• 제어로봇시스템학회:학술대회논문집
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• 1992.10a
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• pp.1060-1066
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• 1992

One of the undesirable nonlinear phenomenon called 'wind up' occours when the integrator in the controller and the saturated actuator interact. Large overshoot, slow response, instability, limit cycle and jump resonance are the characteristics of wind up phenomenon. Several 'anti-windup' compensators have been developed to prevent some of the aforementioned nonlinear characteristics such as instabilituy and limit cycle, but none has studied the effect of antiwindup compensator on the jump resonance. In this paper, we developed an analyitcal method to design the compensator to prevent not only limit cycle but also jump resonance. An illustrative example is included to show the compensator eliminates jump resonance of effectively.

• Journal of the Korean Society for Aviation and Aeronautics
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• v.21 no.4
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• pp.47-52
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• 2013

In this paper, limit cycle flutter induced by Hopf bifurcation is studied with nonlinear system analysis approach and observed for the critical slowing down phenomenon. Considering an attractor of the dynamics of a system, when a small perturbation is applied to the system, the dynamics converge toward the attractor at some rate. The critical slowing down means that this recovery rate approaches zero as a parameter of the system varies and the size of the basin of attraction shrinks to nil. Consequently, in the pre-bifurcation regime, the recovery rates decrease as the system approaches the bifurcation. This phenomenon is one of the features used to forecast bifurcation before they actually occur. Therefore, studying the critical slowing down for limit cycle flutter behavior would have potential applicability for forecasting those types of flutter. Herein, modeling and nonlinear system analysis of the 2D airfoil with torsional nonlinearity have been discussed, followed by observation of the critical slowing down phenomenon.

• Journal of the Korean Society for Railway
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• v.17 no.5
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• pp.342-348
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• 2014

Contact between wheel and rail leads to the creep phenomenon. Linear creep theory, assuming linear increase in the creep force vs creep, results in a critical speed at which the vibration of a railway vehicle goes to infinity. However, the actual creep force converges to a limited value, so that the vibration of a railway vehicle cannot increase indefinitely. In this study, the dynamics of a railway vehicle is investigated with a 6 DOF bogie model includingthe nonlinear creep curves of Vermeulen, Polach, and a newly calculated creep curve with strip theory. Strip theory considers the profiles of the wheel and rail. The results show that the vibration of a railway vehicle results in a limit-cycle over a specific running speed, and this limit-cycle becomes smaller as the slope of the creep-curve steepens. Moreover, a hunting phenomenon is caused due to flange contact, which restricts the magnitude of the limit-cycle.

• Advances in aircraft and spacecraft science
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• v.4 no.2
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• pp.203-218
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• 2017

This paper focuses on the computational study of nonlinear effects of unsteady aerodynamics for non-classical aileron buzz. It aims at a comprehensive investigation of the aileron buzz phenomenon under varying flow parameters using the describing function technique with multiple inputs. The limit cycle oscillatory behavior of an asymmetrical airfoil is studied initially using a CFD-based numerical model and direct time marching. Sharp increases in limit cycle amplitude for varying Mach numbers and angles of attack are investigated. An aerodynamic describing function is developed in order to estimate the variation of limit cycle amplitude and frequency with Mach number and angle of attack directly, without time marching. The describing function results are compared to the amplitudes and frequencies predicted by the CFD calculations for validation purposes. Furthermore, a limited sensitivity analysis is presented to demonstrate the potential of the approach for aeroelastic design.

• Journal of the Institute of Electronics Engineers of Korea SC
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• v.43 no.1 s.307
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• pp.39-44
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• 2006

Pattern noise in the Delta-Sigma modulator is a well Known phenomenon that intrigued many circuit designers. These noise appear as the modulator output falls into a cyclic mode of operation. This paper addresses the dependence of these tone signal upon the system topologies. Among the four well known single-stage DSM topologies, namely Cascade of Integrators with Feedback Form(CIFB), Cascade of Integrators with Feedforward Form(CIFF), Cascade of Resonators with Feedback Form(CRFB), and Cascade of Resonators with Feedforward Form(CRFF), resonator type DSMs turn out to be more susceptible to the pattern noise than the integrator type. Noise transfer functions of the investigated topologies are also presented.

