No description available.

No description available.

No description available.

No description available.

No description available.

Numerous mechanical systems contain surfaces in partial or full sliding contact, and therefore, prone to friction-induced vibration instability. These include systems containing mechanical switches, brakes, clutches, gears, rolling contact bearings, journal bearings, robot end-effector grasp and motion, oil drills, etc. The prominent dynamic features of a mechanical system, subject to friction-induced vibration, can be captured by an appropriate equivalent mass-on-belt model. It is the goal of this research to provide a comprehensive study of friction-induced vibrations in mechanical systems by using their equivalent mass-on-belt models. Friction-induced vibration is manifested through three mechanisms termed Stribeck effect, mode-coupling and sprag-slip. Mechanical systems prone to vibrations by one or more of the three mechanisms of instability are considered and studied in detail. The mechanical systems fall into one of two groups. A system in the first group is the pseudo-rigid-body mass-on-belt representation of a compliant bistable linkage mechanism characterized by substantial geometric nonlinearity and nonlinear elasticity. A system in the second group is a mass-on-belt model that accounts for mass-belt contact stiffness. Such a system is excited primarily through mode coupling. In the first group of mechanical systems super and subcritical pitchfork bifurcation as well as Hopf bifurcation are observed. The normal force and spring pre-compression are bifurcation parameters leading to the subcritical pitchfork bifurcation and the belt velocity corresponds to the Hopf bifurcation. It is found that for a low damping and negligible spring nonlinearity, one equilibrium point dominates the steady-state response. Otherwise, the phase plane is split into two separate planes associated with the corresponding fixed point. The boundary is dictated by structural damping and spring nonlinearity. It is shown that the destabilizing mechanism in the bistable mechanisms is the Stribeck effect of friction. The dominant mode of instability for the second group of mechanical system is mode coupling instability. In this group intermittent loss of contact between the mass and the moving belt within a periodic cycle is allowed. Addition of a vibration absorber consisting of a second mass suspended from the first mass by a spring provides effective passive control of friction-induced instability due to mode-coupling. The research concludes with the study of a two mass system in which both masses are in contact with a belt and the friction force is characterized by the three regimes of lubricated contact that include boundary lubrication, mixed boundary and hydrodynamic lubrication and full hydrodynamic lubrication as sliding speed is increased. It is shown that such systems can experience periodic, quasi-periodic and chaotic vibration response.

Typical aero-engine assemblies have at least two nested rotors mounted within a flexible casing via squeeze-film damper (SFD) bearings. As a result, the flexible casing structures become highly sensitive to the vibration excitation arising from the High and Low pressure rotors. Lowering vibrations at the aircraft engine casing can reduce harmful effects on the aircraft engine. Inverse problem techniques provide a means toward solving the unbalance identification problem for a rotordynamic system supported by nonlinear SFD bearings, requiring prior knowledge of the structure and measurements of vibrations at the casing. This thesis presents two inverse solution techniques for the nonlinear rotordynamic inverse problem, which are focused on applications where the rotor is inaccessible under operating conditions, e.g. high pressure rotors. Numerical and experimental validations under hitherto unconsidered conditions have been conducted to test the robustness of each technique. The main contributions of this thesis are:• The development of a non-invasive inverse procedure for unbalance identification and balancing of a nonlinear SFD rotordynamic system. This method requires at least a linear connection to ensure a well-conditioned explicit relationship between the casing vibration and the rotor unbalance via frequency response functions. The method makes no simplifying assumptions made in previous research e.g. neglect of gyroscopic effects; assumption of structural isotropy; restriction to one SFD; circular centred orbits (CCOs) of the SFD. • The identification and validation of the inverse dynamic model of the nonlinear SFD element, based on recurrent neural networks (RNNs) that are trained to reproduce the Cartesian displacements of the journal relative to the bearing housing, when presented with given input time histories of the Cartesian SFD bearing forces.• The empirical validation of an entirely novel approach towards the solution of a nonlinear inverse rotor-bearing problem, one involving an identified empirical inverse SFD bearing model. This method is suitable for applications where there is no adequate linear connection between rotor and casing. Both inverse solutions are formulated using the Receptance Harmonic Balance Method (RHBM) as the underpinning theory. The first inverse solution uses the RHBM to generate the backwards operator, where a linear connection is required to guarantee an explicit inverse solution. A least-squares solution yields the equivalent unbalance distribution in prescribed planes of the rotor, which is consequently used to balance it. This method is successfully validated on distinct rotordynamic systems, using simulated data considering different practical scenarios of error sources, such as noisy data, model uncertainty and balancing errors. Focus is then shifted to the second inverse solution, which is experimentally-based. In contrast to the explicit inverse solution, the second alternative uses the inverse SFD model as an implicit inverse solution. Details of the SFD test rig and its set up for empirical identification are presented. The empirical RNN training process for the inverse function of an SFD is presented and validated as a part of a nonlinear inverse problem. Finally, it is proved that the RNN could thus serve as reliable virtual instrumentation for use within an inverse rotor-bearing problem.

