An in situ modal-based method for structural dynamic joint parameter identification

Shield machine plays an indispensable role in the mining, transportation, underground engineering, hydraulic engineering and municipal construction. Shield cutters of shield construction process often appears serious deformation, damage that leads to engineering accident, or even a threat to human life and safety. In order to provide high precise data for shield machine cutter tool dynamic modification and to diagnose the shield machine cutter tool fault, the dynamic characteristic of the shield machine cutter tool system, which is the main component of a shield machine cutter tool, has to be obtained precisely. The compute modal parameters identification method base on the finite element method is proposed to identify the modal parameters of the shield machine cutter tool. By means of Solidwords software, the knife ring structure of the shield machine tool and the tool is designed; then build the tool the finite element model, modal analysis, obtained the dynamic characteristics, and find out the weak link, put forward the improvement measures and prolong its life. Therefore, the study on dynamic characteristics of shield machine cutter, for the optimization design of domestic tool, has an important significance improve tool life.

A synthesis of peak picking method and wavelet packet transform for structural modal identification

In this study, a combined finite element (FE)–boundary element (BE) method is presented to investigate the dynamic characteristics of shell and plate structures in contact with fluid. The numerical procedure consists of two parts. In the first part, the dynamic characteristics of structures are obtained under in-vacuo condition by using the finite element method. Then, in the second part, fluid-structure interaction effects are computed in terms of generalized added mass coefficients by using the boundary element method. In analyses, surrounding fluid is assumed ideal, i.e. inviscid, incompressible and irrotational, in the context of linear hydroelasticity theory. In order to show the applicability of the proposed method, the dynamic characteristics of two different structures—a vertical rectangular plate in contact with fluid on one side and a horizontal cylindrical shell partially filled with fluid—are investigated and compared with the results obtained with a commercial software, ANSYS.

Cognitive Causality Modelling for Human Behavior: The Method and Its Applications

In this study, the dynamic model for the herringbone planetary gear transmission system is established by the lumped parameter method based on the system dynamics and the Lagrange equation, and the impact of the support stiffness and the torsional stiffness on dynamic characteristics is studied. The research results have a guiding significance for the design of the herringbone gear transmission system. In this model, the herringbone gear is treated as a special gear coupled by 2 opposite helical gears, where the stagger angle, comprehensive meshing error, support stiffness, support damping, and load inertia are considered in the analysis of dynamics. Moreover, the dynamic characteristic of the carrier is considered as well. By calculating the meshing force curve of the transmission system, the impact of the stagger angle, supporting stiffness, and the torsional stiffness on meshing force and load sharing coefficient is analyzed. The results show that the stagger angle has an obvious impact on load sharing coefficient while it has little impact on maximum meshing force. And the support stiffness has a more obvious impact on the dynamic characteristics of the system. The recommendary support stiffness of the system is that all of the support stiffness of the sun gear, planetary gear, ring gear, and carrier is 107 N/m. The torsional stiffness has little impact on the dynamic characteristics of transmission system, except the torsional stiffness of planetary gear, and carrier has an obvious impact on load sharing coefficient. The commercial software ADAMS carried out dynamics analysis of the transmission system to verify the necessity validity of the theoretical analysis.

Algorithms of exact restoration in an analytical form of dynamic characteristics of non-stationary dynamic systems are constructed. Non-stationary continuous dynamical systems modeled by Volterra integral equations of the first kind and non-stationary discrete dynamical systems modeled by discrete analogues of Volterra integral equations of the first kind are considered.The article consists of an introduction and three sections: 1) The exact restoration of the dynamic characteristics of continuous systems, 2) The restoration of the transition characteristics of discrete systems, 3) Conclusions. The introduction provides a statement of the problem and provides an overview of dynamical systems for which algorithms for exact reconstruction in ananalytical form of the impulse response (in the case of continuous systems) and the transition characteristic (in the case of discrete systems) are constructed. In the first section, the algorithm is constructed for the exact reconstruction of the impulse response of an non-stationary continuous dynamic system from three interconnected input signals. The first signal may be arbitrary, the second and third signals are associated with the first signal by integral operator. The exact formula for the Laplace transform of the impulse response, represented by an algebraic expression from the Laplace transform of the system output signals, is given. A model example illustrating the effectiveness of the algorithm is given. The practical application of the presented algorithm isdiscussed. In the second section, an algorithm is constructed for the exact reconstruction of the transition response of a non-stationary discrete dynamical system from three input signals that are interconnected. The first signal may be arbitrary, the second and third signals are associated with the first summing operator. The exact formula of the Z-transform of the transition characteristic is presented, which is represented by an algebraic expression from the Z-transform of the system output signals. A model example is given. The “Conclusions” section provides a summary of the results presented in the article and describes the dynamic systems to which the proposed algorithms can be extended.

