Active vibration absorber

The principle structure of the active vibration absorber corresponds approximately to the structure of the passive vibration absorber shown in Fig. 6.9 whereby the passive elastic material between mi and m2 has been replaced by a piezoelectric actuator and a displacement amplification system to increase the achievable displacement of m2. The amplification system is given in this example by elastic joints, similar to those illustrated in Fig. 6.9. 

Fig. 6.11. Active vibration absorber, a Amphtude and phase response, b compensation effect within the time domain (Gfc disturbance frequency response)...

Another space related topic is the reduction of noise impacts on pay-loads through active noise control using technologies such as an acoustic foam with embedded PVDF material, distributed active vibration absorbers (DAVA) made from acoustic foam hnked to a metallic plate and in a latest version even added by a very small electrodjmamic shaker that allows to cover lower frequencies [103]. This sandwich of acoustic foam, PVDF and electrodynamic shakers is then used as an active coating on the fairings of space vehicles. 

O Regan, S.D. Burkewitz, B. Fuller, C.R. Lane, S. and Johnson, M. Payload noise suppression using distributed active vibration absorbers. Proc. SPIE Vol. 4698 (2002) pp. 150-159... 

Consider next the water molecule. As we have seen, it has a dipole moment, so we expect at least one IR-active mode. We have also seen that it has CIt, symmetry, and we may use this fact to help sort out the vibrational modes. Each normal mode of iibratbn wiff form a basis for an irreducible representation of the point group of the molecule.13 A vibration will be infrared active if its normal mode belongs to one of the irreducible representation corresponding to the x, y and z vectors. The C2 character table lists four irreducible representations A, Ait Bx, and B2. If we examine the three normal vibrational modes for HzO, we see that both the symmetrical stretch and the bending mode are symmetrical not only with respect to tbe C2 axis, but also with respect to the mirror planes (Fig. 3.21). They therefore have A, symmetry and since z transforms as A, they are fR active. The third mode is not symmetrical with respect to the C2 axis, nor is it symmetrical with respect to the ojxz) plane, so it has B2 symmetry. Because y transforms as Bt, this mode is also (R active. The three vibrations absorb at 3652 cm-1, 1545 cm-1, and 3756 cm-, respectively. 

As the frequency range is scanned the various infrared active vibrations (i.e. those involving a dipole moment change) will sequentially absorb radiation as the energy equivalence of the radiation and the particular vibrational mode is met, giving rise to a series of absorptions. 

These are analytical tools since the character of the interaction is related to the structure and composition of the materials under test. When IR radiation goes across a sample, some photons are absorbed or suffer an inelastic scattering process caused by the active vibrations of the atoms, molecules, and ions, which compose the test material. The frequencies of the absorbed, or scattered, radiation are exclusively related to a particular vibration mode. Consequently, the process reveals attributes of the test material. Subsequently, IR (absorption) and Raman (scattering) are vibration-based spectroscopic methods widely used for characterizing materials, because they allow qualitative structural information to be obtained. 

Suppose that a compound has a Raman-active vibration at vM. If it is illuminated by a probe laser (v) simulataneously with a pump continuum covering the frequency range from v to v + 3,500 cm-1, one observes an absorption at v + vM in the continuum together with emission at v. Clearly, the absorbed energy, h(v + vM), has been used for excitation (/zvM) and emission of the extra energy (hv). This upward transition is called the inverse Raman effect since the normal anti-Stokes transition occurs downward. Because the inverse Raman spectrum can be obtained in the lifetime of the pulse, it may be used for studies of shortlived species (Section 3.5). It should be noted, however, that the continuum pulse must also have the same lifetime as the giant pulse itself. Thus far, the inverse Raman effect has been observed only in a few compounds, because it is difficult to produce a continuum pulse at the desired frequency range. 

A molecule without Raman-active modes absorbs a photon with the frequency v0. The excited molecule returns to the same basic vibrational state and emits light with the same frequency v0. This type of interaction is called an elastic... 

The selection rule for a vibration to be infrared active is that the molecule must undergo a vibrational motion that changes the dipole moment of that vibrational mode. Hence. IR light of a frequency matching that of an IR-active vibrational motion will be directly absorbed by the molecule. This quantum of light will then promote the molecule to a higher vibrational energy state. 

