Non Linear System Identification of Offshore Floating Structures

2008 ◽  
Author(s):  
Nadathur P. Varadarajan ◽  
Satish Nagarajaiah
Keyword(s):  
Frequency Response ◽  
Hilbert Transform ◽  
Low Frequency ◽  
Nonlinear Behavior ◽  
Nonlinear Effects ◽  
Floating Structure ◽  
Floating Structures ◽  
Spar Platforms ◽  
The Time Domain ◽  
Non Linear System

Floating structures such as spar platforms are typically designed to be compliant or move with environment loadings, rather than resisting them. Hence they are designed so that there is no dynamic amplification in wave frequency response. However, higher order nonlinear effects are produced in low frequency wind excited regions, especially in Spar platforms. It is difficult to separate the nonlinear behavior of the model response from the loading using conventional methods. In this paper, Empirical Mode Decomposition and Hilbert Transform (EMD/HT) is used to identify the nonlinear response of a spar from the model test results. From the measured response the dynamic parameters are estimated as follows: 1) The multi-component response of the floating structure is decomposed into IMF components. 2) Hilbert transform of the input and the IMF signal in the time domain is done to extract the instantaneous dynamic characteristics. 3) Amplitude and frequency dependent frequency response function is used to represent the result of HT identification. The EMD method can identify any changes in system properties in real time and can be effectively used for repair and retrofit.

Characterization of Electro-Active Paper Vibration Sensor by Impact Testing and Random Excitation

2015 ◽  
Vol 07 (04) ◽  
pp. 1550065 ◽  
Author(s):  
Zafar Abas ◽  
Dong Ho Yang ◽  
Heung Soo Kim ◽  
Moon Kyu Kwak ◽  
Jaehwan Kim
Keyword(s):  
Frequency Response ◽  
Response Function ◽  
Polyvinylidene Fluoride ◽  
Frequency Response Function ◽  
Low Frequency ◽  
Random Excitation ◽  
Impact Testing ◽  
Vibration Sensor ◽  
Dynamic Strain ◽  
The Time Domain

We characterized a vibration sensor made of piezoelectric paper by measuring the frequency response function of an aluminum cantilever that was subjected to impulse loading and random excitation. The dynamic characteristics of the device were measured by recording the transient response of the smart cantilever beam with a pair of electro-active paper (EAPap) and polyvinylidene fluoride (PVDF) sensors located at a 5 mm distance from the clamped end as well as from a second pair of piezoelectric sensors located at a distance of 140 mm. The responses were measured by impacting the cantilever at its tip and at its mid-point. A fast Fourier transform was applied on the time domain data to measure the resonant frequencies of the vibrating structure. Both the EAPap and the PVDF sensors were observed to be very sensitive to varying levels of dynamic strain. The EAPap sensor showed a low strain sensitivity that was found to be desirable due to the inherent piezoelectricity and eco-friendly behavior of the material. The results revealed that the dynamic sensing ability of the EAPap at a low frequency range was quite comparable to that of PVDF when monitoring structural vibrations. The frequency response function was also measured via random excitation, piezoelectricity of the EAPap sensor shows potential for sensing vibrations with a dynamic response.

Fractional Control of a Single-Link Flexible Manipulator

2005 ◽  
Author(s):  
Vicente Feliu ◽  
Blas M. Vinagre ◽  
Concepcio´n A. Monje
Keyword(s):  
Low Frequency ◽  
Nonlinear Effects ◽  
Flexible Manipulator ◽  
Phase System ◽  
Low Frequency Vibration ◽  
Control Scheme ◽  
Single Link ◽  
The Time Domain ◽  
Tip Mass ◽  
Fractional Controller

A new method to control single-link lightweight flexible manipulators in the presence of changes in the load is proposed in this paper. The overall control scheme consists of three nested control loops. Once the friction and other nonlinear effects have been compensated, the inner loop is designed to give a fast motor response. The middle loop decouples the dynamics of the system, and reduces its transfer function to a double integrator. A fractional-derivative controller is used to shape the outer loop into the form of a fractional-order integrator. The result is a constant-phase system with, in the time domain, step responses exhibiting constant overshoot, independently of variations in the load. Continuous and discrete approximate implementations of the fractional controller are simulated. Comparison of the responses to a step command of the manipulator controlled with the proposed approximations and with the ideal fractional controller showed that the latter could be accurately approximated by standard continuous and discrete controllers of high order preserving the robustness. An interesting feature of this control scheme is that the overshoot is independent of the tip mass. This allows a constant safety zone to be delimited for any given placement task of the arm, independently of the load being carried, thereby making it easier to plan collision avoidance. Simulations also include comparison with standard PD controller, and verification of the assumption of dominant low-frequency vibration mode.

A SPICE-Compatible Nonlinear CCII Macromodel

2017 ◽  
Vol 26 (09) ◽  
pp. 1750144 ◽  
Author(s):  
C. Sánchez-López ◽  
M. A. Carrasco-Aguilar ◽  
F. E. Morales-López
Keyword(s):  
Dynamic Range ◽  
Nonlinear Function ◽  
Low Frequency ◽  
Nonlinear Behavior ◽  
Chaotic Signal ◽  
Nonlinear Circuits ◽  
Current Conveyors ◽  
Active Device ◽  
Dc Gain ◽  
The Time Domain

