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1.
In this paper we investigate the non-linear dynamics of a two-degree-of-freedom system with symmetries subject to random parametric excitation. The study of this non-linear near-Hamiltonian system is simplified by using the symmetry and separation of scales present in the problem. To this end, we study the equations as a random perturbation of a four-dimensional weakly dissipative Hamiltonian system. We achieve the model-reduction through stochastic averaging and the reduced process is simply a Markov process on a line. Examination of the reduced Markov process on the line yields many important results, namely, probability density functions, and stochastic bifurcations. The steady state dynamics is computed explicitly. Phenomenological and dynamical bifurcations are investigated. The approach adopted in this paper can in principle, be applied to any four-dimensional integrable system. 相似文献
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J.C. JIA.Y.T. LEUNG 《Journal of sound and vibration》2002,253(5):985-1000
The primary, superharmonic, and subharmonic resonances of a harmonically excited non-linear s.d.o.f. system with two distinct time-delays in the linear state feeback are studied. The two different time-delays are presented in the proportional feedback and the derivative feedback respectively. The method of multiple scales is utilized to obtain the first order approximation of response. The effect of the feedback gains and time-delays on the steady state responses of three types of resonances is investigated. It is found that a proper selection of the feedback gains and time-delays can enhance the control performance. 相似文献
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Yuan J 《The Journal of the Acoustical Society of America》2008,124(1):241-246
Feedforward control is a popular strategy of active noise/vibration control. In well-damped noise/vibration systems, path transfer functions from actuators to sensors can be modeled by finite impulse response (FIR) filters with negligible errors. It is possible to implement noninvasive model independent feedforward control by a recently proposed method called orthogonal adaptation. In lightly damped noise/vibration systems, however, path transfer functions have infinite impulse responses (IIRs) that cause difficulties in design and implementation of broadband feedforward controllers. A major source of difficulties is model error if IIR path transfer functions are approximated by FIR filters. In general, active control performance deteriorates as model error increases. In this study, a new method is proposed to design and implement model independent feedforward controllers for broadband in lightly damped noise/vibration systems. It is shown analytically that the proposed method is able to drive the convergence of a noninvasive model independent feedforward controller to improve broadband control in lightly damped noise/vibration systems. The controller is optimized in the minimum H2 norm sense. Experiment results are presented to verify the analytical results. 相似文献
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Adaptive control of laser modal properties 总被引:1,自引:0,他引:1
Quetschke V Gleason J Rakhmanov M Lee J Zhang L Franzen KY Leidel C Mueller G Amin R Tanner DB Reitze DH 《Optics letters》2006,31(2):217-219
An adaptive optical system for precise control of a laser beam's mode structure has been developed. The system uses a dynamic lens based on controlled optical path deformation in a dichroic optical element that is heated with an auxiliary laser. Our method is essentially aberration free, has high dynamic range, and can be implemented with high average power laser beams where other adaptive optics methods fail. A quantitative model agrees well with our experimental data and demonstrates the potential of our method as a mode-matching and beam-shaping element for future large-scale gravitational wave detectors. 相似文献
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Some efficient strategies for the active control of vibrations of a beam structure using piezoelectric materials are described. The control algorithms have been implemented for a cantilever beam model developed using finite element formulation. The vibration response of the beam to an impulse excitation has been calculated numerically for the uncontrolled and the controlled cases. The essence of the method proposed is that a feedback force in different modes be applied according to the vibration amplitude in the respective modes i.e., modes having lesser vibration may receive lesser feedback. This weighting may be done on the basis of either displacement or energy present in different modes. This method is compared with existing methods of modal space control, namely the independent modal space control (IMSC), and modified independent modal space control (MIMSC). The method is in fact an extension of the modified independent space control with the addition that it proposes to use the sum of weighted multiple modal forces for control. The proposed method results in a simpler feedback, which is easy to implement on a controller. The procedure is illustrated for vibration control of a cantilever beam. The analytical results show that the maximum feedback control voltage required in the proposed method is further reduced as compared to existing methods of IMSC and MIMSC for similar vibration control. The limitations of the proposed method are discussed. 相似文献
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E. Battistelli P. Bruscaglioni A. Ismaelli L. Lo Porto G. Zaccanti 《Il Nuovo Cimento C》1985,8(6):893-906
Summary A variable field-of-view transmissometer has been used in order to distinguish the contribution of forward-scattered radiation
to the power transmitted through a diffusing medium. Measurements were made in fog and in the laboratory, where the medium
was a suspension of polystyrene spheres in water. The analysis of scattered radiation gave information on the diffusing medium.
