Staff

Abstract:
This paper proposes and tests a semi-active method for mitigation of random and harmonic forced vibrations of frame structures. The method is based on the Prestress Accumulation-Release (PAR) strategy, and it stimulates the transfer of vibration energy from low-order into high-order natural modes of vibration. Due to their high-frequency, the target high-order modes are efficiently mitigated by standard material damping mechanisms. The control is based on local reconfiguration of nodal ability to transfer moments between adjacent beams, which might be momentarily suppressed for selected nodes: performed at the maximum of the local bending strain, such a suppression stimulates a sudden release of the accumulated strain energy into high-frequency local and global vibrations. The effectiveness of the approach is confirmed numerically and experimentally in mitigation of low-frequency vibrations, including resonance conditions, of a slender planar frame structure subjected to harmonic, sweep and random forced excitations.

Keywords:
damping of vibrations, smart structures, semi-active control, decentralized control, truss–frame nodes

Abstract:
In this study, a novel modal control strategy by means of semi-actively lockable joints is proposed. The control strategy allows for a directed flow of energy between vibrational modes, which makes it suitable not only for vibration attenuation purposes but also for energy scavenging driven by electromechanical energy harvesters. The proposed control strategy is an extension of the prestress-accumulation release (PAR) technique; however, it introduces also new concepts that increase the efficiency of the overall control system. Contrary to the PAR, the proposed method requires measurement of both strains in the vicinity of the semi-active joints and translational velocities that provide global information about system behavior. The latter aspect requires the control system to be organized within a hierarchical feedback architecture. The benefit from this higher complexity of the control system is its better performance compared to the PAR. The proposed semi-active modal control not only attenuates structural vibration faster, but it also achieves this goal with a smaller number of switches implemented in the joints. The effectiveness of the proposed methodology has been demonstrated on structures equipped with two lockable joints. Two practical examples have been investigated: one employs the concept of vibration-based energy harvesting for a two-story frame structure, while the second one reduces vibration of an eight-story frame structure subjected to kinematic excitation.

Keywords:
energy harvesting, lockable joint, modal coupling, semi-active control, vibration attenuation

Abstract:
This paper presents an experimental approach to assessment of semi-active vibration control systems based on the Prestress-Accumulation Release concept. The objectives are threefold: 1) to introduce an experimental validation method for control algorithms based on switchable transfer of moments, 2) to propose a method to assess experimentally the control effects on structural dynamic response under several types of excitation, and 3) to propose an approach for adequate sensor placement. A laboratory frame demonstrator equipped with dedicated semi-active nodes is used. The proposed approach is based on spectral responses and modal analysis. According to the presented findings, the investigated control is effective in reducing the vibration level while keeping the structural dynamic stiffness at a proper level. The investigation is conducted in the case of free response, as well as responses to impact loading and random excitation. The results confirm the accuracy of the adopted algorithm parameters and reveal the sensor locations that provide the best control effectiveness.

Abstract:
This paper proposes an on/off semi-active control approach for mitigation of free structural vibrations, designed for application in 2D smart frame structures. The approach is rooted in the Prestress-Accumulation Release (PAR) control strategies. The feedback signal is the global strain energy of the structure, or its approximation in the experimental setup. The actuators take the form of on/off nodes with a controllable ability to transfer moments (blockable hinges). Effectiveness of the approach is confirmed in a numerical simulation, as well as using a laboratory experimental test stand.

Keywords:
structural reconfiguration, structural control, semi-active control, frame structures, controllable nodes‎

Abstract:
This paper proposes a quantitative criterion for optimization of actuator placement for the Prestress–Accumulation Release (PAR) strategy of mitigation of vibrations. The PAR strategy is a recently developed semi-active control approach that relies on controlled redistribution of vibration energy into high-order modes, which are high-frequency and thus effectively dissipated by means of the natural mechanisms of material damping. The energy transfer is achieved by a controlled temporary removal of selected structural constraints. This paper considers a short-time decoupling of rotational degrees of freedom in a frame node so that the bending moments temporarily cease to be transferred between the involved beams. We propose and test a quantitative criterion for placement of such actuators. The criterion is based on local modal strain energy that can be released into high-order modes. The numerical time complexity is linear with respect to the number of actuators and potential placements, which facilitates quick analysis in case of large structures.

