Dangerous Oscillations Research Articles

Reduce the sway of the crane payload using on-off damping radial spring-damper

Operating cranes is challenging because payloads can experience large and dangerous oscillations. One of the solutions to reduce the sway oscillation of the crane payload is to use a passive damper. This method is simple but has limited effectiveness because without adaptability and flexibility to prevent adverse movements and amplify the favorable ones. This paper proposes an on-off damping controller for the semi-active damper to improve the passive damper. The on-off damping control aims to amplify the radial motion of the damper to increase the energy dissipated, then reduce the sway vibration. The effectiveness of the proposed controller is verified by numerical simulations of a 2D crane.

Improved crowbar protection technique for DFIG using fuzzy logic

<span lang="EN-US">A doubly-fed induction generator is the most widely used as a wind turbine generator. Due to its drawbacks, doubly fed induction generator (DFIG) is extremely sensitive to grid disturbances, and the fragility of some components which are costly to the producer. Also, its acquisition value is very high in terms of maintenance time or component cost, causing substantial harm to both the energy production and power supplier. It is required that the DFIG components must be protected, especially power electronics devices and DC-Link capacitor. Therefore, this paper presents an improved crowbar strategy for DFIG. This method is based on the AI technique concept of utilizing a fuzzy logic controller. The main goal of this project is to improve the system performance by reducing the dangerous oscillations of electromagnetic torque, DC-link voltage, and rotor current during fault. This work consists of replacing the hysteresis control for the crowbar with fuzzy logic to realize crowbar-FLC. The proposed crowbar is based on free light chain (FLC) depending on rotor currents and DC-link voltage measurements. The control strategy is simulated in the MATLAB Simulink platform to evaluate the efficiency of the suggested technique.</span>

Robust active disturbance rejection control for systems with internal uncertainties: Multirotor UAV application

AbstractActive Disturbance Rejection Control (ADRC) has recently stood out as a viable alternative to the proportional–integral–derivative controllers. An interesting field of application of this approach is the control of multirotor unmanned aerial vehicles (UAVs) which are inevitably subject to various force and torque disturbances. What makes ADRC attractive is the enhanced trajectory tracking and disturbance rejection capabilities that it allows while requiring minimal knowledge about the system. Although in theory, large uncertainties on the few required system parameters do not affect the stability of the ADRC's closed‐loop, they cause performance deterioration and can lead to dangerous oscillations that result in instability in practice. In this study, we design a robust ADRC for multirotor UAVs that allows maintaining the performance despite large parameter variations, without changing the initial control gains tuning. Simulation and experimental results support the theoretical findings.

Optimal adaptive control of electromechanical oscillations modes in power systems

The paper proposes methods and algorithms for optimal tuning of power system stabilizers (PSS). The methods are based on the representation of the power system in the form of a graph and the construction for the graph of the power system of an equivalent multi-connected system of automatic control of modes of electromechanical oscillations. The node dynamics is represented by a simplified model of an oscillator together with a proportional-differential regulator, which is fair for describing a real generator with a PSS only in the region of low frequencies of dangerous oscillations. The problem of optimal tuning PSS parameters is formulated and methods for its solution based on heuristic intelligence algorithms and the method of tuning with implicit reference model are proposed. The effectiveness of the proposed methods is demonstrated on the model of a modified classical two-area Kundur test system with tuning parameters of PSS of three generators.

Vehicle ride analysis considering tyre-road separation

Quarter-car model is examined here from a new viewpoint to consider tyre-road separation. A nonlinear model is formulated using nondimensionalisation methods which would incorporate the tyre-road discontinuities to minimise the number of parameters and generalise the analysis. The investigation of the frequency responses and fraction of the separation time has been sensitively implemented. The separation dynamics are numerically simulated to identify which values of suspension characteristics can postpone and avoid the contact-free phenomenon, which results in eliminating the potentially dangerous oscillation. Converting between the nondimensional and dimensional formulations is beneficial for investigating the tyre separation with a wide variety of vehicle suspensions. Comparing the outcomes of traditional models without separation modelling demonstrates the proposed approach has achieved more precise frequency responses that contain more realistic system dynamics. Finally, model validation is carried out in the MSC-ADAMS medium.

Mechanics of robot inspector on electrical transmission lines conductors with unequal heights supports

The aim of this paper is to create a mathematical model of the motion of robot inspector on electrical transmission lines conductors when the towers are not in one level i.e. when the tension through the conductor is not constant. In this model, we consider the electrical line as a thread with two supports at the ends with different elevations and the robot considered as a moving object on the thread. The obtained equation of motion has been solved by using the separating variables method. As the result, dangerous oscillations have been seen although the motion of the inspection robot on the line is steady. These vibrations may cause parametric vibration in the perpendicular plane of motion and can cause inertial load on the construction of the robot. To study this affect we consider the robot as a pendulum with movable suspension base. This article shows the possibility of a kind of parametric resonance.

