A conventional electrodynamic exciter
(shaker) that is used in vibration testing has a shaker head that is suspended
on a rigid housing through a flexible diaphragm. The shaker head is excited by
the electromagnetic force (as in the case of a DC motor) generated in the drive
coil, which is wound around the core of the head.
Consider a shaker of this type, with a test
object mounted on it. An accelerometer is mounted on the shaker head. The coil
is excited by a known transient current from the drive amplifier. This current
is proportional to the electromagnetic force that is generated. The
acceleration of the shaker head is measured. The frequency-response function
between the drive force and the head acceleration is computed using a spectrum
analyzer. Its magnitude is found to have two prominent resonances that are
separated by a flat region. Explain this characteristic shape of the frequency
response of the shaker, indicating the sources of the two resonances.