Comparison of FRF Estimates Obtained Using Various SIMO and MIMO Pneumatic Excitation Configurations

The frequency response function (FRF) is the most important measurement in the field of experimental modal analysis. Theoretically, for linear systems, the natural frequencies stay constant irrespective of using various numbers, types, and locations of inputs. In practice, there is some difference between natural frequencies estimated using various input configurations due to different numbers/locations of exciters (shakers) and force transducers. Historically, these anomalies have been attributed to local changes in mass, stiffness, and damping at the connection points. In this chapter, FRF estimates obtained with various single-input multi-output (SIMO) and multi-input multi-output (MIMO) configurations using pneumatic excitation have been compared. While testing a circular steel plate and a rectangular steel plate using pneumatic exciters with force transducer on structure (FTS) mounting, it was discovered that natural frequencies vary slightly in different input configurations. Unlike shaker excitation, pneumatic excitation decouples the exciter from the structure, which results in identical mass, stiffness, and damping matrices of the structure in different configurations. In addition, all the force transducers were kept mounted throughout these tests, even when force was not applied/measured through them, to ensure the invariability of the structure. Despite taking these precautions, it was found that the natural frequencies obtained in various configurations are different. With further investigation, it was discovered that the cause of variation in natural frequencies is the variable mass loading effect of force transducers in different input configurations.