frequently changing senders, requirements for interoperability with existing standards, extensibility and flexibility for future requirements, etc. No currently proposed technique addresses all of the requirements for message and entity authentication in VANETs. After analyzing the requirements for viable VANET message authentication, we propose a modified version of TESLA, TESLA++, which provides the same computationally efficient broadcast authentication as TESLA with reduced memory requirements. To address the range of needs within VANETs we propose a new hybrid authentication mechanism, VANET authentication using signatures and TESLA++ (VAST), that combines the advantages of ECDSA signatures and TESLA++. Elliptic curve digital signature algorithm (ECDSA) signatures provide fast authentication and non-repudiation, but are computationally expensive. TESLA++ prevents memory and computation-based denial of service attacks. We analyze the security of our mechanism and simulate VAST in realistic highway conditions under varying network and vehicular traffic scenarios. Simulation results show that VAST outperforms either signatures or TESLA on its own. Even under heavy loads VAST is able to authenticate 100% of the received messages within 107ms. VANETs use certificates to achieve entity authentication (i.e., validate senders). To reduce certificate bandwidth usage, we use Hu et al.'s strategy of broadcasting certificates at fixed intervals, independent of the arrival of new entities. We propose a new certificate verification strategy that prevents denial of service attacks while requiring zero additional sender overhead. Our analysis shows that these solutions introduce a small delay, but still allow drivers in a worst case scenario over 3 seconds to respond to a dangerous situation.

• Progress in Superconductivity and Cryogenics
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• v.18 no.4
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• pp.25-29
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• 2016

The high temperature superconducting (HTS) contra-rotating propulsion (CRP) systems comprise two coaxial propellers sited on behind the other and rotate in opposite directions. They have the hydrodynamic advantage of recovering the slipstream rotational energy which would otherwise be lost to a conventional single-screw system. However, the cooling systems used for HTS CRP system need a high cooling power enough to maintain a low temperature of 2G HTS material operating at liquid neon (LNe) temperature (24.5 - 27 K). In this paper, a single thermo-syphon cooling approach using a Gifford-McMahon (G-M) cryo-cooler is presented. First, an optimal thermal design of a 1.5 MW HTS motor was conducted varying to different types of commercial 2G HTS tapes. Then, a mono-cryogenic cooling system for an integration of two 1.5 MW HTS motors will be designed and analyzed. Finally, the 3D finite element analysis (FEA) simulation of thermal characteristics was also performed.

• The Transactions of The Korean Institute of Electrical Engineers
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• v.64 no.1
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• pp.136-142
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• 2015

In the paper, a design method of knee and pelvis joint in the biped robot is proposed for shock absorption and gravity compensation. Similarly to the human's body, the knee joints of the biped robot support most body weight and get a shock from the landing motion of the foot on the floor. The torque of joint motor is also increased sharply to keep the balance of the robot. Knee and pelvis joints with the spring are designed to compensate the gravity force and reduce the contact shock of the robot. To verify the efficiency of the proposed design method, we develope a biped robot with the joint mechanism using springs. At first, we experiment with the developed robot on the static motions such as the bent-knee posture both without load and with load on the flat ground, and the balance posture on the incline plane. The current of knee joint is measured to analyze the impact force and energy consumption of the joint motors. Also, we observe the motor current of knee and pelvis joints for the walking motion of the biped robot. The current responses of joint motors show that the proposed method has an effect on shock reduction and gravity compensation, and improve the energy efficiency of walking motions for the biped robot.

• Journal of the Korean Society of Propulsion Engineers
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• v.20 no.1
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• pp.20-27
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• 2016

The prediction of combustion instability is important to avoid an obvious threat to the structural safety and the motor performance because it affects the apparent response function of the propellant, the burning rate, and a mean flow Mach number at the local surface. The combustion instability occurs in case acoustic waves were coupled with the combustion/flow dynamic frequency. In this paper, an acoustic instability model is derived from the nonlinear wave equation for analysing acoustic dynamics in solid rocket motors. The chamber pressure and burning rate effects on combustion instability have been investigated.

