Static, modal and dynamic behaviour of a stress ribbon footbridge : experimental and computational results

Congreso celebrado en la Escuela de Arquitectura de la Universidad de Sevilla desde el 24 hasta el 26 de junio de 2015.Response for the static, modal and dynamic problem corresponding to a stress ribbon footbridge is studied. The equilibrium equations describing the problem are coupled nonlinear differential equations which are numerically solved using the finite element method. The objective of this work is to present a proper computational model for such a structure and to check its applicability in predicting not only the static behaviour but also modal parameters and estimate its dynamic response. As the footbridge is continuously monitored, it has been possible to measure the sag and to identify natural modes. This experimental data has been used for updating the finite element model

Piezo-electromechanical smart materials with distributed arrays of piezoelectric transducers: Current and upcoming applications

This review paper intends to gather and organize a series of works which discuss the possibility of exploiting the mechanical properties of distributed arrays of piezoelectric transducers. The concept can be described as follows: on every structural member one can uniformly distribute an array of piezoelectric transducers whose electric terminals are to be connected to a suitably optimized electric waveguide. If the aim of such a modification is identified to be the suppression of mechanical vibrations then the optimal electric waveguide is identified to be the 'electric analog' of the considered structural member. The obtained electromechanical systems were called PEM (PiezoElectroMechanical) structures. The authors especially focus on the role played by Lagrange methods in the design of these analog circuits and in the study of PEM structures and we suggest some possible research developments in the conception of new devices, in their study and in their technological application. Other potential uses of PEMs, such as Structural Health Monitoring and Energy Harvesting, are described as well. PEM structures can be regarded as a particular kind of smart materials, i.e. materials especially designed and engineered to show a specific andwell-defined response to external excitations: for this reason, the authors try to find connection between PEM beams and plates and some micromorphic materials whose properties as carriers of waves have been studied recently. Finally, this paper aims to establish some links among some concepts which are used in different cultural groups, as smart structure, metamaterial and functional structural modifications, showing how appropriate would be to avoid the use of different names for similar concepts. © 2015 - IOS Press and the authors

Adaptive control of large space structures using recursive lattice filters

The use of recursive lattice filters for identification and adaptive control of large space structures is studied. Lattice filters were used to identify the structural dynamics model of the flexible structures. This identification model is then used for adaptive control. Before the identified model and control laws are integrated, the identified model is passed through a series of validation procedures and only when the model passes these validation procedures is control engaged. This type of validation scheme prevents instability when the overall loop is closed. Another important area of research, namely that of robust controller synthesis, was investigated using frequency domain multivariable controller synthesis methods. The method uses the Linear Quadratic Guassian/Loop Transfer Recovery (LQG/LTR) approach to ensure stability against unmodeled higher frequency modes and achieves the desired performance

Piezoelectric and Magnetoelastic Strain in the Transduction and Frequency Control of Nanomechanical Resonators

Stress and strain play a central role in semiconductors, and are strongly manifested at the nanometer-scale regime. Piezoelectricity and magnetostriction produce internal strains that are anisotropic and addressable via a remote electric or magnetic field. These properties could greatly benefit the nascent field of nanoelectromechanical systems (NEMS), which promises to impact a variety of sensor and actuator applications. The piezoelectric semiconductor GaAs is used as a platform for probing novel implementations of resonant nanomechanical actuation and frequency control. GaAs/AlGaAs heterostructures can be grown epitaxially, are easily amenable to suspended nanostructure fabrication, have a modest piezoelectric coefficient roughly twice that of quartz, and if appropriately doped with manganese, can form dilute magnetic compounds. In ordinary piezoelectric transducers there is a clear distinction between the metal electrodes and piezoelectric insulator. But this distinction is blurred in semiconductors. An integrated piezoelectric actuation mechanism is demonstrated in a series of suspended anisotype GaAs junctions, notably pin diodes. A dc bias was found to alter the resonance amplitude and frequency in such devices. The results are in good agreement with a model of strain based actuation encompassing the diode’s voltage-dependent carrier depletion width and impedance. A bandstructure engineering approach is employed to control the actuation efficiency by appropriately designing the doping level and thickness of the GaAs structure. Actuation and frequency are also sensitively dependent on the device’s crystallographic orientation. This combined tuning behavior represents a novel type of depletion-mediated electromechanical coupling in piezoelectric semiconductor nanostructures. All devices are actuated piezoelectrically, whereas three techniques are demonstrated for sensing: optical interferometry, piezoresistance and piezoelectricity. Finally, a nanoelectromechanical GaMnAs resonator is used to obtain the first measurement of magnetostriction in a dilute magnetic semiconductor. Resonance frequency shifts induced by field-dependent magnetoelastic stress are used to simultaneously map the magnetostriction and magnetic anisotropy constants over a wide range of temperatures. Owing to the central role of carriers in controlling ferromagnetic interactions in this material, the results appear to provide insight into a unique form of magnetoelastic behavior mediated by holes

