Carmen Ramiro

Se ha observado un notable y creciente interés, tanto en el mundo académico como en el empresarial, en el uso de materiales piezoeléctricos en estructuras para la cancelación de las vibraciones mecánicas que se produzcan en ellas cuando se encuentran en servicio. Si bien se encuentra documentación abundante en la literatura para el uso de estos dispositivos, la mayor parte de los estudios encontrados se centran en la utilización de dispositivos piezoeléctricos, bien de forma pasiva o semiactiva a través de circuitos de derivación (shunts) con diferentes parámetros. La excitación activa del piezoeléctrico se contempla en un número limitado de casos, y es todavía más escasa la consideración de arquitecturas de control avanzado como el control predictivo basado en modelo (CPBM, o más frecuentemente MPC, del inglés Model Predictive Control). El control mediante MPC es de indiscutible interés en la industria y la academia y es una línea de investigación que ha permanecido continuamente activo desde sus primeros avances en los años 70 y 80, debido a que permite calcular las acciones de control óptimas a lo largo de un horizonte futuro de control para llevar el sistema controlado a la referencia deseada, ofreciendo gran flexibilidad en sus propiedades. En este trabajo se presenta el análisis de la respuesta de un controlador predictivo simple en una viga en voladizo excitada mediante aceleraciones en su base. La actuación se realiza por medio de la incorporación de un parche piezoeléctrico fijado cerca de la base, y el objetivo es atenuar la vibración en el extremo libre. La medida de efectividad en la cancelación de vibraciones se cuantifica a través de la reducción en el cociente de las PSD (Power Spectral Density) de la aceleración en el extremo libre frente a la de la base del sistema controlado en comparación con el sistema sin controlar. El enfoque de control predictivo utilizado resuelve el sistema para todo el horizonte de control en cada instante de tiempo, lo que resulta costoso de procesar e imposible de ejecutar en tiempo real, lo que motiva que en este trabajo se presenten únicamente resultados de simulación y se propongan otras alternativas.

En las últimas décadas, se ha observado un notable aumento en el volumen de publicaciones académicas que utilizan actuadores piezoeléctricos en estructuras con el propósito de atenuar vibraciones. En general, se utilizan de forma pasiva o semiactiva mediante la conexión a circuitos de derivación o shunt; aunque también pueden utilizarse de forma activa aplicando directamente tensiones en el actuador piezoeléctrico. En este trabajo se presenta un caso de estudio simple, consistente en la aplicación de un control de tipo LQR/LQG en una estructura consistente en una viga en voladizo a la que se ha fijado un parche piezoeléctrico muy cercano a la raíz del empotramiento. Esta viga es perturbada a través de su base (con aceleración de magnitud acotada pero desconocida) y la misión de la actuación del piezoeléctrico es atenuar la vibración del extremo libre de la viga en voladizo. Como es habitual en este tipo de estudios, la capacidad del control para cancelación de vibraciones se mide mediante el cociente de las PSD del extremo libre frente a la excitación de la base. En el trabajo se comprueba que el desempeño del control LQR/LQG es razonable para esta aplicación.

Rolling stock manufacturers face the challenge of manufacturing lightweight high-speed trains without deteriorating comfort. One of the difficulties is to mantain or increase structural stiffness and damping as the car bodies become lighter. Leaving aside active solutions, which are expensive and generally complex to implement, increasing structural damping by means of viscoelastic patches (via Constrained Layer Damping) seems to be a viable solution which is in fact already used for acoustic insulation in automotive, aerospace and even railway applications. Although there are works in the literature that try to optimise viscoelastic panels, this work presents an experimental study with two essential contributions: (i) to analyse the influence of a broad set of design parameters such as type of the constraining layer (uniform or honeycomb), thickness of the viscoelastic layer, location, covered area and continuity between patches; and (ii) to consider absolute and specific (per unit mass) damping depending on the design scenario. To locally increase the structural damping of an existing lightweight structure without compromising its weight, partial application of thin viscoelastic and constraining layers turned out to be the best solution. To enhance structural damping from the design stages, disregarding constraining layer mass by incorporating its stiffness into the overall stiffness of the structure, full coverage with thick viscoelastic layer and a honeycomb constraining layer with a high cross-section moment of inertia turned out to be the best option, reaching modal damping ratios up to 22 times higher than structures without viscoelastic materials.

