Semi-active Tuned Vibration Absorbers Research Articles

Comparison of Floating Offshore Wind Turbine Tower Deflection Mitigation Methods Using Nonlinear Optimal-Based Reduced-Stroke Tuned Vibration Absorber

Tower fatigue and strength are crucial operational concerns of floating offshore wind turbines (FOWTs) due to the escalation of the vibration phenomena observed on these structures as compared to land-based ones. FOWT towers are excited by wave and wind polyperiodic disturbances yielding continual transient states of structural vibration that are challenging for vibration mitigation systems. Thus, the paper investigates a novel implementation of nonlinear optimal-based vibration control solutions for the full-scale, tension leg platform (TLP)-based, NREL 5MW wind turbine tower-nacelle model with a 10-ton tuned vibration absorber (TVA), equipped with a magnetorheological (MR) damper, located at the nacelle. The structure is subjected to excessive wave and wind excitations, considering floating platform motions derived from model experiments in a wave tank. The MR damper operates simultaneously with an electromagnetic force actuator (forming a hybrid TVA) or independently (a semiactive TVA). The study includes both actuators’ nonlinearities and dynamics, whereby the former are embedded in the Hamilton-principle-based nonlinear control solutions. The TVA is tuned either to the NREL 5MW tower-nacelle 1st bending mode frequency (TVA-TN) or to the TLP surge frequency (TVA-TLP). The optimal control task was redeveloped concerning the TVA stroke and transient vibration minimisation, including the implementation of the protected structure’s acceleration and relative displacement terms, as well as the nonzero velocity term in the quality index. The regarded model is embedded in a MATLAB/Simulink environment. On the basis of the obtained results, the TVA-TN solution is by far superior to the TVA-TLP one. All the regarded TVA-TN solutions provide a tower deflection safety factor of ca. 2, while reference systems without any vibration reduction solutions or with a passive TVA-TLP are at risk of tower structural failure as well as the hybrid TVA-TLP system. The obtained TVA stroke reductions of 25.7%/22.0% coincide with 3.6%/10.3% maximum tower deflection reductions for the semiactive/hybrid TVA-TN case (respectively) with regard to the previously developed approaches. Moreover, these reductions are obtained due to the sole control algorithm enhancement; thus, no additional resources are necessary, while this attainment is accompanied by a reduction in the required MR damper force. The lowest obtained TVA stroke amplitude of 1.66 m is guaranteed by the newly introduced semiactive control. Its hybrid equivalent ensures 8% lower primary structure deflection amplitude and reduced nacelle acceleration levels thanks to the utilisation of the force actuator of the relatively low power (ca. 6 kW); the trade-off is an increased TVA stroke amplitude of 2.19 m, which, however, is the lowest among all the tested hybrid solutions. The analysed reference passive TVA systems, along with a modified ground-hook hybrid solution, can hardly be implemented in the nacelle (especially along the demanding side–side direction). The latter, being the well-proven hybrid solution for steady-state tower deflection minimisation, yielded unsatisfactory results. The achievements of the study may be used for an effective design of a full-scale vibration reduction system for the TLP-based floating wind turbine structure.

Optimal Semiactive Damping Control for a Nonlinear Energy Sink Used to Stabilize Milling

Improving product quality of machining components has always met with problems due to the vibration of the milling machine’s spindle, which can be reduced by adding a vibration absorber. The tuned vibration absorber (TVA) has been studied extensively and found to have a narrow bandwidth, but the cutting force possesses wide bandwidth in the process of machining parts. Introducing nonlinearity into the dynamic vibration absorber can effectively increase the bandwidth of vibration suppression and can significantly improve the robustness of the vibration absorber. In addition, a semiactive TVA has proved to be more effective than a passive TVA for many applications, so the main purpose of this study is to find some appropriate semiactive control methods for a nonlinear energy sink (NES), a nonlinear vibration absorber, in structural vibration applications. Two semiactive control methods are considered in this study: continuous groundhook damping control based on velocity and on-off groundhook damping control based on velocity. To fairly compare these vibration absorbers, the optimal parameters of a passive TVA, a passive NES, and two semiactive NESs are designed using numerical optimization techniques to minimize the root-mean-square acceleration. Two cutting forces are introduced in this study, a periodic force and an aperiodic force, and the four vibration absorbers are compared. When the primary structure is excited with aperiodic cutting force, the amplitude of the primary structure decreased by 17.73% with the passive TVA, by 72.29% with the passive NES, by 73.54% with the on-off NES, and by 87.54% with the continuous NES. When the primary structure is excited with periodic cutting force, the amplitude of the primary structure decreased by 49.01% with a passive TVA, by 86.93% with a passive NES, by 96.38% with an on-off NES, and by 99.23% with a continuous NES. The results show that the passive NES is better than the passive TVA; the semiactive NES provides more effective vibration attenuation than the passive NES, and the continuous control is more effective than the on-off control.