FTIR spectroscopy, used as a detector in GC, has many advantages. First, FTIR is the most universal of all detectors currently used in GC, because all organic compounds exhibit IR-active vibrations and, thus, absorb IR radiation. Moreover, these absorptions obey Beer s law so that the data can be used directly for quantification. Furthermore, FTIR can be used both as a selective and as a specific detector. Finally, the non-destructive character of GC/ FTIR is an important advantage when compared with other detectors it offers the opportunity to investigate GC eluates after FTIR analysis. However, despite these advantages, FTIR detection is not widely applied in GC analysis at present This is, particularly, due to its lack of sensitivity a second reason is the complexity of IR spectra. Yet, the power of GC/FflR is the unique structural information that can be extracted from the spectral data and that cannot be obtained from any other method. It is, therefore, very likely that, in the next few years, this technique will attract much closer attention from analytical chemists. 

The symmetric stretch does not produce an instantaneous dipole moment and is IR inactive. The asymmetric stretch and hend motions do produce instantaneous dipole moments and are IR active. It is these IR active vibrations that make CO2 a strong IR absorber and greenhouse gas (see Chapter 13). 

An infrared vibration is active, or absorbs infrared energy, when the molecular vibration induces a net change in dipole moment during the vibration. 

The mathematical model of the mechanical actuator system can be developed directly from the CAD design drawing by means of commercial FEM software tools, e.g. ANSYS [3]. This model is fundamental for the calcula-tional modal analysis which serves to find out the systems natural frequencies. Figure 6.10 shows the FEM model of the active piezo absorber and gives an impression of the third vibration mode of the structure which is used in this example for the vibration absorption. 

Adaptronic solutions like active mounts, adaptive vibration absorbers, or distributed in-plane actuators can optimize these structural djmamic and vibroacoustic characteristics [149]. Typical target functions are the active control of introduction and transfer of disturbances and/or damping of elastic modes. Basically, three different conceptual approaches can be considered ... 

For vibration reduction of panels mostly, semi-active vibration damping systems based on piezoelectric transducers have been studied. The piezos are attached to the structure and electrically shunted with resistor-inductor circuits. By proper tuning of the circuit elements, effects compared to the application of mechanical vibration absorbers can be observed, however only laboratory experiments have been performed yet [147]. 

Conventional dynamic vibration absorbers are composed of a mass, spring, and damper. Although dynamic vibration absorbers do not have sensors or controllers, they can provide vibration mitigation similar to that of actively controlled systems with a complicated sensor, control, and actuator system. Since an absorbers mass/spring/damper forms a single degree of freedom (DOF) vibration system, it consequently has a single resonant frequency and can exhibit an amplified response at this frequency. Dynamic absorbers behave similar to a system with a sensor to detect the specific frequency and a controller to amplify the vibration. Therefore, the absorbers natural frequency should be carefully tuned to a specific frequency for which the vibration amplitude of the host structure is to be reduced. The tuned frequency usually corresponds to natural modes and harmonically excited vibrations of a system. 

Most of the above studies have related to the use of dynamic vibration absorbers in mechanical systems. While application ofthese systems in civil engineering structures, frequently known as tuned mass damper (TMD) systems, is expected to be different. In last decades, many research studies have been performed to show the effectiveness of the TMD system and the other control devices, derived from TMD, such as semi-active tuned mass dampers (STMDs) and active tuned mass dampers (ATMDs) in civil engineering community in reducing the structural responses, some of which are reviewed in the following. 

Koo, J. H., Ahmadian, M., Setareh, M., Murray, T. (2003). In search of suitable control methods for semi-active tuned vibration absorbers. Journal of Vibration and Control, 10, 163-174. doi 10.1177/1077546304032020... 

Active Vibration ControV - Fully active vibration control systems employ sensors to measure the vibration of concern, actuators to provide forces that act to reduce this vibration, and signal processors to provide appropriate control signals to the actuators. The actuators, which may be electrodynamic or piezoelectric, may react against a support (often advantageously in parallel with conventional isolators) or against an inertial mass. In semiactive vibration control systems, the characteristics of some elements of the vibrating system—such as the stiffness of isolators, the resonance frequency of a dynamic absorber, or the positions of masses—are adjusted automatically on the basis of sensed vibrations. 

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