Single Response Measurement Research Articles

Conditional linear approximation of nonlinear systems using Operational Modal Analysis

Dynamic characteristics of structures in operational conditions are commonly identified from measured responses using Operational Modal Analysis (OMA). The OMA techniques are, however, confined to the principle of linearity. To overcome some of this limitation, this paper proposes a method for an OMA-based conditional linear approximation of a type of nonlinear systems, by which two or more sets of linear modes are estimated that together describe the behaviour of the true system. These sets of modes can be used to update a nonlinear numerical model that fits each linear estimate in relation to the associated conditions. Additionally, the method can alleviate the issue of varying approx. natural frequencies of nonlinear systems, when employing Structural Health Monitoring to detect damages based on changes of these. The method is demonstrated on both a numerical and an experimental study. Specifically, the numerical study consists of a cantilever beam with a clearance and a stopper at the tip, and it is shown, based on a single response measurement with multiple channels, that the method enables identification of both the underlying linear system and a linear system with modal properties affected by the nonlinearity. The experimental study consists of two simple, friction-coupled, offshore platform-like models, for which two sets of modes are estimated from one measurement, each set characterising the dynamic behaviour in coupled and uncoupled state, respectively. The paper also demonstrates that the proposed method can relieve the said complications of conducting Structural Health Monitoring of structures with changing natural frequencies due to nonlinearity.

Tuned-Sinusoidal Method for the Operational Modal Analysis of Small and Light Structures

Small and light structures have some distinctive features that intensify the difficulties of measuring their modal parameters. The mass that is added to the structure by the sensors cannot be neglected and the resonant frequencies are usually relatively high, therefore a wide frequency range of measurement is needed. There are also difficulties with assuring the proper excitation, so that all the measured modes are excited well and that at the same time the excitation level is not too large, which would cause larger response of the structure than the measuring ranges of the sensors can cover. In this study an innovative method for operational modal analysis of small and light structures is presented. The method is non-contact, therefore there is no added mass of the sensors to the structure. The structure is acoustically excited with a pure sine signal that is tuned to each resonant frequency. A single response measurement with the Laser Doppler Vibrometer in individual points is needed to determine the modal parameters. A mass-change strategy is used for the mass-normalisation of the measured mode shapes. The main contribution of the presented method compared to other similar methods is that the mode shapes are better accentuated (due to sine excitation), which can improve the results of the modal analysis on small and light structures, where the response of the structure is weak. The method is also easy to perform, because only a single response measurement is needed for each point and the excitation force does not need to be measured. The presented method gives accurate results, which was confirmed with the comparison of the experimental and the numerical results on a sample of simple geometry.

Behavioral methods in infancy: pitfalls of single measures

This paper outlines the principal behavioral methods used to study music processing in infancy. The advantages of conditioning procedures are offset by high attrition rates and restrictions on the stimuli that can be used. The head-turn preference procedure is more user-friendly but poses greater interpretive challenges. In view of the multidimensional nature of infant attention, no single response measure, whether behavioral, physiological, or neural, can provide unambiguous information about music processing in infancy. Greater use of ecologically valid stimuli is likely to generate increased cooperation from infants and greater generality of the findings.

A Decrement Method for Quantifying Nonlinear and Linear Damping in Multidegree of Freedom Systems

A method is presented which can estimate the linear and nonlinear damping parameters in a lightly damped multidegree of freedom system which allows the system to be decomposed into a set of single degree of freedom nonlinear systems. Only a single response measurement from a free decay test is required as input ensuring that the magnitude of the damping parameters is not compromised by phase distortion between measurements. The response is band-pass filtered in the time domain, around each of the natural frequencies. While this provides a free response measurement for each mode, it introduces a restriction as the natural frequencies must be distinct and separated. The instantaneous energy of each trace is used to describe the long-term evolution of the mode. This is achieved by using only the peak amplitudes in each period, and so the stiffness and inertial forces are effectively ignored, and only the damping forces are considered. Thus, the method is not unlike the familiar decrement method, which can estimate the viscous damping in linear systems. The method is developed for a weakly nonlinear, lightly damped two-degree-of-freedom system, with both linear and Coulomb damping. Simulated response data is used to demonstrate the accuracy of the technique.