In the transmission process of gear system, the change of load will make the system in different states of motion, which affects the transmission efficiency of gear system. It is important to investigate the nonlinear dynamic characteristics of gear system under different load states. Using straight cylindrical gears as the object of study, the concentrated mass method is used to establish a dynamic model that takes into account nonlinear factors such as tooth side clearance, time-varying meshing stiffness and transmission errors. The differential equations of the system are solved by the Longe-Kutta method to obtain the bifurcation diagram, the maximum Lyapunov exponent diagram and the phase plane diagram of the gear system to analyze the effect of the meshing damping ratio on the dynamic characteristics of the system under different load states. The results show that the influence of the engagement damping ratio on the dynamic characteristics of the system is greater under light load conditions, showing different states of motion as the engagement damping ratio gradually increases. Under heavy load conditions, the effect of the engagement damping ratio on the dynamic characteristics of the system is small. Appropriately increasing the mesh damping ratio is beneficial to the gear system to avoid the chaotic zone and maintain a stable cyclic motion state. The results of the study provide a reference for the design of gear systems with variable loads.

Modal analysis of complex rotor-bearing systems using the finite element method (FEM) may become controversial when considering the tradeoff between geometric complexity and elastodynamic integrity of the FEM model. In this paper, the modal characteristics of an actual multi-impeller rotor-bearing system with complicated geometry are estimated and verified using two modeling schemes. The first scheme invokes a developed finite element elastodynamic model that accounts for gyroscopic effects, torsional-bending inertia coupling, internal material damping, shear and anisotropic bearings. In this model, the intricate details of the impeller were ignored, and only the inertial properties of the impeller disk are considered. The second scheme employs a general-purpose finite element code, wherein the complicated impeller geometry is included in lieu of ignoring some of the rotational effects and inertia coupling effects. The obtained results shed the light on the tradeoffs involved in modal analysis of complicated rotor systems using the FEM. Comparisons with experimental values showed that the first scheme, which adopts simplified impeller geometry while accounting for all the rotational effects is more accurate in estimating the modal characteristics of such complex rotor-bearing systems.

Using force signals to monitor mechanical systems

Studies on the planetary gear have attracted considerable attention because of its advantages, such as compactness, large torque-to-weight ratio, vibrations, and high efficiency, which have resulted in its wide applications in industry, wind turbine, national defense, and aerospace fields. We have established a novel dynamic model of the planetary gear transmission by using Newton’s theory, in which some key factors such as time-variant meshing stiffness, phase relationships, and tooth contact characteristics are considered. The influences of gear axial tipping, operating conditions, and the meshing phase on the contact characteristics and the dynamic characteristics were researched systematically. It was found that the contact area of the tooth surface was moved due to the axial gear tipping, which obviously affected the meshing stiffness. With the increase in the inclination angle of the sun gear, the meshing stiffness decreases, which produces an evident influence on the high natural frequency in the planetary transmission system. In terms of the dynamic characteristics of the system, the component of rotating frequency appeared in the dynamic meshing force of the sun gear and the planetary gear. Moreover, the floating track of the center wheel varied significantly and exhibited an oval distribution as the inclination angle of the sun gear changed. When the inclination angle of the sun gear increased, the rotating frequency component increased significantly, but the other meshing frequency components remained unchanged; meanwhile, the deformation of the floating track also increased. If the inclination angle of the sun gear changes, the vibration state of the system and the collision impact could become more serious, and the lifetime of the planetary transmission system will reduce. Furthermore, when the load was increased, we found that the gear-tooth contact zone transformed from line contact to surface contact, the meshing stiffness increased, the effect of high natural frequency on the planetary transmission system became more evident, but its low-order natural frequency remained stable. With regard to the dynamic characteristics of the system, the components of the major frequency at the external gearing remained unchanged, but the rotation frequency of the sun gear and the meshing frequency amplitude increased linearly with the increase in load. In conclusion, the variation in the meshing stiffness of the planetary gear system had minor impact on the low-order natural frequency, but had a significant impact on the high natural frequency of the planetary transmission system due to the phase variation of the gear.

