Vibration reduction

When all possible vibration reduction has been obtained, the machine must be isolated from the structure. Some form of spring mounting achieves this. Spring mounts have a resonant frequency dependent on the stiffness of the spring and the weight of the object placed on it. It will be apparent that the static deflection of the spring will also be proportional to the resonant frequency. 

The principal benefits of rubber lining are its excellent resistance to corrosive and abrasive chemicals and materials, e.g., acids, alkalies, salt water, slurries, sand, shot blast media, crushed ores etc. In addition to this, rubber linings provide other benefits including noise and vibration reduction, electrical and thermal insulation and product protection. 

Monner, H. P. 2005. Smart materials for active noise and vibration reduction. Proceedings of the Novem - Noise and Vibration Emerging Methods, Saint-Raphael, France. 

The test results showed that, on average, the felt pads transmitted over six times more vibration to the foundation than the proper isolators. At the right front foot, the pads transmitted about 10 times more vibration than the isolators. In other words, up to 90% additional reduction in foundation vibration occurred after removing the pads and reinstalling the press on isolators. (See Fig. 2.) On average, an 84% additional vibration reduction occurred 15]. 

Test Shows Micro/Level Isolators Provide Superior Vibration Reduction and Support Compared to Felt Pads. Technical Bulletin M/L 438. Broadview, IL Vibro/Dynamics Corporation, 1990. 

Active Vibration Reduction. An active vibration control system consists of a hydraulic or electrodynamic actuator, vibration sensor, and electronic controller designed to maintain the seat pan stationary irrespective of the motion of the seat support. Such a control system must be capable of reproducing the vehicle motion at the seat support, which will commonly possess large displacement at low frequencies, and supply a phase-inverted version to the seat pan to counteract the vehicle motion in real time. This imposes a challenging performance requirement for the control system and vibration actuator. Also, the control system must possess safety interlocks to ensure it does not erroneously generate harmful vilxation at the seat pan. While active control systems have been employed commercially to adjust the static stiffness or damping of vehicle suspensions, to improve the ride comfort on different road surfaces, there are currently no active seat suspensions. 

A 3D positioning actuator with a stacked structure as pictured in Figure 4.1.21 was proposed by a German company, where shear strain was utilized to generate thex andy displacements [38]. Polymer-packed PZT bimorphs have been commercialized by ACX for vibration reduction/control applications in smart structures [39]. 

As a matter of fact, from the numerical results one observes that further vibration reduction was accomplished. This is reasonable, as the new system uses four measurements, instead of the previous one that was using only two. Nevertheless the problems concerning velocities and accelerations remain. 

To perform the robust optimum design, the OF mean and standard deviation are numerically evaluated with a new procedure based on a Lyapunov type equation. Robustness is formulated as a multiobjective optimization problem, in which both the mean and the standard deviation of the deterministic OF are minimized. The results show a significant improvement in performance control and OF real values dispersion limitation if compared with standard conventional solutions. Some interesting conclusions can be reached with reference to the results obtained for the adopted examples. With reference to TMD efficiency in vibration reduction, the real structural performance obtained by using conventional optimization has a reduced efficiency compared to those obtained when system uncertainty parameters is properly considered. With reference to the obtained robust solutions, it can be noted that they can control and limit final OF dispersion by limiting its standard deviation. Moreover, this goal is achieved by finding optimal solutions in terms of DV that induce an increase in OF mean value. 

Fig. 2.4. MD 900 helicopter hingeless blade displaying the planned trim tab for in-flight tracking and active control flap for noise and vibration reduction [10]...

One adaptronic approach to reduce torsional vibrations was to integrate actuators within the diagonal stiffening struts of the car underbody (Fig. 8.24) [150,151]. Different actuators have been investigated, piezoceramic stacks, hydraulic cylinders and hydraulic muscles. Control approaches like adaptive feed-forward and feedback were implemented and significant vibration reductions were achieved (Fig. 8.24). To commercialize this affirmative active concept system cost, size, complexity and power consumption must be further reduced. 

Fig. 8.24. AVC-application within a convertible a convertible and concept of active struts, b achievable vibration reduction...

Fig. 8.26. Vibration reduction of a VW Bora roof a test vehicle equipped with electronics and actuator locations on the roof, b results of control in comparison to passive behaviour...

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]. 

Fenn, R.C. et al. Terfenol-D Driven Flaps for Helicopter Vibration Reduction. Smart Mater. Struct., 5 (1996), pp. 49-57... 

Thomaier, M. Atzrodt, H. Herold, S. Mayer, D. Melz, D. Simulation of a Complete System Using the Example of an Active Interface for Vibration Reduction. Proc. Virtual Product Development in Automotive Eng., Miirzzuschlag / Austria (2005)... 

Vibration reduction based on an eneigy transfer from a main system to a passive absorber with sufficient energy dissipation. 

To produce significant vibration reduction, isolators must be soft enough to produce trans-missibihty T of 0.1 or less. The total stiffness k (that is, the sum of the stiffness of all of the isolators that support the machine and base) of the isolators that isolate a mass m (of the machine and base) at a transmissibility Tmay be estimated from... 

Figure 4.106 is a convenient chart for estimating the major isolation system parameters needed to achieve a desired vibration reduction or transmissibility.The dashed line in the figure illustrates the case where there exists a disturbing frequency of 3000 cpm (corresponding to 50 Hz), at which it is desired to reduce the vibration by 99.9 percent, i.e., to 0.01 times its original value. The chart shows that here a value for k/m of about 2.5 (lb/iu)/lb is needed thus, for a 2000-lb mass, a total stiffness of 2.5 x 2000 = 5000 Ib/in is required. (A lesser stiffness would provide better isolation than that prescribed, whereas a greater stiffness would result in poorer isolation.) One may also read from the chart that to k/m = 2.5 (lb/in)/lb there corresponds a static deflection of about 0.4 in and a resonance frequency of about 300 cpm or 5 Hz. 

Semi-active tuned mass dampers are similar to TMDs, but with the capability of varying then-level of damping. They are mainly used for wind vibration reduction. Another type of semi-active TMD is the semi-active variable stiffness tuned mass damper (SAIVS-TMD), where the stiffness is also controllable. Their performance is similar to that of AMDs but with less power consumption. 

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