Moment Excitation Research Articles

J044042 非接触レーザー加振技術を用いたモーメント加振法による周波数応答関数の推定

This paper addresses a method of moment excitation using non-contact laser excitation. In order to generate couple of forces using laser ablation, each laser beam divided by half mirror with equally laser pulse energy is irradiated on target structure's two excitation points. The synchronous laser ablation generates pure moment of force at the center of excitation points on the structure. In this paper, the pulsed-laser-ablation moment of force is calibrated via Newton's second law using a rigid block. Then, rotational frequency response function (input: moment of force, output: acceleration) of the structure can be measured. The results are compared to finite element analysis and the effectiveness of this method is examined.

The Third Superharmonic Resonance of Diesel Shafting Subjected to Harmonic Moment Excitation

The Third Superharmonic Resonance of Diesel Shafting Subjected to Harmonic Moment Excitation

Vibration of L-shaped plates under a deterministic force or moment excitation: a case of statistical energy analysis application

Analytical and closed form solutions are presented in this paper for the vibration response of an L-shaped plate under a point force or a moment excitation. Inter-relationships between wave components of the source and the receiving plates are clearly defined. Explicit expressions are given for the quadratic quantities such as input power, energy flow and kinetic energy distributions of the L-shaped plate. Applications of statistical energy analysis (SEA) formulation in the prediction of the vibration response of finite coupled plate structures under a single deterministic forcing are examined and quantified. It is found that the SEA method can be employed to predict the frequency averaged vibration response and energy flow of coupled plate structures under a deterministic force or moment excitation when the structural system satisfies the following conditions: (1) the coupling loss factors of the coupled subsystems are known; (2) the source location is more than a quarter of the plate bending wavelength away from the source plate edges in the point force excitation case, or is more than a quarter wavelength away from the pair of source plate edges perpendicular to the moment axis in the moment excitation case due to the directional characteristic of moment excitations. SEA overestimates the response of the L-shaped plate when the source location is less than a quarter bending wavelength away from the respective plate edges owing to wave coherence effect at the plate boundary.

720 分離型モーメント印加装置に関する研究

In the experimental modal analysis, the treatment device has to be fixed on the target structure in order to obtain the vibration response to the moment excitation. However, if the treatment device is used, the vibration property must be changed because of the additional mass of the device. Hence, it is difficult to obtain the accurate vibration response to the moment excitation. A moment excitation device that a step moment can be impacted on the structure without using treatment device was proposed. The proposed device is easy to handle and realizes the accurate measurement without additional weight. But, in this way, excitation frequency is limited to the low frequency band. In this study, a new moment excitation device that a impulse moment can be impacted on the structure is proposed. First of all, the principle of the proposed device is shown. Then, the experimental results are shown to confirm the validity of the proposed technique.

119 吸着型モーメント印加装置の性能向上に関する研究(加振実験・計測,OS-9 モード解析とその応用関連技術,総合テーマ「伝統を,未来へ!」)

119 吸着型モーメント印加装置の性能向上に関する研究(加振実験・計測,OS-9 モード解析とその応用関連技術,総合テーマ「伝統を,未来へ!」)

A study of vibration and vibration control of ship structures

This paper examines the vibration characteristics and vibration control of complex ship structures. It is shown that input mobilities of a ship structure at engine supports, due to out-of-plane force or bending moment excitations, are governed by the flexural stiffness of the engine supports. The frequency averaged input mobilities of the ship structure, due to such excitations, can be represented by those of the corresponding infinite beam. The torsional moment input mobility at the engine support can be estimated from the torsional response of the engine bed section under direct excitation. It is found that the inclusion of ship hull and deck plates in the ship structure model has little effect on the frequency-averaged response of the ship structure. This study also shows that vibration propagation in complex ship structures at low frequencies can be attenuated by imposing irregularities to the ring frame locations in ships. Vibration responses of ship structures due to machinery excitations at higher frequencies can be controlled by structural modifications of the local supporting structures such as engine beds in ships.