Digital control of a continuoiis-time system implies discretization in time of the system. The discretization is likely to bring about changes in the dynamic characteristics of the continuous-time system. Two approaches to the design of a digital control system can be considered: 1) discretization of a previously designed continuoustime control system and 2) direct design in the discrete-time domain. The difference between these two approaches and their effects on a closed-loop control system employing a Luenberger-type observer is demonstrated. The first approach consists of simulating a continuous-tim e control system on a digital computer. Because the possibility exists that a given set of parameters of the continuous-time system, such as gains and observer poles, will produce a change in the dynamic characteristics of the system during digital simulation, the question is how large the sampling time can be without affecting adversely the discrete simulation. An example of an unstable digital simulation of a closed-loop control system employing a deterministic observer that is stable in continuous-ti me is presented. The second approach inherently guards against changes in the dynamic characteristics that may be caused by discretization. The two approaches are compared by using a second-order dynamic system.

The coupled heave and pitch motions of a ship sailing in head waves affect the stability of the marine rotor-bearing system. Based on the theory of analytical mechanics, this study establishes a dynamic model of the rotor-bearing system subjected to the coupled motions of heave and pitch, considering nonlinear oil film moments produced by the tilting of the rotor in the bearings. The nonlinear dynamic behaviours of the system are analysed using numerical methods to obtain Poincaré sections, bifurcation diagrams, and the largest Lyapunov exponents. The results show that dynamic bifurcation characteristics reveal complex quasi-periodic motion of upper and lower branches after the initial instability of the system, and the speed of second instability increase markedly. At high speeds, the amplitude of the rotor system increases sharply, which can cause the rotor to touch the inner wall of the bearings in the quasi-periodic state and a failure to transition to the chaotic state. Additionally, the effects of heave and pitch amplitude variations on the dynamic characteristics of the system are also discussed.

Based on energy conservation principle and heat transfer mechanisms, a set of dynamic model was established for an indirect district heating system. Dynamic start-up process under ideal condition was simulated and the influence of heat loss from piping network, make-up water and intermediate heat exchange station, and radiator area surplus on the dynamic characteristics of the system was discussed. Dynamic process without control was simulated under the hourly outdoor temperature measured in the days from February 13 to 19, 2012. To overcome the obvious indoor temperature variation with outdoor temperature, four PI control strategies were proposed. The dynamic characteristic and energy consumption of the system operated under these four control strategies were discussed. It is found that the control strategy 3, controlling the fuel flow rate, and water flow rate in the first loop not only produce stable working performance but also obtain considerable energy saving potentials. These results are very helpful to realize the dynamic characteristics of the indirect district heating system and to select suitable control strategy and optimal performance.

Fluid-structure interaction method is performed to predict the dynamic characteristics of axial fan system. A fluid-structure interface physical environment method (monolithic method) is used to couple the fluid flow solver with the structural solver. The integration of the three-dimensional Navier-Stokes equations is performed in the time Doman, simultaneously to the integration of the three dimensional structural model. The aerodynamic loads are transfer from the flow to structure and the coupling step is repeated within each time step, until the flow solution and the structural solution have converged to yield a coupled solution of the aeroelastic set of equations. Finite element method is applied to solve numerically the Navier-Stockes equations coupled with the structural equations The first ten eigenvalue (natural frequency), the first ten eigenvector (mode shape) and effective stress for each part of a rotor system and complete system assembly are predicted.
 The validity of the predicted dynamic characteristics of duct fan system was confirmed experimentally by investigating geometrically similar fan system test rig. Good agreement of dynamic characteristics is observed between experimental and numerical results.
 

China Spallation Neutron Source (CSNS) is a high intensity proton accelerator-based facility. Its accelerator complex includes two main parts: an H- linac and a rapid cycling synchrotron (RCS). The RCS accumulates an 80 MeV proton beam and accelerates it to 1.6 GeV, with a repetition rate of 25 Hz. The AC dipole of the CSNS/RCS is operated at a 25 Hz sinusoidal alternating current which causes severe vibration. The vibration will influence the long-term safety and reliable operation of the magnet. The CSNS/RCS AC dipole-girder system takes vibration isolator to decrease the vibratory force and the vibration amplitude of the dipole. For the long-term safety and reliable operation of the dipole, it is very important to study the dynamic characteristics of the dipole-girder system. This paper takes the dipole-girder as a specific model system. A method for studying the dynamic characteristics of the system is put forward by combining theoretical calculation with experimental testing. The modal parameters with and without vibration isolator of the dipole-girder system are obtained through ANSYS simulation and testing. Then, the dynamic response of the system is calculated with modal analysis and vibration testing data. With the simulation and testing method, the dynamic characteristics of the AC dipole-girder are studied.