40m QIL Cryo_Lab CTN SUS_Lab TCS_Lab OMC_Lab CRIME_Lab FEA ENG_Labs OptContFac Mariner WBEEShop
  PSL  Not logged in ELOG logo
Entry  Mon Jan 7 13:54:29 2013, Kristen, Summary, PMC, Modal Frequency Testing for PMC PMC_Measurement_Diagram.JPGExperimental_Modal_Data_-_Dec_18.JPGANSYS_PMC_Mode_441.JPG
    Reply  Wed Jan 16 16:48:39 2013, rana, Summary, PMC, Modal Frequency Testing for PMC 
Message ID: 1088     Entry time: Mon Jan 7 13:54:29 2013     Reply to this: 1089
Author: Kristen 
Type: Summary 
Category: PMC 
Subject: Modal Frequency Testing for PMC 

The PMC was tested & lowest resonant frequency was 330 Hz;  FEA model was adjusted to new frequency of 441 Hz

Results from December 18, 2012

The PMC in 058B W. Bridge was secured with several dog clamps to the laser table.  This table is not as stiff as the table in the Modal Lab in Downs, but was thought to be sufficient for this test.  Testing was done with the B&K system, using a laser vibrometer for the accelerometer and the small 8206 B&K hammer for excitation.  Below is a representation of the axis for this test, to understand where the PMC was excited and measured.

PMC_Measurement_Diagram.JPG

 

Measurements and excitations were approximately at the center of the corresponding face, as indicated in the image.

Below is a graph of the results of these measurements.  You can see that the lowest resonant frequency is at 330 Hz.

Experimental_Modal_Data_-_Dec_18.JPG 

 

Next Steps:

Next I will update the ANSYS model to be more accurate, hopefully showing about 330 Hz as the lowest mode.

FEA Model in ANSYS:

Briefly: the previous model reported in the elog was changed by refining the mesh on the slots/cones and the bearings.  This would allow for that portion to behave more accurately.  The contacts were left as Program-controlled (any other control seemed to overestimate the contact, raising the predicted resonant frequencies). 

Below is an image of the lowest mode, at 441 Hz.  The arrow indicates the motion - the mode is roughly a transverse flagpole mode.

ANSYS_PMC_Mode_441.JPG

Now that the model has been made more accurate, steps can be taken to raise the resonant frequency. While the initial goal that was mentioned to me of 1kHz is improbable, there are certainly ways to raise the frequency and damp the modes that are problematic.

 

 

ELOG V3.1.3-