Force Excitation by Random Noise and Impact

Direct application of force excitation for instant frequency response functon is discoused. For mobility measurements the structure must be excited by a measurable dynamic force. Here the term random applies to the amplitude of the excitation force which, in statistical terms, has a normal or Gaussian probability distribution. On average however, the spectrum is flat and continuous, containing energy at approximately the same level for all frequencies. Due to the random characteristic of the signal, the structure is excited over a wide force range at each frequency. This randomizes any nonlinear effects, and averaging then gives a best linear approximation. The most popular excitation technique used for modal analysis is impact, or hammer excitation. The waveform produced by an impact is a transient (short duration) energy transfer event. The spectrum is continuous, with a maximum amplitude at 0 Hz and decaying amplitude with increasing frequency. Advantage and disadvantage of both techniques are discussed. Results from real measurements are presented in graphs.