27 research outputs found
On the dynamics of the Furuta pendulum
No full textThe Furuta pendulum, or rotational inverted pendulum, is a system found in many control labs. It provides a compact yet impressive platform for control demonstrations and draws the attention of the control community as a platform for the development of nonlinear control laws. Despite the popularity of the platform, there are very few papers which employ the correct dynamics and only one that derives the full system dynamics. In this paper, the full dynamics of the Furuta pendulum are derived using two methods: a Lagrangian formulation and an iterative Newton-Euler formulation. Approximations are made to the full dynamics which converge to the more commonly presented expressions. The system dynamics are then linearised using a Jacobian. To illustrate the influence the commonly neglected inertia terms have on the system dynamics, a brief example is offered.Benjamin Seth Cazzolato and Zebb Prim
Lyapunov-based control strategies for the global control of symmetric VTOL UAVs.
No full textThe last decade has seen significant advances in the development of Vertical takeoff and landing (VTOL) unmanned aerial vehicles (UAVs). The emergence of enabling technologies, in addition to the practical usefulness of such systems has driven their development to a point where numerous technology demonstrators and commercial products are now in existence. Of particular interest has been the development of small scale, VTOL UAVs commonly referred to as mini and micro-VTOL UAVs. The versatility and agility of such vehicles offers great potential for the use in clustered, urban environments.
Despite recent advancements, the autonomous navigation of VTOL UAVs remains a very challenging research area. The dynamics of VTOL UAVs are heavily nonlinear, underactuated and non-minimum phase. This, coupled with the aggressive maneuvers that such vehicles are expected to execute provides a stimulating problem in dynamic control. This is particularly true in the case of micro-VTOL UAVs. The fast, nonlinear nature of these systems render classical, linear control approaches inadequate.
The past twenty years has seen great interest in the development of nonlinear control strategies. This has led to the emergence of a number of standard design tools, most notably feedback linearisation and Lyapunov-based, backstepping approaches. Such design techniques offer a framework for the derivation of model based control laws capable of achieving global stabilisation and trajectory tracking control for heavily nonlinear systems. Recently, there has been significant interest in the application
of such nonlinear control paradigms for the stabilisation and control of VTOL UAVs.
The aim of this thesis is to further the application and analysis of nonlinear control design techniques for the control of VTOL UAVs. In particular, focus is placed on Lyapunov-based, backstepping-type control approaches. The first half of this thesis investigates Lyapunov-based control strategies that cast the closed-loop VTOL dynamics into a globally stable, cascade structure. This work was directly inspired by, and builds on, a variety of previously published works. Firstly, an alternative design approach to that previously published is presented, resulting in an improved closed-loop dynamic structure. Although inspired by the VTOL system, this idea may be generalised for the control of a broad class of systems, and is presented as such. A singularity issue arising in the cascade control of VTOL vehicles is then investigated, and a novel approach to overcome this issue is formulated. The second half of this thesis is dedicated to the trajectory tracking control of VTOL UAVs at velocities where the influence of aerodynamics is significant. In general, the aerodynamic models of VTOL UAVs are heavily nonlinear and poorly known. The use of such models in a backstepping framework that uses explicit differentiation of these models for dynamic inversion is questioned, due to the potential sensitivity of such nonlinear models. Consequently, an alternative approach utilising coupled filters to avoid such sensitivity issues is proposed. All control designs formulated in this thesis are accompanied by proofs guaranteeing their global stability, and numerical simulations demonstrating their time domain response characteristics.Thesis (Ph.D.) -- University of Adelaide, School of Mechanical Engineering, 200
The application of Tikhonov regularised inverse filtering to digital communication through multi-channel acoustic systems.
