• International Journal of Aeronautical and Space Sciences
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• v.8 no.1
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• pp.21-31
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• 2007

Relaxed Static Stability (RSS) concept has been applied to improve aerodynamic performance of modern version supersonic jet fighter aircraft. The T-50 advanced supersonic trainer employs the RSS concept in order to improve the aerodynamic performance. And the flight control system stabilizes the unstable aircraft and provides adequate handling qualities. The T-50 longitudinal control laws employ a proportional-plus-integral type controller based on a dynamic inversion method. The longitudinal dynamic modes consist of short period with high frequency and phugoid mode with low frequency. The design goal of longitudinal control law is optimization of short period damping ratio and frequency using Lower Order Equivalent System (LOES) complying the requirement of MIL-F-8785C. This paper addresses phugoid mode characteristics such as damping ratio and natural frequency that is affected by the nonlinear control laws such as angle of attack limiter, auto pitch attitude command system and autopilot of pitch attitude hold.

• Proceedings of the Korean Society of Marine Engineers Conference
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• 2011.10a
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• pp.75-75
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• 2011

This paper has been studied that ocean wave vibration power generator is composed of buoy and vibration generator to make effective use of ocean wave energy. We designed buoy to can occur resonance for dominant frequency with ocean wave. And then we fitted the natural frequency of vibration system with vibration power generator to buoy's natural frequency. And we can show that the amplitude of ocean wave up and down motion is decreased, on the other hand, the displacement of vibration system with vibration power generator is increased. Therefore, ocean wave vibration power generator which is proposed in this paper has merits not only securing its stability from surroundings but also producing more electronic power by using ocean wave energy.

• Journal of KSNVE
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• v.3 no.4
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• pp.383-394
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• 1993

An automotive suspension system generally behaves like a low frequency band-pass filter(0.5 - 10 Hz). Passengers are very sensitive to this frequency range in terms of ride quality and road holding ability. In this paper, a LQG/ LTR controller is suggested to improve the ride quality and road holding ability in the specified frequency rage. It has been found by numerical simulation that the ride quality and road holding ability can be improved in the frequency ranges of 0.5 - 3.0 Hz and 0.3 - 2.1 Hz respectively. In addition, a new approach using root locus to evaluate the stability robustness of the active suspension system is studied. It is shown that the stability robustness of the LQG/LTR controller designed in this paper is improved, compared to the passive system.

• Proceedings of the KSME Conference
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• 2001.06b
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• pp.99-104
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• 2001

Discrete-time controller design is proposed using feedback system identification in frequency domain. System Stability imposed by a new controller is checked in the function of a conventional closed-loop system, instead of a poorly modeled plant due to non-linearity and disturbance as well as unstable components, etc. The stability of the system is evaluated in view of Popov criterion. All the equations are formulated in the framework of the discrete-time system. Simulation results are shown on the plant with input saturation components, DC disturbance and a pure integration.

• Transactions of the Korean Society for Noise and Vibration Engineering
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• v.21 no.5
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• pp.447-454
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• 2011

This paper reports a filtered velocity feedback(FVF) controller, which is an alternative to direct velocity feedback(DVFB) controller. The instability problems due to high frequency response under DVFB can be alleviated by the suggested FVF controller. The FVF controller is designed to filter out the unstable high frequency response. The FVF controller and the dynamics of clamped beams under forces and moments are first formulated. The effects of the design parameters(cut-off frequency, gain, and damping ratio) on the stability and the performance are then investigated. The cut-off frequency should be selected not to affect the system stability. The magnitude of the open loop transfer function(OLTF) at the cut-off frequency should be small. As increasing the gain of the FVF controller, the magnitude of the OLTF is increased, so that the closed loop response can be reduced more. The enhancement of the OLTF at the cut-off frequency is reduced but the phase behavior around the cut-off frequency is distorted, as the damping ratio is increased. The control performance is finally estimated for the clamped beam. More than 10 dB reductions in velocity response can be achieved at the modal frequencies from the first to eighth modes.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2011.04a
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• pp.264-270
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• 2011

