Comparative study between three methods of measuring of natural frequencies and modal damping

The determination of the natural frequencies and modal damping is very important for the survival of structures and rotating machinery. The precise knowledge of these frequencies allowed avoiding the appearance of the resonance phenomenon, which represents a real danger for these structures. In practice, the measurement of natural frequencies is influenced by several parameters, such as the mass of the sensor, the type of excitation and the boundary conditions. In this work, we present the results of natural frequencies measurements by three different methods in order to see which of them is closest to that obtained by numerical simulation. The first method consists of exciting the structure by a shock hammer and measuring the vibration signal by an accelerometer glued to the beam. In the second method, a vibration exciter equipped with an impedance head is used, which allowed to directly obtain the Frequency Response Function (FRF). The third method consists of fixing, on the vibration exciter, a force sensor with a metal rod to excite the structure and the vibratory signal is measured by an accelerometer glued to the beam. The results of the three methods show that the natural frequencies obtained by the impedance head are closer to those of the numerical simulation, which is explained by the absence of the mass added by the accelerometer. For modal damping, the results obtained by shock hammer excitation are better than those of the two other methods. The absence of stress on the free vibrations of the beam due to this type of excitation gives resonance frequencies with a bandwidth at -3 dB wider than in the case of the other two methods where the vibration exciter blocks the free vibrations of the beam.