Piezoelectric energy harvesting has become a feasible method for powering micro portable electronics and
wireless sensor networks by converting ambient vibration energy into electrical energy. As a thumb of rule, it is
critical to tune the resonant frequency of the generator to the frequency of the environmental vibrations in order
to induce the maximum structural deformation and then the maximum converted electrical energy through
piezoelectric effect. However, it is well-known that the ambient vibrations are not usually fixed in only one
single frequency and could span over a limited frequency band. In this paper, a band-pass design optimization of
piezoelectric cantilever bimorph (PCB) energy harvester is presented based on the system transfer function of the
PCB generator presented in a previous literature. For such an energy harvester, a group of PCB with dimensions
appropriately selected can be integrated into a band-pass energy harvester working over a limited frequency band
if the dimensions of piezoelectric bimorphs and proof masses are appropriately chosen. Further, the finite
element analysis (FEA) of such a band-pass energy harvester is performed in ANSYS to validate the theoretical
proposal. The result shows that the band-pass design optimization leads to a piezoelectric generator working over
a certain frequency band while keeping outputting the relatively stable open-circuit voltage.
With the popular use of wireless sensor networks, the replacement of the batteries becomes more concerned in the
research communities. Piezoelectric materials (PZT) can be used to convert ambient vibration energy into electrical
energy for use in powering microelectronics. Because the generated power and voltage depend on not only the amplitude
and frequency of the ambient vibration, but also the geometric configuration of the piezoelectric bimorph generator, it is
of great interest to study the relationships between the harvester's geometric configuration and its resonant frequency,
and between the driving frequency and the harvested voltage and power. In this paper, a piezoelectric bimorph cantilever
beam with a proof mass on the free end is adopted as the basic configuration, since it is simple and widely used as a low
resonance frequency energy harvester. The previous model in the literature is described at the beginning, and then the
Euler-Bernoulli beam model is created and calculated numerically. ANSYS simulation is performed to compare the
results of the two different models. The results show that the Euler model has better predication of the resonant
frequency of the PEH than the model in the literature. Finally, the design optimization of PEH is presented.
Semi-active control of torsional vibration can be realized through the use of variable friction brakes or clutches applied to a primary system acted on by oscillating torques. The performance of a given vibration control approach will depend greatly on the bandwidth of the actuators used to realize control. Three commercially available torsional actuators, a dry friction brake, a magneto-rheological fluid brake, and a magnetic particle brake have been tested and analyzed to assess their applicability for use in semi-active torsional vibration control. A test stand was constructed and used to run specific tests including step responses to determine "on" and "off" response times, open-loop bandwidth determination via swept-sine tests, and friction as a function of rotating speed. The data can be used to create general mathematical models to predict the behavior of the different actuators when excited with different control signals. The results indicate some of the limitations of the different actuators and will be used to provide a basis for determining the actuators' applicability to general torsional vibration control problems.
Analytical and experimental work has been performed on the use of a magneto-rheological (MR) fluid brake for controlling torsional vibrations in rotating systems. In this paper, two different strategies are examined for controlling the MR brake in such applications. First, implementation of the MR fluid brake as a passive friction damper with a variable friction torque is presented. In that application, fixed currents were applied to the electromagnets in the MR brake. As a result, the dominant behavior of the brake was as a dry friction damper whose friction was a function of applied current. The second approach was the implementation of the MR brake in a modified skyhook damping control approach. In that application, the friction in the MR brake was adjusted according to a comparison between the sign of absolute velocity of the primary system and the sign of the relative velocity between the MR brake and the primary system in order to add effective damping to the system. Characterization of the MR brake was an essential part of both control strategies. This work includes the results of system identification performed on the MR fluid brake, along with experimental performance results of the system under the different control strategies.
Crankshaft dampers are a common approach for controlling engine crankshaft vibration. The optimum damper parameters are relatively easy to determine for the case of single-mode systems and multi-mode systems with a dominant mode, provided that the primary system is undamped and the system response is linear. For nonlinear systems such as internal combustion engines that experience complex periodic inputs, the true optimum damper parameters may not be apparent. The crank kinematics introduce nonlinear torques acting on the crankshaft. In addition, the gas torque is, in some sense, a state-dependent input, as it is a function of not only the energy addition per cycle, but also of the crank angle. It is reasonable to expect that truly optimal damper parameters may not be obtained using classical approaches. As an alternative, genetic algorithms may be used to determine optimum crankshaft damper settings for this complex system. This paper will present the modeling of an internal combustion engine from the perspective of determining crankshaft vibrations. Optimum damper settings are then determined using a genetic algorithm. Simulation results are shown that compare the achievable vibration reduction in an engine equipped with a GA-tuned damper and the reduction achieved with a conventional passive damper.