Experimental implementation of switching and sweeping tuneable vibration absorbers for broadband vibration control

This paper describes the laboratory implementation of two semi-active tuned vibration absorbers (TVAs): a switching TVA and a sweeping TVA. The two absorbers are designed to control the low frequency total flexural response of a cylindrical duct excited by a stationary broadband random force. The two TVAs are composed by a seismic mass mounted on a axial spring. Both TVAs are equipped with a relative displacement and relative velocity feedback control system, which is used to vary the characteristic stiffness and damping, that is the characteristic natural frequency and damping ratio, of the TVA. The switching TVA cyclically tunes its characteristic natural frequency and damping ratio to iteratively control the resonant responses due to three targeted flexural natural modes of the duct. Alternatively the sweeping TVA continuously varies its characteristic natural frequency and damping ratio within given ranges to blindly control the resonant responses due to the same three flexural natural modes of the duct. The paper presents both simulation and experimental results regarding the feedback loop used to iteratively or continuously tune the TVAs and about the reduction of vibration produced by the two semi-active TVAs. The simulation and the experimental results show that both TVAs effectively control the flexural response of the duct in the low frequency band, which is characterised by the resonances of the three targeted flexural natural modes of the duct.

A Novel Semiactive Variable Stiffness Device and Its Application in a New Semiactive Tuned Vibration Absorber

An innovative design for a semiactive variable stiffness (SAVS) device is presented in this paper. This beamlike device is capable of altering its stiffness in a smooth manner between minimum and maximum levels by using the variations of moment of inertia of an area as it rotates around a normal axis passing through its centroid. Analytical expressions for the stiffness of the proposed device have been derived. As an application of the SAVS device in engineering, a semiactive tuned vibration absorber (SATVA) is developed that is capable of real-time retuning and operates effectively in broadband frequency excitations. A single-degree-of-freedom (SDOF) system coupled with the SATVA is considered. The excitation force is assumed to arise from a rotating unbalance whose frequency varies with a constant acceleration and then reaches a steady-state operating condition. The absorber stiffness is varied to tune the absorber’s natural frequency to the forcing frequency in real time. The effectiveness of SATVA is evaluated by comparing the system’s responses with those of the system with passive vibration absorber. The results show the salient features of the proposed SATVA in transient response reduction compared with the traditional passive vibration absorber.

Optimal active absorber with internal state feedback for controlling resonant and transient vibration

An active, standalone vibration absorber utilizing the state feedback taken from the absorber mass is proposed. Expressions of the optimum absorber parameters are obtained both by optimizing the Η ∞ norm of the frequency response function. For improved transient response featuring low peak response and fast attenuation, the design procedure utilizes the mode equalization followed by the maximization of the damping. An interesting feature of the proposed absorber is that the performance of the absorber does not require having its natural frequency close to the natural frequency of the primary system as is generally the case for tuned passive absorbers or other active and semi-active tuned vibration absorbers. In fact, the performance of the proposed system can be progressively enhanced by increasing the absorber frequency. Compared to the optimum passive absorber, the optimal active absorber can yield wider bandwidth of operation around the natural frequency of the primary system and lower frequency response within the suppression band. The active absorber also offers better transient response compared to the passive absorber both optimized for the best transient responses. The efficacy of the absorber is analyzed both for a single-degree-of-freedom and beam like primary structure.

Dynamic performance analysis of non-linear tuned vibration absorbers

The main purpose of this study is to evaluate the dynamic characteristics of a damping controlled semi-active tuned vibration absorber (TVA). A base-excited, single-degree-of-freedom structure coupled with a TVA model is adopted as the baseline model for our analysis. A non-model based groundhook control (displacement-based on–off control or “on–off DBG”) was used to control the damping of the TVA. After developing numerical models of a non-linear TVA along with the on–off DBG control and a passive TVA, the optimal tuning parameters of both TVA models were obtained using an optimization routine. Using the two optimally tuned models, parametric studies were preformed by varying the values of both on and off-state damping to evaluate the dynamic performances of the TVAs using the peak transmissibility criteria. The results showed that the peak transmissibility of the semi-active TVA is nearly 21% lower than that of the passive, indicating that the semi-active TVA is more effective in reducing vibration levels. The results further showed that increases in off-state damping decreases the effectiveness of semi-active TVA in reducing maximum vibration levels.