Keywords:
semi-active control, damping of vibrations, actuator placement, smart structures, prestress-accumulation release (PAR)

Abstract:
This paper proposes, tests numerically and verifies experimentally a decentralized control algorithm with local feedback for semi-active mitigation of free vibrations in frame structures. The algorithm aims at transferring the vibration energy of low-order, lightly-damped structural modes into high-frequency modes of vibration, where it is quickly damped by natural mechanisms of material damping. Such an approach to mitigation of vibrations, known as the prestress-accumulation release (PAR) strategy, has been earlier applied only in global control schemes to the fundamental vibration mode of a cantilever beam. In contrast, the decentralization and local feedback allows the approach proposed here to be applied to more complex frame structures and vibration patterns, where the global control ceases to be intuitively obvious. The actuators (truss–frame nodes with controllable ability to transmit moments) are essentially unblockable hinges that become unblocked only for very short time periods in order to trigger local modal transfer of energy. The paper proposes a computationally simple model of the controllable nodes, specifies the control performance measure, yields basic characteristics of the optimum control, proposes the control algorithm and then tests it in numerical and experimental examples.

Keywords:
Damping of vibrations, Smart structures, Semi-active control, Decentralized control, Truss-frame nodes

Abstract:
The paper is aimed at modelling and optimization of a rescue cushion system, which is a device used by fire brigades for evacuation of people from buildings. The goal of the study is an improvement of the system response under various operational conditions. For faster and cheaper analysis of the system performance and further development of the complex structure of the rescue cushion system, a dedicated dynamical model is elaborated. Implemented numerical model is further applied for the parametric study in order to evaluate the influence of selected airbag parameters on the effectiveness of impact mitigation process and to determine possible improvements of the actual system design. In particular, adaptation of the rescue cushion to the mass of landing person, as well as to the initial velocity, is analyzed. Simultaneously, the necessity of meeting all the functional and operational requirements is taken into account. The most promising directions of further research are indicated.

Abstract:
This contribution presents an experimental analysis of the control system configuration for a semi-active frame structure. The structure is equipped with a system that implements a Prestress Accumulation Release strategy for mitigation of vibration. A proper distribution of the sensors for monitoring the actual state of the structure is the key factor that determines the overall effectiveness of the applied strategy. The results and findings presented here reveal a set of basic rules dedicated to solving this crucial issue.

Keywords:
Prestress Accumulation Release, Vibration control, Sensor placement, Semi-active structures, Piezoelectric actuation

Abstract:
Semi-active control systems are investigated for more than 40 years, and despite the great progress in this research area, they are still considered to be a complex topic in both theoretical and technical terms. However, their advantages ensure that these control systems remain an extremely attractive subject of scientific and technological development.
In this contribution, we present a semi-active strategy for mitigation of vibration, which utilizes an energy management approach called Prestress-Accumulation Release and is based on controllable activation and removal of selected structural constraints. Here, it is implemented by means of controllable structural nodes of a specific design that allow the transmission of moments between adjacent structural elements to be controlled in an on/off manner. The developed control strategy turned out to be very effective in damping of free structural vibrations of planar frame structures. Extension of the research to other types of vibrations has shown that the proposed control algorithm is versatile and stays efficient in a range of applications and different configurations of the investigated structures. This work is focused on mitigation of vibrations excited by a randomly generated force load.
Decentralization, understood here as controlling the employed actuators based on locally measured structural response, results in a decisive reduction of the complexity of the data acquisition and control systems, which is crucial for actual implementations in real structures, and which facilitates an ad hoc reconfiguration and expansion of the control system if necessary. It also provides the possibility of considering selected structural elements as separate energy dissipative devices, which in our approach act effectively as vibration dampers. This feature, provided by the decentralization, enables to take the advantage of two complementary, interrelated mechanisms of material damping: global dissipation of vibration energy by the PAR and local dissipation in single involved elements.
Numerical and experimental analyses indicate a high degree of effectiveness in alleviation of the amplitude of vibrations induced by a random transient force excitation. The proposed control strategy can be thus utilized not only in the case of momentary impulsive loads that result in predominantly free vibrations, but also in the case of transient random force excitation. It significantly extends the range of possible modes of operation of a structure equipped with the proposed damping system.

