Emiliano Rustighi

A shape memory alloy adaptive tuned vibration absorber: design and implementation

In this paper a tuned vibration absorber (TVA) is realized using shape memory alloy (SMA) elements. The elastic modulus of SMA changes with temperature and this effect is exploited to develop a continuously tunable device. A TVA with beam elements is described, a simple two-degree-of-freedom model developed and the TVA characterized experimentally. The behaviour during continuous heating and cooling is examined and the TVA is seen to be continuously tunable. A change in the tuned frequency of 21.4% is observed between the cold, martensite, and hot, austenite, states. This corresponds to a change in the elastic modulus of about 47.5%, somewhat less than expected. The response time of the SMA TVA is long because of its thermal inertia. However, it is mechanically simple and has a reasonably good performance, despite the tuning parameters depending on the current in a strongly nonlinear way.

A Magnetorheological Fluid Damper for Rotor Applications

Even though we are still far from industrial applications, in the last decade there has been increasing attention directed toward the employment of electrorheological (ER) and magnetorheological (MR) fluids in active bearings and active squeeze film dampers in rotordynamics. MR fluids react to magnetic fields undergoing reversible changes in their mechanical characteristics, viscosity, and stiffness in particular. In previous literature, some applications of ER fluids in rotor squeeze film dampers can be found; however, on the contrary, little is reported on similar test rigs set up for MR dampers. In this work, the design of an MR squeeze film damper is presented and discussed. A numerical simulation has been carried out in order to evaluate the dynamic behavior of the damped rotor as a function of the magnetic field strength. The test rig is made of a slender shaft supported by two oilite bearings and an unbalanced disk. The damper is interfaced with the shaft through a rolling bearing. Electric coils generate the magnetic field whose field lines cross the MR film. Since the damping characteristics can be varied continuously by controlling the magnetic field, it is possible to have the optimum conditions for each regime of rotational speed. Preliminary tests are encouraging.

Design of a novel magneto-rheological squeeze-film damper

Magneto-rheological (MR) fluids react to magnetic fields undergoing changes in their mechanical characteristics, viscosity in particular. After an analytical and numerical study, an MR squeeze-film damper has been designed and set up on a reduced scale rotor test-rig. Numerical simulations were carried out in order to evaluate the dynamic behaviour of the damped rotor as a function of the current supplied to the adjustable device. A linear model that depicts the main characteristics of the system has been developed as a useful tool in damper and control design. By testing different fluids, an optimal fluid has been singled out. Tests conducted on the selected fluid show that it is possible to have the optimum conditions for each steady rotational speed.

Real-time control of a shape memory alloy adaptive tuned vibration absorber

The tuned vibration absorber (TVA) is a well established vibration control device. An adaptive TVA (ATVA) is one whose properties can be continuously adapted to maintain optimal tuning. This paper concerns an ATVA with shape memory alloy (SMA) elements: heating or cooling the SMA changes its elastic modulus and hence the effective stiffness and tuned frequency of the ATVA. The emphasis of this paper is placed on control algorithms for real-time adaptation of the ATVA. An error signal is defined in terms of the phase between the velocities of the ATVA mass and the host structure. Various control algorithms are discussed, including proportional, proportional-plus-derivative (PD) and fuzzy control. Discrete and continuous-time implementations are considered, together with control parameter optimization. Numerical simulations and experimental results are presented and compared. The SMA ATVA is seen to be able to adaptively re-tune in the face of a changing disturbance frequency within a fixed range, although the adaptation time is limited primarily by the thermal time constant of the system. The PD controller gives close to the optimum performance while being very simple to implement.

Design of a 2DOF Vibrational Energy Harvesting Device

A novel design method for a 2DOF energy harvesting device is studied. The energy harvesting device is modeled as a rigid body supported by two parallel sets of springs and dampers. The impedance expression for the model has been developed by utilizing the concept of the inerter. The proposed design method deals with tuning two resonant peaks and equalizing the harvested power at those frequencies. As a result, the proposed energy harvesting device is particularly effective at two frequencies and has increased bandwidth as well as reduced size and weight in comparison with previous SDOF devices. A numerical design example is provided to show effectiveness of the proposed method.

A unified approach to optimal conditions of power harvesting using electromagnetic and piezoelectric transducers

Power harvesting systems generate electric power from mechanical vibration using electromechanical transducers. To assess the practical performance of these systems, it is important to be able to estimate the maximum power that could be harvested from a specific structure and to derive the optimal conditions for the harvesting. We introduce a two-port network model which can fully describe the electromechanical coupled dynamics of the transducer, including the interaction between the structure and the electrical load in a unified manner. Electromagnetic and piezoelectric transducers are then considered as special cases of this general treatment. The power harvested by both types of transducer on a simply supported beam is derived and the optimal matching conditions and the maximum harvested power are obtained theoretically. In this numerical example, a piezoelectric transducer would have higher potential than an electromagnetic device of equal mass for the harvesting, but only when the transducer parameters are tuned optimally.

