Marcin Maślanka

Frequency and damping adaptation of a TMD with controlled MR damper

This paper describes the new concept of a semi-active tuned mass damper with magnetorheological damper (MR-STMD). The real-time controlled MR damper force emulates controlled damping and a superimposed controllable stiffness force to augment or diminish the force of the passive spring stiffness which enables us to control the MR-STMD natural frequency. Both the damping and natural frequency are tuned according to Den Hartog’s formulae to the actual dominant frequency of the main structure irrespective of whether it is a resonance or a forced frequency. The MR-STMD is experimentally validated on the Empa bridge with a 15.6 m main span for different added masses to shift its resonance frequency 12.2% andC10.4% away from its nominal value. The experimental results are compared to those obtained when the MR-STMD is operated as a passive TMD that is precisely tuned to the nominal bridge. The comparison shows that the MR-STMD outperforms the TMD both in the tuned and all de-tuned cases by up to 63%. Simulations of the MR-STMD concept point out that the proposed semi-active control algorithm is most suitable for MR-STMDs due to the small amount of clipped active forces. A sensitivity analysis demonstrates that the real MR-STMD could be even more powerful if the force tracking errors in the MR damper force due to the current driver and MR fluid dynamics and remanent magnetization effects could be further reduced. The MR-STMD under consideration represents the prototype of the 12 MR-STMDs that have been running on the Volgograd Bridge since late fall 2011. (Some figures may appear in colour only in the online journal)

An adaptive tuned mass damper based on the emulation of positive and negative stiffness with an MR damper

This paper presents a new adaptive tuned mass damper (TMD) whose stiffness and damping can be tuned in real-time to changing frequencies of a target structure. The adaptive TMD consists of a tuned mass, a tuned passive spring and a magnetorheological (MR) damper. The MR damper is used to emulate controlled friction?viscous damping and controlled stiffness. The controlled positive or negative stiffness emulated by the MR damper works in parallel to the stiffness of the passive TMD spring. The resulting overall TMD stiffness can therefore be varied around the passive spring stiffness using the MR damper. Both the emulated stiffness and friction?viscous damping in the MR damper are controlled such that the resulting overall TMD stiffness and damping are adjusted according to Den Hartogs formulae. Simulations demonstrate that the adaptive TMD with a controlled MR damper provides the same reduction of steady state vibration amplitudes in the target structure as a passive TMD if the target structure vibrates at the nominal frequency. However, if the target structure vibrates at different frequencies, e.g.?due to changed service loads, the adaptive TMD with a controlled MR damper outperforms the passive TMD by up to several 100% depending on the frequency change.

Precise stiffness and damping emulation with MR dampers and its application to semi-active tuned mass dampers of Wolgograd Bridge

This paper investigates precise stiffness and damping emulation with MR dampers when clipping and a residual MR damper force constrain the desired control force. It is shown that these force constraints lead to smaller equivalent stiffness and greater equivalent damping of the constrained MR damper force than desired. Compensation methods for precise stiffness and damping emulations are derived for harmonic excitation of the MR damper. The numerical validation of both compensation methods confirms their efficacy. The precise stiffness emulation approach is experimentally validated with the MR damper based semi-active tuned mass damper (MR-STMD) concept of the Wolgograd Bridge . The experimental results reveal that the precise stiffness emulation approach enhances the efficiency of the MR-STMD significantly when the MR-STMD is operated at reduced desired damping, where the impact of control force constraints becomes significant.

Semi-Active TMD Concept for Volgograd Bridge

The Volgograd Bridge in Russia is known not only for its record length but also for the large amplitude vibrations induced by wind in May 2010. This paper describes the development of a new semi-active TMD with a magnetorheological damper (MR-STMD) that was installed on the Volgograd Bridge in fall 2011. The main feature of the MR-STMD concept is that the real-time controlled MR damper emulates a controllable stiffness force and a controllable friction force. The controllable stiffness force augments or diminishes the stiffness of the passive springs and thereby tunes the MR-STMD frequency to the actual frequency of the bridge. The controllable friction force generates frequency dependent energy dissipation. The small-scale prototype was experimentally tested on the 19.2 m long Empa bridge for various modal masses and disturbing frequencies. After that, the full-scale MR dampers were tested at Empa by hybrid testing for the expected frequencies and amplitudes of the bridge. Finally, the frequency controllability of one full-scale MR-STMD was verified at the University of the German Armed Forces, Munich. All tests confirm that the new technology can compensate for the frequency sensitivity of passive TMDs and works at high efficiency.

Measured performance of a semi-active tuned mass damper with acceleration feedback

Semi-active tuned mass dampers (STMDs) with magnetorheological (MR) dampers are becoming promising alternative to passive tuned mass dampers (TMDs) and active tuned mass dampers (ATMDs). In this paper, a new control algorithm for STMDs with acceleration feedback is experimentally evaluated in a laboratory wind tower - nacelle model equipped with a prototype STMD. The control algorithm adopts an existing acceleration feedback control approach which was originally proposed for ATMDs. The STMD consists of a mass, passive springs and an MR damper. The fail-safe operation of the STMD is reported due to both an internal friction of the STMD and a residual force of the MR damper at its off-state. The paper compares the simulated performance of the STMD with the measured performance of the fail-safe STMD under harmonic force excitation and discusses the major deteriorating factors that limit the measured performance. Despite the limitations, the paper reports that at low excitation the fail-safe STMD acts similarly to the TMD with same mass, while already at moderate excitation its performance is almost equally good as that of the TMD with two times larger mass.

Precise Stiffness Control with MR Dampers

Mag can be used not only as controllable damping devices but also to emulate a controllable positive or negative stiffness in combination with the dissipative force. However, the dissipative nature of MR dampers constrains the stiffness control. This work formulates the problem of combined stiffness and damping control with MR dampers if the damper is subjected to pure harmonic motion. A new method is presented that ends up in precise stiffness emulation with MR dampers, also when the sum of the stiffness and dissipative forces is constrained by the semi-active nature and residual force of MR dampers. The new control concept is applied to a semi-active tuned mass damper with an MR damper (MR-STMD). The numerical and experimental results demonstrate that the MR-STMD outperforms the passive TMD significantly.

Microstructure of AgNi and AgSnBi Powders Consolidated by CEC

Powder metallurgy is widely used to the production of AgNi and AgSnBi powders employed for electrical contacts. In the work AgNi and AgSnBi powders were consolidated by the cyclic extrusion compression (CEC) method enabling cyclic unlimited deformation. In the initial stage the AgNi powder contained the two phases Ag and Ni, recognized by the EDX technique using scanning electron microscopy (SEM). The investigations shown that the Ni phase is distributed in the form of small granules around larger Ag granules. In the AgSnBi powder phases Ag + Bi + Ag3Sn (ξ) were distributed uniformly. It was found that after the CEC consolidation phases were excellently joined without cavities and cracks. Detailed observations of microstructure have been performed by the transmission electron microscopy (TEM) and revealed inside the consolidated granules nanometric grains with the nanometric twins inside.