Measuring vibrations: Why?

1
Measuring vibrations Why?

• Vibration level to check for safety limits in
  human exposure cases or other mechanical limits
  for induced vibrations (noise, resistance)
• Excitation measurement to assess forces or
  displacement that may induce undesired vibrations
• Response to excitation to assess the frequency
  response function, mode shapes or natural
  frequencies of a body or assembly

2
Measuring vibrations how?

• Low frequency lt 1Hz DISPLACEMENT
• High frequency gt100Hz ACCELERATION
• exceptions
• Human exposure to vibrations ACCELERATION
• Building structural integrity VELOCITY

3
Measuring vibrations how?

• Peakmaximum absolute value
• Peak-to-peakmaximum variation
• Peak-to-averageor crest-factor, useful for
  impacts damage evaluations
• RMSenergy content of vibration

peak
rms
0
4
Measuring vibrations how?
PSD
TFA
FFT
5
Measuring vibrations FRF

• If measuring both the input X and the output Y is
  a viable solution, H(?)Y(?)/X(?) could be
  experimentally determined using spectral tools

Displacement/Force Velocity/Force Acceleration/Force
Admittance, Compliance Mobility Accelerance, Inertance
Force/Displacement Force/Velocity Force/Acceleration
Dynamic Stiffness Mechanical Impedance Apparent Mass
6
Modal Analysis Approaches
INPUT MEASURED INPUT NOT MEASURED
SINGLE SHOT Impact, sine sweep FRF EXPERIMENTAL MODAL ANALYSIS ImpactFFT/PSD NATURAL FREQUENCIES IDENTIFICATION
REPEATED MEASUREMENTS Pseudorandom FRF/PSD EXPERIMENTAL MODAL ANALYSIS No inputAveraged PSD OPERATIONAL MODAL ANALYSIS
7
Measuring vibrations Why?

• Vibration level PSD/RMS
• Excitation measurement PSD/RMS
• Response to excitation
• Mechanical properties FRF
• mode shapes FRF
• natural frequencies PSD/FFT/FRF
• In most cases the analysis follow an incremental
  complexity approach, starting from a simple
  natural frequency identification to a full
  experimental modal analysis