No full textCommunication between underwater vessels such as submarines is difficult to achieve over long distances using radio waves because of their high rate of absorption by water. Using underwater acoustic wave propagation for digital communication has the potential to overcome this limitation. In the last 30 years, there have been numerous papers published on the design of communication systems for shallow underwater acoustic environments. Shallow underwater acoustic environments have been described as extremely difficult media in which to achieve high data rates. The major performance limitations arise from losses due to geometrical spreading and absorption, ambient noise, Doppler spread and reverberation from surface and seafloor reflections (multi-path), with the latter being the primary limitation. The reverberation from multi-path in particular has been found to be very problematic when using the general communication systems that have been developed for radio wave communication systems. In the early 1990s, the principal means of combating multi-path in the shallow underwater environment was to use non-coherent modulation techniques. Coherent of obtaining a phase-lock and also that the environment was subject
to fading. Designs have since been presented that addressed both of these problems by using a complex receiver design that involved a joint update of the phase-lock loop and the taps of the decision feedback filter (DFE). In recent years a technique known as time-reversal has been investigated for use in underwater acoustic communication systems. A major benefit of using the time-reversal filter in underwater acoustic communication systems is that it can provide a fast and simple method to provide a receiver design of low complexity. A technique that can be related to time-reversal and possibly used in underwater acoustics is Tikhonov regularised inverse filtering. The Tikhonov regularised inverse filter is a fast method of obtaining a stable inverse filter design by calculating the filter in the frequency domain using the fast
Fourier transform, and was originally developed for use in audio reproduction systems. Previous research has shown that the Tikhonov regularised inverse filter design outperformed time-reversal when using a Dirac impulse transmission within a simulated underwater environment. This thesis aims to extend the previous work by examining the implementation of Tikhonov regularised inverse filtering with communication signals. In addressing this goal, two topics have been examined: the influence of the sensitivities in the filter designs, and an examination of various design implementations for Tikhonov regularised inverse filtering and similar filtering techniques.
The influence of transducer sensitivities on the Tikhonov regularised inverse filter:
During the implementation of the Tikhonov regularised inverse filter it was observed that both the Tikhonov regularised inverse filter and the timereversal filter were influenced by the sensitivity of the transducers to the acoustic signals, which is determined by the transducer design and the amplifying stages. Unlike single channel systems, setting the sensitivities of the transducers to their maximum value for multi-channel systems does not always maximise the coherence between the input and output of the entire system consisting of the inverse filter, the sensitivities and the electro-acoustic system where the channel is the electro-acoustic transfer function between the transmitter and receiver. The influence the sensitivities have on the performance of the multi-channel Tikhonov regularised inverse filters and the timereversal filter was examined by performing a mathematical examination of the system. An algorithm was developed that adjusted gains to compensate
for the decrease in performance that results from the poor sensitivities. To test the algorithm, a system with an inappropriate set of sensitivities was examined. The performance improvement of the communication system was examined using the generated gains to scale the signal. The algorithm was found to reduce the signal degradation and cross-talk. If the gains were used in the digital domain (after the analog to digital and before the digital to analog converters) then the quality of the signal was improved at the expense of the signal level. During this examination it was found that the time-reversal filter is equivalent to the Tikhonov regularised inverse filter with infinite regularisation.
Variations of the Tikhonov regularised inverse filter and performance comparisons:
In this thesis, various design structures for the implementation of the Tikhonov inverse filter were proposed and implemented in an experimental digital communication system that operated through an acoustic environment in air. It was shown that the Tikhonov inverse filter and related filter design structures could be classified or implemented according to three different classifications. The Tikhonov inverse filter was implemented according to each of these classifications and then compared against each other, as well as against two other filter designs discussed in the literature: time-reversal filtering, and the two-sided filter developed by Stojanovic [2005]. Due to the number of parameters that could be varied, it was difficult to identify the influence each parameter had on the results independently of the other parameters. A simulation was developed based on a model of the experiment to assist in identifying the influences of each parameter. The parameters examined included the number of transmitter elements, carrier frequency, data rate, and the value of the regularisation parameter. When the communication system consisted of a signal receiver, the Stojanovic two-sided filter generally outperformed the Tikhonov regularised inverse filter designs when communicating. However, at higher data rates, the Stojanovic two-sided filter required the addition of a regularisation parameter to allow it to continue to operate. However, given an appropriately selected regularisation parameter, the difference between the performance of the Tikhonov filter and the Stojanovic two-sided filter was minimal. When performing multi-channel communications, the full MIMO implementation of the Tikhonov regularised inverse filter design was shown to have the best performance. For the environment considered, the Tikhonov regularised inverse filter was the only design that was able to eliminate all symbol errors.Thesis (Ph.D.) -- University of Adelaide, School of Mechanical Engineering, 201