No description available.

The urgent need for a clean and sustainable power supply for wireless sensor nodes and low-power electronics in various monitoring systems and the Internet of Things has led to an explosion of research in substitute energy technologies. Traditional batteries are still the most widely used power source for these applications currently but have been blamed for chemical pollution, high maintenance cost, bulky volume, and limited energy capacity. Ambient energy in different forms such as vibration, movement, heat, wind, and waves otherwise wasted can be converted into usable electricity using proper transduction mechanisms to power sensors and low-power devices or charge rechargeable batteries. This dissertation focuses on the design, modeling, optimization, prototype, and testing of novel piezoelectric energy harvesters for extracting energy from human walking, bio-inspired bi-stable motion, and torsional vibration as an alternative power supply for wireless monitoring systems. To provide a sustainable power supply for health care monitoring systems, a piezoelectric footwear harvester is developed and embedded inside a shoe heel for scavenging energy from human walking. The harvester comprises of multiple 33-mode piezoelectric stacks within single-stage force amplification frames sandwiched between two heel-shaped aluminum plates taking and reallocating the dynamic force at the heel. The single-stage force amplification frame is designed and optimized to transmit, redirect, and amplify the heel-strike force to the inner piezoelectric stack. An analytical model is developed and validated to predict precisely the electromechanical coupling behavior of the harvester. A symmetric finite element model is established to facilitate the mesh of the transducer unit based on a material equivalent model that simplifies the multilayered piezoelectric stack into a bulk. The symmetric FE model is experimentally validated and used for parametric analysis of the single-stage force amplification frame for a large force amplification factor and power output. The results show that an average power output of 9.3 mW/shoe and a peak power output of 84.8 mW are experimentally achieved at the walking speed of 3.0 mph (4.8 km/h). To further improve the power output, a two-stage force amplification compliant mechanism is designed and incorporated into the footwear energy harvester, which could amplify the dynamic force at the heel twice before applied to the inner piezoelectric stacks. An average power of 34.3 mW and a peak power of 110.2 mW were obtained under the dynamic force with the amplitude of 500 N and frequency of 3 Hz. A comparison study demonstrated that the proposed two-stage piezoelectric harvester has a much larger power output than the state-of-the-art results in the literature. A novel bi-stable piezoelectric energy harvester inspired by the rapid shape transition of the Venus flytrap leaves is proposed, modeled and experimentally tested for the purpose of energy harvesting from broadband frequency vibrations. The harvester consists of a piezoelectric macro fiber composite (MFC) transducer, a tip mass, and two sub-beams with bending and twisting deformations created by in-plane pre-displacement constraints using rigid tip-mass blocks. Different from traditional ways to realize bi-stability using nonlinear magnetic forces or residual stress in laminate composites, the proposed bio-inspired bi-stable piezoelectric energy harvester takes advantage of the mutual self-constraint at the free ends of the two cantilever sub-beams with a pre-displacement. This mutual pre-displacement constraint bi-directionally curves the two sub-beams in two directions inducing higher mechanical potential energy. The nonlinear dynamics of the bio-inspired bi-stable piezoelectric energy harvester is investigated under sweeping frequency and harmonic excitations. The results show that the sub-beams of the harvester experience local vibrations, including broadband frequency components during the snap-through, which is desirable for large power output. An average power output of 0.193 mW for a load resistance of 8.2 kΩ is harvested at the excitation frequency of 10 Hz and amplitude of 4.0 g. Torsional vibration widely exists in mechanical engineering but has not yet been well exploited for energy harvesting to provide a sustainable power supply for structural health monitoring systems. A torsional vibration energy harvesting system comprised of a shaft and a shear mode piezoelectric transducer is developed in this dissertation to look into the feasibility of harvesting energy from oil drilling shaft for powering downhole sensors. A theoretical model of the torsional vibration piezoelectric energy harvester is derived and experimentally verified to be capable of characterizing the electromechanical coupling system and predicting the electrical responses. The position of the piezoelectric transducer on the surface of the shaft is parameterized by two variables that are optimized to maximize the power output. Approximate expressions of the voltage and power are derived by simplifying the theoretical model, which gives predictions in good agreement with analytical solutions. Based on the derived approximate expression, physical interpretations of the implicit relationship between the power output and the position parameters of the piezoelectric transducer are given. / Doctor of Philosophy / Wireless monitoring