Modal Survey Techniques and Theory

The evolution of multiple shaker induction of pure modes has nonetheless retained a key assumption and consequent test procedure a quarter of a century old -- mode vectors are real, i.e., in phase except for polarity and therefore an in phase force vector should be applied at a single frequency along the jω axis. The validity of the assumption and procedure is limited to undamped or proportionally damped structures. Modern, complex and inhomogeneous hardware seems less likely to satisfy the limiting assumption. The complex modes of non-proportionally damped structures can be induced with two improvements on existing procedure made possible by modern electronics and digital computers -- apply a force vector controlling AMPLITUDE and PHASE and allow COMPLEX FREQUENCY EXCITATION anywhere in the s plane.