7 Replies Latest reply on Sep 14, 2009 4:29 PM by Kirby Meyer

    Using Base Excitations

    Mike Lowinske
      I am wondering what exactly base excitations are and how they are applied and what the meaning of the results that come out of Simulation in a dynamic analysis are?
        • Using Base Excitations
          Kirby Meyer
          Hello Mike

          Have you ever seen a shaker table used for creating Random Vibration? You'll notice that the system being shaken is bolted down to the table. The bolts and fixture have to be bulky and big, otherwise they too can contribute to the modal frequencies of the system being shook.

          In Simulation, the system is therefore to be given a fixed geometry at the interface to the destined shaker table. Basically, at the mounting points. You don't model the shaker table. These fixed geometry points are then excited to the PSD map you provide.

          Global excitation means that all fixed points are excited in the direction you specify. Base excitation means you specify which fixed geometric surfaces are excited. In the case above, if the system is only anchored to the shaker table, then you can use Global excitation as there is no other fixed geometry. However, say that one end of the system is bolted against a wall, e.g., an actuator. Here you can't excite that additional fixed spot, so Base Excitation is needed.

          The other reason to use Base Excitation is if you have different PSDs to apply at each fixed geometry, as if you had accelerometers physically measuring the accelerations at those points.

          I have addressed an anomaly in Simulation 2009 v2.1 that Base Excitations always invoke the Sparse Solver. You cannot flag the FFEPlus solver. This leads to hopelessly chewed up memory allocations for ~ 900,000 DOF systems with ~ 8 GB of memory. Always use Global Excitation unless you absolutely need it.

          Regards
          Kirby
            • Re: Using Base Excitations
              Ashish Jaiswal

              Hi Kirby,

               

              I have a questiong regarding Uniform Base Excitation. I have been given the "Y Displacement data" (Displacement Vs Hz data) to be applied on an enclosure fixed at 3 ends (i.e. its base) to perform the Linear Dynamic analysis and study the response. What value should I specify under my Displacements (with respect to directions in Y direction) after I add the given curve?

               

              Ashish