Keywords:
Semi-active damping, Vibration damping, Random vibration, Forced vibration

Abstract:
Vibration mitigation in space structures creates a unique class of a technical problem where resistant for outgassing and non-fluidic solutions are preferable. Additionaly, a vibration induced by time-varying excitations needs to be effectively reduced. The vibration mitigation task is speciffically difficult in the case of light, slender and inherently flexible structures of various types, such as supporting structures, deployable structures, modular structures or wide-span skeletal roofing structures. This study presents a concept of a vibration attenuation method based on semi-active joints and dedicated to frame structures under forced vibration excitation. The presented investigation contains an analysis of the problem of the optimal control of a structure fitted with semi-active structural members. Furthermore, an adequate model of the semi-active joints is developed and a numerical example is presented. Finally, the research provides an experimental verification of the developed control algorithms, which is conducted on a test stand in a laboratory environment.

Abstract:
This contribution proposes a quantitative criterion for optimization of actuator placement for the Prestress–Accumulation Release (PAR) strategy of mitigation of vibrations. The PAR strategy is a semi-active control approach that relies on controlled redistribution of modal energy into high-frequency high-order modes, where it is effectively dissipated by means of the natural mechanisms of material damping. The energy transfer is achieved by a controlled temporary removal of selected structural constraints. An example is a short-time decoupling of rotational degrees of freedom in a frame node, so that the bending moments are no longer transferred between the involved beams. If it such a decoupling is performed at the maximum of the shear/bending strain energy of adjacent beams, it results in an almost instantaneous energy release into high-frequency local vibrations and quick dissipation of energy. We propose and test a quantitative criterion for placement of such actuators. The criterion is based on local modal strain energy that can be released into high-order modes. The numerical time complexity is linear with respect to the number of actuators, which facilitates quick selection of placements in large structures.

Keywords:
semi-active control, damping of vibrations, actuator placement, smart structures, Prestress-Accumulation Release (PAR)

Abstract:
Semi-active systems for mitigation of vibrations proved to be effective in many applications. Their prominent advantage is that they combine strong points of passive and active damping systems. Proper design can ensure their reliability, which is what passive systems are praised for. A high effectiveness in vibration damping links them with active systems. At the same time they do not have many deficiencies of active systems. They are adaptive, so they can stay effective in different environmental conditions, which is the factor that eliminates passive systems from many implementations. Their mass and energy consumption is very low, and the controlled structure can stay in the safe configuration even in case of power supply failure, which puts them in contrast to many active systems. The mentioned attributes make them a good choice for many structures subjected to vibrations, especially when there is a strong emphasis on maximization of the efficiency/mass ratio of the damping system.
This contribution presents a decentralized closed-loop control strategy and applies it in a frame structure equipped with controllable truss-frame nodes. Such nodes can be switched between frame-like and truss-like states in a controllable manner. In the frame-like state the node transmits all forces and moments, while in the truss-like state only axial and shearing forces are transmitted. These nodes allow for structural reconfiguration, which can be utilized by semi-active control strategies for the purpose of vibration damping. The implemented control algorithm applies the Prestress-Accumulation Release (PAR) strategy based on the transmission of the accumulated potential energy to high modes of vibration, which are highly dissipative. Strain measurements are conducted locally on selected elements. A similar strategy proved its effectiveness in mitigation of free structural vibrations. This research studies the concept of its application to mitigation of forced structural vibrations, caused by variable external conditions.

Keywords:
Semi-active damping, Truss-frame nodes, Prestress-Accumulation Release (PAR), Decentralized control

Abstract:
This contribution proposes a decentralized closed-loop control algorithm for semi-active mitigation of free vibrations in frame structures. The control uses dedicated dissipative devices, which consist of two controllable structural nodes placed pairwise in both ends of selected structural beams. The nodes are capable of a controlled transition between the standard frame mode of operation (full moment-bearing ability) and the truss mode in which they do not bear any moments and constitute in fact structural hinges. Synchronous switching is equivalent to reconfiguration of the global structure by (dis)allowing the involved beams to transmit moments and to accumulate vibration energy in the form of their bending strain. Upon switching to the truss mode, the accumulated energy is released into high-frequency local vibrations, which undergo quick dissipation by standard mechanisms of material damping. The approach is illustrated in a numerical example and verified in a preliminary experimental test.