Design and test of an adjustable quasi-zero stiffness device and its use to suspend masses on a multi-modal structure

In some applications, such as ground vibration testing in the aerospace industry, it is of interest to observe the modal behaviour of a slender structure while it is statically loaded. One way of statically loading such a structure is to suspend masses using very soft springs. If the springs are linear, then this results in an extremely large static deflection of the springs. This problem could be overcome by dynamically isolating the masses using quasi-zero stiffness (QZS) springs. This paper describes the design, construction and experimental testing of a device that can exhibit QZS. A novel design is proposed that allows the stiffness and the symmetry of the device to be adjusted independently using separate adjustment mechanisms. Quasi-static and dynamic testing of the device show that it can be adjusted to have an extremely low stiffness within the limits of measurement. The main trend of the force-displacement curve shows that it has a cubic stiffness characteristic, and that friction is responsible for its hysteretic behaviour. Dynamic testing shows that the device locks-up due to friction at low frequencies, but at high frequencies the device acts as an efficient linear isolator. An experiment was also performed where a mass was suspended on a multi-modal beam structure via the QZS device. It was shown that a static load could be applied to the beam without the attached mass appreciably affecting the dynamic response of the beam, even though the suspended mass was about 12% of that of the host structure

Attitude dynamics and shape control of reflectivity modulated gossamer spacecraft

The utilisation of space provides many opportunities to deliver pioneering innovations during the 21st century. One of these opportunities is the gossamer spacecraft, an emerging technology to achieve very low mass, large area and low stowage volume. Examples include large ultra-lightweight membrane reflectors and distributed tethered formations. Gossamer spacecraft offer the potential to deliver innovative new science and applications missions to aid our growing globalised societies: high-performing communications antennae, scientific telescopes and space-based solar power collectors. However, the ability to control such large structures in space is essential for their successful operation. To this aim, this thesis investigates a novel means to control large gossamer spacecraft by exploiting modulated solar radiation pressure (SRP), thus by modifying the nominal light pressure acting on the structure in space. Various concepts have been proposed in the past to control the attitude of a gossamer spacecraft, employing complex mechanical systems or thrusters. Furthermore, methods to control the surface shape of a large membrane reflector using, for example, piezoelectric actuators, are being developed. Since on-board control systems need to be high-performance, reliable and importantly lightweight, this thesis investigates the use of thin-film reflectivity control devices across the spacecraft surface. Controlling the reflectivity modulates the Sun's light pressure acting on a thin membrane thus controlling its shape. In addition, body forces and torques become available to control the attitude of such a large structure 'optically', without using traditional mechanical systems. The concept is demonstrated first by controlling a two-mass tethered formation in a Sun-centred orbit, showing that the spacecraft attitude can be stabilised around new equilibria created by controlling the surface reflectivity of the masses. Subsequently, the concept is applied to control the attitude of a large membrane reflector, which confirms the viability of reflectivity modulation by generating variable optical torques in the membrane plane. In particular, the nominal SRP forces are modified by introducing different surface reflectivity distributions across the membrane. It is shown that through these optical torques, the reflector can be steered, for example, to a Sunpointing attitude from an arbitrary initial displacement. The analysis also considers the variation of the SRP force magnitude with changing light incidence angle towards the Sun during the manoeuvre, thereby presenting solutions to a challenging attitude control problem. Furthermore, by adopting a highly-integrated multi-functional design approach, the concept of reflectivity modulation is also employed to control the surface shape of a large membrane reflector. First, the nominal (non-parabolic) deflection shapes due to uniform SRP across the surface are presented. Subsequently, a closed-form solution for the reflectivity function across the membrane required to create a true parabolic deflection shape is derived. In order to improve the quite large focal lengths of the deflected shapes that can be generated for a tensioned membrane, shape control of a slack suspended surface is also considered. The achievable (shorter) focal lengths support the feasibility of exploiting modulated SRP for controlled surface deflection. In summary, this thesis demonstrates the potential of using surface reflectivity modulation to control the attitude and morphology of large gossamer spacecraft without using complex mechanical systems or thrusters. Therefore, the concept of optical control represents a major step towards highly-integrated adaptive gossamer structures and supports the development of this promising key-technology to deliver advanced space applications.The utilisation of space provides many opportunities to deliver pioneering innovations during the 21st century. One of these opportunities is the gossamer spacecraft, an emerging technology to achieve very low mass, large area and low stowage volume. Examples include large ultra-lightweight membrane reflectors and distributed tethered formations. Gossamer spacecraft offer the potential to deliver innovative new science and applications missions to aid our growing globalised societies: high-performing communications antennae, scientific telescopes and space-based solar power collectors. However, the ability to control such large structures in space is essential for their successful operation. To this aim, this thesis investigates a novel means to control large gossamer spacecraft by exploiting modulated solar radiation pressure (SRP), thus by modifying the nominal light pressure acting on the structure in space. Various concepts have been proposed in the past to control the attitude of a gossamer spacecraft, employing complex mechanical systems or thrusters. Furthermore, methods to control the surface shape of a large membrane reflector using, for example, piezoelectric actuators, are being developed. Since on-board control systems need to be high-performance, reliable and importantly lightweight, this thesis investigates the use of thin-film reflectivity control devices across the spacecraft surface. Controlling the reflectivity modulates the Sun's light pressure acting on a thin membrane thus controlling its shape. In addition, body forces and torques become available to control the attitude of such a large structure 'optically', without using traditional mechanical systems. The concept is demonstrated first by controlling a two-mass tethered formation in a Sun-centred orbit, showing that the spacecraft attitude can be stabilised around new equilibria created by controlling the surface reflectivity of the masses. Subsequently, the concept is applied to control the attitude of a large membrane reflector, which confirms the viability of reflectivity modulation by generating variable optical torques in the membrane plane. In particular, the nominal SRP forces are modified by introducing different surface reflectivity distributions across the membrane. It is shown that through these optical torques, the reflector can be steered, for example, to a Sunpointing attitude from an arbitrary initial displacement. The analysis also considers the variation of the SRP force magnitude with changing light incidence angle towards the Sun during the manoeuvre, thereby presenting solutions to a challenging attitude control problem. Furthermore, by adopting a highly-integrated multi-functional design approach, the concept of reflectivity modulation is also employed to control the surface shape of a large membrane reflector. First, the nominal (non-parabolic) deflection shapes due to uniform SRP across the surface are presented. Subsequently, a closed-form solution for the reflectivity function across the membrane required to create a true parabolic deflection shape is derived. In order to improve the quite large focal lengths of the deflected shapes that can be generated for a tensioned membrane, shape control of a slack suspended surface is also considered. The achievable (shorter) focal lengths support the feasibility of exploiting modulated SRP for controlled surface deflection. In summary, this thesis demonstrates the potential of using surface reflectivity modulation to control the attitude and morphology of large gossamer spacecraft without using complex mechanical systems or thrusters. Therefore, the concept of optical control represents a major step towards highly-integrated adaptive gossamer structures and supports the development of this promising key-technology to deliver advanced space applications