Guidance for time efficient path following of under-actuated differential thrust AUVs
No full textAutonomous Underwater Vehicles (AUVs) are relatively new underwater devices developed to execute missions in water without human operators. The advancements made in AUV technology have significant implications for a wide range of underwater applications. Guidance and control of AUVs plays an important role in many applications, and it is a challenging research topic, not only because of the significant nonlinearities and couplings in the AUV’s dynamics, but the under-actuation found in typical AUVs. This thesis presents new work contributing to time efficient path following of under-actuated AUVs. Different from the conventional fin-manoeuvred AUVs, the prototype vehicle considered in this thesis is a differential thrust manoeuvred AUV devoid of fins or rudders. Such a manoeuvring feature makes the vehicle very agile, but brings challenges in guidance and control. A model of the prototype AUV is constructed based on the vehicle dynamics and manoeuvring features. In order to achieve time efficient path following, the AUV should operate at its motion limits. To derive the motion limits, a Monte Carlo analysis is conducted using the AUV model, which provides a numerical solution to derive the maximum admissible motion of the vehicle with respect to the curvatures along given paths. Thus, a curvature-based guidance system is developed. The strategy alters the AUV path following speed according to the path curvature, hence increasing the overall time efficiency. The effectiveness of the proposed method is demonstrated through simulations of the AUV following a range of different paths.Thesis (M.Phil.) -- University of Adelaide, School of Mechanical Engineering, 2016
Spatially fixed and moving virtual sensing methods for active noise control.
No full textLocal active noise control systems generate a zone of quiet at the physical error sensor location. While significant attenuation is achieved at the error sensor, local noise control is not without its problems, chiefly that the zone of quiet is generally small and impractically sized. It may be inconvenient to place the error sensor at the desired location of attenuation, such as near an observer’s ear, preventing the small zone of quiet from being centered there. To overcome the problems encountered in local active noise control, virtual acoustic sensors have been developed to shift the zone of quiet away from the physical sensor position to a spatially fixed desired location.
The general aim of the research presented in this thesis is to improve and extend the spatially fixed and moving virtual sensing algorithms developed for active noise control thus far and hence increase the scope and application of local active noise control systems. To achieve this research aim, a number of novel spatially fixed and moving virtual sensing algorithms are presented for local active noise control.
In this thesis, a spatially fixed virtual sensing technique named the Stochastically Optimal Tonal Diffuse Field (SOTDF) virtual sensing method is developed specifically for use in pure tone diffuse sound fields. The SOTDF virtual sensing method is a fixed gain virtual sensing method that does not require a preliminary identification stage nor models of the complex transfer functions between the error sensors and the sources. SOTDF virtual microphones and virtual energy density sensors that use both pressure and pressure gradient sensors are developed using the SOTDF virtual sensing method. The performance of these SOTDF virtual sensors is investigated in numerical simulations and using experimental measurements made in a reverberation chamber. SOTDF virtual sensors are shown to accurately estimate the pressure and pressure gradient at a virtual location and to effectively shift the zone of quiet away from the physical sensors to the virtual location. In numerically simulated and post-processed experimental control, both virtual microphones and virtual energy density sensors achieve higher attenuation at the virtual location than conventional control strategies employing their physical counterpart.
As it is likely that the desired location of attenuation is not spatially fixed, a number of moving virtual sensing algorithms are also developed in this thesis. These moving virtual sensing algorithms generate a virtual microphone that tracks the desired location of attenuation as it moves through a three-dimensional sound field. To determine the level of attenuation that can be expected at the ear of a seated observer in practice, the performance of the moving virtual sensing algorithms in generating a moving zone of quiet at the single ear of a rotating artificial head is investigated in real-time experiments conducted in a modally dense three dimensional cavity. Results of real-time experiments demonstrate that moving virtual sensors provide improved attenuation at the moving virtual location compared to either fixed virtual sensors or fixed physical sensors.
As an acoustic energy density cost function spatially extends the zone of quiet generated at the sensor location, a fixed three-dimensional virtual acoustic energy density sensing method is also developed for use in a modally dense three-dimensional sound field. The size of the localised zone of quiet achieved by minimising either the acoustic energy density or the squared pressure at the virtual location with the active noise control system is compared in real-time experiments conduced in a modally dense three-dimensional cavity. Experimental results demonstrate that minimising the virtual acoustic energy density provides improved attenuation in the sound field and a larger 10 dB zone of quiet at the virtual location than virtual microphones.Thesis (Ph.D.) -- University of Adelaide, School of Mechanical Engineering, 201
Robust scheduling control of aeroelasticity.
