General Mechanical

General Mechanical

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Extracting eigen vectors

    • Md_Salem

      Hello everyone,

      I did modal analysis on a beam in order to select the most independent modes that is going to be tested in the laboratory.

      The basic criteria for my selection will be (Orthogonality Check-Modal Independence Factor MIF-Modal Independence Index MII-Modal Assurance Criterion MAC)

      So, I extracted the mass and stiffness matrices for the beam using the next APDL code at the workbench:

      *SMAT, MatKS, D, IMPORT, FULL, file.full, STIFF
      *SMAT, MatMS, D, IMPORT, FULL, file.full, MASS
      *Export, MatKS, MMF, matK_MMF.txt 
      *Export, MatMS, MMF, matM_MMF.txt 
      Then I treated the MMF files with MATLAB and finally solved the eigenvalue problem (M,K) .
      Unfortunately, the obtained eigen values, which are supposed to be the natural modes, were very different from the obtained natural modes of the solved problem in ANSYS, which compromised the realability of obtained Eigen vectors (mode shapes) !!
      Is there any way to do the shown independent checks (Orthogonality Check-Modal Independence Factor MIF- Modal Independence Index MII- Modal Assurance Criterion MAC), directly on ANSYS?
      OR, is there any way to extract eigen vectors for designated modes directly from the ANSYS model?


    • Dave Looman
      Ansys Employee

      APDL Math Example 4.1, "Verify Orthogonality of Eigenmodes after a Modal Analysis", of Section 4.8 of the Ansys Parametric Design Language Guide does this.  Or the RSTMAC command could be used to write the correlation matrix.

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