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:
The paper presents an analysis of the state-dependent path-tracking method devoted to mitigation of dynamic response of systems and structures under impact excitations. The objective of the study is an evaluation of the adaptive performance and robustness of the novel control method. Robust and adaptive control methods are intensively developed by researchers and control engineers. Progress in the field influences various areas including mechanical engineering, within which these methods are applied for control of industrial processes as well as mitigation of structure dynamic response. Commonly solved problems relate especially to mitigation of vibrations, e.g. for protection of seismically excited structures. Another closely related area is the field of impact absorption, which is still challenging because of short time periods of energy absorption and number of process uncertainties. Nevertheless, due to higher and higher performance of smart sensors and actuators, as well as increasing efficiency of data processing systems, novel high- performance solutions also for impact mitigation problems can be proposed. This fact is reflected in the paper and important contribution to the field of Adaptive Impact Absorption is demonstrated. The importance of presented study results from the fact that applied smart absorber controlled with the use of kinematics-based approach ensures efficient mitigation of the impact excitation and automatic adaptation to various loading conditions. In contrast to shock-absorbers developed so far, the system implemented in laboratory provides adaptation to unknown impact conditions and compensates the influence of unpredictable perturbations. Within the paper an experimental validation of the novel control method is discussed and the system robustness to contact conditions, as well as to different values of operational medium parameters, is demonstrated. Possible extension of the method is analyzed and directions of further research are indicated.

Keywords:
adaptive impact absorption, experimental study, kinematic feedback control, pneumatic absorber, self-adaptive system, smart shock-absorber

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:
Many of mechanical energy absorbers utilized in engineering structures are hydraulic dampers, since they are simple and highly efficient and have favourable volume to load capacity ratio. However, there exist fields of applications where a threat of toxic contamination with the hydraulic fluid contents must be avoided, for example, food or pharmacy industries. A solution here can be a Pneumatic Adaptive Absorber (PAA), which is characterized by a high dissipation efficiency and an inactive medium. In order to properly analyse the characteristics of a PAA, an adequate mathematical model is required. This paper proposes a concept for mathematical modelling of a PAA with experimental verification. The PAA is considered as a piston-cylinder device with a controllable valve incorporated inside the piston. The objective of this paper is to describe a thermodynamic model of a double chamber cylinder with gas migration between the inner volumes of the device. The specific situation considered here is that the process cannot be defined as polytropic, characterized by constant in time thermodynamic coefficients. Instead, the coefficients of the proposed model are updated during the analysis. The results of the experimental research reveal that the proposed mathematical model is able to accurately reflect the physical behaviour of the fabricated demonstrator of the shock absorber.

Abstract:
Adaptive Impact Absorption focuses on adaptation of energy absorbing structures to actual dynamic loading by using system of sensors detecting and identifying impact in advance and embedded semi-active dissipaters with controllable mechanical properties. Application of such devices allows to modify dynamic characteristics of the structure during the period of impact and to precisely control the process of energy dissipation. The paper presents an overview of research conducted at the Department of Intelligent Technologies of the Institute of Fundamental Technological Research dedicated to design and applications of various systems of Adaptive Impact Absorption. Wide range of presented examples covers adaptive hydraulic and pneumatic landing gears, skeletal systems equipped with controllable elements and detachable joints as well as adaptive inflatable structures.

Keywords:
adaptive impact absorption, safety engineering, smart structures, optimal control

Abstract:
This paper describes a pneumatic valve based on a multilayer piezoelectric actuator and Hörbiger plates. The device was designed to operate in an adaptive pneumatic shock absorber. The adaptive pneumatic shock absorber was considered as a piston–cylinder device and the valve was intended to be installed inside the piston. The main objective for the valve application was regulating the gas flow between the cylinder's chambers in order to maintain the desired value of the reaction force generated by the shock absorber. The paper describes the design constraints and requirements, together with results of analytical modelling of fluid flow verified versus experimentally obtained data. The presented results indicate that the desired performance characteristics of the valve were obtained. The geometrical constraints of the flow ducts were studied and the actuator's functional features analysed.

Abstract:
Within this contribution a challenging problem of adaptive impact absorption is considered and studied in detail. The paper is focused on practical implementation of the self-adaptive system and experimental assessment of its performance. For this purpose a novel kinematics feedback control method is applied and used to adjust in real-time the opening of piezo-electric valve, which is an important part of the smart pneumatic shock-absorber devel-oped in the Institute of Fundamental Technological Research Polish Academy of Sciences (IPPT PAN). As a result, an outstanding shock-absorbing system, capable to adaptively mitigate the impact, is obtained and decelerations acting on the amortized object are significantly reduced for varying parameters of the dynamical excitation. Within the paper the control system im-provement based on proportional control of the piezo-electric valve opening is considered. This improvement may provide much better response of the system in terms of reaction force, which is transferred to the amortized object. Indeed, such control in real-time is very hard to be realized in practice. Nevertheless, the authors make an effort to develop the electronic system allowing for proportional adjustment of the valve opening and replacing the on-off control, which gives worse performance and higher control cost.

