Software for Operational
Modal Analysis
ARTeMIS |
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Modal Analysis in the Presence of Harmonic
Excitation
The case study presented here demonstrates how the
harmonic detection and compensation in the Enhanced Frequency Domain
Decomposition algorithm makes it easy to estimate modes from of a ship. It
also demonstrate an alternative analysis using the Crystal Clear SSI
algorithm.
The measurements and analysis presented here are
performed by University of Rostock,
Faculty of Mechanical engineering. Chair of Technical Mechanics.
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The ship tested has the following technical specifications:
- Build at Flensburger Schiffbau
Gesellschaft.
- Dimensions: length over all: 199.8
m
- Speed: 22.5 kn
- Main engine: 9L 60 MC-C (MAN B&W,
9 cylinder)
- Power: 20 070 kW
- Speed: 123 rpm
- Working process: two-stroke
- 4-bladed propeller with
controllable pitch
The ship was driven by variable pitch
propeller, therefore the rotational speed of the main engine was
more or less constant. Several harmonic excitation lines is present
in the data because of the dynamic characteristic of the engine.
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Estimation of Harmonic Indicators
The automatic detection of harmonics was applied to
the data. In the display below the Singular Value Decomposition of the
spectral densities of the measurements are shown in the lower window. In the
top window the average Kurtosis of projection channels at each discrete
frequency is shown as the gray curve. Most Kurtosis values are located
around 3, which is the Kurtosis for normally distributed data. However, at
the frequencies of the deterministic sinusoidal signal (the harmonic and its
over tones) the Kurtosis drops to 1.5, that is the value for randomly
sampled sinusoidal data. A threshold, indicated by the blue horizontal line,
marks what is considered to be "harmonics". At all frequencies where the
Kurtosis is below this threshold a harmonic indicator is activated. This
indicator is shown as a green vertical line below. All information at these
green frequencies should be disregarded, when modal parameters are being
estimated.
Utilizing Harmonic Indicators in the Enhanced
Frequency Domain Decomposition
In the Enhanced Frequency Domain Decomposition
shown below, it is seen how the green "harmonic" zones are disregarded, when
the SDOF spectral density is estimated.
The spike caused by the forced "harmonic"
excitation is not included the spectral density estimation, and is therefore
not destroying the estimation of the natural frequency and damping ratio.
Estimation using Crystal Clear Stochastic Subspace
Identification
An alternative way of estimating modal parameters
from measurement including deterministic sinusoidal forces, from e.g.
rotating machinery, is to apply the Crystal Clear Stochastic Subspace
techniques available in the ARTeMIS Extrator Pro. In this case the
"harmonics" are fitted just like all other physical information contained in
the data. The "harmonics" can then easily be disregarded by looking at the
damping, which typically is unreasonable low, and by the harmonic indicator.
See below:
Related Information
S.E. Rosenow, P. Andersen
Operational Modal Analysis of a Wind
Turbine Mainframe using Crystal Clear SSI
To be presented at the 28th International Modal Analysis Conference
(IMAC) Jacksonville, Florida USA, 2010.
N.J. Jacobsen, P. Andersen, R. Brincker
Applications of Frequency Domain Curve-fitting in the EFDD Technique
Proceedings of the 26th International Modal Analysis Conference
(IMAC) Orlando, Florida USA, 2008.
N.J. Jacobsen, P. Andersen, R. Brincker
Using EFDD as a
Robust Technique to Deterministic Excitation in Operational Modal
Analysis
Proceedings of The 2nd International Operational Modal Analysis
Conference (IOMAC)
Copenhagen, Denmark, 2007.
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