Tuned vibration absorbers and tuned vibration dampers are structurally similar, but there are significant differences in their implementation and tuning laws. Lightly damped absorbers are typically applied to primary systems experiencing tonal excitation and achieve a "near-zero" in the frequency response of the primary system. An additional resonant peak is added to the system, such, transients or a mismatch in the absorber's tuned condition may result in poor performance. In contrast, a mini-max approach is used in the design of tuned vibration dampers. The stiffness and damping are designed such that the maximum frequency response across a band of frequencies is minimized. The vibration damper is insensitive to changes in the excitation frequency, but does not generally achieve high performance, due to the added damping in the vibration damper. This paper presents a novel approach whereby the characteristics of both a vibration absorber and a vibration damper are utilized in a single device. During transients, the damping is increased with the goal of helping the system to settle quickly and to avoid operation at resonant conditions. As the vibration becomes tonal, the damping is reduced and good steady state performance is achieved.
Control of torsional vibrations in an automotive crankshaft is a classical vibration control problem. The most common solution is to mount a crankshaft damper at one end of the crankshaft. Typical crankshaft dampers are composed of parallel stiffness and damping elements connecting a rotational inertia to the crankshaft. Appropriate design of the damper elements may result in substantial crankshaft vibration. Conventional couplings include elastomeric spring-damper elements and purely viscous fluid couplings. While those approaches result in satisfactory reduction of crankshaft vibration, it may be that a semi-active approach can achieve improved performance. To that end, an investigation of a semi-active crankshaft damper using magneto-rheological (MR) fluid has been initiated. A torsional MR fluid brake was obtained and applied to a scale model of a crankshaft for a common eight-cylinder engine. Experiments were performed with the MR brake as a fixed-friction device. In addition, a simple stick-slip control algorithm was developed such that the MR brake became an on-line variable friction device. While a good deal of work remains to be performed in future efforts, the preliminary experimental results have demonstrated that a torsional damper composed of an MR fluid brake has potential application in the field of torsional vibration control.
Adaptive tuned vibration absorbers for attenuation of harmonic vibration may be realized through the use of active materials in absorber designs. For example, the variable elastic modulus of shape memory alloy may be used to incorporate variable springs in an absorber, such that the absorber natural frequency may be on-line tuned. The difficulty then becomes the control of the tuned condition of the absorber. One method for controlling the tuned condition of the absorber is to drive the relative phase between the primary system and the absorber to a desired value. The classical phase-locked loop is one method that might be used to achieve this control goal. An alternative method is to utilize a simple PI controller that uses the relative phase as an error signal. The map between the control input to the active material elements and the resulting relative phase can be modeled as a first-order linear system in series with a static nonlinearity with certain characteristics. This paper presents an analysis of the stability of the phase tracking of an adaptive tuned vibration absorber made of active material. The necessary characteristics for the map between the control signal and the resulting relative phase are presented. A control algorithm is developed that results in asymptotic stability of the system in the presence of an uncertain fixed excitation frequency. Results of implementation of an experimental ATVA on a primary system are presented.
KEYWORDS: Shape memory alloys, Control systems, Device simulation, Signal attenuation, Vibration control, Data acquisition, Temperature metrology, Power supplies, Current controlled current source, Nonlinear filtering
Controlled continuous tuning of the stiffness of shape memory alloy (SMA) spring elements of an adaptively tunable vibration absorber (ATVA) is a novel concept for adaptive-passive vibration control. Minimization of the vibration of a primary system is achieved indirectly via stiffness control of the SMA structural elements supporting a secondary mass. Stiffness control is further achieved via the heating of the SMA elements. In this paper a control law to achieve phase- tracking by controlling the heating of the SMA elements is developed and implemented. Successful analytical and experimental results demonstrate the feasibility of continuous control of the SMA ATVA. Performance of the SMA ATVA is compared to the performance of comparable passive tuned vibration absorbers (TVA). The comparison shows that substantial improvements in vibration attenuation can be achieved through the implementation of the SMA ATVA.
The passive-adaptive approach to vibration control shows promise in its ability to combine the robust stability and low-complexity of passive tuned absorbers with the adaptability of active control schemes. Previous tunable vibration absorbers have been complex and bulky. Shape memory alloys (SMA) with their variable material properties, offer an alternative adaptive mechanism. Heating an SMA causes a change in the elastic modulus of the material by a factor as high as three. Incorporating SMA in parallel with traditional spring materials creates an absorber with a variable spring stiffness and a corresponding variable tuning frequency. Using on-off actuation of the SMA, discrete frequencies of tuning are obtained through the use of multiple SMA elements.
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