5 Replies Latest reply on Jul 12, 2014 2:15 PM by Jared Conway

    Modal analysis and Steinberg Decoupling

    Mark Wylie

      Hi guys,

      I am performing modal analysis of an electronics enclosure and using Steinberg’s Octave rule to decouple the chassis and the PCBs. According to Steinberg’s book “Vibration analysis for electronic equipment”, the chassis fundamental and the PCBs fundamental frequencies  ONLY should be separated by an octave (i.e. Mode 1  = 200 Hz chassis, PCBs should be Mode 1 400 Hz or Mode 1=  100 Hz). This rule does not mention matching the frequencies which coincide on the same axis.

       

      My question is:

       

      Steinberg only states that the fundamental frequency be compared and mentions nothing of coincidental axes (i.e. I may have a chassis fundamental frequency in Z direction and a PCB board  mounted perpendicularly in the chassis who’s fundamental frequency is in the X direction).

       

      In my opinion I should be comparing the chassis Mode (Hz) in the X axis with the “critical” axis of the PCB, the X-axis in this case. This may be, for example, Mode 3 of the chassis, which has dominant mass participation in the X axis with the PCB fundamental (X axis also).

       

      Any input?

       

      Many thanks,

       

      Mark

        • Re: Modal analysis and Steinberg Decoupling
          Shaun Densberger

          On a fundamental level, you're argument makes sense, but that fact that Steinberg specifically mentions the fundamental mode and never mentions anything about coincidental eigenvactors makes me think that it might not matter. I was only able to find basic information on Steinberg's Octave Rule via a Google search, so maybe there is some information in his textbook that addresses this. However, I'd be shocked if Steinberg didn't take this effect (how the eigenvectors of the chassis and PCB compare) into account or not mention that he didn't. Sometimes these rules/standards have large factors of safety in them to account for a wide range of design conditions, and this could be the case with this.

           

          Is there a reason why you can't simulate both the chassis and the PCB together?

            • Re: Modal analysis and Steinberg Decoupling
              Mark Wylie

              Thanks Shaun,

               

              I am thinking the same. I can simulate the PCB and chassis together but I would need to measure relative transmissability (Q) I guess to see if they are coupled or decoupled, or do you have another method? I have modelled the chassis with the mass of the PCBs only (as per Steinbergs book) and compared that to the PCB structures on their own with the appriopriate fixtures as they would be in the chassis.

               

              Regards,

               

              Mark

            • Re: Modal analysis and Steinberg Decoupling
              Jared Conway

              i'm with shaun, your logic is good. a consideration, why not make a comparison with both your method and the assumed method from steinberg? maybe they both line up?

               

              and similarly, i'm with shaun, why not analyze them together in a dynamic analysis? if the direction is a factor, it should be really obvious.