In this setup, bearings are modeled as isotropic linear
spring and damper. The imbalance is modeled as concentrated
mass on the rigid shaft. Two coordinate systems are
used: the body-fixed coordinate oxyz and the inertial coordinate
OXYZ. The body-fixed y-axis is the rotating axis of
the shaft, and x and z axes are defined by the other two
principal inertia axes of the rotor. The origin of xyz is
selected as the geometric center of the shaft. The XYZ
coordinate system is the stationary coordinate and coincides
with the xyz coordinate system when the body is at
rest. The transverse motion of the rotor is described by the
position of the geometric center [RX RZ] and by the orientation
of the rigid shaft with respect to the X and Z axes [θ
ψ]. A simplified governing equation in state space form is
shown as follows (Zhou and Shi, 2000):