Selectively Tuning a Buckled Si/SiO\u3csub\u3e2\u3c/sub\u3e Membrane MEMS through Joule Heating Actuation and Mechanical Restriction

This research followed previous work and attempted to modify the spring in two ways. First, a Ti/Au meander resistor was deposited atop the membrane in an effort to actuate the membrane and change the spring constant. Secondly, a series of overhanging cantilevers were attached to the bulk substrate surrounding the membrane in an effort to constrain the membrane buckling deflection to the negative stiffness region. Membrane buckling was investigated through Finite Element analysis (FEA) and analytical equations. Deflections were measured using an interferometric microscope (IFM) and force/deflection measurements were captured using a unique measurement scheme. The results concluded that by introducing a thermal stress, the membrane could be actuated with a corresponding 3x increase in spring constant. Additionally, the overhanging beams restricted the membrane deflection by up to 30%, but, because of a lack in beam stiffness, failed to restrict the membrane to the negative stiffness region. This research laid the ground work for future work in this area

Proceedings of the Belgian-Dutch IABSE Young Engineers Colloquium 2019:YEC2019

The proceedings contain 35 papers. The topics discussed include: fatigue monitoring of railway bridges by means of virtual sensing; steel-supported glazed atrium roof between two adjacent existing buildings; the Boekelose bridge: an innovative structure; case study of rail-bridge interaction of a large span railway viaduct in riga; probabilistic approach to evaluate fatigue safety status in steel railway bridges; buckling design approach for unstiffened curved plates in uniform shear; finite element modeling of residual welding stresses in an orthotropic steel bridge component; uniformly loaded tensegrity bridge design via morphological indicators method; tensile and shear resistance of bolted connectors in steel-FRP hybrid beams; and parametric analysis of rib distortion induced stress concentration at rib-to-crossbeam joint.</p