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Attach modal shaker at stiff or flexible area of structure?

Experimenter
Experimenter

Curious what people find to be best practice.

 

Is it better to attach a modal shaker and stinger on the stiffest part of a structure, or on the most flexible part of a structure?

 

Our simulation engineers do a pretest analysis and usually advise attaching the shaker/stinger on a pretty active/flexible part of the test structure.

2 REPLIES

Re: Attach modal shaker at stiff or flexible area of structure?

Siemens Phenom Siemens Phenom
Siemens Phenom

Hello Ed,

 

that is not actually not one question, but two:

 

1)  Should we attach a shaker to a flexible (compliant) or stiff part of the structure?  I would attach to a stiff part of the structure that does not have compliance or localized deformation when the force is applied.

 

2)  Should I attache the shaker to a location that moves in most of the modes or is a node and does not move at one or more of the modes?  Normally we would do a driving point survey during the initial test setup.  We would move the hammer and accelerometer around the structure measuring a series of driving points.  We would then overlay them and choose the location that excites the greatest number of resonances.    In a Pretest analysis using CAE modes they would do something similar by ranking the driving point residues of all possible excitation locations and then choosing one that had a high driving point residue.  Both methods ensure that all modes are properly excited.  If you choose an excitation location that is a node of a mode it will not excite then the force at that location will not excite the mode properly and either will not be found or it will most likely be noisy.

 

There is a good article on a driving point survey on the Siemens Simcenter Testing Knowledge Base at https://community.plm.automation.siemens.com/t5/Testing-Knowledge-Base/Modal-Testing-Driving-Point-S...

Re: Attach modal shaker at stiff or flexible area of structure?

Siemens Legend Siemens Legend
Siemens Legend

In my experience, seems like there are often issues using a modal shaker on a very flexible part of the structure.  Seems to create force drop outs, etc.