Hidden order in URu2Si2 originates from Fermi surface gapping induced by dynamic symmetry breaking

Spontaneous, collective ordering of electronic degrees of freedom leads to second-order phase transitions that are characterized by an order parameter driving the transition. The notion of a 'hidden order' has recently been used for a variety of materials where a clear phase transition occurs without a known order parameter. The prototype example is the heavy-fermion compound URu(2)Si(2), where a mysterious hidden-order transition occurs at 17.5 K. For more than twenty years this system has been studied theoretically and experimentally without a firm grasp of the underlying physics. Here, we provide a microscopic explanation of the hidden order using density-functional theory calculations. We identify the Fermi surface 'hot spots' where degeneracy induces a Fermi surface instability and quantify how symmetry breaking lifts the degeneracy, causing a surprisingly large Fermi surface gapping. As the mechanism for the hidden order, we deduce spontaneous symmetry breaking through a dynamic mode of antiferromagnetic moment excitations.

The Effect of Off-Neutral Axis Excitation on the Vibration Response of Finite Ribbed-Plates

In this paper, an analytical solution is derived and used for studying the effect of off-neutral axis loading (point force excitation applied off the neutral axis of a rib) on ribbed-plate responses. Effects of off-neutral axis loading on ribbed-plate responses, in terms of input power and kinetic energy distribution in the component plates, are found to be significant except at frequencies where the beam flexural impedance is small (near the natural frequencies of the uncoupled beam). The negative input power component due to the point force or moment excitation is observed and explained in this analysis. Energy flow from the beam to the plates is dominated by the shear force coupling at the beam/plate interface if the force excitation is applied on the neutral axis of the beam. As the point force excitation location shifts away from the neutral axis, the beam is excited by a force and a moment. These dual excitations involve both shear force and moment couplings at the interface. Thus, changes in the energy flow into the component plates are dependent on the relative phase between the individual induced responses by the point force and moment excitations. This paper also discusses changes in the kinetic energy of the component plates due to changes in the energy flow.

Investigation of Rotor Blade Structural Dynamics and Modeling Based on Measured Airloads

The work presented herein treats measured airloads from the UH-60A Airloads Program as prescribed external loads to calculate the resulting structural loads and motions of a rotor blade. Without the need to perform any aerodynamic computations, the coupled aeroelastic response problem is reduced to one involving only structural dynamics. The results, computed by RCAS and CAMRAD II, are compared against measured results and against each other for three representative test points. The results from the two codes mostly validate each other. Seven more test points, with responses computed by RCAS, to form thrust and airspeed sweeps are evaluated to better understand key issues. One such issue is an inability to consistently predict pushrod loads and torsion moments well, and this is found to be amplified at the two test points with the highest thrust coefficient. For these two test points, harmonic analysis reveals that the issue is due to excessive amounts of 5/rev response that stem from high levels of 5/rev pitching moment excitation. Another issue that concerns all test points is that the phase of the 1/rev blade flapping motion is not predicted well, which reflects the high sensitivity of this quantity that is developed due to having a first flap frequency of approximately 1/rev. Results also show that current force-velocity relationships, used in describing the lead-lag damper, are not satisfactory to consistently yield accurate inboard chordwise bending moment predictions. Overall, the investigation here, conducted with numerous test points, further confirms the methodology of prescribing measured airloads for assessing the structural dynamics capability of a computational tool.

Measurements of the bending moment at boundaries of a structure

In the vibration transmission process, the part due to the moment excitation is often neglected, because of the difficulty to measure them, even if these terms are sometimes important. Indeed, several studies show that the influence of the moment in structure borne power transmission can be higher than simple force. Moment measurement or identification is an old problem especially at boundaries. It has been mainly investigated in mobility methods. In fact, bending moment expression can be seen as spatial derivatives of displacements. These derivatives can be approximated from measured displacements but two major difficulties appear: derivatives are highly sensitive to measurement errors and the usual methods used to obtain them (finite differences, modal approach, etc.) are not well adapted at boundary points. In this paper, three different moment identification methods are investigated where the considered structure is a beam. Both of these approaches are already developed by authors, the third method is based on the use of particular mechanical and electric setups of piezoelectric patches. These 3 methods are numerically and experimentally implemented allowing us to compare and discuss on results.

