We use this data to understand how a structure vibrates and to design the changes required to minimize the vibration under normal operating conditions.
Here are a few key facts about experimental modal analysis
- Modal analysis is performed on a structure when it is not operating.
- Modal analysis reveals the information that proves where the resonant frequencies will be located, and how the structure will move/deform when it is excited.
- Modal analysis can be used as the basis to compute the required structural modifications required to alter the structure.
- Change the mass, stiffness or damping to move the resonances to a frequency that will not be excited by our machine’s forcing frequencies.
There are a number of steps required to perform the modal analysis test:(1) Determine the geometry/test points
- Enter the plan into the modal software
- Take data at each test location
- Feed it to the analysis software
- Analyze the results
- Find the resonant frequencies
- Curve fit the frequency response function to extract damping
- Animate the structure
- Make the required modifications
- Modify the structure so resonances won’t be excited
Impact Testing (Modal Testing)
Modal testing involves the injection of a known force and measuring the response.
Methods used to excite the structure: (Input)
Impact Hammer (Equipped with load cell)
Usually hammers tips are designed to impart the force in a band of frequencies, Hard tips excite higher frequency band, soft tips excite lower frequency band.
Shaker
Methods used to measure the structure response to excitation: (Output):- Single or Triaxial accelerometer
Test Setup
- Set Fmax to cover the frequencies of interest
- Resolution to set the length of the time record
- Get average result of four times repeated coherence test
- Force-response (exponential) window
Test output data
- Impact time waveform
- Spectrum of impact
- Response time waveform
- Spectrum of response
- Coherence
- Transfer function & phase