Michele Zilletti

Integrated tuned vibration absorbers: A theoretical study

This article presents a simulation study on two integrated tuned vibration absorbers (TVAs) designed to control the global flexural vibration of lightly damped thin structures subject to broad frequency band disturbances. The first one consists of a single axial switching TVA composed by a seismic mass mounted on variable axial spring and damper elements so that the characteristic damping and natural frequency of the absorber can be switched iteratively to control the resonant response of three flexural modes of the hosting structure. The second one consists of a single three-axes TVA composed by a seismic mass mounted on axial and rotational springs and dampers, which are arranged in such a way that the suspended mass is characterized by uncoupled heave and pitch-rolling vibrations. In this case the three damping and natural frequency parameters of the absorber are tuned separately to control three flexural modes of the hosting structure. The simulation study shows that the proposed single-unit absorbers produce, respectively, 5.3 and 8.7 dB reductions of the global flexural vibration of a rectangular plate between 20 and 120 Hz.

Adaptive control of tonal disturbance in mechanical systems with nonlinear damping

This paper describes an adaptive system for controlling the tonal vibration of a single-degree-of-freedom system with nonlinear damping. The adaptive control system consists of a force actuator in parallel with the suspension, which includes the nonlinear damper, and a velocity sensor mounted on the mass. The adaptation of the controller is done once every period of the excitation. Because the response of the nonlinear system changes with excitation level, conventional adaptive algorithms, with a linear model of the plant, can be slow to converge and may not achieve the desired performance. An on-line observer is used to obtain a describing function model of the plant, which can vary with the excitation level. This allows the adaptive control algorithm to converge more quickly than using a fixed plant model, although care has to be taken to ensure that the dynamics of the observer do not interfere with the dynamics of the adaptive controller.

Switching and sweeping vibration absorbers: Theory and experimental validation

Abstract This paper investigates the principal properties of time varying operation modes for tuneable vibration absorbers mounted on distributed structures to reduce vibrations produced by stationary broad frequency band disturbances. The study considers a practical application where an electro-mechanical tuneable vibration absorber, comprising a seismic coil-magnet linear transducer, is fixed on a thin walled circular duct flexible structure. The absorber is commanded to either periodically switch or continuously vary the stiffness and damping of the elastic suspension holding the moving magnet. As a result, the tuneable vibration absorber fundamental natural frequency and damping ratio are respectively switched or swept to cyclically reduce the resonant response of multiple flexural natural modes of the duct structure. The study analytically shows, and confirms with simulations and experiments, that the vibration control produced by a simple “blind sweep” operation mode of the tuneable vibration absorber does not differ significantly from the (sub)optimal performance attainable via a switching operation mode.

Sweeping tunable vibration absorbers for structural vibration control

Tuneable vibration absorbers (TVAs) have been successfully implemented in distributed thin structures (e.g., panels shells) to control either tonal vibration, produced, for example, by unbalanced machinery, or low-frequencies broadband vibration, generated, for example, by stochastic pressure distributions due to diffuse sound fields or turbulent boundary layer fluid flows. This study is focused on the broadband control of structural vibration where often multiple TVAs are used to control the response of the natural modes that resonate in the controlled frequency band. Normally, each TVA is tuned to minimize the resonant response of a specific natural mode of the hosting structure by tuning the TVA natural frequency to the resonance frequency of the mode and setting the TVA damping ratio to properly dissipate energy. The proposed sweeping tuneable vibration absorbers (STVAs) have been conceived to operate on the multiple natural modes of the hosting structure where they are mounted by sweeping the TVA tun...

Adaptive vibration control of a mechanical system with nonlinear damping excited by a tonal disturbance

In this study, an adaptive control system to reduce the tonal vibration of a mechanical system characterized by nonlinear damping is considered. Since the response of the system to a sinusoidal excitation is however mainly sinusoidal, nonlinear controllers are not required for the control of a tone in such a mechanical system. The adaptation and stability of an adaptive control algorithm, however, depends on the accuracy of the plant model. Since the tonal response of the nonlinear system changes with excitation level, conventional adaptive algorithms, with a fix linear model of the plant, can be slow to converge and may not achieve the desired performance. The use of an online observer is proposed, to estimate the describing function model of the plant, which will vary with excitation level. This allows the adaptive control algorithm to converge more quickly than using a fixed plant model, although care has to be taken to ensure that the dynamics of the observer do not interfere with the dynamics of the ...

Two-mass MEMS velocity sensor feedback control loop design

This paper presents theoretical and experimental results about the design of an internal velocity feedback loop in a new capacitive micro-electro-mechanical system (MEMS) velocity sensor. The sensor comprises two mass-spring systems connected in series, termed the principal and control sensors. The control sensor output is fed to an electrostatic actuator that acts between the sensors frame and the principal proof mass. The aim of the internal feedback control loop is to generate a sky-hook damping effect on the principal sensor, so that, in the frequency band of interest, the output of the sensor is proportional to the base velocity. The sensor is fabricated on a Silicon-on-Isolator (SOI) wafer. The sensor interface and the controller are implemented on a printed circuit board (PCB). The design of the control loop is carried out offline; using measured frequency response functions (FRFs) between the displacements of the two proof masses with respect to i) the base acceleration, measured with a reference accelerometer mounted on the sensors frame, and ii) the voltage signal driving the electrostatic actuator for the velocity feedback loop.