Higher-order spectra appear often in the analysis and identification of nonlinear systems. The auto-bispectral density is one example of a higher-order spectrum and may be used in the analysis of ...stationary structural response data to detect the presence of certain types of structural nonlinearities. In this work a closed-form expression for the auto-bispectral density, derived previously by the authors, is used to find the bispectral frequency most sensitive to the nonlinearity. The properties of nonlinearity detectors based on estimates of the magnitude of the auto-bispectral density at this frequency are then explored. Estimates of the auto-bispectral density are obtained using the direct method based on the discrete Fourier transform. The bias associated with this estimator is derived here and combined with previously derived expressions for the estimator variance to give both Type-I and Type-II errors for the detector. Detector performance is quantified using a receiver operating characteristic (ROC) curve illustrating the trade-off between false positives (Type-I error) and power of detection (1.0-Type-II error). Theoretically derived ROC curves are compared to those obtained via numerical simulation and show excellent agreement. Results are presented for different levels of nonlinearity in both the stiffness and damping terms for a spring–mass system. Possible consequences are discussed with regard to the detection of damage-induced nonlinearities in structures.
This paper reports on the design of FRESCA2, a dipole magnet model wound with Nb 3 S n Rutherford cable. This magnet is one of the deliverables of the High Field Magnets work package of the European ...FP7-EuCARD project. The nominal magnetic flux density of 13 Tesla in a 100 mm bore will make it suitable for upgrading the FRESCA cable test facility at CERN. The magnetic layout is based on a block coil, with four layers per pole. The mechanical structure is designed to provide adequate pre-stress, through the use of bladders, keys and an aluminum alloy shrinking cylinder.
Damage detection and structural health monitoring techniques based on vibration data have seen increased attention in recent years. Among the different vibration-based methods, the ones based on ...random vibrations are of particularly interest, especially when they do not require measurement of the input(s). In this work, several frequency and time domain signal processing techniques are explored in their respective abilities to detect damage in a bolted composite structure. First, a joint loosening model is developed and used to simulate the dynamic response to a stationary Gaussian excitation. Informed by the model, two signal processing techniques are used to assess the connection strength. The first method relies on basic statistical properties of the measured strains and their time derivatives, while the second is based on the signal power in different frequency bands. Both approaches are then used to assess progressive bolt loosening on an experimental composite-to-metal joint. All strain response data were obtained using a fiber optic strain sensing system. Results are presented in the form of Receiver Operating Characteristic (ROC) curves, showing both Type-I and Type-II errors associated with the proposed detection schemes.
Higher-order spectral analysis techniques are often used to identify nonlinearities in complex dynamical systems. More specifically, the auto- and cross-bispectrum have proven to be useful tools in ...testing for the presence of quadratic nonlinearities based on knowledge of a system's input and output. In this paper, analytical expressions for the auto- and cross-bispectrum are developed using a Volterra functional approach under the assumption of a zero-mean, stationary Gaussian input; proper simplifications are presented when the whiteness of the input signal is also imposed. These formulae show the contributions of the bispectrum in terms of the system frequency response function and elementary physical properties of the system. Simulations based on a stochastic numerical integration technique accompany the analytical solutions for a mechanical mass–spring–damper system possessing quadratic damping and stiffness coefficients and subjected to Gaussian white noise excitation. Subsequent estimates of the bispectrum based on the simulated signals show excellent agreement with theory. These results show how modes may interact nonlinearly producing intermodulation components at the sum and/or difference frequency of the fundamental modes of oscillation. The presence and extent of nonlinear interactions between frequency components are identified. Advantages of using higher-order spectra techniques will be revealed and pertinent conclusions will be outlined.
Higher-order spectra have become a useful tool in spectral analysis, particularly for identifying the presence and type of system non-linearity. Two such spectra that have figured prominently in ...signal processing are the bispectrum and trispectrum. In a previous work, the authors developed an analytical solution for the bispectrum for multi-degree-of-freedom systems. Here this analysis is extended to the trispectrum. Specifically, an expression is developed for the trispectrum of a multi-degree-of-freedom system subject to Gaussian excitation applied at an arbitrary location. The analytical expression is compared to those obtained via estimation using the direct method.
Background and purpose
Drugs with anticholinergic properties might have a negative impact on cognition, but findings are still conflicting. The association was evaluated between anticholinergic drugs ...and cognitive performance in primary care patients with first cognitive complaints.
Methods
From April 2013 to March 2014, 353 general practitioners administered the Mini‐Mental State Examination (MMSE) to patients presenting with first cognitive complaints. Drug history was collected and the anticholinergic cognitive burden (ACB) was scored and categorized as ACB 0, ACB 1 and ACB 2+. A mixed effect linear regression model was used to assess the association between ACB and MMSE score.
Results
Of 4249 subjects entering the study (mean age 77 ± 8.2 years, 66.4% women and mean years of schooling 8.9 ± 4.5), 25.8% received at least one drug with anticholinergic action. According to multivariate analysis, and after adjustment for several confounders, subjects with ACB 2+ had a statistically significant lower MMSE score compared with those with ACB 0 (β −0.63; 95% confidence interval −1.19; −0.07). Subjects with ACB 1 had a non‐statistically significant lower MMSE score than those with ACB 0 (β −0.11; 95% confidence interval −0.37; 0.15).
Conclusions
Anticholinergic medication might affect cognitive function in people with first cognitive complaints. Alternatives should be taken into account when possible, balancing the benefits and harms of these medications.
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