Experimental Robustness Analysis of Magneto-Rheological Tuned Vibration Absorbers Subject to Mass Off-Tuning

This paper offers an experimental robustness analysis of a semi-active tuned vibration absorber (TVA) subject to structural mass off-tuning, which occurs frequently in practical applications of TVAs. One of the critical problems of a conventional TVA is off-tuning or miss-tuning because off-tuned TVAs may amplify the vibrations of the primary structure. This study intends to evaluate how well semi-active TVAs are able to adapt to structural mass changes (mass off-tuning) as compared with passive TVAs. To this end, a test apparatus was built to represent a two-degree-of-freedom structure model coupled with a TVA. The semi-active TVA considered in this study employed a magneto-rheological (MR) damper as its damping element to enhance overall performance. Using this test apparatus, a series of tests were conducted to identify the optimal tuning parameters of the TVAs. After obtaining each TVA’s optimal tuning parameters based on equal peak transmissibility criteria, the mass off-tuning tests were then performed on the optimally tuned TVAs. In order to off-tune the system, the mass of the primary structure varied from −19% to +19% of its nominal mass using a set of steel plates. Overall, the experimental results show that the semi-active MR TVA outperforms the passive TVA in reducing maximum vibrations. Moreover, the results show that the semi- active TVA is more robust to changes in the primary mass (mass off-tuning) The results further show that the semi-active MR TVA offers larger performance gains as the structure mass increases.

Improving Robustness of Tuned Vibration Absorbers Using Shape Memory Alloys

A conventional passive tuned vibration absorber (TVA) is effective when it is precisely tuned to the frequency of a vibration mode; otherwise, it may amplify the vibrations of the primary system. In many applications, the frequency often changes over time. For example, adding or subtracting external mass on the existing primary system results in changes in the system’s natural frequency. The frequency changes of the primary system can significantly degrade the performance of TVA. To cope with this problem, many alternative TVAs (such as semiactive, adaptive, and active TVAs) have been studied. As another alternative, this paper investigates the use of Shape Memory Alloys (SMAs) in passive TVAs in order to improve the robustness of the TVAs subject to mass change in the primary system. The proposed SMA-TVA employs SMA wires, which exhibit variable stiffness, as the spring element of the TVA. This allows us to tune effective stiffness of the TVA to adapt to the changes in the primary system's natural frequency. The simulation model, presented in this paper, contains the dynamics of the TVA along with the SMA wire model that includes phase transformation, heat transfer, and the constitutive relations. Additionally, a PID controller is included for regulating the applied voltage to the SMA wires in order to maintain the desired stiffness. The robustness analysis is then performed on both the SMA-TVA and the equivalent passive TVA. For our robustness analysis, the mass of the primary system is varied by ± 30% of its nominal mass. The simulation results show that the SMA-TVA is more robust than the equivalent passive TVA in reducing peak vibrations in the primary system subject to change of its mass.

Development of a new damper to reduce resonant vibrations in lightweight steel joist floors

Floor vibrations annoying to humans often occur in lightweight constructions. A number of methods to solve the problem of resonant vibrations are reported in the literature. Tuned mass damper, semi-active tuned vibration absorber and active control system are all examples of existing methods. A new method has been tested in laboratory environment on a prefabricated floor containing a resilient ceiling with a size up to 6.8×4.8 m 2. The method takes advantage of small pieces of visco-elastic material connected between the ceiling joists and the primary beams. A finite element model is used to calculate the correct amount of visco-elastic material. The new damper is especially effective in damping mode shapes where the ceiling oscillates out of phase relative to the floor but shows improvements for other mode shapes as well.

Passive, Adaptive and Active Tuned Vibration Absorbers—A Survey

An overview of the recent development of tuned vibration absorbers (TVAs) for vibration and noise suppression is presented. The paper summarizes some popular theory for analysis and optimal tuning of these devices, discusses various design configurations, and presents some contemporary applications of passive TVAs. Furthermore, the paper also presents a brief discussion on the recent progress of adaptive and semi-active TVAs along with their on-line tuning strategies, and active and hybrid fail-safe TVAs.

Passive, Adaptive and Active Tuned Vibration Absorbers—A Survey

An overview of the recent development of tuned vibration absorbers (TVAs) for vibration and noise suppression is presented. The paper summarizes some popular theory for analysis and optimal tuning of these devices, discusses various design configurations, and presents some contemporary applications of passive TVAs. Furthermore, the paper also presents a brief discussion on the recent progress of adaptive and semi-active TVAs along with their on-line tuning strategies, and active and hybrid fail-safe TVAs.