This paper presents a new formulation for analyzing a beam on elastic supports traveling on irregular profiles. The model is a first approximation of a passenger railway vehicle car body. The main difference with previous works is the use of a complex modulus to represent structural damping rather than relying on equivalent viscous terms. The formulation groups rigid body modes with flexible modes and proposes a matrix form that is easy to interpret and solve in the frequency domain. Comfort indexes are readily obtained from weighted response spectral densities. The model is used to assess the influence of structural damping and stiffness on comfort. It will be shown that the evolution of comfort with stiffness is non-monotonic and, therefore, comfort does not always improve as stiffness increases.

It has been recently pointed out that railway vehicle passengers do not behave as inert masses but as dissipative elements: they increase modal damping without significantly affecting the natural frequencies of the carbodies. The phenomenon is more prominent in current lightweight high-speed railway vehicles, because weight reduction is usually attained at the expense of reduced stiffness. The few works that deal with the influence of passengers in actual structures are confronted with a large number of coupled structural vibration modes with cumbersome mode shapes, and this hinders the understanding of the physical mechanisms that make passengers attenuate vibrations. This work sheds some light on this topic from three points of view: new experimental tests on an actual carbody to corroborate and broaden existing results; simplified experimental tests, with small living beings (toddlers and pets) on a simplified pinned-pinned beam without modal coupling, to better determine the physical phenomena involved in the interaction between the passenger and the flexible structure; and improved analytical beam-cushion-passenger models to clarify the experimental results. All results suggest that passengers do indeed behave as classical dynamic vibration absorbers the positions of which with respect to the anti-nodes of mode shapes is key in vibration attenuation.

There are many engineering design problems that call for rubber components as the best solution. Vulcanized rubber has found its way into all sorts of devices, from the universal automobile pneumatic tire to the ubiquitous compliant bushing. Some high-speed rail vehicle suspensions make use of rubber, not only in the air spring itself, but also in the auxiliary spring. The mechanical characteristics of this component influence vehicle dynamics and, therefore, accurate spring models with which to conduct dynamic analysis would make for powerful design tools. Nevertheless, the mechanical behavior of rubber defies simple modeling on account of stress relaxation, creep, set, viscosity, internal friction, and nonlinear stress–strain relations. Despite the advances in the micromechanical understanding of these phenomena, as well as in the macroscopic modeling of rubber spring behavior, there is ample room for refinement, and this is precisely the goal of this paper. The mechanical behavior of a particular rubber spring for high speed rail vehicles has been characterized. The results reveal the necessary components of the model, and suggest the appropriate procedure for parameter extraction. Our model proposal consists of three elements in parallel: a nonlinear elastic spring; a “soft friction” element; and a Maxwell viscous component. The characterization procedure takes into account both stress relaxation and nonlinear elasticity. The proposed model accurately reproduces experimental results and may then be used with confidence in any type of numerical simulation. Nevertheless, for this statement to be true, the problem of “numerical softening” potentially induced by soft friction models should be resolved. The paper will show that a trailing moving average filter, seamlessly tied to the model, wipes out the softening effect.

The ubiquity of pneumatic vibration isolators, especially in heavy road vehicle suspensions and in passenger rail vehicle secondary suspensions, has boosted research interest in these systems. Thermodynamic models have been used in order to inform and support engineering design decisions. Most models assume that the air inside the system undergoes a polytropic transformation during the compression/expansion cycle, but the necessary polytropic index value is often guessed at and seldom measured. This paper presents measured polytropic indexes that contradict the traditional assumptions with respect to its value. On a different note, the work presented in this paper also investigates the air flow through the restrictor (a component that is often used in suspension applications to introduce damping and adapt the frequency response function). The results show that this flow can be analyzed with utterly simple models.