AST Consulting Ltd.

Auckland, New Zealand

AST Consulting Ltd.

Auckland, New Zealand
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Steinwolf A.,AST Consulting Ltd.
Test Engineering and Management | Year: 2011

Time Variable Replication is added to the Time Waveform Replication or Time History Reproduction (TWR) shaker testing technique. A time history where 20 runs on the same road section are joined together is presented, demonstrating that there is an inherent variability which is lost if we do what is a common TWR practice. The road data is subjected to Fourier transform and from the power spectral density (PSD) obtained, any number of different time history blocks can be recreated by the inverse fast Fourier Transform (IFFT) if the phase spectrum is prescribed randomly for each next block. The height and also positions of high peaks in our test profile realistically from block to block. This. It should be emphasized that the test profile obtained does not have the quasi-cyclic block-to-block pattern of the subsequent road runs where the highest peak in the data block.


Steinwolf A.,AST Consulting Ltd. | Steinwolf A.,University of Auckland
Experimental Techniques | Year: 2010

Reducing the crest factor of a random shaker excitation by sigma clipping is a well-known practice, but it may upset the shaker controller operation. Clipping brings about frequency distortions which result in substantial losses of the controller's dynamic range. This outcome is unavoidable as long as changes are made to the drive signal after it has been IFFT-generated, no matter if these changes are truncation by clipping, or polynomial re-shaping the peaks, or anything else. The phase selection method is different since it decreases kurtosis and crest factor not after but in the process of the IFFT signal generation. © 2009, Society for Experimental Mechanics.


Steinwolf A.,AST Consulting Ltd
International Journal of Vehicle Noise and Vibration | Year: 2015

Non-Gaussian random vibration testing with kurtosis control is considered in the paper as a way of introducing realistic high peaks into shaker drive signals, thereby increasing the excitation crest factor. This is required for more accurate simulation of ground vehicle vibrations. Implementing kurtosis, not the crest factor itself, as an additional test specification along with the PSD, leads to closed-form expressions for the non-Gaussian simulation criteria and allows for analytical solutions as a result. An approach of subjecting Gaussian signals to polynomial transformation is simple but has an inherent tendency of introducing frequency distortions jeopardising the PSD simulation. There is no such difficulty with the IFFT phase manipulation approach because the power spectrum is not influenced by the phases. A phase selection procedure capable of modelling random excitations with high kurtosis has been developed and it also works for the opposite case of decreasing kurtosis. Because of the analytical solution advantage, the methods proposed in the paper meet time restrictions critical for closed-loop operation of shaker controllers. Numerical simulations of automobile vibrations with severe time history peaks and experimental work with a complete shaker testing setup were carried out. Copyright © 2015 Inderscience Enterprises Ltd.


Steinwolf A.,AST Consulting Ltd.
Journal of Testing and Evaluation | Year: 2013

Non-Gaussian random vibration testing with kurtosis control is considered in the paper as a way of increasing or decreasing the excitation crest factor. An increase of crest factor is required for more accurate simulation of ground vehicle vibrations and the opposite action of crest factor decrease is useful in other applications, such as modal testing. Implementing kurtosis as an additional test specification leads to closed-form solutions for the requirement of the excitation high peak behavior being controlled simultaneously with the traditional power spectral density (PSD) control. A method of subjecting Gaussian signals to polynomial transformation is simpler but has an inherent tendency of introducing frequency distortions jeopardizing the PSD simulation. There is no such difficulty with another approach of phase manipulation in the inverse fast Fourier transform since the power spectrum is not influenced by the phases. A universal phase selection procedure capable of modeling non-Gaussian random excitations with a high or low kurtosis has been developed. Because of the analytical solution advantage, the proposed phase method can be implemented in automatic shaker testing systems with closed-loop control. This paper is the first in a series of two publications. Part II will present numerical and experimental results. © by ASTM Int'l (all rights reserved).


Steinwolf A.,AST Consulting Ltd
Journal of Testing and Evaluation | Year: 2013

This paper is a follow-up to a preceding paper (Part I) in which two methods of non-Gaussian random vibration testing with adjustable kurtosis were introduced and motivation for kurtosis control as a way of increasing or decreasing the excitation crest factor was discussed. The current paper (Part II) adds numerical examples of automobile vibration simulation and experimental results for a kurtosis upgrade implemented in the same form of closed-loop control as in industrial shaker controllers. It was observed in experiments that the dynamic range of a kurtosis controller based on the polynomial transformation method was reduced and the handling of resonances worsened notably. These problems also arise with the sigma clipping technique of crest factor limiting. However, there are no such difficulties with the non-Gaussian method of phase manipulation in the inverse fast Fourier transform (IFFT). When using this method, the signal-to-noise ratio, the controller's dynamic range, and the stabilization time are as good as in standard Gaussian random testing. Evaluation of the performance of the proposed phase selection algorithm has shown that for increased kurtosis it ensures realistic variability of high peaks in terms of their amplitudes and positions, as well as the number of severe peaks per data block. Because of the analytical solution advantage, both methods, the polynomial transformation and the phase selection, meet time restrictions critical for the operation of shaker testing controllers. © ASTM Int'l (all rights reserved); Wed May 21 23:57:57 EDT 2014.


Steinwolf A.,AST Consulting Ltd.
Mechanical Systems and Signal Processing | Year: 2012

Non-Gaussian random shaker testing with kurtosis control is introduced in the paper as a way of increasing or decreasing the excitation crest factor (CF). The CF increase is required for more accurate simulation of ground vehicle vibrations and the CF decrease is useful in other applications such as modal testing. Using kurtosis as a measure of CF behavior leads to closed-form solution for making the IFFT generation non-Gaussian by special phase manipulation. A universal phase selection procedure capable of modeling random excitations with a high or low kurtosis has been developed. Because of the analytical solution advantage, the proposed phase method meets time restrictions critical for shaker controller operation. Low CF values achieved by the suggested analytical kurtosis-based solution are close enough to those obtained by the known method of sequential moment minimization, which considers moments of higher order but by numerical optimization only. In the case of CF increase, the developed phase manipulation algorithm randomizes high peaks in terms of their amplitude, position, and the number of severe peaks per data block. This ensures realistic variability of high peak behavior distinct from having just one high peak of narrow height variation in all data blocks as in another known approach of on-band phase limiting that is also a numerical technique, not an analytical solution as the proposed method. © 2011 Elsevier Ltd. All rights reserved.

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