Error due to two-force excitation in moment mobility measurement

In direct moment mobility measurement, one difficulty is the moment excitation. Two-force configuration excitation is widely used to generate moment in practice. Previous error analysis mainly focused on the initial load of exciters, the matching of the two forces, and the system resonance. In this paper, the distance requirement of two-force configuration in moment excitation is investigated. Based on the numerical calculation, other factors that affect the error in moment mobility measurement are discussed. The influence of the deduced distance requirement on moment mobility measurement is also explained.

Measurement of Vibrational Energy and Point Mobility of a Beam Subjected to Moment Excitation Using a Finite-Difference Approximation

Moment excitation is often neglected in structural vibration analysis because of difficulties in measuring the applied moment and the resulting wave motion in the structure. Further, it is often assumed that moment-induced vibrational energy is significant only in the high frequency region. However, recent studies have shown that moment excitation should be included in vibrational analysis at all frequencies, when the source location is in close proximity to a structural discontinuity. In this article, a novel method is presented to measure the point mobility and resulting vibrational energy of a beam subjected to moment excitation. The proposed method utilizes a finite-difference approximation to calculate the rotational motion of the beam at the point of excitation. Moment excitation is induced by a specially designed impact rig which applies two equal and opposite forces on two moment arms that are perpendicularly attached to the beam. It is shown that, using the newly developed technique, the measured point mobility follows the trend of the equivalent theoretical structure. The technique also showed good agreement over a wide frequency range between the measured input energy and the measured transmitted flexural wave energy along the beam.

A closed form solution for the dynamic response of finite ribbed plates

A simple and closed form solution for the vibration response of finite ribbed plates to point force/moment excitations is presented in this paper. This solution shows that input mobilities of finite ribbed plates are bounded by the input mobilities of the uncoupled plate and beam that form the ribbed plate. It is found that point force input mobilities of a finite ribbed plate are controlled by the plate bending stiffness when the excitation force is more than a quarter wavelength away from the beam. The input mobilities are mainly dominated by the beam flexural stiffness when the force acts on or very close to the beam, and when the beam flexural stiffness is far greater than the plate bending stiffness. A similar result is found in the moment excitation case when the moment axis is perpendicular to the beam neutral axis (bending moment excitation). In contrast, the input mobilities of the ribbed plate do not vary much from that of the corresponding uncoupled plate when the moment axis parallels to the beam’s neutral axis (torsional moment excitation) where the input mobilities are mainly dominated by the plate bending stiffness. The reductions in plate kinetic energy due to beam insertions are discussed.

Proposed piezoceramic excitation for translational and rotational mobility measurements

A piezoceramic excitation approach is proposed that allows for a convenient measurement of translational and rotational structural mobility. This article focuses on verifying the ability of piezoceramic patches, when aligned in certain formations, to mimic normal force and moment excitation and allow easy measurement of translational and rotational structural mobility. Depending on the orientation of the patches on a thin, clamped-clamped beam, a normal force, bending moment or torsional moment is created and accelerometers are used to measure the mobility at the center point of the beam. The experimental measurements are compared to numerical results from a finite element program to verify the efficacy of the piezoceramic patches in exciting a structure. The specific arrangements of the patches are able to excite the desired modes and measurements agree well with the numerical model. This method combined with the proposed array could be placed at any point on a structure to excite the structure and measure the translational and rotational point structural mobility.

Estimation of the Rotational Terms of the Dynamic Response Matrix

The dynamic response of a structure can be described by both its translational and rotational receptances. The latter ones are frequently not considered because of the difficulties in applying a pure moment excitation or in measuring rotations. However, in general, this implies a reduction up to 75% of the complete model. On the other hand, if a modification includes a rotational inertia, the rotational receptances of the unmodified system are needed. In one method, more commonly found in the literature, a so called T‐block is attached to the structure. Then, a force, applied to an arm of the T‐block, generates a moment together with a force at the connection point. The T‐block also allows for angular displacement measurements. Nevertheless, the results are often not quite satisfactory. In this work, an alternative method based upon coupling techniques is developed, in which rotational receptances are estimated without the need of applying a moment excitation. This is accomplished by introducing a rotational inertia modification when rotating the T‐block. The force is then applied in its centroid. Several numerical and experimental examples are discussed so that the methodology can be clearly described. The advantages and limitations are identified within the practical application of the method.

