SUMMARYTime delays are ubiquitous in control systems. They usually enter because of the sensors and actuators used in them. Traditionally, time delays have been thought to have a deleterious e ect on both the stability and the performance of controlled systems, and much research has been done in attempting to eliminate them, compensate for them, or nullify their presence. In this paper we take a di erent view. We investigate whether purposefully injected time delays can be used to improve both the system's stability and performance. Our analytical, numerical, and experimental investigation shows that this can indeed be done. Analytical results of the e ects of time delays on collocated and non-collocated control of classically damped and non-classically damped systems are given. Experimental and numerical results conÿrm the theoretical expectations. Issues of system uncertainties and robustness of time delayed control are addressed. The results are of practical value in improving the performance and stability of controllers because these characteristics (performance and stability) improve dramatically with the intentional injection of small time delays in the control system. The introduction of such time delays constitutes a 'minimal change' to a controller already installed in a structural system for active control. Hence, from a practical standpoint, time delays can be implemented in a nearly costless and highly reliable manner to improve control performance and stability, an aspect that cannot be ignored when dealing with the economics and safety of large structural systems subjected to strong earthquake ground shaking.
Fault detection is desirable for increasing machinery availability, reducing consequential damage, and improving operational efficiency. Many of these faulty situations in three-phase induction motors originate from an electrical source. Vibration signal analysis is found to be sensitive to electrical faults. However, conventional methods require detailed information on motor design characteristics and cannot be applied effectively to vibration diagnosis because of their nonadaptability and the random nature of the vibration signals. This paper presents the development of an online electrical fault detection system that uses neural network modeling of induction motor in vibration spectra. The short-time Fourier transform is used to process the quasi-steady vibration signals for continuous spectra so that the neural network model can be trained. The electrical faults are detected from changes in the expectation of modeling errors. Experimental observations show that a robust and automatic electrical fault detection system is produced whose effectiveness is demonstrated while minimizing the triggering of false alarms due to power supply imbalance.
Non-classically damped structural systems do not easily lend themselves to the modal superposition method because these systems yield coupled second-order differential equations. In this paper, a variety of new computationally efficient iterative methods for determining the response of such systems are developed. The iterative approaches presented here differ from those presented earlier in that they are computationally superior and/or are applicable to the determination of the responses of broader classes of structural systems. Numerical examples, which are designed to evaluate the efficacy of these schemes, show the vastly improved rates of convergence when compared to earlier iterative schemes. INTRODUCTIONRecently, there has been a strong revival of interest in determining the response of non-classically damped structural systems. These systems are modelled by the following linear second-order differential equations of motion:where the constant N x N matrices M , K and C are the mass, the stiffness and the damping matrices, respectively. The vectors x(t) and a(t) are N x 1 vectors of displacement and force, respectively. For most of the physical systems arising in the area of structural dynamics, the mass matrix M is real, symmetric and positive-definite, and the stiffness matrix K is real, symmetric and positive-semidefinite. Under these circumstances, we can find a transformation matrix T which simultaneously diagonalizes M and K; for this transformation to diagonalize C also, the matrix C has to be of a special form.'.* In the literature this kind of damping is referred to as classical damping or proportional damping. The response of classically damped systems is obtained by the modal superposition method.Yet in practice, proportional damping is usually a rare occurrence rather than a common one. This is because most large-scale, real-life, dynamic systems, are comprised of different subcomponents. Even if we were to ascribe a viscous damping character to each of these subcomponents, the final damping matrix C, constructed through, say, a finite element model for the whole system, would generally be of the nonproportional type. This would of course be more so true when these subcomponents themselves are comprised of widely differing materials, as is found, for example, in the area of soil-structure interaction, and in the area of aerospace structures (which are usually optimized for their weight).In this paper, we assume that C is a real general matrix. When the matrices M , K and C cannot be simultaneously diagonalized by a suitable matrix transformation, one is left with the following coupled set of second-order linear differential equations where h(t) = T'a(t). The matrix T has columns which are the eigenvectors of the undamped system; the Professor. *Graduate student.
This paper presents some of the results of an experimental and analytical study of a controllable electrorheological device configured to induce an adjustable amount of dynamic shear force in response to an applied voltage. Maps of the force-deformation characteristics of the aluminosilicate based ER material are developed over a relatively wide frequency range, and approximating analytical expressions are obtained for the force-defonation-frequency-voltage characteristics of the material. Subsequently, an evaluation is made of the efficiency of using on line control of an electrorheological actuator to emulate the operation of an optimally tuned auxiliary mass damper attached to a primary system subjected to arbitrary dynamic environments. It is shown through numerical simulation studies that the proposed parameter control algorithm provides an efficient means for the on line control of the primary system under a wide range of excitations. An experimental study is presented in which an ER device i s used, in conjunction with a small laboratory building model, as a semiactive element in an on line structural control approach using pulse control techniques.
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