• Journal of Biosystems Engineering
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• v.36 no.6
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• pp.476-483
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• 2011

The aerial application using an unmanned helicopter has been already utilized and an attitude controller would be developed to enhance the operational convenience and safety of the operator. For a preliminary study of designing flight controller, a state space model for an RC helicopter would be identified. Frequency sweep flight tests were performed and time history data were acquired in the previous study. In this study, frequency response of the flight test data of a small unmanned helicopter was analyzed by using the CIFER software. The time history flight data consisted of three replications each for collective pitch, aileron, elevator and rudder sweep inputs. A total of 36 frequency responses were obtained for the four control stick inputs and nine outputs including linear velocities and accelerations and angular velocities in 3-axis. The results showed coherence values higher than 0.6 for every primary control inputs and corresponding on-axis outputs for the frequency range from 0.07 to 4 Hz. Also the analysis of conditioned frequency response showed its effectiveness in evaluating cross coupling effects. Based on the results, the dynamic characteristics of the model helicopter can further be analyzed in terms of transfer functions and the undamped natural frequency and damping ratio of each critical mode.

• Journal of the Korean Society for Precision Engineering
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• v.19 no.2
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• pp.203-212
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• 2002

Active magnetic hearings allow much high surface speed than conventional ball bearings and therefore greatly suitable for high speed cutting. This paper describes a design and test of an active magnetic bearing system with 16-pole radial magnets. The spindle is originally designed for a CNC lathe and driven by outer motor with 5.5 kW power and maximum speed 10,000 rpm. Considering static load condition and geometric restrictions, radial magnet is designed 16-pole type for smaller outer diameter of the spindle system. Dynamic system characteristics such as natural frequency, critical speed, stiffness, damping and system stabilities are simulated with a rigid rotor model including direct feedback controller. The designed spindle system is realized with digital PIDD controller to compensate phase lag of PWM amplifier and magnet coils. With levitation and step response experiment the control system characteristics are tested, and the spindle is rotated up to 10,000 rpm stab1y.

• Smart Structures and Systems
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• v.14 no.5
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• pp.869-881
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• 2014

This paper presents the materials analysis for combination of working modes of Magnetorheological (MR) damper. The materials were selected based on the optimum magnetic field strength at the effective areas in order to obtain a better design of MR damper. The design of electromagnetic circuit is one of the critical criteria in designing MR dampers besides the working mechanism and the types of MR damper. The increase in the magnetic field strength is an indication of the improvement in the damping performance of the MR damper. Eventually, the experimental test was performed under quasi-static loading to observe the performances of MR damper in shear mode, squeeze mode and mixed mode. The results showed that the increment of forces was obtained with the increased current due to higher magnetic flux density generated by electromagnetic coils. In general, it can be summarized that the combination of modes generates higher forces than single mode for the same experimental parameters throughout the study.

• Wind and Structures
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• v.7 no.4
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• pp.251-264
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• 2004

The effects of a class of nonlinear Tuned Mass Dampers on the aeroelastic behavior of SDOF systems are investigated. Unlike classical linear TMDs, nonlinear constitutive laws of the internal damping acting between the primary oscillator and the TMD are considered, while the elastic properties are keept linear. The perturbative Multiple Scale Method is applied to derive a set of bifurcation equations in the amplitude and phase and a parametric analysis is performed to describe the postcritical scenario of the system. Both cubic- and van der Pol-type dampings are considered and the dependence of the limit-cycle amplitudes on the system parameters is studied. These new results, compared with the previously obtained bifurcation scenario of a SDOF aeroelastic oscillator equipped with a linear TMD, show a detrimental effect on the maximum limit-cycle amplitude reduction of the nonlinear TMD. However, the analyses evidence that in the parameter region away from the perfect tuning condition the nonlinear connection can be used to tune the system with an enhancement of the limit-cycle amplitude reduction.

• Wind and Structures
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• v.8 no.2
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• pp.89-106
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• 2005

For estimating the vortex excited vibrations of overhead transmission lines, the Energy Balance Principle (EBP) is well established for spans damped near the ends. Although it involves radical simplifications, the method is known to give useful estimates of the maximum vibration levels. For very long spans, there often is the need for a large number of in-span fittings, such as in-span Stockbridge dampers, aircraft warning spheres etc. This adds complexity to the problem and makes the energy balance principle in its original form unsuitable. In this paper, a modified version of EBP is described taking into account in-span damping and in particular also aircraft warning spheres. In the first step the complex transcendental eigenvalue problem is solved for the conductor with in-span fittings. With the thus determined complex eigenvalues and eigenfunctions a modified energy balance principle is then used for scaling the amplitudes of vibrations at each resonance frequency. Bending strains are then estimated at the critical points of the conductor. The approach has been used by the authors for studying the influence of in-span Stockbridge dampers and aircraft warning spheres; and for optimizing their positions in the span. The modeling of the aircraft warning sphere is also described in some detail.