• Journal of Institute of Control, Robotics and Systems
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• v.22 no.5
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• pp.346-352
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• 2016

This paper proposes an adaptive robust control method for an acrobot system in the presence of input disturbance. The acrobot system is a typical example of the underactuated system with complex nonlinearity and strong dynamic coupling. Also, disturbance can cause limit cycle phenomenon which appears in the acrobot system around the desired unstable equilibrium point. To minimize the effect of the disturbance, we apply a fuzzy disturbance estimation method for the swing-up and balancing control of the acrobot system. In this paper, both disturbance observer and controller for the acrobot system are designed and verified through mathematical proof and simulations.

• 제어로봇시스템학회:학술대회논문집
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• 1997.10a
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• pp.1864-1866
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• 1997

Uncertainties in the aerodynamic coefficients or time delay effects in implementing an autopilot algorithm can make a Flight Control System(FCS) unstable. When a FCS enters unstable state, the actuator or sensor limiters in FCS make the unstable system not diverge but be in the state of stable limit cycle. If an autopilot recognize the FCS to be in the stable limit cycle phenomenon, it woudl be better to adjust autopilot gains to stabilize the FCS. A novel method to stabilize a FCS using parameter estimation and maintenance of given phase margin is proposed. The method is applied to roll control loop and verified its performance.

• The Transactions of The Korean Institute of Electrical Engineers
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• v.60 no.11
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• pp.2017-2020
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• 2011

Flicker, which is also known as voltage fluctuation, is an electromagnetic phenomenon generated by large scale nonlinear loads, such as arc furnaces and welding machines. Since a severe and continuous flicker can cause to some damages to electrically sensitive loads as well as human's visual irritations, it needs to be appropriately managed by being accurately measured, quantified and assessed. In Korea, an equivalent 10-Hz flicker index, shortly ${\Delta}V10$, is used to determine the permission limit of flicker. This paper presents an efficient calculation of the flicker index by using a half-cycle sliding window and a recursive method, showing a concrete calculating procedure of ${\Delta}V10$ from the viewpoint of signal processing.

• Journal of Korean Academy of Nursing
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• v.47 no.4
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• pp.562-574
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• 2017

Purpose: This study attempts to explore the subjective experience of frailty among elderly individuals in Korea. Methods: From June to August in 2014, 11 elderly persons who had experienced frailty in a community were interviewed. For data analysis, the method suggested by Colaizzi was applied as a phenomenological method. Results: According to the analysis, the study participants' frailty process was structured in seven categories: (a) 'natural phenomenon with ageing,' (b) 'life force comes to an end,' (c) 'the light in my heart turns off,' (d) 'unavoidable situation,' (e) 'continuous and connected vicious cycle,' (f) 'the limit of recovery energy already passes,' and (g) 'life is supported by someone's help.' Conclusion: The frailty experience in the participants is a natural process of aging, which cause vicious cycle acting with each other among physical, psychological, and social health. It is said that the cycle of frailty was started from weight loss and insufficient sleep, and boostered by pain. The participants from repetition of the vicious cycle become exhausted and pass the threshold of their recovery energy at some points. If they meet with sudden accidents such as falling, traffic accident and so on, they become to live a dependent life supported by someone's help in a moment. To prevent frailty and worsening conditions in Korean elderly individuals, it is recommended to provide a interventional programs using this study's results.

• Journal of KSNVE
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• v.3 no.4
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• pp.373-382
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• 1993

Active micro-vibration control of a structure, which simulates a stepper device, is performed using a pair of piezolectric actuators. The control aims at reducing the translational and rotational vibrations of the upper plate when the base is subject to seismic disturbance and the upper plate undergoes impulsive transient motion. Using the experimentally determined model, derivative control scheme is adopted so that the damping of the closed-loop system is effectively increased. It is found that the predicted control performance is in good agreement with the experimental results. Finally, the limit cycle phenomenon due to the controller voltage saturation is compared with the simulation.