No full textAeroelasticity is a broad term describing the often complex interactions between structural mechanics and aerodynamics. Aeroelastic phenomena such as divergence and flutter are potentially destructive, and thus must be avoided. Passive methods to avoid undesirable aeroelastic phenomena often involve the addition of mass and/or limiting the achievable performance
of the aircraft. However, active control methods allow both for the suppression of undesirable aeroelastic phenomena, and for utilisation of desirable aeroelastic phenomena using actuators, thus increasing performance without the associated weight penalty of passive systems. The work presented in this thesis involves the design, implementation and experimental validation of novel active controllers to suppress undesirable aeroelastic phenomena over a range of airspeeds. The controllers are constructed using a Linear Parameter Varying (LPV) framework, where the plant and controllers can be represented as linear systems which are functions of a parameter, in this case airspeed. The LPV controllers are constructed using Linear Matrix Inequalities (LMIs), which are convex optimisation problems that can be used to represent many linear control objectives. Using LMIs, these LPV controllers can be constructed
such that they self-schedule with airspeed and provide upper performance bounds during the design process. The aeroelastic phenomena being suppressed by these controllers are Limit-Cycle Oscillations (LCOs), which are a form of flutter with the aeroelastic instability bounded by a structural nonlinearity in the aeroelastic system. In this work, the aeroelastic system used is the Nonlinear Aeroelastic Test Apparatus (NATA), an experimental aeroelastic test platform located at Texas A&M University. Three and four degree-of-freedom dynamic models were derived for the NATA, which include second-order servo motor dynamics. These servo motor dynamics are often neglected in literature but are sufficiently slow that their dynamics are significant to the controlled response of the NATA. The dynamic model also incorporates quasi-steady aerodynamics, which are accurate for low Strouhal numbers calculated from the oscillation frequency of the wing. Is it shown how the dynamics of the NATA can be represented in LPV form, with a quadratic dependence on airspeed and linear dependence on torsional stiffness. Using a variety of techniques the parameters of the NATA are identified, and shown through nonlinear simulation to provide excellent agreement with experimental results. It is also argued that structural nonlinearity, in the way of a nonlinear torsional spring connecting the wing section to the base, generally improves stability due to its largely quadratic stiffness function, and hence in many instances it is safe to
linearise this nonlinearity when designing a controller. Using a H₂ generalised control problem representation of a Linear Quadratic Regulator (LQR) state-feedback controller, LPV synthesis LMIs are constructed using a standard transformation which render the LMIs affine in the transformed controller and Lyapunov matrices. These matrices have the same quadratic dependence on airspeed as the NATA model. To reduce conservatism the parameter space of airspeed versus airspeed squared is gridded into triangular convex hulls over the true parameter curve, and the LMIs are numerically optimised to give an upper bound on the H₂ norm across the design airspeed. The resulting state-feedback controller is constructed from the transformed controller and Lyapunov matrices, and can be solved symbolically as a function of airspeed, however it forms a high-order rational function of airspeed, hence it is quicker to solve for the controller gains numerically on-line. The controller is analysed for the classical measures of robustness, namely gain and phase margins, and maximum sensitivity. While not providing the guarantees of these measures that a conventional LQR controller provides, the controller is shown to be sufficiently robust across the airspeed design range. Experimental results for this controller were performed, and the results show excellent LCO suppression and disturbance rejection, the results from which are published in Prime et al. (2010). Following the above work based on a scalar performance index, the upper bound on the H₂ norm is allowed to vary with airspeed using the same quadratic dependence on airspeed as the NATA model, and the transformed controller and Lyapunov matrices. A simple method
of solving the LMIs is shown such that the LPV H₂ upper bound is as close to optimal as possible, and using this method a new controller is synthesised. This new controller is compared against the LPV LQR controller with the scalar performance index, and is shown to be closer to optimal across the airspeed design range. Nonlinear simulations of the controlled NATA using this new controller are then presented. Based on Prime et al. (2008), a Linear Fractional Transformation (LFT) is applied to the NATA model to render the dynamics dependent upon the feedback of the linear value of airspeed. This allows the LMIs to be constructed at only two points, the extreme values of the linear design airspeed, rather than gridding over the parameter space as was performed above. An output-feedback controller that itself depends upon the feedback of a function that is linearly dependent upon airspeed is constructed using an induced L₂ loop-shaping framework. The induced L₂ performance objective is based upon a Glover-McFarlane H∞ loop-shaping process where the NATA singular values are shaped using pre- and post-filters, and minimising the induced L₂ norm from both the input and output to both the input and output. An LFT controller is synthesised, and simulations are performed showing the suppression of LCOs.Thesis (Ph.D.) -- University of Adelaide, School of Mechanical Engineering, 201
Trajectory design for a very-low-thrust lunar mission.