Keywords:
self-adaptive impact absorber, adaptive control system, real-time control, pneu-matic absorber, drop tests, piezo-electric valve, braking system

Abstract:
The discussed study is focused on implementation of a novel kinematics-based control technique. Presented results are based on theoretical and numerical analyses as well as on experimental investigations, which are focused on elaboration of the efficient self-adaptive energy absorption system. The developed control method has been originally dedicated to the impact mitigation problem, but it can be adjusted to other types of dynamic excitations. Superior performance of the method results from the fact that proposed system adapts automatically to unidentified dynamic excitations and compensates possible unexpected disturbances during the impact absorption process. The analyzed self-adaptive impact absorption system is based on the pneumatic shock-absorber with piezoelectric valve and real-time control system. This contribution is focused on chosen factors which can lead to undesired imperfections in practical implementation of the control method.

Abstract:
Small drones with total mass of a few kilograms are becoming more and more popular in many applications increasing the probability of occurrence of emergency situations caused by an equipment failure or a human error. In case of a fall from a high altitude very often it is possible to use parachute rescue systems, which however require relatively long time for deployment and development of braking forces. The touchdown velocity may be large enough to exceed limit accelerations for UAV equipment. The paper presents the concept of deployable airbag systems, in particular with adaptive flow control, which provides a possible solution to the above-mentioned problems. The paper discusses the overall control and adaptation strategy. Simplified methods for mathematical modeling are proposed and formulated for an example on a cylindrical airbag. The conceptual part is concluded with the presentation of the methodology of experimental verification and results of initial tests of the integrated airbag system.

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:
The paper is aimed at discussion of various control techniques developed for adaptive impact-absorbers protecting structures and machines. Different approaches to the problem of optimal damper design are presented and systems comparison is provided with the example of pneu-matic shock-absorber. The influences of selected control strategy on the absorber characteris-tics, its efficiency and adaptation capabilities are shown. The contribution includes both numerical and experimental examples. The authors highlight the fact that the final design of the device should be elaborated simultaneously with the development of dedicated control system. In some cases properly assumed architecture of the control system enables significant simplifi-cation of the absorber. The paper covers analyses of semi-passive devices with single reconfig-uration to identified excitation conditions and semi-active absorbers capable of adaptation to unknown impact loading. Adaptation mechanisms of such devices and their robustness are com-pared in reference to volatility of system parameters and variety of loading conditions. Limita-tions of smart devices (e.g. piezo-electric valve in pneumatic absorbers) used in practice for absorbers' control are described in relevant mathematical models. Technological challenges in the design and manufacturing of absorbers are identified and methods of their overcoming are proposed.

Keywords:
Adaptive Impact Absorption, adaptive control, adaptable system, damper

Abstract:
The objective of this work was to develop a mathematical model of coupled thermodynamic and mechanical processes proceeding in pneumatic, adaptive absorbers under cyclic loadings. The results of the modelling were to be verified versus experimentally obtained data. The analysis was divided into sections devoted to: forces acting on the piston, thermodynamics of the gas in the absorber’s chambers, gas flow through the piezoelectric valve. Three control volumes were distinguished within the absorber’s structure in order to analyze the thermodynamic processes. For each control volume analysis of energy balance, thermodynamic state parameters and heat transfer were performed. A set of equations was formulated for each control volume in order to determine: (1) motion of the piston in relation to the acting forces, (2) the gas state evolution, (3) energy balance within each control volume and (4) heat transfer to the surroundings. The obtained results revealed that the proposed approach to modeling was in good agreement with the data obtained experimentally. The controllability of the absorber was successfully reflected by means of the numerical model outcome.

Abstract:
An adaptive pneumatic shock absorber with a piezo-valve was designed for real-time impact energy dissipation. The device was a piston-cylinder type with a fast actuated (less than 2 ms) piezo-valve positioned inside the piston. The principle of operation of the device was to keep the reaction force on a predefined level by means of managing of the gas flow between the internal chambers of the shock absorber. The internal chambers were defined by spaces in the cylinder on both sides of the piston. The proper control of the valve, which connected the two chambers, allowed to adjust the instantaneous pressure drop between them. The pressure drop was a decisive factor that influenced the total reaction force of the shock absorber.
The presented investigation was conducted using the MTS Test System experimental setup in order to perform measurements of stiffness and viscous effects in the domain of frequency of excitation. The shock absorber under investigation was fixed between a stiff base and a piston rod of the hydraulic actuator that was used for the mechanical excitation.
The conducted set of experimental tests included measuring of the following set of quantities: frequency of harmonic excitation, reaction force of the absorber, displacement of the piston, velocity of the piston, gas pressure in both chambers.
The presented research was focused on characterization of the response of the device to harmonic excitation. The study was aimed at identification of limits of the device in terms of its controllability and adaptability.

