Nonlinear Dynamics Introduced by Modal Shakers

Many methods for nonlinear structural dynamics testing have been developed, some of which utilize modal shakers for excitation. One such technique is force appropriation, where the structure’s excitation is maintained at resonance using closed-loop control as the forcing amplitude is increased. This technique has been shown to provide both valuable insights into the true physics of structures and rich data sets from which to perform system identification; however, there is a potential for these results to be distorted if any nonlinear physics is inadvertently introduced to the system by the modal shaker either in the measured input or structural response. This work investigates—via experimentation—the nonlinear electrodynamic effects produced by the shaker and injected into the structure. This nonlinearity is evaluated by conducting force appropriation on a linear structure up to sufficiently high levels such that harmonic distortions appear in the excitation force.