Effect of gripping conditions and strip oscillations on vibrations originating at cold rolling

The process of continuous rolling is subjected to non-stationarity, accompanied by oscillations of not only basic technological and power parameters, but also parameters in the hearth of deformation. Non-stationarity at the rolling accounting high dynamic of the process results in origination of negative vibration effects. To prevent the vibrations originations the rolling speed is decreased, which prevents the reaching of the designing parameters of rolling mills operation. The study of influence of non-stationarity of the rolling technological process on gripping conditions and origination of dangerous oscillations was carried out. Characteristics of rolling mode of strip of 2.0–0.45 × 970 mm dimension, accompanied by vibration was presented, as well as graphs of the strip oscillations in the process of stationary rolling and under conditions of vibrations origination presented. An assumption made, that during vibrations origination the gripping conditions of the strip by rollers are disturbed. It means that provision of the grip stationarity will decrease the risk of the vibrations originating at the rolling. The assumption was confirmed by the studies of the cold rolling at five-stand mill 1700 of Cherepovets steel-works. The calculations of the hearth of deformation parameters were made by application of an elaborated dynamic model of the rolling process. The higher probability of vibrations originating at the rolling of strips of smaller thickness explained. It was shown, that due to the strip length, the rolling of the thinner metal is accomplished at higher speeds, comparing with the rolling of thicker metal. In addition, it was shown that the friction coefficient value at that would promote increase of risk of non-stationary rolling mode originating. The support of stable friction conditions in the hearth of deformation is promoted by usage of chrome-plated working rollers and asymmetrical rolling.

Mechanical challenges of electrical transmission lines inspection robot

In this article, the dynamics of electrical transmission lines inspection robot while moving on the line has been investigated and mathematical modeling issues of its movement along the conductors have been considered. In order to achieve dynamical stability during the movement of inspection robot on the electrical line variation methods for problems of stretched string has been considered and solved. However, line inspection robot operation might be subject to failures because of conductor vibration. In this article, the robot-inspector considered as moving load and pendulum. As a result of even steady motion of inspection robot on the line, saw-tooth oscillations have been observed in the vertical plane that causes parametric oscillations in the perpendicular plane. In order to avoid dangerous oscillation, dynamic vibration absorber has been designed. The behavior of electrical line inspection robot motion through transmission lines has been studied.

Earthquake resistance of buildings with suspended structures

There are different approaches to providing earthquake resistance for buildings. One implies selection of efficient structural systems with high dynamic characteristics, the use of which can significantly reduce seismic loads on buildings. One of such solutions suggests the use of suspended buildings. Due to various structural solutions in the buildings, a significant increase in the natural period of oscillations and, therefore, decrease in seismic loads on buildings can be achieved. Structural solutions of these buildings were analysed and dynamic characteristics of solutions under review were examined. Design investigation was performed with involvement of software applications SCAD and SOFiSTiK based on a technique using finite elements. The most efficient structural solution for a suspended building was defined. It was demonstrated that a higher ductility may lead to a dangerous oscillation of a building, especially under low-frequency seismic impacts. To limit dangerous shifts, it is suggested to introduce special damper elements in the flooring level to improve reliable and safe behaviour of buildings with suspended structures during earthquakes.

Large eddy simulation of thermal mixing with conjugate heat transfer at BWR operating conditions

Thermal fatigue occurs in most metals under cyclic heat loads and can threaten the structural integrity of metal parts. Detailed knowledge of these loads is of utter importance to prevent such issues. In this study, a large eddy simulation (LES) with wall-adapting local eddy viscosity (WALE) subgrid model is performed to better understand turbulent thermal mixing in an annulus with a pair of opposing cold inlets at a low axial level (z=0.15m) and with a pair of opposing hot inlets at a higher axial level (z=0.80m). Each inlet pair is 90° from each other in the azimuthal direction. Conjugate heat transfer between fluid and structure is accounted for. The geometry simplifies a control-rod guide tube (CRGT) in a boiling water reactor (BWR). LES results are compared with measurement data. This is one of the first times BWR conditions are met in both experiments and LES: pressure equals 7.2 MPa, while the temperature difference between hot and cold inlets reaches 216 K. LES temperatures at the fluid-structure interface are fairly correlated with their experimental equivalents, with regard to mean values, local variances, and dangerous oscillation modes in fatigue-prone areas (z=0.65-0.67m). An elastic analysis of the structure is performed to evaluate stress intensities there. From them, cumulative fatigue usage factors (CUFs) are estimated and used as screening criteria in the subsequent frequency analysis of temperature time series at the fluid-structure interface. The likelihood of initiating a fatigue crack is linked to the maximum CUF, which is 3.2 × 10−5 for a simulation time of ~10s.

