ISO 8608: get the time history from PSD of road roughness?

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SUMMARY

The discussion focuses on generating a time history of vertical road displacement from the Power Spectral Density (PSD) data as specified in ISO 8608 for vehicle multibody modeling. The process involves using the Fast Fourier Transform (FFT) to obtain the PSD and then performing the inverse operation to retrieve the time history. It is crucial to note that the PSD lacks phase information, which can lead to non-unique results. To achieve realistic simulations, users should either assign random phases to Fourier components or research methods for selecting phases that control the crest factor, which is the ratio of peak displacement to RMS displacement.

PREREQUISITES
  • Understanding of Power Spectral Density (PSD) and its application in road roughness analysis.
  • Proficiency in Fast Fourier Transform (FFT) for converting time domain data to frequency domain.
  • Knowledge of inverse FFT operations to retrieve time history from PSD.
  • Familiarity with crest factor concepts in signal processing.
NEXT STEPS
  • Research methods for selecting phase values in inverse FFT to optimize crest factor.
  • Explore advanced techniques in multitone signal generation for realistic simulations.
  • Study the implications of phase information on time domain signals in vehicle dynamics.
  • Investigate additional resources on ISO 8608 and its applications in vehicle modeling.
USEFUL FOR

Vehicle dynamics engineers, multibody simulation specialists, and researchers focused on road roughness modeling and its impact on vehicle performance.

serbring
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Hi all,

I have a vehicle multibody model and I want to excite it in different conditions. So I'm thinking to use the PSD or road roughness as reported in the ISO 8608. What I don't understand is, how can I get an equivalent time history of the vertical road displacement from the PSD?

thanks
 
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Think how you would get the PSD from the time history (using an FFT). Then do the inverse operation.

That won't give you a unique answer, because the PSD only contains the amplitude information from the FFT, not the phase information. So you can choose any values for the phases when you do the inverse.

If you set all the phases = 0, you will probably get a big "spike" in the time history at one point, which may or may not be realistic depending on what you are simulating. Either choose random phases for each Fourier component, or do some research into how to select the phases to control the "crest factor", i.e. the peak displacement / RMS displacement (or acceleration if that is more interesting) in the time domain. For example http://www.stanford.edu/~boyd/papers/pdf/multitone_low_crest.pdf
 
AlephZero said:
Think how you would get the PSD from the time history (using an FFT). Then do the inverse operation.

That won't give you a unique answer, because the PSD only contains the amplitude information from the FFT, not the phase information. So you can choose any values for the phases when you do the inverse.

If you set all the phases = 0, you will probably get a big "spike" in the time history at one point, which may or may not be realistic depending on what you are simulating. Either choose random phases for each Fourier component, or do some research into how to select the phases to control the "crest factor", i.e. the peak displacement / RMS displacement (or acceleration if that is more interesting) in the time domain. For example http://www.stanford.edu/~boyd/papers/pdf/multitone_low_crest.pdf

Many thanks for your help I'll take a look at the linked document
 

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