Keywords:
Mitigation of vibrations, Semi-active control, Decentralized control, Structural reconfiguration

Abstract:
Extensive research efforts have been recently devoted to semi-active structural control with its paradigms of smart self-adaptivity and low consumption of energy, which is used for local adaptation rather than to generate external control forces. Considered application areas include adaptive landing gears, seismic isolation systems, vehicle-track/span systems, power train electro-mechanical systems, damping of flexible space structures, vehicle crashworthiness, arctic engineering, wind turbines, etc. A part of the research concerns semi-active management of strain energy for damping of structural vibrations. Early works considered truss structures with stiffness-switched bars. They later evolved into either standalone one degree of freedom stiffness-switched dampers and isolators or investigations in triggering modal energy transfer to highly-damped high-order modes. The latter researches seem all to study the fundamental vibration mode of a cantilever beam with two detachable layers and differ mainly in the actuator technologies; the main idea is to employ actuators for a quick release of the vibration-related strain energy. This research extends the problem to general 2D frames. Controllable truss-frame nodes are incorporated into the structure. Thanks to their controllable ability to transmit moments, they allow for a quick transition between truss and frame modes. We propose a new, decentralized, closed-loop control strategy based on local energy measures. Vibration damping is more effective than in the previously studied control scheme based on a global energy measure, especially for higher vibration modes. Mitigation of vibrations will be presented in representative numerical examples, including a comparison to the global energy-based control strategy. Finally, results of experimental study, conducted on a structure analogous to the one from numerical simulations, will be demonstrated.

Keywords:
Vibration damping, Smart structures, Semi-active control, PAR strategy, Decentralized damping strategy

Abstract:
This paper discusses passive and semi-active techniques of structural control by means of smart joints, and then it proposes a specific smart joints system for frame structures and tests its capability in mitigation of free vibrations. Basically, the proposed solution modifies frame beams by addition of truss-type hinges, and its effectiveness relies on the softening effect that occurs in compression due to geometric nonlinearities and which triggers the highly-damped high-frequency response modes of the structure. First, the finite element (FE) model of the specific frame structure with geometrical nonlinearities is derived, and the proposed passive joints are described and incorporated into the model. Then, their principle of operation and effectiveness is examined numerically for the first two natural modes of vibrations with various initial displacement amplitudes. An objective function is proposed to assess joints placement, based on the efficiency in mitigation of the excited vibrations.

Keywords:
Vibration Damping, Structure Response, Smart Structure, Structural Control

Abstract:
In recent years, vibration damping strategies based on semi-active management of strain energy have attracted a large interest and were proven highly effective. However, most of published research considers simple one degree of freedom systems or study the same basic example (the first vibration mode of a cantilever beam) with the same control strategy. This contribution focuses on truss-frame nodes with controllable moment-bearing ability. It proposes and tests an approach that allows the control strategy to be extended to more complex structures and vibration patterns.

Keywords:
adaptive impact absorption, smart structures, semi-active control, safety engineering

Abstract:
This contribution reviews the challenges in adaptive self-protection of structures. A proper semi-active control strategy can significantly increase structural ability to absorb impact-type loads and damp the resulting vibrations. Discussed systems constitute a new class of smart structures capable of a real-time identification of loads and vibration patterns, followed by a low-cost optimum absorption of the energy by structural adaptation. Given the always surging quest for safety, such systems have a great potential for practical applications (in landing gears, road barriers, space structures, etc.). Compared to passive systems, their better performance can be attributed to the paradigm of self-adaptivity, which is ubiquitous in nature, but still sparsely applied in structural engineering. Being in the early stages of development, their ultimate success depends on a concerted effort in facing a number of challenges. This contribution discusses some of the important problems, including these of a conceptual, technological, methodological and software engineering nature.

