I made a simulation for PMC body mode, and found out that for Al PMC, the first body mode is 1kHz. And 780 Hz for stainless steel pmc.
November 27, 2012
It is desirable for the first body mode of the PMC to be at or above 1000 Hz in order to provide consistent length for the cavity.
ANSYS Summary
- Geometry
- For any given test, all parts of the assembly were changed to be the same material. Materials tested include fused silica, aluminum, and stainless steel, and material will be specified per test.
- Connections > Contacts
- Most contacts determined by ANSYS were sufficient. However, the contacts between the press-fit slots or cones and the ball bearings were adjusted for accuracy. The "Pinball Region" was changed from "Program Controlled" to "Radius", and the radius set to between 0.1 and 0.3 meter. This is to ensure that ANSYS recognizes these two objects are resting on each other.
- Analysis Settings - Supports
- The PMC assembly was modified so that the constraints could be more accurately modeled. Split lines were added to the bottom of the PMC base to model the force applied by the dog clamps, and these small 0.5"x0.5" squares were defined as "Fixed Supports" in the ANSYS model.
- The entire base was labeled as a frictionless support, because it is sitting on a table.
- Results
- The first body mode when the PMC is made entirely out of stainless steel is 865Hz
- The first body mode when the PMC is made entirely out of aluminum is 881Hz
Above you can see the first mode shape of the PMC. The colors represent the displacement - deep blue indicates no motion, while red indicates the greatest amount of motion. The animation of this mode shape shows the PMC spacer rocking transversely on the PMC base. The PMC base does not move at all.
- Changes to geometry (December 3rd, 2012)
- Holes were cut through the PMC spacer to try to increase the first body mode frequency. New geometry is shown in the figure below
 - There was no increase in the first body mode frequency - when made out of stainless steel, ANSYS reported the first body mode of this to be at 855 Hz
One question that came up is whether ANSYS is importing the geometry file at the correct size. According to the scale on the screen, it is the right size. However, when the material is changed to resemble fused silica, the lowest body mode is 998 Hz, which is about an order of magnitude lower than expected. This indicates some other error, possibly in importing the structure into ANSYS.
/more to come
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