International Journal of Structural Stability and Dynamics

In this paper, we propose a novel concept of a harvester-absorber system. The idea is based on usage of the dynamic absorber for energy harvesting. The device consists of three elements: a main system (damped body), a pendulum (tuned mass absorber) and an electromagnetic harvester. The ultimate goal of this investigation is to achieve simultaneous mitigation of vibrations and energy harvesting. We show the dynamics of the system and ranges of parameters where we observe good damping properties and high energy recovery. Additionally, an optimization of the harvester device parameters has been performed.

This paper presents results on adaptive control strategies for a sensory system to identify (unknown) ground excitations which force the sensor and its seismic masses, so that acting forces can be measured and identified. The sensor system is modeled as a spring-mass-damper system within a rigid frame with two degrees-of-freedom. The seismic masses are under the load of internal control forces, which shall ensure stabilization of the mass point rest positions despite the continuing ground excitations. Using these (measured) resulting regulating forces, we are able to identify the excitation force. The control strategies are designed referring to the natural behavior of mechanoreceptors from biology. These are able to adapt their sensitivity to the environment, so that they filter the important information out of the flood of information. Mimicking this behavior, adaptive control strategies are used with time-varying controller gains. In this way, we are able to design controllers which are still sensitive while a constant stimuli affects. So new incoming information can be identified with a high quality. Further on, the sensor has to be universal and shall consume less energy as possible. Therefore, control strategies from literature are analyzed and modified, so that the most effective ones are used for the sensor system in this paper. Finally, the best working control strategies are tested for both their long-term behavior to an excitation which simulates different situations and for their response to different system parameters, chosen randomly.

In this paper, the effectiveness of a saturation control strategy in suppressing vibration of a rotating composite thin walled beam is studied. The mathematical model of the flexible beam takes into account a shear deformation effect, a warping function, a centrifugal force and the Coriolis acceleration. To extend the generality of the proposed formulation an inertia of the hub is also considered. Adaptive capability of the beam is achieved through the implementation of the saturation control algorithm.

Within the performed tests, the discussed control strategy is applied for different magnitudes of flexural–torsional vibration modes resulting from different orientations of beam laminate-reinforcing fiber’s. The obtained results prove the applied nonlinear control to be the effective method for beam vibration suppression in near-by resonance zones for all studied cases. Parametric studies considered different rotating speeds of the system. It is shown that the vibration of the beam can be suppressed to similar levels independently of the transportation motion rotating speed. However, significant differences in the width of vibration suppression zones are observed.

The model-based motion control of underactuated, multiple degree-of-freedom, complex multibody systems is in focus. Underactuated mechanical systems possess less number of independent control inputs than degrees-of-freedom. The main difficulty in their control is caused by the dynamics of the uncontrolled part of the system. The complexity of multibody systems makes the dynamical and control formulation difficult. The direct application of traditional control techniques available in the literature can lead to unstable dynamic behavior in many cases. In order to avoid instability, these general methods are usually adapted for specific problems in an intuitive way. Here, we present a direct, more algorithmic approach, and propose the use of periodic servo-constraints to overcome stability problems and enhance the dynamic behavior. An exact, stability analysis-based method is also proposed for tuning the control parameters. A stability analysis procedure is developed which is directly applicable for investigating the dynamics of mechanical systems described by dependent coordinates and mathematically formulated as a set of algebraic differential equations.

In this paper, the control strategies are studied for the fractional order uncertain wave equation subject to persistent external disturbances in Hilbert spaces. The twisting and super-twisting fractional order sliding mode controllers (SMCs) are designed for the infinite dimensional setting and they are applied for addressing the asymptotic state tracking of the fractional order perturbed wave equation. Furthermore, by introducing the adaptive control law to the twisting controller, the bound of the external disturbances which is unknown is dealt with, and for the design of the super-twisting SMC, a fractional order sliding mode manifold is utilized which results in a continuous input control and a chattering free signal. Both of the controllers are associated with the fractional order parameter, which influences the convergence rate of the proposed control algorithms. In addition, the relative theorem involved in the paper for the proof of the stability is proved. Then, the control algorithms are extended to globally asymptotically stabilize the fractional order uncertain wave equation by choosing the appropriate Lyapunov functional. Finally, numerical simulations are presented to verify the viability and efficiency of the proposed fractional order controllers.

