Hi Ahmad,
Input PSD curves are often provided based on engineering specifications or criteria that must be met. You might need to refer design hand books or data from test engineers. Data loggers which record and process the data from accelerometers provide the time history or frequency data and I am not sure if you can use the same directly for PSD input.
Please see if the following course is of any help to you: Random Vibration Analysis in Ansys Mechanical | Ansys Courses
Regards,
Ashish Khemka
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