Multiple Dominant Modes Research Articles

Research on the Residual Vibration Suppression of Delta Robots Based on the Dual-Modal Input Shaping Method

The Delta robot is a high-speed and high-precision parallel robot. When it is in function, the end effector generates residual vibration, which reduces the repeat positioning accuracy and positioning efficiency. The input shaping method has previously been shown to suppress the residual vibration of the robot, but the vibration suppression effect of the single-modal input shaper is not good for the delta robot, which has multiple dominant modes for the residual vibration. To solve this problem, this paper proposes an effective method for residual vibration suppression of Delta robots based on dual-modal input shaping technology. Firstly, the modal analysis of the Delta robot is performed using finite element software, and the dominant modal of its residual vibration is determined. Secondly, six dual-modal input shapers are designed according to the obtained modal parameters. Finally, Simulink is used for simulation analysis to verify the robustness and vibration suppression performance of the designed six dual-modal input shapers and traditional single-modal input shapers. The simulation results show that the designed ZVD-EI dual-modal input shaper has good robustness, can effectively suppress the residual vibration of the Delta robot, and can effectively improve the repetitive positioning accuracy and work efficiency of the Delta robot when it is running at high speed.

Real-Time Damping Estimation on Nonlinear Electromechanical Oscillation

Real-time damping estimation for a dominant inter-area mode is important for situational awareness of potential angular instability in power systems. Electromechanical oscillations energized by large disturbances often manifest obvious nonlinearities in measurements on first several swings. Traditional methods based on linear system theory often discard first several swings intentionally to avoid nonlinearity; if not, the estimated damping ratios often vary with the length and starting point of the measuring window. By identifying a nonlinear oscillator to fit a dominant mode, this paper proposes a new measurement-based approach utilizing complete post-disturbance data for robust damping estimation independent of the measuring window. Case studies on the IEEE 9-bus system and a 48-machine Northeast Power Coordinating Council system validate the proposed approach for providing accurate and robust damping estimation compared with existing methods including the Prony's method. Meanwhile, three factors influencing damping estimation in practical applications are also addressed, including measurement noises, limited coverage of PMU measurements, and existence of multiple dominant modes.

Passive vibration control of the SCOLE beam system

We investigate the optimization of multiple tuned mass dampers (TMDs) to reduce vibrations of flexible structures described by partial differential equations, using the non-uniform SCOLE (NASA Spacecraft Control Laboratory Experiment) beam system with multiple dominant modes as an illustrative application. We use multiple groups of TMDs with each group being placed at the antinode of the mode shape of a dominant mode of the SCOLE beam. We consider both the harmonic and random excitations, for which we employ frequency-limited H ∞ and H 2 optimizations respectively to determine the parameters of the TMDs. Our optimization scheme takes into account the trade-off between effectiveness and robustness of the multiple TMDs to suppress the multiple dominant modes of the SCOLE beam. Simulation studies show that our scheme achieves substantial improvements over the traditional methods in terms of both effectiveness and robustness and that with equal total mass, TMD systems with each group having multiple TMDs are more effective and more robust than the ones with each group having a single TMD.

三次元翼における遷音速バフェット特性のスパン方向変化

Transonic buffet is self-induced oscillation of shock waves appeared on a suction side of a wing. We simulated the transonic buffet over the NASA Common Research Model (CRM) wing, which is an open geometry, generic transport model, and characterized its variation in spanwise direction. The numerical method employed in this simulation was a zonal detached eddy simulation (ZDES), which is a RANS/LES hybrid method. The numerical results indicate that the frequency spectra of oscillation on the surface pressure were varied in spanwise direction and were categorized into three patterns. In the inboard region, buffet exhibits a periodical oscillation having a single dominant frequency equivalent to the 2D wing. At the mid-span of the wing, the wavefront of shock moves due to the existence of the angle of sweepback. In the outboard, the spectrum exhibits multiple dominant modes due to superposition of two difference sources. One is a periodical oscillation similar to the 2D wing. The other is pressure oscillation propagating from the inboard region.

