The parametric instability of a beam under electromagnetic excitation was i
nvestigated experimentally and analytically. In experiment an electromagnet
ic device, acting like a spring with alternating stiffness, was designed to
parametrically excite the beam. The frequency and the amplitude of the exc
itation force were accurately controlled by the AC current flowing 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 dir
ection, the disturbances induced by the geometric imperfection of the beam,
by the eccentricity of the usual axial excitation force, and the coupling
effects between the excitation mechanism and the beam were effectively avoi
ded. The dynamic system was analysed based on the assumed-modes method. The
instability regions of the system were found to be the functions of the mo
dal parameters of the beam and the position, the stiffness of the electroma
gnetic device for various cantilevered beams. The modal damping ratios of t
he beam specimens were also identified. The experimental results were found
to agree well with the analytical ones. (C) 2001 Academic Press.