Rotors can be provided with sensors and actuators that, through a suitable control system, can perform different tasks, from relieving the loads from the bearings to performing an active control of the dynamic behavior of the system. The control system, with its sensors and actuators, can act on only a few of the degrees of freedom of the rotor, for example to supply an additional damping, or it can fully control its position in space, as it occurs in a fully active magnetic suspension.

Magnetic bearings are perhaps the most common example of active control applied to the dynamics of rotors, but there are also many other applications, like controlled hydrodynamic supports, electrostatic bearings used in micromachines, or controlled pneumostatic bearings. In the following sections, only a brief introduction to the dynamics of controlled rotors will be presented, with particular reference to magnetic bearings, but the relevant equations can be applied to other devices.

A simplified linearized study of the behavior of rotors on magnetic bearings was presented in Section 8.5. There each magnetic bearing was considered as a linear spring-damper system, neglecting the dynamics of the sensors, actuators, amplifiers, and controller and neglecting the fact that in most applications, the sensors and the actuators are not colocated; i.e., the force exerted by the latter is applied in a point that does not coincide with the point where the sensor reads the displacement. These assumptions will be dropped in the present section.