This paper reports a filtered velocity feedback (FVF) controller, which is an alternative to direct velocity feedback (DVFB) controller. The instability problems due to high frequency response under DVFB can be alleviated by the suggested FVF controller. The FVF controller is designed to filter out the unstable high frequency response. The FVF controller and the dynamics of clamped beams under forces and moments are first formulated The effects of the design parameters (cut-off frequency, gain, and damping ratio) on the stability and the performance are then investigated. The cut-off frequency should be selected not to affect the system stability. The magnitude of the open loop transfer function (OLTF) at the cut-off frequency should be small. As increasing the gain of the FVF controller, the magnitude of the OLTF is increased, so that the closed loop response can be reduced more. The enhancement of the OLTF at the cut-off frequency is reduced but the phase behavior around the cut-off frequency is distorted, as the damping ratio is increased The control performance is finally estimated for the clamped beam. More than 10dB reductions in velocity response can be achieved at the modal frequencies from the first to eighth modes.

• Journal of the Ergonomics Society of Korea
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• v.25 no.3
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• pp.1-15
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• 2006

Loss of postural balance can possibly lead to increased risk of slips and falls in work places. Present study was performed to investigate the effects of noisy environments on postural stability during standing. It is known that a sound is characterized by the frequency and pressure level of the sound. Therefore, effects of the frequency and pressure level on postural stability were of primary concern. Ten male subjects participated in the experiment. Subject's center of pressure(COP) position was collected on a force plate while they were exposed to different frequency and pressure levels of the sound. Measured COP was then converted into the length of postural sway path in both anterior-posterior(AP) and medio-lateral(ML) axis. Results showed that the length of sway path in AP axis was significantly affected by the frequency of sound. The length of sway path was lowest at frequency level of 2000Hz and increased below and above this frequency range. The sound pressure level, however, did not significantly affect the postural sway length in both AP and ML axis. The results imply that industrial workers in noisy environments should be aware that their abilities of postural balance can be disturbed significantly.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.33 no.3
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• pp.210-217
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• 2009

This paper presents the analytical prediction of stability lobes in milling. The stability lobes are obtained by measuring the frequency response function (FRF) of a machining center at the cutting point of the end mill cutter, identifying cutting constants, and approximating cutting force coefficients. The stability lobes are experimentally verified through cutting tests.

• The Journal of the Acoustical Society of Korea
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• v.4 no.1
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• pp.16-21
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• 1985

Both shifiting the input signal's frequencies by a fixed frequency and compressing the input signal's bandwidth have been known to be effective in improving the stability margin of public adress systems operating in reverberant spaces. This paper describes the effect of an alternative approach of improving the acoustic-feedback stability and yet maintaining speech inteligibility by bandwidth compression and expansion. Conditions are derived for this technizue to be realized and an experimental system has been made - up. A series of experiments has been performed in small spaces and the results have shown that more than 5dB improvement can be obtained in the stability margin.

• Journal of Electrical Engineering and Technology
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• v.10 no.2
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• pp.443-451
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• 2015

The stability of a multi-terminal direct current (MTDC) system is often influenced by its control strategy. To improve the stability of the MTDC system, the control strategy of the MTDC system must be appropriately adopted. This paper deals with estimating stability of a MTDC system based on the line-commutated converter based high voltage direct current (LCC HVDC) system with an inverter with constant extinction angle (CEA) control or a rectifier with constant ignition angle (CIA) control. In order to evaluate effects of two control strategies on stability, a MTDC system is tested on two conditions: initialization and changing DC power transfer. In order to compare the stability effects of the MTDC system according to each control strategy, a mathematical MTDC model is analyzed in frequency domain and time domain. In addition, Bode stability criterion and transient response are carried out to estimate its stability.