systems with embedded wireless sensor nodes have been widely applied in human health care, structural health monitoring, home security, environment assessment, and wild animal tracking. One distinctive advantage of wireless monitoring systems is to provide unremitting, wireless monitoring of interesting parameters, and data transmission for timely decision making. However, most of these systems are powered by traditional batteries with finite energy capacity, which need periodic replacement or recharge, resulting in high maintenance costs, interruption of service, and potential environmental pollution. On the other hand, abundant energy in different forms such as solar, wind, heat, and vibrations, diffusely exists in ambient environments surrounding wireless monitoring systems which would be otherwise wasted could be converted into usable electricity by proper energy transduction mechanisms. Energy harvesting, also referred to as energy scavenging and energy conversion, is a technology that uses different energy transduction mechanisms, including electromagnetic, photovoltaic, piezoelectric, electrostatic, triboelectric, and thermoelectric, to convert ambient energy into electricity. Compared with traditional batteries, energy harvesting could provide a continuous and sustainable power supply or directly recharge storage devices like batteries and capacitors without interrupting operation. Among these energy transduction mechanisms, piezoelectric materials have been extensively explored for small-size and low-power generation due to their merits of easy shaping, high energy density, flexible design, and low maintenance cost. Piezoelectric transducers convert mechanical energy induced by dynamic strain into electrical charges through the piezoelectric effect. This dissertation presents novel piezoelectric energy harvesters, including design, modeling, prototyping, and experimental tests for energy harvesting from human walking, broadband bi-stable nonlinear vibrations, and torsional vibrations for powering wireless monitoring systems. A piezoelectric footwear energy harvester is developed and embedded inside a shoe heel for scavenging energy from heel striking during human walking to provide a power supply for wearable sensors embedded in health monitoring systems. The footwear energy harvester consists of multiple piezoelectric stacks, force amplifiers, and two heel-shaped metal plates taking dynamic forces at the heel. The force amplifiers are designed and optimized to redirect and amplify the dynamic force transferred from the heel-shaped plates and then applied to the inner piezoelectric stacks for large power output. An analytical model and a finite model were developed to simulate the electromechanical responses of the harvester. The footwear harvester was tested on a treadmill under different walking speeds to validate the numerical models and evaluate the energy generation performance. An average power output of 9.3 mW/shoe and a peak power output of 84.8 mW are experimentally achieved at the walking speed of 3.0 mph (4.8 km/h). A two-stage force amplifier is designed later to improve the power output further. The dynamic force at the heel is amplified twice by the two-stage force amplifiers before applied to the piezoelectric stacks. An average power output of 34.3 mW and a peak power output of 110.2 mW were obtained from the harvester with the two-stage force amplifiers. A bio-inspired bi-stable piezoelectric energy harvester is designed, prototyped, and tested to harvest energy from broadband vibrations induced by animal motions and fluid flowing for the potential applications of self-powered fish telemetry tags and bird tags. The harvester consists of a piezoelectric macro fiber composite (MFC) transducer, a tip mass, and two sub-beams constrained at the free ends by in-plane pre-displacement, which bends and twists the two sub-beams and consequently creates curvatures in both length and width directions. The bi-direction curvature design makes the cantilever beam have two stable states and one unstable state, which is inspired by the Venus flytrap that could rapidly change its leaves from the open state to the close state to trap agile insects. This rapid shape transition of the Venus flytrap, similar to the vibration of the harvester from one stable state to the other, is accompanied by a large energy release that could be harvested. Detailed design steps and principles are introduced, and a prototype is fabricated to demonstrate and validate the concept. The energy harvesting performance of the harvester is evaluated at different excitation levels. Finally, a piezoelectric energy harvester is developed, analytically modeled, and validated for harvesting energy from the rotation of an oil drilling shaft to seek a continuous power supply for downhole sensors in oil drilling monitoring systems. The position of the piezoelectric transducer on the surface of the shaft is parameterized by two variables that are optimized to obtain the maximum power output. Approximate expressions of voltage and power of the torsional vibration piezoelectric energy harvester are derived from the theoretical model. The implicit relationship between the power output and the two position parameters of the transducer is revealed and physically interpreted based on the approximate power expression. Those findings offer a good reference for the practical design of the torsional vibration energy harvesting system.