Experimental results of a SPICE-compatible macromodel to model the nonlinear behavior of second generation current conveyors (CCIIs) at low frequency are presented. The derived macromodel includes not only those real physical performance parameters more important for CCII like the dynamic range, slew rate, DC gain and gain–bandwidth product, but parasitic resistors and capacitors associated to the input and output terminals are also included. To validate the derived macromodel, a saturated nonlinear function series (SNFS) was built by using AD844AN commercially available active device configured as CCII and embedded in a chaotic system. After that, an experimentally generated chaotic signal was applied as excitation signal to SNFS built with AD844AN foundry-provided macromodel and the proposed herein. Our results indicate that the derived macromodel can be used in the time-domain for forecasting the behavior of nonlinear circuits without worsening the accuracy and at less time compared with the foundry-provided macromodel.

scholarly journals Understanding the low frequency response of carbon nanotube thermoacoustic projectors

2021 ◽  
Vol 498 ◽  
pp. 115940
Author(s):  
Prashant Kumar ◽  
Rammohan Sriramdas ◽  
Ali E. Aliev ◽  
John B. Blottman ◽  
Nathanael K. Mayo ◽  
...  
Keyword(s):  
Carbon Nanotube ◽  
Frequency Response ◽  
Low Frequency

A waveform inversion technique for measuring elastic wave attenuation in cylindrical bars

Geophysics ◽  
1992 ◽  
Vol 57 (6) ◽  
pp. 854-859 ◽  
Author(s):  
Xiao Ming Tang
Keyword(s):  
Elastic Wave ◽  
Wave Attenuation ◽  
Waveform Inversion ◽  
Finite Size ◽  
Low Frequency ◽  
Frequency Range ◽  
Multiple Reflections ◽  
Low Frequencies ◽  
The Time Domain ◽  
Attenuation Values

A new technique for measuring elastic wave attenuation in the frequency range of 10–150 kHz consists of measuring low‐frequency waveforms using two cylindrical bars of the same material but of different lengths. The attenuation is obtained through two steps. In the first, the waveform measured within the shorter bar is propagated to the length of the longer bar, and the distortion of the waveform due to the dispersion effect of the cylindrical waveguide is compensated. The second step is the inversion for the attenuation or Q of the bar material by minimizing the difference between the waveform propagated from the shorter bar and the waveform measured within the longer bar. The waveform inversion is performed in the time domain, and the waveforms can be appropriately truncated to avoid multiple reflections due to the finite size of the (shorter) sample, allowing attenuation to be measured at long wavelengths or low frequencies. The frequency range in which this technique operates fills the gap between the resonant bar measurement (∼10 kHz) and ultrasonic measurement (∼100–1000 kHz). By using the technique, attenuation values in a PVC (a highly attenuative) material and in Sierra White granite were measured in the frequency range of 40–140 kHz. The obtained attenuation values for the two materials are found to be reliable and consistent.

Collective oscillations in bubble clouds

2011 ◽  
Vol 680 ◽  
pp. 114-149 ◽  
Author(s):  
ZORANA ZERAVCIC ◽  
DETLEF LOHSE ◽  
WIM VAN SAARLOOS
Keyword(s):  
Frequency Response ◽  
Acoustic Field ◽  
Low Frequency ◽  
Acoustic Energy ◽  
Viscous Damping ◽  
Edge States ◽  
Bubble Cloud ◽  
Collective Oscillations ◽  
The Individual ◽  
Bubble Oscillations

In this paper the collective oscillations of a bubble cloud in an acoustic field are theoretically analysed with concepts and techniques of condensed matter physics. More specifically, we will calculate the eigenmodes and their excitabilities, eigenfrequencies, densities of states, responses, absorption and participation ratios to better understand the collective dynamics of coupled bubbles and address the question of possible localization of acoustic energy in the bubble cloud. The radial oscillations of the individual bubbles in the acoustic field are described by coupled linearized Rayleigh–Plesset equations. We explore the effects of viscous damping, distance between bubbles, polydispersity, geometric disorder, size of the bubbles and size of the cloud. For large enough clusters, the collective response is often very different from that of a typical mode, as the frequency response of each mode is sufficiently wide that many modes are excited when the cloud is driven by ultrasound. The reason is the strong effect of viscosity on the collective mode response, which is surprising, as viscous damping effects are small for single-bubble oscillations in water. Localization of acoustic energy is only found in the case of substantial bubble size polydispersity or geometric disorder. The lack of localization for a weak disorder is traced back to the long-range 1/r interaction potential between the individual bubbles. The results of the present paper are connected to recent experimental observations of collective bubble oscillations in a two-dimensional bubble cloud, where pronounced edge states and a pronounced low-frequency response had been observed, both consistent with the present theoretical findings. Finally, an outlook to future possible experiments is given.

Simulation of Engine Vibration on Nonlinear Hydraulic Engine Mounts

2005 ◽  
Author(s):  
A. R. Ohadi ◽  
G. Maghsoodi
Keyword(s):  
Equations Of Motion ◽  
Low Frequency ◽  
Governing Equations ◽  
Engine Mounts ◽  
Engine Vibration ◽  
Engine Mount ◽  
Rotational Motions ◽  
The Time Domain ◽  
Nonlinear Equations Of Motion ◽  
Four Cylinders

In this paper, vibration behavior of engine on nonlinear hydraulic engine mount including inertia track and decoupler is studied. In this regard, after introducing the nonlinear factors of this mount (i.e. inertia and decoupler resistances in turbulent region), the vibration governing equations of engine on one hydraulic engine mount are solved and the effect of nonlinearity is investigated. In order to have a comparison between rubber and hydraulic engine mounts, a 6 degree of freedom four cylinders V-shaped engine under inertia and balancing masses forces and torques is considered. By solving the time domain nonlinear equations of motion of engine on three inclined mounts, translational and rotational motions of engines body are obtained for different engine speeds. Transmitted base forces are also determined for both types of engine mount. Comparison of rubber and hydraulic mounts indicates the efficiency of hydraulic one in low frequency region.

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