1,721,316 research outputs found
Active control of parametrically excited systems
No full textThis article discusses active control of parametrically excited systems. Parametric resonance is observed in a wide range of applications and can lead to high levels of unwanted motion. For example, in cable-stayed bridges, the vibration of the deck excites the cables axially, inducing a periodically time-varying tension. If the frequency of deck vibration is about twice the natural frequency of the cable, a parametric resonance occurs and leads to a large-amplitude swinging motion of the cable. To tackle the consequences of parametric instability, active vibration control employing a piezoelectric actuator is proposed in this article. We consider a beam subjected to an axial harmonic load that represents a parametrically excited system with a periodically varying stiffness. Using both analytical and experimental methods, we assess the stability of the beam and propose active control aimed at relocating the transition curves and hence stabilising the system via velocity feedback and pole placement. Analytical relationship between the transition curves and the poles of the system is derived. Transition curves can be assigned to a prescribed location using appropriate velocity and displacement control gains. Finally, we demonstrate the proposed approach with experiments on a beam equipped with a macro fibre composite patch
Nonlinear vibrations of a stroke-saturated inertial actuator
No full textProof-mass actuators are typically used to supply an external control force to a structure,for the purpose of vibration suppression. These devices comprise a proof-mass suspended in a magnetic field that is accelerated in order to provide a reaction force on the actuator casing and the structure itself. If the actuator stroke length is reached or exceeded, the proof-mass will hit the end stops, resulting in a nonlinear phenomenon known as stroke saturation. In this paper, a theoretical and experimental investigation into the actuator’s dynamical behaviour is undertaken. First, the blocked inertial force of the actuator in response to an input voltage was measured experimentally using a variety of excitation amplitudes and frequencies. An analysis was conducted in the time- and frequency-domains, and the first-order force-voltage FRF of the actuator was ascertained for each excitation amplitude. The information provided by the analysis was then used to estimate the parameters for a linear piecewise stiffness model of the actuator, in order to simulate the time-domain response. Finally, a comparison of the simulated and measured signals is conducted to establish the accuracy of the model
Gust load alleviation using nonlinear feedforward control
No full textA strategy based on feedforward control for gust loads alleviation of a nonlinear aeroelastic model is considered. The model is representative of a typical wing section with a trailing-edge flap and with a polynomial nonlinearity in the structural model. The aerodynamics is given by thin aerofoil theory. First, the effects of structural nonlinearity in the dynamic response of the open-loop system are evaluated. Then, it is shown that the performance of the controller is greatly affected by the approximations made in the internal model of the controlled plant. To suppress gust-induced vibrations of an intrinsically nonlinear plant, the control performance is degraded when using a linear representation for the internal model. The control strategy performs well when including all nonlinearities in the mode
Periodic and chaotic response of a macro-scale tuning fork gyroscope
No full textIn this paper, the dynamic behaviour of a macro-scale tuning fork gyroscope is presented. The gyroscope consists of two inverted pendulums on a suspension mass. The suspension mass is subjected to force excitation generated by an electromagnetic shaker. The dynamics of the shaker are included in the analysis. It is shown that the system is a parametrically excited system. Parametric excitation can lead to vibration in the horizontal motion of the suspension mass, when the two pendulums are in phase. The problem is particularly interesting for energy harvesting. Due to the interaction between the system’s degrees of freedom, the energy is transferred from vertical (base excitation) to horizontal direction. Initial parametric studies are carried out to analyse the dynamic behaviour of the system by varying the initial conditions, base excitation frequency and amplitude. It is demonstrated that under certain parameters the system can exhibit complex dynamic behaviour such as chaotic motion
Adaptive control of tonal disturbance in mechanical systems with nonlinear damping
No full textThis paper describes an adaptive system to control the tonal vibration of a single-degree-of-freedom system with nonlinear damping. The adaptive control system consists of a force actuator in parallel with the suspension, which includes the nonlinear damper, and a velocity sensor mounted on the mass. The adaptation of the controller is done once every period of the excitation. Since the response of the nonlinear system changes with excitation level, conventional adaptive algorithms, with a linear model of the plant, can be slow to converge and may not achieve the desired performance. An on-line observer is used to obtain a describing function model of the plant, which can vary with the excitation level. This allows the adaptive control algorithm to converge more quickly than using a fixed plant model, although care has to be taken to ensure that the dynamics of the observer do not interfere with the dynamics of the adaptive controller
Nonlinear damping and quasi-linear modelling
No full textThe mechanism of energy dissipation in mechanical systems is often nonlinear. Even though there may be other forms of nonlinearity in the dynamics, nonlinear damping is the dominant source of nonlinearity in a number of practical systems. The analysis of such systems is simplified by the fact that they show no jump or bifurcation behaviour, and indeed can often be well represented by an equivalent linear system, whose damping parameters depend on the form and amplitude of the excitation, in a “quasi-linear” model. The diverse sources of nonlinear damping are first reviewed in this paper, before some example systems are analysed, initially for sinusoidal and then for random excitation. For simplicity, it is assumed that the system is stable and that the nonlinear damping force depends on the n-th power of the velocity. For sinusoidal excitation, it is shown that the response is often also almost sinusoidal, and methods for calculating the amplitude are described based on the harmonic balance method, which is closely related to the describing function method used in control engineering. For random excitation, several methods of analysis are shown to be equivalent. In general, iterative methods need to be used to calculate the equivalent linear damper, since its value depends on the system’s response, which itself depends on the value of the equivalent linear damper. The power dissipation of the equivalent linear damper, for both sinusoidal and random cases, matches that dissipated of the nonlinear damper, proving both a firm theoretical basis for this modelling approach and clear physical insight. Finally, practical examples of nonlinear damping are discussed; in microspeakers, vibration isolation, energy harvesting, and the mechanical response of the cochlea
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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