Stinger Effects on Measured Frequency Response Functions in Vibration Testing

In performing a modal test the most commonly used excitation technique is shaker excitation. The shaker head should never be directly attached to the structure. If the shaker were to be directly attached to the structure, there would be significant dynamic effects of the shaker imposed onto the structure, resulting very poor frequency response measurements. Basically, the stinger decouples the shaker system from the structure and applies force to the structure. Force transducers measure axial force therefore, any side loads are unmeasured and contribute noise on the measurement. A stinger that is properly designed, selected and aligned will reduce or eliminate this. This paper overviews the effects of stinger dimensions and stinger misalignment on the measured FRF. The results show that too short stinger cannot decouple the structure from the excitation system perfectly and there will be some dynamic effects of the shaker on the dynamics of the structure. On the other hand, too long stinger may buckle and causes insufficient force be transmitted to the structure. Also, it is demonstrated that stinger misalignment can cause distortion of the measured FRF.