Active Control of the Longitudinal-Lateral Vibration of a Shaft-Plate Coupled System

The coupled longitudinal-lateral vibration of a shaft-plate system and its suppression by means of a feedback control scheme are discussed. A simplified model of the system is established through synthesis of frequency response functions (FRFs) and verified with the finite element method (FEM). This analytical model describes the coupled longitudinal-lateral vibration of the system induced by longitudinal periodic excitation at the free end of the shaft. Based on this model, vibration control via longitudinal actuation on the shaft and active vibration cancellation are studied. The active control scheme is based on an adaptive feedback scenario and a novel mechanism of adaptation of the controller’s gain, which is proposed for time-varying dynamics induced by the variation of the axial spring stiffness. Simulation results have demonstrated that the control scheme is effective in attenuating vibration of the system. Furthermore, axial actuation on the shaft is able to cancel the effect of the longitudinal disturbance acting at the free end of the shaft and consequently reduces the internal forces as well as the vibration in the plate. However, deviation of the actuation force from the shaft axis will deteriorate control of the lateral vibration and sufficiently small deviation needs to be guaranteed

An adaptive anechoic termination for active vibration control

The active broadband control of the flexural vibration of a slender structure, in particular a beam, is obtained by the use of an adaptive anechoic termination. The anechoic termination, which absorbs any energy incident upon it, is implemented by applying a force close to one end of the structure. The force is determined by a feed-forward adaptive control that uses estimates of the incident and reflected waves as reference and error signals. Digital filters are implemented to estimate, in real-time, the amplitudes of these waves by filtering the outputs of an array of sensors. The reflected wave is used as the cost function in a filtered-X LMS adaptive control. The use of the propagating waves as reference and error signals also allows the method to be effective for resonant structures, a situation in which conventional approaches fail to be reliable. In order to compare the method with a conventional approach an anechoic termination that uses the primary excitation as reference is also considered. Numerical and experimental results demonstrate the method applied to semi-infinite and finite resonant structures. A broadband reduction of up to 20 dB in the ratio of the reflected and incident powers is demonstrated both numerically and experimentally. The use of the adaptive anechoic termination to reduce the vibration levels in structures is shown to be more effective than other typical feed-forward active control systems. Furthermore, it can be applied to cases where no reference signal, such as the primary excitation, is directly available.

Active vibration control of periodic disturbances using a DEAP damper

A potential problem that could possibly restrict the application of dielectric electro-active polymer (DEAP) actuators for active vibration damping is highlighted in this contribution. If a periodic electric field is applied to a DEAP actuator to counteract a periodic vibratory disturbance, a very common vibration attenuation problem, then the mechanical output will be the square of the periodic input. This will result in an actuator output with several harmonics. Therefore from a vibration damping perspective not only does the first harmonic of the periodic disturbance need to be considered but also additional harmonics, introduced by the actuator itself. Feedforward active damping of periodic vibratory disturbances using a tubular DEAP actuator is addressed in this contribution. Initially the addition of a d.c. bias offset to the periodic voltage signal applied to the actuator is investigated to try and reduce the effect of the higher harmonics. The use of a linearizing gain schedule is then also examined. Using a comparatively large d.c. bias voltage offset has a linearizing affect on the voltage-strain characteristics of the tubular actuator thereby reducing the influence of the higher harmonics on the resulting vibration damping characteristics. The disadvantage of this approach is that the operating range, in terms of the actuator stroke that can be achieved, is decreased. The use of a linearizing gain schedule also reduces the influence of the higher harmonics but provides less of a constraint on the operating range of the actuator.

Isometric force generated by locust skeletal muscle: responses to single stimuli

A mathematical model of the locust hind leg extensor muscle is described. The model accounts for the force response of the muscle to well-separated input stimuli under isometric conditions. Experimental data was collected by stimulating the extensor muscle and measuring the force generated at the tibia. In developing a model it was assumed that the response to a single isolated stimulus was linear. A linear model was found to fit well to the response to an isolated stimulus. No assumptions were made about the model order and models of various order were fitted to data in the frequency domain, using a least squares fit. The stimulus can be approximated as an impulse, with the response to each stimulus well described by a linear second-order system. Using a third-order model provided a better fit to data, but the improvement in fit was marginal and the model uses one extra parameter. A fourth-order model, which is often used to describe the behaviour of isometric muscle was found to overfit the data. Using a second-order model provides a simpler way of describing the behaviour of an isometric twitch.