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[FAQ]

Please take a look at the attached .mechdat file that compares y-displacement at a vertex: #1. “Response PSD” object gives RMS value of 6.8219e-3 mm #2. “Response PSD tool > Response PSD 2” gives RMS value of 6.7137e-3 mm – note frequency point resolution increased compared with method #1 above #3. “Directional Deformation” gives 6.7149e-3 mm Methods #1 and #2 are performing numerical integration to get RMS value. Method #2 uses more frequency points, which gives closer result to method #3. Method #3 uses closed-form integration and is most accurate (as long as input PSD curve is represented well). Note that, with Method #3, in the Details view, you can specify min/max RMS value (1-sigma value) as an output parameter. In summary, use a regular 1-sigma result and parametrize its output to get the RMS value, as it is most accurate. Don’t try to do it via the response PSD curve integration, as that is less accurate. (If, for whatever reason, the user wants to get the RMS value from the response PSD curve, we need to use APDL commands to do this. See FAQ KM 2011556 for an example of the APDL syntax in /POST26.)