No full textThe University of Stuttgart is conducting a research program to build a succession of small satellites. The ultimate goal of this program is to build and launch a craft named Lunar Mission BW-1 (after the federal state that Stuttgart is situated in, Baden-Wurttemberg) into lunar orbit, for eventual impact with the Moon. As with the majority of space missions, launch cost is a severely limiting factor so it is necessary to carefully plan the trajectory before launch, to ensure lunar capture and minimise the amount of fuel needed by the spacecraft. This thesis outlines work conducted to find a robust fuel-optimal trajectory for Lunar Mission BW-1 to reach the Moon. Several unique aspects of this craft require a novel approach to that optimisation. Firstly, the spacecraft uses a new low-cost propulsion system, severely limiting manoeuvrability and accessibility of transfer trajectories. Secondly, to reduce the mass and complexity of moving parts, the solar panels are fixed to the body; consequently, the craft must rotate itself to point its solar panels towards the Sun to recharge. No thrusting can occur during this time. This magnifies the effect of the third design decision, which is to restrict the dry mass of the craft by giving it very little on-board power storage. After approximately an hour of accelerating it is expected to need to coast for several hours to recharge its batteries, resulting in a relatively high frequency stop-go-stop thrust profile. Due to these constraints, the trajectory optimisation is one of the most complex ever attempted. Since the craft will be built and launched, many simplifications made in purely theoretical studies could not be utilised, such as neglecting the weaker forces acting on the spacecraft in cis-lunar space. The very low thrust results in very long transfer times, during which even small magnitude forces acting on the spacecraft can significantly perturb its trajectory. However, including these forces creates non-linearities in the equations of motion associated with spacecraft trajectories, limiting the optimisation methods that could be used, and increasing computational complexity. Optimisation methods for low-thrust spacecraft trajectories have been the subject of much research, but most studies conclude that knowledge is still lacking in this area. Furthermore, many optimisation methods investigated in existing literature are incompatible with the intermittent thrust profile required by the Lunar Mission BW-1 thrusters. For this reason it was necessary to thoroughly review available optimisation methods and determine which may be adapted to this scenario. The resulting optimisation method was applied to the Lunar Mission BW-1 scenario to determine an efficient thrusting profile that will get the craft to the Moon. It was found that very few established optimisation algorithms can support the number of variables required for such a complex, long duration trajectory. The Sparse Optimal Control Software (SOCS) marketed by The Boeing Corporation was used via an interface developed at the University of Stuttgart called the the Graphical Environment for Simulation and Optimisation (GESOP). Due to unknown constraints such as launch date, the phases defined by the mission architecture were modelled and optimised independently. This approach allows mission planning flexibility while still providing reliable estimates for optimal fuel use, mission duration and power limitations. A trajectory is presented for each of the phases, ascending from the intial geosynchronous transfer orbit (GTO) to the eventual low lunar orbit (LLO). The resulting science phase is propagated forward in time to ensure orbital lifetime meets the mission requirements. Recommendations are subsequently made for the continuing development of the mission architecture. The primary outcome of this study is a procedure for developing an operational trajectory for Lunar Mission BW-1 after launch details are known. Given the current mission architecture and assumed launch details, the thermal arcjet requires 1205 hours (50.2 days) of operation while consuming 93 kg of ammonia propellant, and the pulsed plasma thrusters require 29177 hours (3.3 years) of operation while consuming 19 kg propellant. Power constraints were not found to be mission limiting for the current spacecraft configuration. Consequently, although the laboratory testing burden on the PPTs is already quite heavy, it is recommended that the mission architecture be adjusted to shorten arcjet phases and lengthen PPT phases. Furthermore, this project found that the optimisation package SOCS was the best commercially available option for low-thrust trajectory optimisation, but that it would benefit greatly by adaptation to a parallel shooting algorithm that may be distributed amongst multiple computer processors.Thesis (Ph.D.) -- University of Adelaide, School of Mechanical Engineering, 201
An investigation into the mechanics of sound propagation through turbulent non-isothermal exhaust jets in cross-flow