• Journal of the Korean Geotechnical Society
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• v.19 no.5
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• pp.327-333
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• 2003

Characteristics of vibration propagation of borehole blasting were analyzed with 578 borehole vibration data obtained from 23 sites which were used in tunnel and underground space design, and 221 tunnel vibration data fron 4 sites of tunnel under construction. Analysis results on the damping of vibration velocity show that site factors in borehole blasting were higher than those in tunnel blasting. And the critical charge calculated from regression equations at large scaled distance was lower in borehole blasting. Dominant frequency was in the range of 30∼60Hz for the borehole blasting and 60∼90Hz for the tunnel blasting. As a conclusion, the borehole blasting data should not be used on the tunnel blasting design without careful statistical analysis.

• The Transactions of The Korean Institute of Electrical Engineers
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• v.59 no.5
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• pp.860-865
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• 2010

When a circuit breaker is opened, a large capacitance around the buses, the circuit breaker and the potential transformer (PT) might cause PT ferroresonance. During PT ferroresonance, the iron core repeats saturation and unsaturation even though the supplied voltage is a rated voltage. This paper describes an analytical analysis of PT ferroresonance in the transient-state. To analyze ferroresonance analytically, the iron core is modelled by a simplified two-segment core model in this paper. Thus, a nonlinear ordinary differential equation (ODE) for the flux linkage is changed into a linear ODE with constant coefficients, which enables an analytical analysis. In this simplified model, each state, which is either saturated or unsaturated state, corresponds to one of the three modes, i.e. overdamping, critical damping and underdamping. The flux linkage and the voltage in each state are obtained analytically by solving the linear ODE with constant coefficients. The proposed transient analysis is effective in the more understanding of ferroresonance and thus can be used to design a ferroresonance prevention or suppression circuit of a PT.

• The KSFM Journal of Fluid Machinery
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• v.14 no.5
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• pp.69-74
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• 2011

This paper deals with the detail rotordynamic analysis for the vertical rotor system as development of vertical sea water lift pump for FPSO deep well. In a vertical rotor system, since linearized stiffness and damping coefficients of fluid film bearing are no longer be valid, hence the transient response analysis considering a fluid film force for every journal position in the bearing needs to be required. In this study, the transient response analysis of the proposed vertical pump rotor system was carried out in dry-run and wet-run conditions, respectively. The results show that orbital vibration responses of the rotor system remain stable at rated speed and thereby operating reliability of the vertical rotor system is confirmed. To overcome complexity of calculation pr ocedure and time consuming calculation of transient analysis, the calculating technique of steady-state response analysis is also proposed. The results of steady-state response obtained by applying the proposed technique to the rotor system are good agreement with the reference results, that is, transient responses.

• Transactions of the Korean Society for Noise and Vibration Engineering
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• v.15 no.3 s.96
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• pp.354-363
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• 2005

Squeeze film dampers (SFDs) have been commonly used to effectively enhance the dynamic behavior of the rotating shaft supported by rolling element bearings. However, due to the recent trends of high operating speed, high load capacity and light weight in rotating machinery, it is becoming increasingly important to change the dynamic characteristics of rotating machines in operation so that the excessive vibrations, which may occurparticularly when passing through critical speeds or unstable regions, can be avoided. Semi-active type SFDs using magneto-rheological fluid (MR fluid), which responds to an applied magnetic field with a change in rheological behavior, are introduced in order to find its applications to rotating machinery as an effective device attenuating unbalance responses. In this paper, a semi-active SFD using MR fluid is designed, tested, and identified to investigate the capability of changing its dynamic properties such as damping and stiffness.In order to apply the MR-SFD to the vibration attenuation of a rotor, a systematic approach for determining the damper's optimal location is investigated, and also, a control algorithm that could improve the unbalance response characteristics of a flexible rotor is proposed and its control performance is validated with a numerical example.

• Journal of Advanced Marine Engineering and Technology
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• v.9 no.2
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• pp.159-169
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• 1985

The calculation of torsional vibration for marine diesel engine propulsion shafting is normally carried out by equalizing exciting energy to damping energy, or using the dynamic magnifier. But, with these methods, the vibration amplitudes are calculated only for resonance points and vibration amplitudes of other running speeds of engine are determined by the estimation. Recently, many energy-saving ships have been built and on these ships, two-stroke, supercharged, super-long stroke diesel engines which have a small number of cylinders are usually installed. In these cases, the first order critical-torsional vibrations of these engine shaftings appear ordinarily near the MCR speed and the stress amplitudes of their vibration skirts exceed the limit stress defined by the rules of classification society. To predict the above condition in the design stage, the synthesized vibration amplitudes of all orders which are summed up according to their phase angles must be calculated from the drawings of propulsion shaft systems. In this study, a theoretical method to fulfill the above calculation is derived and a computer program is developed according to the derived method. And a shafting system of two-stroke, super-long stroke diesel engine which was installed in a bulk carrier is analyzed with this method. The measured values of this engine shafting are compared with those of calculated results and they show a fairly good agreement.