Industries

Executing Modal Analysis on High Damping Structure

Why we choose this structure?
This structure is made of acrylic .It has three layers: the low layer, the middle layer and the high layer which is fixed on the platform of the shaker. We can add different mass block to modify the natural frequency of the structure (but for this time we just want to see the modes without mass modification). The whole structure is really very flexible and directional. In a word, this is a structure with high damping and low frequencies.
Though this demo, we will see that our system preforms excellent in the low frequency and high damping structure. So we can use our system in something like bridge and building modal test.
Test Instrument Configuration
•Dynamic Signal Analyzer (AVANT series MI-7008 from Econ Technologies (Hangzhou) Co.,Ltd )
•Modal Genius Software
•Hammer
•Acrylic structure
•PC
 
How to acquire measurement data?
In order to avoid mass loading effect, I use revolving hammer method and just use one acceleration sensor. This means I fixed the acceleration sensor in the same point and revolve the hammer in different points to acquire different FRFs.
I just impacted 8 points on the different bolt tips. If I impact other location, a double impact would occur. We could use interpolation function in order to see a smoothly simulation.
Create the model in Modal Genius.
Though extrude function, we can create the model easily. In this demo, I create more point than test, because some of the point is hard to test and we can use interpolation function to execute a good simulation.
 
Execute modal analysis in Modal Genius.
Import the measurement data we test before into the Modal Genius software to execute modal analysis. For the picture under below, we can see a very good curve-fitting .The black trace is the original test data and the red trace is the curve-fitting result. In our software, we can use double cursor to define the frequency band we interested in. In this demo, I just set the analysis frequency band as 0-10Hz. We can also see that the MAC is really very good.
 
Simulate with modal shapes
After matching the point and test data, we can see the movement of the structure clearly. Although we just use 8 test points, we still acquired a smooth simulation by interpolation function. The first 3 frequencies of this structure are lower than 10Hz and some of damping are higher than 4%.From this demo, we get the exact dynamic characteristics of this structure.
 
All the steps were finished in just one hour.
All in all, our system is very effective in doing modal test and it is also very excellent in modal testing of structures with high damping and low natural frequency.