No full textOpen cycle gas turbines (OCGT) are increasingly being used for power generation as they are able to provide base loads in peaking or uncertain conditions and can respond quickly to power grid demands. However, it has been recorded in literature that OCGT (also known as single cycle gas turbines) have exhibited higher levels of low-frequency noise levels in communities near these plants than orthodoxy predicts. High levels of low-frequency noise within a community can cause the following effects: audible low frequency ‘rumble’, ‘beating’ sensations in the chest, nausea and acoustic excitation in structures with low resonant frequencies, such as glazing. It is hypothesised that the low-frequency noise affecting neighbouring communities is due to the sound emitted from the stack being refracted by hot turbulent exhaust gases and cross-winds. It is known that the shear layer, counter-rotating vortex pairs and horseshoe vortices that are generated from this type of flow scenario can affect the propagation of sound with effects such as refraction, diffraction and scattering. This thesis investigates the effects of this fluid flow mechanism on the propagation of low-frequency sound from a plane-wave through a heated mean flow. The investigation is undertaken numerically with a commercial computational fluid dynamics (CFD) package using ANSYS FLUENT, and with an acoustic finite element model (FEM) using ANSYS Mechanical. Subsequent investigations were completed experimentally using a reduced scale exhaust rig within a wind tunnel at the University of Adelaide. The numerical and experimental results have shown that downwind of the exhaust stack the plume causes an increase in the sound pressure level (SPL) of up to 11 dB when compared to acoustic radiation from an exhaust stack in a homogeneous medium. Furthermore, the experimental and numerical results have shown qualitative similarities, that the sound propagation path is drastically altered by the non-isothermal, turbulent exhaust jet that is deflected by a cooler cross-flow. The results of the research have shown that the measured sound radiation from exhaust stacks in cross-flow is highly dependent on the following: acoustic frequency, distance from the exhaust stack, temperature of the exhaust jet, thermal and velocity gradient in the plume, and the jet to cross-flow momentum flux ratio. The change in the sound propagation path, which impacts the downwind SPL, can also be significantly altered by changing the outlet of the exhaust stack. A prototype exhaust nozzle was developed to reduce SPLs downwind. An investigation of a straight duct section with walls that are acoustically transparent, but relatively impervious to flow, has been completed both numerically and experimentally. The investigation has shown that the SPL downwind of the stack is reduced in the presence of the nozzle. Two other prototype nozzles of different geometries have also been designed and experimentally tested. The two additional prototype nozzle geometries have walls that are both acoustically transparent and impervious to flow but are implemented to change the fluid dynamics of the plume. All three nozzles show a reduction in the acoustic refraction downwind of the exhaust stack. It is expected that the results of the research will be used to develop new noise control measures for the electrical power generation industry.Thesis (Ph.D.) -- University of Adelaide, School of Mechanical Engineering, 202
Application of micro perforated and impervious membranes for noise barriers
No full textMembrane materials have been commonly used for decades in buildings. When acoustic environments are concerned, the acoustic properties of these membrane structures are of special interest. This thesis aims to investigate acoustic properties of micro perforated membranes (MPMs) and impervious membranes and enhance the sound insulation of double layer impervious membranes by combining these with MPMs, thereby increasing the internal loss mechanisms of what is essentially a reactive wall. This thesis firstly develops a new model of an impervious membrane, taking into consideration the tension and the internal damping due to the membrane curvature. The sound absorption of MPMs inserted between the impervious layers has been studied by introducing a new boundary condition where the particle velocity at the hole wall boundary is assumed to be equal to the membrane vibration velocity. The comparison between the predicted and measured results demonstrates that MPM 1 to 3 can be considered impervious due to their sufficiently small perforation radii, and MPM 4 is sound absorbing due to its larger perforations. Non-linear sound absorption of MPM 4 has been observed in the experiments. It was found that the non-linear sound absorption coefficient is strongly dependent on both the magnitude of the SPLs and the waveform of the excitation. Two analytical models were developed for the non-linear acoustic impedance of MPMs. In the first model, the non-linear impedance of MPMs is considered to