Galvanometer scanners (GSs) are the most utilized optomechatronic systems for laser scanning. A GS consists of an oscillatory element (which includes the galvomirror) in a motor structure equipped with a positioning servo-system built usually in a closed-loop structure and controlled by different algorithms. Such structures have to provide speed and accuracy in the positioning of the laser beam or for raster scanning. Although tackled by numerous studies, on different aspects, this is still an open problem. The aim of this study is to approach it in a simple way and to provide a low cost solution to increasing accuracy by developing a structure which is more immune to disturbances than the classical one. In order to do this, a basic closed-loop GS which consists of a proportional-derivative (PD-L1) controller and a servo-motor is considered first. The mathematical model and the GS parameters are identified using a theoretical approach followed by experimental identifications. The extended control solution is developed starting from this basic GS structure by introducing a proportional-integrative (PI) controller in series with the PD-L1 controller of the GS; further on, a P-L1 reference filter is added to this new structure. In the absence of disturbances, both the basic and the extended control solutions show good tracking performances, but the basic solution is faster. In the presence of constant disturbances which can affect the servo-system, the latter structure (extended and filtered) has the best dynamical and tracking performances.

Intelligent and adaptive control systems are naturally suitable to deal with dynamic uncertain systems with non-smooth nonlinearities; they constitute an important advantage over conventional control approaches. This control technology can be used to design powerful and robust controllers for complex vibration engineering problems such as vibration control of civil structures. Fuzzy logic based controllers are simple and robust systems that are rapidly becoming a viable alternative for classical controllers. Furthermore, new control devices such as magnetorheological (MR) dampers have been widely studied for structural control applications. In this paper, we design a semi-active fuzzy controller for MR dampers using an adaptive neuro-fuzzy inference system (ANFIS). The objective is to verify the effectiveness of a neuro-fuzzy controller in reducing the response of a building structure equipped with a MR damper operating in passive and semi-active control modes. The uncontrolled and controlled responses are compared to assess the performance of the fuzzy logic based controller.

A novel concept of a nonlinear oscillator is proposed, based on a bistable element, which operates around an unstable equilibrium point. Contrary to the Quasi-Zero Stiffness oscillators, a totally different redistribution of the stiffness elements is followed, so that any level required static stiffness for the entire system can be maintained. This oscillator is designed to present the same overall (static) stiffness around the system equilibrium point, the same mass and to use the same damping element as a reference classical linear SDOF oscillator. Once such an oscillator is optimally designed, it is shown to exhibit an extraordinary apparent damping ratio, which is several orders of magnitude higher than that of the original SDOF system, especially in cases where the original damping of the SDOF system is extremely low. This damping behavior is not a result of a novel additional extraordinary energy dissipation mechanism, but a result of the phase difference between the positive and the negative stiffness elastic forces; this is in turn a consequence of the proper redistribution of the stiffness and the damping elements. This fact ensures that an adequate level of elastic forces exists throughout the entire frequency range, able to counteract the inertial and the external excitation forces. Consequently, a resonance phenomenon, which is inherent in the original linear SDOF system, cannot emerge in the proposed oscillator.

This work presents a dynamic model of the piston-crankshaft assembly of the Stirling engine with three degrees of freedom combined with the isothermal thermodynamic submodel. The model allowed for consideration of the working gas pressure from the thermodynamic submodel, whose working space was divided into partial volume units and subjected to analysis. Performing the analysis of the physical model and adopting the data from the real object, with the assumption of the static mass reduction, enabled developing of a simulation model of the piston-crankshaft assembly. Subsequently, the model was extended by the part describing pressure changes in the cylinder, theoretical work, theoretical power of the working gas with the assumption of the isothermal heat exchange in the compression and expansion spaces. On the basis of the motion equations shown in the work, the influence of the model’s chosen parameters on the operation of the integrated simulation model was presented and analyzed. The results of the conducted simulations were also additionally derived from an analysis of the displacement, velocity, and piston acceleration curves, as well as the curves of displacement, velocity, and crankshaft angular acceleration. The presented results convey the information about the dynamic operation of the simulated real object working at the preset thermodynamic parameters of the working gas.

The paper is focused on the kinematic, dynamic and power consumption analysis of the constructed prototype of the hexapod robot walking with tripod gait on a flat and hard ground. The movements of the robot legs are controlled by different well known oscillators working as central pattern generators (CPGs). The mentioned models, as well as those proposed in our previous paper, are employed and compared from the viewpoint of fluctuations of the robot gravity center both in vertical and movement direction, contact forces between the robot legs and the ground as well as energy demand of the whole robot during walking process. Time histories of the key kinematic and dynamic quantities describing locomotion of the robot are numerically studied and experimentally verified. Power consumption of the whole robot is experimentally investigated based on the current consumption in the applied servo motors which drive the robot legs. We show that the proposed CPG model is more efficient regarding acceleration/deceleration of the whole robot, contact forces and overload acting on the robot legs as well the energy demand during walking of the robot. The appropriate choice of the robot leg movements, depending on the actual situation, can have a positive influence on the investigated issues, i.e. avoid unnecessary acceleration/deceleration of the robot, decrease the contact forces between the legs and the ground and reduce energy demand of the whole robot. Consequently, it also allows to improve structural stability of the robot during walking process.

