4 Replies Latest reply on Nov 16, 2017 8:51 PM by Ranny Meier

    Is it possible to get harmonic frequency response results that include rigid body modes?

    Ranny Meier

      I know that we can use Frequency Shift or Result Options – Selected Modes to eliminate the rigid body modes and get a Harmonic Frequency Response result to list values.  I am afraid however that doing that to get a result captures nearly none of the inertia actually in the model.  I think that we need a result where as the frequency approaches zero, the response becomes much greater in magnitude than at the higher frequencies.

        • Re: Is it possible to get harmonic frequency response results that include rigid body modes?
          Bill McEachern

          To me at any rate I think you need a better explanation of why you need the rigid body modes and I did not understand anything that was said and I have done quite of bit of FEA vibration work. What are you trying to do or model?

            • Re: Is it possible to get harmonic frequency response results that include rigid body modes?
              Ranny Meier

              Hey Bill,

              I imagine a good example might be a satellite in orbit with a variable speed electric motor onboard that turns a rotor with an unbalance.  Our application is not so interesting however.  We want to use SolidWorks to simulate the free-free tool and holder to be combined to the measured Receptance at the machine spindle, similar to these examples:

              https://coefs.uncc.edu/tschmit4/files/2012/04/Receptance-Coupling-for-Tool-Point-Dynamics-Prediction-on-Machine-Tools.pd…

              https://coefs.uncc.edu/tschmit4/files/2011/11/spindle_rcsa.pdf

              We already have CAD models of all of the tools, the harmonic response simulations do not take too long to run, and besides theoretically the answer may be more accurate than using beam elements for short donut shaped bodies.

              Another more earthly example might be if we have a machine gun mounted on our speed boat, where the water acts kind of like soft springs on the boat body, without actually fixing the body to ground. We want know how some points in the boat structure respond while running the gun. A machine gun mounted on an airplane is another similar case. An aileron fluttering on a wing possibly?  I am trying to think of another more common example.

              Ranny

                • Re: Is it possible to get harmonic frequency response results that include rigid body modes?
                  Bill McEachern

                  Ok, I get it. Don't rack your brain further for more examples. Well right now I don't understand the problem well enough to be of all that much help. It sounds like you are trying to predict the vibrational modes of the tip in the presence of the whole system as it will provide a more refined estimate of the frequencies that will excite the tip? Is that about right? If any of that is close why wouldn't you use the machine FRF as the base driver in a sine analysis of the rest of the model? Anyway Just a thought.

                  Other wise I think you are into springs and masses to represent the the part you want to exclude to run the modes you want. Those lower order modes (in the free free range) are probably not going to be contributing much at the higher frequencies would be my guess. When you run free free the effective mass for a low number of modes never adds up for much even though you would think it should since the whole thing is moving but in reality none of the mass is really ringing off in the 6 free body modes.

                    • Re: Is it possible to get harmonic frequency response results that include rigid body modes?
                      Ranny Meier

                      Hey Bill,

                      Thanks for keeping a conversation going.  Yes that is what we want to do; predict the result at the tip in the presence of the whole system.

                      I think the RCSA method is outlined like these:

                      1. Use an instrumented hammer and accelerometer to measure FRF at locations on a known shape mounted in the spindle.

                      2. Obtain a free-free FRF of the known shape alone (using SolidWorks hopefully).

                      3. Use inverse RCSA to subtract the free-free result (Step 2) from the measured result (Step 1) to obtain a FRF for the spindle alone.

                      4. Obtain free-free FRF results for other more complicated tool shapes (using SolidWorks hopefully).

                      5. Use RCSA to add these other free-free FRF to the result saved in Step 3, to obtain a predicted FRF result at the tool tip of these other tool shapes.

                      I have read on the Internet that some commercial FEA applications have methods that allow harmonic frequency response with rigid body modes included (free-free).  As we have already paid for SolidWorks application - we hope to find a way to use SW for this.

                             SolidWorks Mass Participation for first 8 modes (including rigid body modes)

                      Mode No.  Freq (Hertz)  X direction  Y direction  Z direction
                      10100
                      20010
                      30001
                      40.00127465.15E-311.48E-198.67E-22
                      50.00171066.34E-211.42E-342.55E-24
                      60.00408083.71E-232.56E-240
                      74833.73.27E-227.51E-212.16E-21
                      84833.92.25E-211.03E-218.83E-21

                      Hopefully there will be a way to use SolidWorks for these types of problems.

                      Ranny