• Proceedings of the Korean Society of Propulsion Engineers Conference
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• 2007.11a
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• pp.252-255
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• 2007

Recently military industrial company, utilizing company funded R&D and goverment and industry contracts, has developed ACTS/DACS technology. This technology can be utilized to rapidly steer "smart" bullets, "smart" rounds, tactical missile, cruise missile and kill vehicles for both endo- and exoatmospheric applications. The ACTS/DACS typically consists of a Smart Bus Controller(SCB), a proprietary network firing bus, Smart Pyrotechnic Devices(SPD), rocket motors, and a structure. The SCB communicates with the SPDs over the propretary network firing bus. Each rocket motor contains an SPD which provides rocket motor ignition. Firing energy is stored locally in the SPD so surge currents do not occur in the system as rocket motors are fired. This approach allows multiple, truly simultaneous firings without the need for large, dedicated batteries. Each SPD also functions as a network tranceiver and high reliability fir set all in the space of a single-sided 10 millimeter diameter circuit. The present work develops a new means for igniting explosive materials. The volume of semiconductor bridge (SCB) is over 30 times smaller than a conventional hot wire. We believe that the present work has a potential for development of a new igniter such as smart pyrotechnic device.

• Nuclear Engineering and Technology
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• v.41 no.1
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• pp.135-142
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• 2009

In this investigation, we prepared a 1 and 3-parallel serpentine single PEMFC, which has an active area of $100\;cm^2$ and a flow channel cross section of $1{\times}1mm$. Distribution and transport of water in a non-operating PEMFC were observed by varying flow types and the flow rates (250, 400, and 850 cc/min). This investigation was performed at the neutron imaging facility at the CO1d Neutron RAdiography facility (CONRAD), HMI, Germany of which the collimation ratio and neutron fluence rate are 250, $1{\times}10^{6}n/s/cm^2$, respectively. The neutron image was continuously recorded by a scintillator and lens-CCD coupled detector system every 10 seconds. It has been observed that although the distilled water was supplied into the cathode channel only, the neutron image showed a water movement from the cathode to the anode channel. The water at the cathode channel was completely discharged as soon as the pressurized air was supplied. But the water at the anode channel was not easily removed by the pressurized air except for the 3-parallel serpentine type with 850cc/min of air flow rate. Moreover, the water at the MEA wasn't removed for any of the cases.

• Journal of Sensor Science and Technology
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• v.24 no.4
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• pp.228-231
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• 2015

Ultrasonic motors provide high torques and quick responses compared to their magnetic counterparts; therefore, they are widely used in small-scale applications such as mobile phones, microrobots, and auto-focusing modules in digital cameras. To determine the feasibility of lead-free piezoceramics for ultrasonic motor applications, we fabricated a ring-type piezoceramic with a KNN-based lead-free piezoceramic (referred to as CZ5), intended for use in an auto-focusing module of a digital camera. The vibration of the lead-free stator was observed at 45.1 kHz. It is noteworthy that the fully assembled lead-free ultrasonic motor exhibited a revolution speed of 5-7 rpm, even though impedance matching with neighboring components was not considered. This result suggests that the tested KNN-based piezoceramic has great potential for use in ultrasonic motor applications, requiring minimal modifications to existing lead-based systems.

• Journal of the Korean Society for Precision Engineering
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• v.30 no.12
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• pp.1303-1312
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• 2013

We report an ultra-precision lathe designed to machine micron-scale features on a large-area roll mold. The lathe can machine rolls up to 600 mm in diameter and 2,500 mm in length. All axes use hydrostatic oil bearings to exploit the high-precision, stiffness, and damping characteristics. The headstock spindle and rotary tooling table are driven by frameless direct drive motors, while coreless linear motors are used for the two linear axes. Finite element method modeling reveals that the effects of structural deformation on the machining accuracy are less than $1{\mu}m$. The results of thermal testing show that the maximum temperature rise at the spindle outer surface is approximately $0.5^{\circ}C$. Finally, performance evaluations of the error motion, micro-positioning capability, and fine-pitch machining demonstrate that the lathe is capable of producing optical-quality surfaces with micron-scale patterns with feature sizes as small as $20{\mu}m$ on a large-area roll mold.