Keywords:
adaptive impact absorption, smart structures, semi-active control, safety engineering

Abstract:
Despite the fact that airbag systems are well-known and used in safety engineering for many years the growth of research and development activities and increasing number of new applications of airbags are observed. Advances in the field includes among others: development of advanced car airbags, analyses of airbag cushioning for landing on Mars, elaboration of emergency landing system for drones. Another field of airbags application and simultaneously the main motivation for the study presented in the paper is the evacuation of people conducted by the fire brigade. When people are forced to leave the building by jump from the window or roof the rescue cushion is placed on the ground in order to mitigate the impact and safe life of evacuated people. The aim of the study is to develop the relevant adaptation strategy for the system and provide efficient operation of the airbag in case of different impact velocities and different masses of the landing person. Several approaches to the system adaptation are analyzed and they include semi-active as well as semi-passive solutions. Also possibility of implementing the concept of self-adaptive impact absorption is assessed. Technical and operational requirements for the rescue cushion are considered and based on them the final adaptation principles are selected. Evaluation of the system performance is conducted with the use of numerical models of dummies provided in the LS Dyna software environment. In Fig. 1. the overloads acting on pelvis of three different dummies dropped from assumed height are shown. The obtained mitigation of the impact loading is significant for all considered cases.

Abstract:
In the recent decades, a significant stream of research in structural control has focused on semi-active control approaches. The two constitutive characteristics of a semi-active system are its low consumption of energy and the capability of smart self-adaptation. The inspiration can be traced back to Nature, where dynamic and energy-efficient self-adaptation to varying external conditions is a ubiquitous mode of operation. These ideas are fundamentally different from the paradigms behind the active control (active counteraction) and the passive approaches (passive absorption). In applications to mitigation of vibrations in structural control, within the spectrum of the semi-active techniques, there are two basic approaches that can be identified as: 1) stimulation of local dissipation in actuators, which basically amounts to maximization of the local force--displacement loops, and 2) local triggering of the global material dissipation mechanisms, which is called the prestress accumulation--release (PAR) control strategy. This contribution reports on a specific control technique from the second group.

Abstract:
Almost all man-made structures are exposed to vibration. Regardless of whether these are large structures such as bridges or skyscrapers, machines with rotating parts such as engine shafts, frame structures or vehicle suspensions, excessive vibrations can be very harmful. From the perspective of their effects they can be seen as very spectacular (e.g., a collapse of a bridge) or not worth much attention (e.g., a failure of a motor shaft), but in each of these cases, the effect is the destruction of the structure and a negative impact on the users of these devices.
Several approaches can be used by the designers to overcome this phenomenon. The most basic, but often sufficient, method is to introduce changes in the mechanical parameters of the system affecting the severity of vibration in operational conditions, i.e., its mass or stiffness. If such design changes cannot be realized, or if vibration problems are detected after the system is manufactured, or if a vibration suppression system must be used for other reasons, one of the three basic types of such systems can be used.
The primary choice is usually a passive vibration damping system. These are relatively simple systems whose mode of operation is the passive dissipation of the energy of structural vibrations. Their design and simple functionality ensures that they are highly reliable, but their simplicity is reflected, unfortunately, in their limited efficiency. Their flexibility may be also considered as insufficient: once configured, even a small change in the specific operating conditions can result in a drastic loss of performance. This indicates a rather narrow spectrum (frequency range) of correct system operation.
Active systems constitute a much more effective damping approach. In this case, vibration attenuation is achieved not by means of dampers, but by actuators integrated into the structure. This approach allows to achieve very good results of vibration mitigation over a wide range of excitation frequencies. High efficiency, however, is burdened with a much higher degree of complexity of such a system as compared to the passive systems. In order to develop such a system, it is necessary to design the controller and install actuators that implement the control algorithm. During the vibration suppression, the actuators themselves require a large energy supply, which can be troublesome in some cases.
The compromise between these damping systems are semi-active systems, where the actuators are used to affect structural properties instead of exerting large external forces. In terms of reliability, semi-active systems can be compared with passive systems, while in terms of the efficiency of damping with active ones. They also do not require large amounts of electric energy to implement the control algorithm. Despite being a relatively new research area with less established design and development procedures, their advantages seem to be large enough to attract a growing number of scientists and engineers.
This contribution presents a strategy for semi-active reduction of forced vibrations in frame structures. Analogous damping technique proved to be effective in damping of free vibrations. The control strategy is based on the Prestress Accumulation–Release (PAR) concept and uses specially designed semi-active rotational nodes. Successive decentralization of the damping system demonstrates that apart from the global mechanism of the energy dissipation based on the PAR, it is also possible to disperse it locally to individual beams that are separate elements of the damping system.