be the sum of the linear impedance, and the non-linear acoustic impedance dependent on the particle velocity within the perforations. The second analytical model presented is inspired by the air motion equation and the mass continuity equation considering the density variation in the time and spatial domains, and provides the most accurate predicted results among the models considered in this study. The analytical models have been developed to predict the STL of double layer impervious membranes separated by a finite-sized air cavity, taking into consideration the fluid-structure coupling on each membrane surface. Comparing the predicted results to the measured STLs, it is found that considering the sound absorbing boundaries of the cavity can enhance the accuracy of the models. STL measurements of double layer impervious membranes with four types of MPMs have been conducted in a diffuse field to quantify the effectiveness of the MPM insertion. The experimental results indicate that the MPM insertion can enhance the STL of the double layer impervious membranes significantly at frequencies above the first resonance frequency of the air cavity. MPMs 1 to 3 have similar main impacts on the STLs, however, MPM 4 has a different effect because of its larger perforations. The normal incidence and diffuse field models for the double layer impervious membranes with inserted MPMs 1 to 3 were developed and the predicted results were compared with the experimental results. The models with MPM 4 were developed by taking into consideration the acoustic impedance of the MPM 4 due to its perforations. These developed models can be used as tools for design of membrane structures.Thesis (Ph.D.) -- University of Adelaide, School of Mechanical Engineering, 2016
Modelling and design of magnetic levitation systems for vibration isolation.
No full textVibration disturbance has a consistent negative impact on equipment and processes. The central theme of this thesis is the investigation of using permanent magnets in the design of a system for vibration isolation. The thesis begins with a comprehensive literature review on the subjects of passive and active vibration isolation, permanent magnetic systems, and the common area between these on nonlinear vibration systems using magnetic forces. The use of cylindrical and cuboid magnets is the primary focus of this work for which analytical solutions are known for calculating forces and torques. Subsequently, the state of the art in analytical modelling of permanent magnet systems is covered, including a contribution in this area for calculating the forces between cylindrical magnets. A range of load bearing designs using simple permanent magnet arrangements are examined, with multiple designs suitable for a variety of objectives. A particular emphasis is placed on a system using inclined magnets, which can exhibit a load independent resonance frequency. Load bearing using multipole magnet arrays is also discussed, in which a large number of magnets are used to generate more complex magnetic fields. A variety of multipole arrays are compared against each other, including linear and planar magnetisation patterns, and an optimisation is performed on a linear array with some resulting guidelines for designing such systems for load bearing. Permanent magnet levitation requires either passive or active stabilisation; therefore, the design of electromagnetic actuators for active control is covered with a new efficient method for calculating the forces between a cylindrical magnet and a solenoid. The optimisation of a solenoid actuator is performed and geometric parameters are found which are near-optimal for a range of operating conditions. Two quasi–zero stiffness systems are introduced and analysed next. These systems are designed with a nonlinearity such that low stiffnesses are achieved while bearing large loads. The first system analysed is a purely mechanical device using linear springs; unlike most analyses of this design, the horizontal forces are also considered and it is shown that quasi–zero stiffness is capable in all translational directions simultaneously. However, a notable disadvantage of such spring systems is their difficulty in online tuning to adapt to changing operating conditions. A magnetic quasi–zero stiffness system is then analysed in detail and design criteria are introduced, providing a design framework for such systems and showing how the complex interaction of variables affects the resulting dynamic behaviour. Although the system is nonlinear, the effects of the nonlinearities on the vibration response are shown to be generally negligible. The thesis concludes with some experimental results of the same quasi–zero stiffness system, constructed as a single degree of freedom prototype. The quasistatic and dynamic behaviour of the system matches the theory well, and active vibration control is performed to improve the vibration isolation characteristics of the device.Thesis (Ph.D.) -- University of Adelaide, School of Mechanical Engineering, 201