The influence of the contact zone on the excitation of wheel/rail noise

Rolling noise is excited by surface roughness at the wheel/rail contact. The contact patch is known to attenuate the excitation at wavelengths that are short in comparison with its length. A distributed point-reacting spring (DPRS) model is used with measured roughness data to determine the contact filter effect, and this result is compared with analytical predictions. It is found that the analytical model gives an attenuation that is too large at short wavelengths but is usable for wavelengths down to somewhat smaller than the length of the contact patch. Additionally, variations in the detailed geometry of the profile can cause the contact point on the wheel and rail to oscillate laterally. This introduces an oscillating moment that can induce additional vibration and noise. The DPRS model and rolling noise prediction model are both extended and used together to allow an estimate of the contribution to the radiated noise. It is found that, while the direct roughness excitation is still more important, the moment excitation can be significant, particularly for conforming profiles.

Estimation of power transmission to a flexible receiver from a stiff source using a power mode approach

A power mode approach to estimating the vibrational power transmitted to a receiver structure by multiple sources is reviewed. This approach is then extended to estimating the power transmitted to a flexible receiver from a stiff source through discrete couplings. There may be both translational and rotational coupling degrees of freedoms, and/or force and moment excitations. Approximations are developed for the upper and lower bounds and the frequency average of the transmitted power. These depend only on the point mobilities of the source and receiver, and thus require much less data than an exact description. The approximations are most accurate when the mobility or stiffness mismatch of the coupled system is large enough, e.g., the source has, on average, low mobility compared to that of the receiver. Numerical examples are presented concerning a multi-point coupled beam/plate model.

Lateral moment and axial force excitation of deep beams; point and point-cross mobilities

The cross-coupling at an excited interface between, on the one hand, a lateral moment and an axial translational velocity and, on the other, an axial force and a lateral rotational velocity is studied for an arbitrarily deep beam. Complete descriptions are developed for the two associated point-cross mobility elements. In addition, the point mobility is established relating to an axial force excitation at an edge of the beam. From the theoretical results it can be concluded that reciprocity with respect to the point-cross mobility elements is strictly, only valid in the hypothetical case of point excitation. The numerical results demonstrate, however, that reciprocity can be invoked in an approximate sense also for realistic excitation conditions. Moreover, it is shown that a reduction of an axial force excitation, remote from the neutral layer, to a combined force and moment excitation at the neutral layer is not applicable except for very small Helmholtz numbers. Finally, it can be concluded that the shear-tensional nearfield affects the point-cross mobilities in the case of two-dimensional beam-like structures. A similar influence is established for the axial point force mobility but an unambiguous description has not been found.

A power mode approach to estimating vibrational power transmitted by multiple sources

A power mode method for the estimation of the power transmitted to a flexible receiver by an array of point force excitations is described. The vibrational power transmitted by N discrete point forces is regarded as the power transmitted by N independent power modes following eigendecomposition of the mobility matrix of the receiving structure. Approximate expressions for the upper and lower bounds and the mean value of the transmitted power are then developed in terms of these power modes. The approach is extended to more general cases, including that where both force and moment excitations are applied to the structure and where there are velocity source excitations. Numerical examples are presented.

Moment excitation of structures using two synchronized impact hammers

Moment mobilities are difficult to measure because there is no simple way to excite a structure with a well-controlled moment. In this paper, a technique to apply a nearly pure moment is presented. It is based on a simple approach that uses two impact forces applied, respectively, by two impact hammers. The limits and the validity of this approach are first examined theoretically on a simply supported plate. Comparisons between the excitation by two distant opposite forces and by a pure moment are performed. Then, the coupling between the two hammers and the structure during the impact phase is considered in order to analyze practical limits of obtaining identical forces impacting at different locations. Finally, a detailed description of the experimental system developed to obtain identical synchronized forces is presented. Two hammers driven by electric motors controlled by a Digital Signal Processor are used. Experimental results are compared to theoretical predictions. It is shown that the technique yields accurate measurements of moment frequency response functions.