In laboratory measurements it was possible to separate the contributions of different orders of scattering.
Paper presented at the 1o Congresso del Gruppo Nazionale per la Fisica dell'Atmosfera e dell'Oceano, June 19–22, 1984, Rome. 相似文献
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《Solid State Communications》1986,57(4):277-281
We propose experimental evidences for the sd character of the occupied band states of f.c.c. Yb metal. Photoionization cross-section σ1 in the range 24–65 eV, covering the 5p core electron energies have been measured for both Yb (4ƒ146s25d0) and Lu (4ƒ146s25d1). Both the hv dependence of σ1 for the Yb valence band and the presence of photoemission resonances above the 5p edges of Lu and Yb indicate the presence of occupied 5d states at the top of the valence band of solid Yb. The Yb 4ƒ σ1 is presented in order to distinguish the 5d band resonances from the 5p-5d giant dipole autoionization decays. L2, 3 X-ray absorption white lines for Yb provide a measure of the 5d bandwidth. 相似文献
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K. Kapoun 《Czechoslovak Journal of Physics》1971,21(12):1246-1249
By means of a time resolution it was found that the electric breakdown of air at atmospheric pressure between a charged plate and a spike, on which a torch discharge is burning, gaves rise to comparatively stable ellipsoidal plasma formations — plasmoids. Their dimensions stay within 2÷10 mm, velocity 0÷4 ms, acceleration –210÷1500 ms–2. Their life time is 10–2 s in magnitude. They decay either at the spike electrode, or when the actual breakdown takes place, or they desintegrate in the inter-electrode space. 相似文献
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The hyperfine structure and the isotope shift of transitions between metastable levels of the configuration 4f 75d6s and levels of the configuration 4f 75d6p of151Eu and153Eu were studied by means of the high resolution laser-atomic-beam technique. The metastable states were populated by an arc discharge burning in the atomic beam. The measured hyperfine constantsA andB of the 4f 75d6s configuration allow to perform a parametric analysis using the effective tensor operator formalism. For the first time the nuclear quadrupole moments are deduced from the quadrupole interaction parameters of the 5d electron in Eu I. The values uncorrected for the Sternheimer effect are151 Q(5d)=1.53(5)b and153 Q(5d)=3.92(12)b. A comparison with the quadrupole moments derived from the 6p electron and the intrinsic quadrupole moment of153Eu gives information about Sternheimer's correction factors. The value found for the hyperfine anomaly due tos electrons in the 4f 75d6s configuration agrees very well with previous results in other Eu configurations. The change in the mean-square nuclear charge radius is derived from the isotope shift measurements and compared with the results obtained from other optical transitions in Eu. The mean value isgd〈r 2〉151, 153 =0.577(25) fm2. 相似文献
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We investigate the modal losses and field distributions of different order transverse modes supported by the photonic crystal Bragg structure using a transfer matrix method. We find that only the fundamental transverse mode has a single-lobed near field and far field and there exists a trade-off between ensuring lasing in the fundamental transverse mode and reducing the threshold. Employing these design principles, we experimentally demonstrate a large-area, edge-emitting, and single-mode semiconductor photonic crystal Bragg laser with a single-lobed, diffraction-limited far field under continuous wave condition. 相似文献
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We present what we believe are the first experimental results obtained with modal linear predictive control of an adaptive optics system operating on sky at a visible wavelength on a 1.52-m telescope (Observatoire de Haute-Provence, France). This system yields substantially better results than optimized integral control: a 30% relative gain in Strehl ratio is measured, even in conditions of low wind speed. 相似文献
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Vijay K. Arora Carlos Munera Mahmoud Jaafarian 《Journal of Physics and Chemistry of Solids》1977,38(5):469-475
Thermomagnetic coefficients appropriate to a many-valley model of a semiconductor of the n -Germanium type are evaluated in the framework of the density matrix formalism developed earlier. These coefficients are determined for arbitrary values of the magnetic field, within the effective mass approximation. The phonon-drag contribution is not included in this work. An application is made to the four-ellipsoidal model of n-Ge under the conditions where elastic-acoustic phonon scattering is the predominant mechanism of scattering and the high-temperature limit of the phonon distribution is valid. The thermoelectric power (magneto-Seebeck effect) is found to increase with increasing values of the magnetic field except for a small region of low magnetic field values where it decreases in the longitudinal configuration only. 相似文献