M.-K. Yeh

Bio: M.-K. Yeh is an academic researcher from National Tsing Hua University. The author has contributed to research in topics: Instability & Vibration. The author has an hindex of 4, co-authored 4 publications receiving 51 citations.

Parametric instability of a beam under electromagnetic excitation

Abstract: The parametric instability of a beam under electromagnetic excitation was investigated experimentally and analytically. In experiment an electromagnetic device, acting like a spring with alternating stiffness, was designed to parametrically excite the beam. The frequency and the amplitude of the excitation force were accurately controlled by the AC current flowing through the coil of the electromagnetic device. Since the excitation force is a non-contact electromagnetic force which acts on the beam in the transverse direction, the disturbances induced by the geometric imperfection of the beam, by the eccentricity of the usual axial excitation force, and the coupling effects between the excitation mechanism and the beam were effectively avoided. The dynamic system was analyzed based on the assumed-modes method. The instability regions of the system were found to be the functions of the modal parameters of the beam and the position, the stiffness of the electromagnetic device for various cantilevered beams. The modal damping ratios of the beam specimens were also identified. The experimental results were found to agree well with the analytical ones.

Parametric instability of a column with an axially oscillating mass

Abstract: This study investigates the dynamic instability behavior of a column carrying a concentrated mass with oscillating motion along the column axis. The dynamic equation of the column was derived based on the assumed-modes method. The derived dynamic equation, which contains parametrically excited terms associated with modal accelerations, modal velocities, and modal displacements, is a general form of Mathieu's equation. A new analytical method used to determine the instability regions of the column was directly applied to the transition state. This method is different from the traditional perturbation method in which a criterion, involving the determination of the characteristic exponents, is used to yield the transition curves. The principal vibration frequencies, the ratio of principal amplitudes, and the phase difference between the parametrically excited force and the principal frequency response on the transition state were obtained systematically. The parametric instability behavior of a column carrying a periodically moving concentrated mass is different from that of a column subjected to a periodic tangential inertia force. The present case contains the simple resonances and combination resonances of sum type only, while the case with tangential inertia force may contain the combination resonances of the difference type additionally. Four examples are given to demonstrate the instability behavior of various columns carrying concentrated oscillating mass along the column axis at varying positions.

Parametric instability of a cantilevered column under periodic loads in the direction of the tangency coefficient

Abstract: Parametrically excited instability in a cantilevered column, under a periodic load in the direction of the tangency coefficient at its free end, was investigated analytically. The tangency coefficient was obtained by dividing the slope in the direction of the applied periodic load by the tangent slope at the free end of the column. The assumed modes method was used to convert the original continuous column to a lumped one. The Lagrangian equation was used to derive the dynamic equation of the system, which contained a term for parametric excitation with the tangency coefficient, and became Hill’s equation of multiple degrees of freedom. For a sinusoidal periodic load, it becomes Mathieu’s equation. Parameters for the instability bandwidth were calculated to find systematically the transition curves separating regions of stability and instability, in which the simple and combination resonances of the parametric instability regions varied with the tangency coefficient. Physical explanations are given for the behavior of simple and combination resonances qualitatively.

Dynamic instability of a general column under a periodic load in the direction of the tangency coefficient at any axial position

Abstract: The instability behavior of a parametrically excited column subjected to a periodic load at any axial position in the direction of the tangency coefficient was investigated analytically. For an inextensional neutral axis, the lateral and axial deflections of the column can be expressed as functions of natural mode shapes and the corresponding mode deflections of the column. The components of modal excitation force induced by the periodic load were found as well. The dynamic equation of the system was obtained by incorporating the modal excitation forces and the modal equations of the free transverse vibration of the column into the virtual work equation. The instability bandwidth of simple and combination resonances of a general column can be described systematically by the natural frequencies of the column, the amplitude of the excitation force and a set of instability bandwidth parameters. A general formula was obtained to determine directly the instability regions of the column system, while bypassing the procedures for reducing and solving the dynamic equation of the system. Physical explanations are given for the behavior of simple and combination resonances. Examples for columns with various boundary conditions were described to indicate their instability regions and were found to agree quite well with the results by previous researchers.

A tunable electromagnetic vibration absorber: Characterization and application

Abstract: The paper presents a newly designed electromagnetic vibration absorber (EMVA), whose stiffness is on-line tunable. The EMVA is capable of suppressing vibration of the primary system excited by a harmonic force with a variable frequency. The EMVA consists of a clamped–clamped aluminum beam and a permanent magnet that is embedded in the center of the beam and placed between two poles of a C-shaped electromagnet. By varying the current of the electromagnet, stiffness of the EMVA can be adjusted instantaneously such that the absorber frequency can be tuned. A detailed characterization of the EMVA is presented. The effective stiffness of the absorber is determined numerically and validated experimentally. To test its effectiveness in vibration suppression, the EMVA is used to track two types of the exciting frequency variations: multi-step and linear. The response of the absorber mass is used to tune the EMVA to ensure that the absorber frequency equals the exciting frequency.

Quantum Amplification by Superradiant Emission of Radiation

Abstract: Light amplification in lasers usually relies on populating higher energy levels with more light emitters than lower energy levels. Scientists propose a new way of amplifying light without such population inversion, based on their discovery of resonant superradiant emission from an atomic ensemble interacting with a driving light field.

Identification of pseudo-natural frequencies of an axially moving cantilever beam using a subspace-based algorithm

Abstract: This study focuses on extraction of frequency information of a linear time-varying system using free response data. Frequency information is obtained from the pseudo-modal parameters that were defined in a previous study. A subspace-based identification algorithm is introduced. An improved version is proposed to make the algorithm less sensitive to measurement noise. An axially moving cantilever beam is used as the experimental system. A dynamic model is presented to show that lateral vibration of the axially moving cantilever beam is governed by a linear time-varying model. A computer simulation is conducted to compare the true pseudo-modal parameters and approximate ones that can be identified using the improved algorithm. The experimental study focuses on the capabilities of the algorithm and the factors that affect the identification results. A method of grouping identified structural pseudo-natural frequencies is proposed. Limitations of the algorithm are discussed.

Quantum amplification by superradiant emission of radiation

Abstract: Light amplification in lasers usually relies on populating higher energy levels with more light emitters than lower energy levels. Scientists propose a new way of amplifying light without such population inversion, based on their discovery of resonant superradiant emission from an atomic ensemble interacting with a driving light field.

Parametric instability of a cantilever beam with magnetic field and periodic axial load

Abstract: The present work deals with the parametric instability regions of a cantilever beam with tip mass subjected to time-varying magnetic field and axial force. The nonlinear temporal differential equation of motion having two frequency parametric excitations is solved using second-order method of multiple scales. The closed-form expressions for the parametric instability regions for three different resonance conditions are determined. The influence of magnetic filed, axial load, damping constant and mass ratio on the parametric instability regions are investigated. These results obtained from perturbation analysis are verified by solving the temporal equation of motion using fourth-order Runge–Kutta method. The instability regions obtained using this method is found to be in good agreement with the experimental result.