An intrusion response decision-making model based on hierarchical task network planning

An intrusion response decision-making model based on hierarchical task network (HTN) planning is presented in the paper. Compared with other response decision-making models, the response decision-making model consists of not only the response measure decision-making process but also response time decision-making process that is firstly proposed in the paper. The response time decision-making model is able to determine response time for different response HTN subtasks. Owing to the introduction of the response time decision-making, the intrusion response system can apply different response strategies to achieve different response goals set by administrators. The proposed response measure decision-making model can optimize a response plan by balancing the response effectiveness and the response negative impact in both a single response measure and a set of response measures. The response decision-making model is self-adaptive and has the ability of tolerating to false positive IDS alerts. The proposed model has been used in the intrusion detection alert management and intrusion response system (IDAM&IRS) developed by us. The functions and architecture of IDAM&IRS are introduced in this paper. In addition, the intrusion response experiments of IDAM&IRS are presented, and the features of the response decision-making model are summarized.

A decrement method for quantifying nonlinear and linear damping parameters

A method is presented which can estimate the linear and nonlinear damping parameters in a lightly damped system. Only a single response measurement from a free decay test is required as input. This ensures that the magnitude of the damping parameters is not compromised by phase distortion between measurements. The method uses the instantaneous energy to describe the long-term evolution of the system. Practically this is achieved by using only the peak amplitudes in each period. In this way the stiffness is effectively ignored, and only the damping forces are considered. For this reason, the method is not unlike the familiar decrement method, which can be used to estimate the apparent linear damping. The method is developed in the context of a weakly nonlinear, lightly damped system, with both linear and cubic damping. Simulated response data is used to demonstrate the accuracy of the technique. The nonlinear damping parameter is extracted from the response data to within 5% of the exact value, even though the nonlinear term contributes less than 1% to the total force in the system.

Playback experiments: design and analysis

The scientific value of the outcome of an experiment is closely related to its design and analysis. This article deals with the design issues of pseudoreplication (whether the experimental design has the statistical features needed to answer the question as posed) and execution errors (problems arising from how the experiment was conducted). Three issues of analysis are also dealt with: the number and type of response measures to record; how measures should, and should not, be combined into a single response measure; and how to interpret an apparent lack of response. Interactive playback is considered separately because it raises its own specific design and analysis issues. Although the examples generally refer to video playback, these issues are common to all experiments in behaviour.

Global Active Noise Control of a One-Dimensional Acoustic Duct Using a Feedback Controller

Active noise control of acoustic enclosures is a classical engineering problem. The active noise control of a one-dimensional hard-walled duct with a partially dissipative boundary condition is addressed in this paper. Previous techniques have attacked this problem by developing adaptive filters designed to cancel acoustic noise at a single measurement location. The work presented here applies modern, state space, control theory to globally reduce noise levels in a one-dimensional acoustic enclosure rather than at a single location. This global control requires only the addition of a single response measurement microphone and control speaker to the open-loop system. Pressure measurements are taken at the microphone location and passed to an observer, which generates estimates of the system states. Using these state estimates, a pole placement feedback control algorithm is used to lower the noise level. Pole placement reduces noise levels globally by increasing the decay rates of all the controlled acoustic states. Experimental results presented here demonstrate reduction in the noise level at all points in the duct by 58 percent when the system is excited by random amplitude pressure input.

Multivariate Analysis Of Intrinsic Individual Differences In Disjunctive Conceptual Behavior.

Since it is known that intrinsic individual differences increase with the complexity of the behavior and since it is suspected that no single response measure can adequately represent the learning process, the present study examined intrinsic individual differences in conceptual behavior with a multivariate model, three-mode factor analysis. Four response measures were selected according to their traditional or apparent importance to the analysis of concept learning. The number of errors, response latency, post-response time, and interresponse time were those selected to reflect the subject's performances on a figural, disjunctive conceptual behavior task. The analyses yielded five individual difference performance factors, three stage factors, and four response components indicating the importance of a multivariate representation of complex behavior.