Keywords:
adaptive devices, Adaptive Impact Absorption, AIA, pneumatic shock-absorbers, piezo stack, piezo-valve

Abstract:
Adaptive Impact Absorption focuses on adaptation of energy absorbing structures to actual dynamic loading by using system of sensors detecting and identifying impact in advance and semi -active dissipaters with controllable mechanical properties which enable change of system dynamic characteristics in real time. The article present s a review of research conducted at the Department of Intelligent Technologies of the Institute of Fundamental Technological Research dedicated to applications of systems for Adaptive Impact Absorption. Wide range of presented examples covers pneumatic landing gears, bumpers for offshore towers, wind turbine blade-hub connections and d protective barriers for automotive applications.

Keywords:
adaptive impact absorption, safety engineering, smart structures, optimal control

Keywords:
Adaptive structures, Shock absorption, Impact energy absorption, Piezoelectric valve

Abstract:
Adaptive Impact Absorption focuses on active adaptation of energy absorbing structures to actual dynamic loading by using system of sensors detecting and identifying impact in advance and controllable semi-active dissipaters with high ability of adaptation. The article presents a review of research carried out in the Department of Intelligent Technologies of Institute of Fundamental Technological Research dedicated to applications of systems for adaptive impact absorption. Wide range of presented examples covers pneumatic landing gears, adaptive crashworthy structures, wind turbine blade-hub connections and flow control based airbags for maritime and aeronautical applications.

Keywords:
smart structures, adaptive structures, Adaptive Impact Absorption, crashworthiness, safety engineering

Abstract:
The class of ultra-light aircraft becomes more and more popular among the enthusiasts of aviation due to low formal requirements of getting the pilot license and low costs of the equipment. Therefore, the training of the pilots starts to be a large-scale task. One of the most difficult operation for the inexperienced pilots is touch-down and it often happens to strike the ground with a high sink speed. In consequence the training machines are endangered of fast structural damage. A potential solution would be to mount a system of adaptive landing gear for light aircraft with a capability of recognition of the actual landing impact and tuning the landing struts in order to conduct the smoothest landing operation possible. In the case of the ultra-light aircraft class the weight of the components is the crucial task and therefore the low-weight pneumatic system is proposed for these application.
The paper presents a concept of an adaptive landing system and adequate control strategy for a small aerial vehicle. The objective of the work was to develop a fully functional model of the landing system and experimental verification of it. The system is based on the new pneumatic impact absorbers actuated via piezo-stacks. The concept assumes designing of the system with the capability of adaptation to actual energy of impact scenario identified by a dedicated sensing system for impact energy recognition.
The designed control system was dedicated to process the data from the system of impact energy recognition in order to perform the optimal landing scenario. The objective of the control strategy was minimization of the structure’s deceleration peaks during the touchdown.
The presented results consist of numerical analysis of the adopted strategy of control and experimental verification of the concept on the dedicated experimental device. The results proved that the proposed method allowed minimization of the maximal deceleration level acting on the demonstrator.

Abstract:
This contribution reviews a recently proposed control strategy for mitigation of vibrations based on the Prestress-Accumulation Release (PAR) approach [1]. The control is executed by means of semi-actively controllable truss-frame nodes. Such nodes have an on/off ability to transfer bending moments: they are able to temporary switch their operational characteristics between the truss-like and the frame-like behaviors. The focus is not on local energy dissipation in the nodes treated as friction dampers, but rather on stimulating the global transfer of vibration energy to high-order modes. Such modes are high-frequency and thus highly dissipative by means of the standard mechanisms of material damping. The transfer is triggered by temporary switches to the truss-like state performed at the moments of a high local bending strain. A sudden removal of a kinematic constraint releases the locally accumulated strain energy into high-frequency and quickly damped vibrations.
The first formulation investigated global control laws [1]. Recent approaches generalized it to decen-tralized control with a local-only feedback, which was tested in damping of free vibrations [2] as well as forced vibrations [3]. Recently, a global formulation was proposed that aims at a targeted energy transfer between specific vibration modes [4], and attempts were made to go beyond skeletal struc-tures [5]. Numerical and experimental results will be presented to confirm the high effectiveness of the approach in mitigation of free, forced random and forced harmonic vibrations.

Abstract:
This contribution reviews a recently proposed semi-active control approach based on the Prestress-Accumulation Release strategy, which aims at damping of structural vibrations by promoting vibration energy transfer from lower- into higher-order modes that have significant material damping. Unlike typical semi-active control, which focuses on local dissipation in actuators, the aim is to trigger natural global damping mechanisms. The actuators are controllable truss-frame nodes: lockable hinges that can change their mode of operation from a frame node (locked hinge) into truss node (free rotation). Sudden removal of such a kinematic constraint releases the accumulated bending energy into high-frequency quickly damped local vibrations. Two formulations are reviewed: decentralized with local-only feedback, and global, which aims at a targeted energy transfer between specific modes. Experimental results confirm the effectiveness using free, forced harmonic and random vibrations.

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.

Keywords:
Smart And Adaptive Structures, Adaptive Impact Absorption, Safety Engineering