Seismic analysis of in-plane loaded walls in unreinforced masonry buildings with flexible diaphragms

It is well recognised that the dynamic response of unreinforced masonry buildings with flexible timber diaphragms typically contains multiple dominant modes associated with the excitations of the diaphragms and the in-plane walls. Existing linear analysis methods for this type of structure commonly account for the multi-mode behaviour by assuming the independent vibrations of the in-plane loaded walls (in-plane walls) and the diaphragms. Specifically, the in-plane walls are considered to be rigid and the unmodified ground motion is assumed to be transmitted up the walls to the diaphragm ends. While this assumption may be appropriate for many low-rise unreinforced masonry buildings, neglecting the dynamic interaction between the diaphragms and the in-plane walls can lead to unreliable predictions of seismic demands. An alternative analysis approach is proposed in this paper, based on the mode properties of a system in which (1) the mass ratios between the diaphragms and the in-plane wall are the same at all levels, and (2) the periods of the diaphragms are the same at all levels. It is proposed that under these conditions, two modes are typically sufficient to obtain the peak seismic demands of the in-plane walls in elastically responding low-rise regular buildings. The applicability of the two-mode analysis approach is assessed for more general diaphragm configurations by sensitivity analysis, and the limitations are identified. The two-mode approach is then used to derive a response modification factor, which may be used in conjunction with a linear static procedure in the seismic assessment of buildings with flexible diaphragms.

Spatial–temporal structure of mixing interface turbulence at two large river confluences

Converging flows at stream confluences often produce highly turbulent conditions. The shear layer/mixing interface that develops within the confluence hydrodynamic zone (CHZ) is characterized by complex patterns of three-dimensional flow that vary both spatially and temporally. Previous research has examined in detail characteristics of mean flow and turbulence along mixing interfaces at small stream confluences and laboratory junctions; however few, if any, studies have examined these characteristics within mixing interfaces at large river confluences. This study investigates the structure of mean velocity profiles as well as spatial and temporal variations in velocity, backscatter intensity, and temperature within the mixing interfaces of two large river confluences. Velocity, temperature, and backscatter intensity data were obtained at stationary locations within the mixing interfaces and at several cross sections within the CHZ using acoustic Doppler current profilers. Results show that mean flow within the mixing interfaces accelerates over distance from the junction apex. Turbulent kinetic energy initially increases rapidly over distance, but the rate of increase diminishes downstream. Hilbert–Huang transform analysis of time series data at the stationary locations shows that multiple dominant modes of fluctuations exist within the original signals of velocity, backscatter intensity, and temperature. Frequencies of the largest dominant modes correspond well with predicted frequencies for shallow wake flows, suggesting that mixing-interface dynamics include wake vortex shedding—a finding consistent with spatial patterns of depth-averaged velocities at measured cross sections. Spatial patterns of temperature and backscatter intensity show that the converging flows at both confluences do not mix substantially, indicating that turbulent structures within the mixing interfaces are relatively ineffective at producing mixing of the flows in the CHZ.

An Investigation of Variable Spindle Speed Face Milling for Tool-Work Structures With Complex Dynamics, Part 1: Simulation Results

A mechanistic dynamic model is used to simulate a face milling process during constant and variable speed machining. The model can be used to predict the optimum speed trajectory that can provide a low level of vibration and consequently a large productivity rate and a small surface error. The model is used to investigate the vibration of face milling processes that have one, or multiple coupled modes of vibration acting throughout the cut. For cutting processes having one dominant mode of vibration, the model predicts that variable speed machining is especially effective over constant speed machining when the tool-work system changes its dominant mode of vibration throughout the cut, or when the tool-work system has several modes of vibration coming from component parts that are cut in the same time. For cutting processes having multiple dominant modes of vibration, the model predicts that variable speed machining is especially effective over constant speed machining when the tool-work modes of vibration are unequal and moderately coupled to each other. Also, the model suggests that for tool-work systems having complex geometries with dynamics hard to predict, variable speed machining is safer to use than constant speed machining when trying to achieve high productivity rates. This is due to the fact that variable speed machining is robust with respect to the dynamics of the tool-work system. Finally, the model predictions are in good agreement with the experiment.