The parametric instability of a beam under electromagnetic excitation was investigated experimentally and analytically. In experiment an electromagnetic device, acting like a spring with alternating sti!ness, was designed to parametrically excite the beam. The frequency and the amplitude of the excitation force were accurately controlled by the AC current #owing through the coil of the electromagnetic device. Since the excitation force is a non-contact electromagnetic force which acts on the beam in the transverse direction, the disturbances induced by the geometric imperfection of the beam, by the eccentricity of the usual axial excitation force, and the coupling e!ects between the excitation mechanism and the beam were e!ectively avoided. The dynamic system was analyzed based on the assumed-modes method. The instability regions of the system were found to be the functions of the modal parameters of the beam and the position, the sti!ness of the electromagnetic device for various cantilevered beams. The modal damping ratios of the beam specimens were also identi"ed. The experimental results were found to agree well with the analytical ones.
2001 Academic Press