Les vibrations qui sont en général source de dérangement, d’usure et même destruction des machines et structures mécaniques doivent être contrôlées ou éliminées. Pour cette raison, la lutte contre les vibrations est devenue depuis des années un enjeu majeur pour les chercheurs de laboratoire et de développement dans l’industrie afin de développer des solutions efficaces contre ces problèmes. De nombreuses technologies ont donc été développées. Parmi ces technologies, les absorbeurs de vibration non linéaires présentent des performances importantes dans l’atténuation de vibration sur une large bande de fréquences. C’est dans ce contexte que cette thèse se focalise sur l’analyse et l’optimisation des absorbeurs de vibration non linéaires. L’objectif de cette thèse est d’analyser le comportement dynamique non linéaire des systèmes présentant des absorbeurs de vibration non linéaires. Pour cela, un modèle dynamique d’un système à deux degrés de liberté est développé mettant en équations le comportement non linéaire. La résolution des équations de mouvement est faite par la Méthode Asymptotique Numérique (MAN). La performance de cette méthode est montrée via une comparaison avec la méthode de Newton-Raphson. L’analyse des modes non linéaires du système ayant une non-linéarité cubique est faite par une formulation explicite des Fonctions de Réponse en Fréquence non linéaires (FRFs) et les Modes Normaux Non linéaires (MNNs). Un démonstrateur sur la base d’un système simple à deux degré de liberté est mis en place afin de recaler les modèles envisagés sur la base des résultats expérimentaux trouvés. / Vibrations are usually undesired phenomena as they may cause discomfort, disturbance, damage, and sometimes destruction of machines and structures. It must be reduced or controlled or eliminated. For this reason, the vibrations attenuation became a major issue for scientists and researchers in order to develop effective solutions for these problems. Many technologies have been developed. Among these technologies, the nonlinear vibration absorbers have significant performance in the vibration attenuation over a wide frequency band. In this context, this thesis focuses on the analysis and optimization of nonlinear vibration absorbers. The objective of the thesis is to analyze the nonlinear dynamic behavior of systems with nonlinear vibration absorbers. For this, a dynamic model of a two degrees of freedom system is developed. The Asymptotic Numerical Method (ANM) is used to solve the nonlinear equations of motion. The performance of this method is shown via a comparison with the Newton-Raphson method. The nonlinear modal analysis system with cubic nonlinearity is made by an explicit formulation of the nonlinear Frequency Response Functions (FRFs) and Nonlinear Normal Modes (MNNs). An experimental study is performed to validate the numerical results.