Mechanics of robot inspector on electrical transmission lines conductors: performance analysis of dynamic vibration absorber

The aim of this paper is to create a mathematical model of the movement of electrical transmission lines inspection robot while moving on the line. In this model, the electrical line is considered as a stretched string while the robot considered as a moving load on the line. The obtained equation of motion has been solved by using the Lagrange equation. Even the motion of the inspection robot on the line is steady, but dangerous vibrations in the vertical plane of the motion have been observed. These types of oscillation may cause parametric oscillations in the perpendicular plane since the robot-inspector considered as moving load and pendulum. To achieve dynamical stability during the movement of inspection robot on the electrical line and avoid dangerous oscillation, dynamic vibration absorber has been designed. Then the performance of dynamic vibration absorber has been investigated.

Transonic Panel Flutter in Accelerating or Decelerating Flow Conditions

Nonlinear panel flutter oscillations at transonic and low supersonic flow speed demonstrate rich panel dynamics, which includes bifurcations of the limit cycle, coexisting of different limit cycles, and nonperiodic oscillations. Passing through the range of transonic Mach numbers to supersonic cruise speed should be sufficiently fast to avoid significant fatigue damage. In this study, the sequence of bifurcations of limit cycles when the flow speed is continuously increasing or decreasing is analyzed. The evolution of limit-cycle oscillations is carefully studied. It is shown that the most dangerous oscillation regimes, high-frequency periodic or nonperiodic oscillations, are suppressed if the flow acceleration is sufficiently fast. However, first-mode limit cycle and limit cycle involving internal 1:2 resonance are not affected by the flow acceleration, such that lower accumulation of the fatigue damage in fast accelerating flow is possible only because of fewer cycles of oscillations, but not because of the decrease of the limit-cycle amplitude.

Ensuring the aerodynamic stability of the long-span bridges through studies in the wind tunnel

The main ideas of the aerodynamic studies of large bridges are presented in present paper. Main types of aero-elastic instability for bridges with spans over 100 meters are considered. A two-step modeling approach is presented. At the first stage, the aerodynamic characteristics of the span fragment are considered, at the second.stage the characteristics of the whole bridge. Methods for investigation of bridge oscillations in a special-purpose experimental facility – the Landscape Wind Tunnel – are described. Examples of tests with elastic similar models of bridges are given, and measurements to mitigate dangerous oscillations early in the bridge design process are described.

Advanced Dynamic Model Development of Dry Gas Seal

Dynamic processes of dry gas seal operation are investigated in this paper. Joint consideration of three models is used to determine stiffness and damping properties of the gas film. There are a two-mass dynamic model, hydraulic model (to determine pressure distribution in the sealing gap) and spiral-grooves model (to determine seal gap). Lifting force of the seal is analyzed under conditions of axial rotor oscillations. For this purpose, single-mass models (varying degrees of complexity) “rotating ring - rotor” are created. An analytical solution in the first approximation can provide the determination of dangerous oscillation levels. The phenomenon of separation of rotating ring and sleeve's face on a rotor is investigated. Causes of this phenomenon are analyzed. Motion characteristics of the rotating ring are calculated for two cases. The first one directly deals with the separation process; the second one examines the impact of inner sleeve on the seal operation. Analytical three-mass model of the seal is created to determine the dynamic characteristics for the case of separation. Results from analytical solution are compared with the results from numerical simulations.

Method of vibration diagnostics of aerodynamic and aeroelastic oscillations in gas turbine engine compressor

The paper analyzes aeroelastic (flutter) and aerodynamic (rotating stall) oscillations in gas turbine engine compressors. We review the external factors that influence these oscillations. We propose a new method of diagnosing hazardous oscillations. The method is based on the analysis of frequency characteristics. The novelty of the method is elements of forecasting introduced in it. We infer the presence of dangerous oscillations by a certain threshold level of the amplitude of the dynamic signal. We forecast this threshold depending on the operation conditions of a gas turbine engine. If the amplitude of the signal reaches the threshold level the engine speed is decreased by changing the rate of fuel consumption. This measure prevents an emergency. The method was implemented during bench tests of an aircraft gas turbine engine.