Results of the simulations of vibrations of collecting electrode system of dry electrostatic precipitator (ESP) are reported. The system of nine electrodes 16m long, suspension beam (upper beam) and anvil beam (bottom beam) is analyzed. First, the variants regarding differ position of the anvil beam are analyzed. Second, the differ position of a point of the impulse force as well as direction of the impulse force are investigated (in this case the anvil beam is in a “classic” position). Discretization of both beams and electrodes is carried out employing the modified rigid finite element method (RFEM). It allows to study an influence of the mass distribution and geometric properties of the analyzed system. Moreover, the method is convenient for the introduction of additional lumped mass parameters modeling the joints that fasten the electrodes to the suspension beam, distance-marking bushes, riveted and screw joints, etc. Position of the anvil beam and position/direction of the point of impulse force application can have essential influence on tangent and normal accelerations at different points of the plates, and thus on the effectiveness of the dust removal process. We aim to find optimal position of an anvil beam, which guaranties the maximum amplitudes of vibrations and their proper distribution in the plates.

The work is devoted to an analysis of the journal bearing dynamics, while factors related to motion and friction of nonrotating elements of the sleeve are taken into account. Based on the originally established numerical model devoted to bearing static simulations, the thickness of gas film was determined as well as the pressure distribution in the bearing and its lift capacity. Moreover, the numerical program developed for dynamic simulations led to an analysis of forced vibrations of the system under consideration. It was possible to visualize a journal trajectory for the dynamic harmonic input function. The relationship between the frequency of excitation force and bearing stiffness has been identified numerically and compared with the experimental results. The Fast Fourier Transform (FFT) analysis of vibrations was implemented. In the spectrum, only a frequency of the input function was observed, whereas and a lack of subsynchronous frequencies pointed to the stable operation of the bearing. The established numerical model yields the basis for simulations carried out in order to determine correctly dynamic characteristics of an oil-free machine rotating system at the early stage of its design and during its operation.

The paper is concerned with the problem of dynamic stability of a rotating elastic shaft supported on rolling bearing at both ends. The material of the shaft is characterized by some dissipative properties which leads to dynamic loss of stability at a certain angular velocity called the critical rotation speed. The shaft is equipped with a system of two perpendicularly arranged pairs of electromagnets which exerts some attractive force upon the shaft in two transverse direction. Presence of such forces, phenomenon of magnetic induction as well as resistance in the electric circuits substantially affect dynamics of the entire system. Application of a semi- and fully active control strategies has proved that the proposed structural solution considerably increases the critical speed and thus enlarges the range of safe operation of the shaft. A growth in the critical threshold by half and even more has occurred to be achievable for quite moderate values of the supply voltage. It has been also shown that the electromagnetic actuators favorably influence near-critical response of the system right after the stability is lost. It has been found that the nonlinear behavior of the system is orbitally stable and the observed self-excited vibration exhibits smooth supercritical nature. Electromagnetic actuation additionally decreases amplitude of the arisen limit cycle.

The beams can be considered as fundamental structural elements applied in many technical structures and equipment. In the operating regimes, these beam structures are very often loaded by the time-dependent forces which cause their undesirable dynamical behavior and the whole system is getting into critical resonant state. It is clear that the reduction of the level of unwanted vibrations or prevention of their occurrence should be one of the important objectives in the design of machine equipment and structures. To achieve these aims, the knowledge of modal properties of beam structures in relation to their internal structure is necessary. The insertion of reinforcing core into a beam body is one of the possible techniques to achieve the required changes in modal properties (mode shapes, natural frequencies) of the beam structure. The dependency of the modal properties of structurally modified beam structure on the geometric parameters and material properties of the inserted reinforcing core is studied in this paper.

Free vibrations of a composite membrane with a hexagonal lattice circular inclusion are investigated. We aim at a study of the lower frequency spectrum, i.e. it is assumed that the minimum space period of the eigenform is essentially larger than the characteristic dimension of the cell periodicity of the analyzed structure. This implies a possibility of approximating the composite structure by the homogenized one with effective characteristics. The latter is yielded by the multi-scale homogenization approach.

Introduction of slow and fast variables yields both counterpart quasistatic local problem regarding the periodically repeated cell and global (homogenized) dynamic problem for homogeneous material with effective properties. The most complicated part of this approach concerns in finding a solution to the local problem. It has been analytically found in the presented paper for relatively large inclusion sizes using lubrication approach. The performed numerical validation of the obtained results shows high accuracy of the implemented approach.