They can adjust the rotational axis of the rigid rotor dynamically and can enforce a rotation around its axis of inertia, where all the unbalance forces cancel each other out. There are two types of active bearings which can be distinguished referring to the utilized actuator type. On one hand active magnetic bearings and on the other hand piezoelectric bearings. The actuators are positioned directly within the load path and can prevent the vibration excitation right at its source.
Piezo bearings are using piezoelectric actuators, which can shift the rotor highly dynamically. This leads to a change of dynamics of the rotor system. The general structure of an active rotor is depicted here. The actuators are kept in place during operation using prestressing springs. The bearing forces are measured with sensors at the actuators locations (collocated) and are used for different control approaches.
Different control approaches are investigated at IMS for piezo bearings. The most challenging part is the change of system properties during operation, which is caused by strong gyroscopic effects.
In active magnetic bearings, electromagnets are used to support the rotor without contact. Compared to conventional roller and friction bearings, these have the advantage that they operate without friction, wear and lubricant. They also require little maintenance and are suitable for very high speeds. The structure of such bearings essentially consists of an electromagnet, the rotor, the sensors for position detection and the control and power electronics. To prevent damage to the system in the event of bearing failure, safety bearings are also required.