MS2.10 - Advances in Control of Structural Vibrations

Wind power is a sustainable and renewable energy source which becomes fundamental in order to meet the worldwide rising demand for energy. New onshore and offshore wind farms will be rapidly constructed in challenging environments, especially in earthquake-prone areas. Although a flexible structure, the wind turbine tower is slender and lightly damped which may exhibit a high susceptibility to earthquake-induced vibration. Whilst past studies have been based on traditional passive control devices, such as tuned mass dampers or tuned liquid column dampers, this study aims to propose a novel approach, the Reduced Column Section (RCS), to design an innovative transition piece to mitigate the vibrations on fixed wind turbines. The novel transition piece of hourglass shape will allow the control of the fundamental period of the wind turbine as well as it will limit the maximum stresses within the device by protecting the tower and the monopile. The proposed approach will be numerically tested on benchmark wind turbines proposed by the International Energy Agency. 3-Dimensional finite element models will be generated using the commercial ADINA software and verified through the 1-dimensional benchmark applications conducted through the OPENFAST code. Therefore, the novel transition piece will be implemented and tested under a combination of multi-hazards such as wind, wave and seismic loading. The benefits of the proposed device will be demonstrated in terms of the mitigation of tower wall and monopile stresses. Finally, practical formulas for modelling the novel device for simplified 1-dimensional models are provided

Due to its low prime costs and stable dynamic behavior, the tuned liquid column damper (TLCD) has been a reliable structural vibration control device since its first adoption for civil engineering structures. The TLCD can mitigate lateral vibrations in uniaxial direction. It consists of two vertical tubes, which are connected by a horizontal conduit and filled with a Newtonian liquid. Besides this prominent uniaxial version, a cross shaped TLCD was also originally proposed which has received less attention so far. This version is established by combining two uniaxial TLCDs in a crossed configuration to form a central joint, where horizontal conduits of both TLCDs are connected and allow liquid passage. Thanks to its layout, the cross shaped TLCD can control lateral vibrations in all transverse directions regardless of the excitation angle. The main reason for its neglected application has been the lack of an appropriate mathematical model. The authors derived the desired model after discovering a mathematical formulation, which couples the liquid deflection in each tube and reduces the damper to a single degree of freedom system. This new representation covers not only the uniaxial and cross shaped TLCDs, but also all omnidirectional TLCDs with star shaped layouts consisting of n>2 tubes. Based on this mathematical model, the authors introduced a semi active frequency tuning mechanism. For this purpose, a test bench, and a prototype of an omnidirectional TLCD were developed and experimental studies conducted. Best performance was observed by dividing the tubes in multiple cells which can be activated and deactivated by actuator driven valves. This approach allows us to modify the area of the tubes yielding a change in the natural frequency of the damper. To estimate the system states of the experimental setup based on the measured liquid deflections in the tubes and the vibration acceleration of the test bench, a moving horizon estimator was developed. Experiments were conducted on the test bench to assess the accuracy of the mathematical model and the semi active frequency tuning. The present contribution summarizes these theoretical and experimental development steps in a collective manner and reports the validation of the damper on a 28 m steel tower. In this recent study, the authors designed and built a second prototype based on the identified structural parameters. Initial tests were conducted on the prototype using the previously developed test bench to identify the damper properties before its installation on the real world structure. Subsequently, the damper and two linear actuators were mounted at the top of the structure, allowing user defined excitation of the structure, and thus simulating different vibration scenarios on the structure. An accelerometer was used to record the structural response in lateral directions. Both the omnidirectional control and semi active frequency tuning capabilities of the damper were investigated together with further state observer approaches. Compared to the case without damper, the measured response of the tower was mitigated independent of the orientation of the damper. Furthermore, using the semi active mechanism, the damper’s restoring force was further optimized.

There is a recognized need to tackle issues of vibration control making use of recent developments of data-driven modelling. In structural dynamics, data driven modelling has been extensively investigated, including with probabilistic approaches to quantify the uncertainty in dynamic systems. However, uncertainty within structural control systems is still an issue, which has become even more problematic, as interest in flexible structures has grown. It is desirable that structural control systems also implement some of these approaches, in order to improve control performance and gain more insight when an informative controller, such as model predictive control (MPC), is in the loop. This work addresses the difficulties imposed by the limitations of the actuator and its dynamics in the range of active vibration control. This paper proposes and examines a data-based Gaussian process NARX model of a proof mass actuator, in a flexible-structure framework, aiming to improve the control performance. This work requires incorporating the identified best nonlinear dynamic model of the actuator, based on data driven modelling, into the control strategy. The proposed method compromises a combination of GP-NARX and MPC to achieve desirable vibration performance. As part of MPC, restrictions on what control actions are permissible can be explicitly considered when determining the best control action. A simulated simply supported beam and nonlinear analysis of the proof mass actuator is shown as a case study for the proposed method. The efficacy of the devised methodology is firstly compared against the standard approximation of the dynamic of the proof mass actuator in a linear benchmark application, where the dynamic of the actuator is assumed linear. Then, the control method is tested on a more complex problem where the actuator dynamic model is assumed to be both uncertain and nonlinear. The outcome of this work encourages further investigation of the developed strategy, especially in real time experimental implementation.

Earthquakes, strong wind or human activity are examples of potential sources of vibrations that might cause damage to structural elements or human discomfort in slender structures. To control vibrations, the mitigation effect of well-known passive control systems such as Tuned Mass Dampers (TMDs) can be used. Nevertheless, the effectiveness of TMDs strongly depends on their mass. With this in mind, the development of innovative auxiliary devices and arrange-ments that can improve the performance of mass-dependent control systems has been a flourishing research area in civil and mechanical engineering in recent years. In this regard, the so-called inerter can be exploited to fictitiously increase the mass of the structure to which it is connected, allowing a mass amplification effect without any physical addition. The most common types are rack and pinion inerters, ball screw inerters, and fluid inerters. The latter are easier to design and develop and can be adapted to implement different damper arrangements, especially for larger civil structures with very low maintenance requirements. In addition, parasitic effects such as backlash and ratcheting are less pronounced than in the mechanical alternatives. For these reasons, the present study aims to identify the dynamic properties of a fluid inerter. To this end, a test campaign is conducted to reveal the actual behaviour of this device, focusing on the effects of its nonlinearities. The prototype consists of a hydraulic cylinder in which the inerter effect is achieved by the flow of the working fluid through an external channel. In the first step, the dry configuration, i.e., without fluid, is investigated, revealing nonlinearity due to the friction of the piston. While simplified models such as Coulomb friction without velocity-dependence of the friction force fail to reproduce the experimental data, a model based on the Stribeck effect successfully captures the dissipation force caused by friction. Subsequent experiments on the complete configuration, i.e., with fluid, reveal further nonlinear effects due to the compressibility of the working fluid and the air trapped in the external channel. An accurate mechanical model of the device is determined and its efficiency is demonstrated by means of comparative studies. The nonlinear layout provides a more realistic numerical prediction than a common linear model and can be implemented in several structural control device configurations. Finally, the experimentally generated inertance shows the large potential of fluid inerters when used for vibration control in civil structures.