Determination of critical flow velocity for tube-bundle with application of numerical investigation method

The work is devoted to the creation and application of mathematical models for the most dangerous oscillation excitation mechanisms of tubes and cylindrical form bluff structures in liquid or gas flow, as well as to the creation of efficient computational methods for description of these models. A numerical investigation method of hydrodynamic forces arising from a separated flow and tube-bundle oscillations excited by these forces was developed by the authors. The method is based on the application of created original tube-bundle hydroelastic oscillation excitation in a cross-flow mathematical model. Hydroelastic excitation problem is reduced to the stability analysis of undisturbed state of elastic tubes. Analysis is conducted with the assumption of linearity of the destabilizing forces. On the basis of the mathematical model, the necessary and sufficient condition for the stability, expressed through the dimensionless system parameters (mass, damping, velocity), was obtained. Numerical identification of the linear hydrodynamic connection matrix algorithm for particular tube-bundles was elaborated. Verification of algorithm and programs based on it was performed by results of simulations and available experimental data correlation. A method for determination of a linear hydrodynamic connection matrix for tube-bundles with a regular arrangement of the cross-section was offered. It is based on computation of a relatively small, but sufficient for reliable results, part of the tube-bundle.

Европа и педагогическая проблема межкультурного взаимодействия: знание и личное участие за пределами утопии

In western states today, and not only in these quarters, there are noteworthy and dangerous oscillations between extreme forms of nationalism and more democratic trends. There is a tendency to shut off towards other nations, to see one’s own nationality as a cultural and traditional dimension in need of protection, currently under threat from invasion by other ethnicities, who cannot easily be assimilated to one’s own values and principles. How, then, is it possible to overcome the appeal of ideologies of resistance of nationalistic, conservative type? In all probability, an excellent first step is offered by the new pedagogical rationality of education for democracy, which is at the head of the current of critical pedagogy of emancipation, calling for responsible choices and direct participation in the democratic life of a civil society.

Control of bridge cranes with distributed-mass payloads under windy conditions

Operating cranes is challenging because payloads experience large and dangerous oscillations, especially when the system is suffering from wind disturbances and the large-size payload is modeled as a distributed-mass model. The payload oscillations induced by both intentional motions commanded by the human operator and by the external wind disturbances make the dynamics more complicated. This paper presents a novel combined control architecture to limit oscillations of the distributed-mass payload caused by both human-operator commands and wind disturbances. While a smoothed command suppressed operator-induced oscillations, a wind-rejection command eliminated the payload swing resulting from the wind gusts. Through simulations, a large range of system parameters and motions are analyzed to investigate the dynamic behavior of bridge cranes with distributed-mass beams and wind disturbances by using the new control scheme. Experimental results obtained from a small-scale bridge crane validate the simulated dynamic behavior and the effectiveness of the proposed method.

Performance of an electronic control system for hydraulically driven forestry tandem trailers

Timber transportation can be a complex operation because variations in timber types and soil characteristics can require changes to the vehicles and techniques used. Furthermore, this operation can be dangerous in unfavourable soil conditions (e.g. frozen and muddy ground). A solution to the problem is the use of mechanically or hydraulically driven trailers. The object of this study was to determine whether an innovative electronic control system for trailers equipped with motor axles, could be adapted to the hydraulic transmissions usually mounted on the forestry tandem trailers. The control system consisted of software that is able to modulate the forward speed of the trailer to that of the tractor as a function of the force presented on the hooked components. The control system mounted on forestry tandem trailers was found to have good performance and the versatility of the forestry tandem trailer was improved. Trailers equipped with hydraulic drive and the control system performed similarly to trailers fitted with mechanical drives normally used in forestry. However, unlike the latter, they had a higher ground clearance and were lighter because the absence of motor axle. It was considered that the control system increased safety levels because the tractor was protected from potentially dangerous oscillations generated by the trailer whilst driving on poor roads. • Evaluation of an electronic control system for hydraulic transmission on a trailer. • System was tested on the forestry tandem trailers using test routes. • System had good performance going up and downhill. • System improved the versatility of the forestry tandem trailers. • Control system could increase the general safety level of forestry trailers.

Radioisotope method of compound flow analysis

The paper presents gamma radiation application to analysis of a multicomponent or multiphase flow. Such information as a selected component content in the mixture transported through pipe is crucial in many industrial or laboratory installations. Properly selected sealed radioactive source and collimators, deliver the photon beam, penetrating cross section of the flow. Detectors mounted at opposite to the source side of the pipe, allow recording of digital signals representing composition of the stream. In the present development of electronics, detectors and computer software, a significant progress in know-how of this field may be observed. The paper describes application of this method to optimization and control of hydrotransport of solid particles and propose monitoring facilitating prevent of a pipe clogging or dangerous oscillations.