Active Bearings

Rotors are always subjected to unbalance due to manufacturing tolerances, which excite the surrounding structure leading to vibrations. Active bearings are able to operate a rotor without unbalance forces when a suitable control is used.

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

Picture: IMS
Structure of an active piezo bearing

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.

Until now many different control approaches such as

  • PDT1, Integral Force Feedback (PT1)
  • H2, H∞ and gain scheduled H∞
  • Adaptive feed forward (e.g. FxLMS)

have been investigated numerically and experimentally. Three test rigs are available at IMS for experimental investigation of control algorithms.

For the past years our main focus was on the implementation of control strategies. However, to make piezoelectric bearings interesting for real-world applications outside the laboratory, the required hardware has to be considered as well. Thus, we are currently exploring and developing an active bearing, which can be treated as a standalone machine element without the need of special knowledge on the technology. Furthermore, a significant reduction of costs is targeted by integrating former control aspects into the hardware functions.

Magnetic Bearings

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.

Due to their lower load capacity compared to conventional bearings, the necessary position detection as well as the safety gear and peripherals, magnetic bearings require a large amount of space and have high acquisition costs. In terms of rotor dynamics, magnetic bearings offer many interesting possibilities of influence, such as active unbalance compensation, adaptation of the system dynamics to the operating point, determination of the rotor's balancing status and system fault diagnosis.

Sensorless magnetic bearings are being researched at the IMS. The position of the rotor is estimated from the system parameters of the magnetic bearing. This makes it possible to dispense with position sensors, which results in a considerable cost saving potential and eliminates the required installation space for the sensors. In addition, the integration of the position detection into the magnetic bearing provides the control technology important property of collocation.

Previous research focused on the implementation of an estimation method that can be used for different magnetic bearing geometries. This estimation method is limited to small bearing sizes due to the non-linearity of the magnetic materials. The aim of further research is to improve the quality of the estimation method so that the use of the method is no longer limited by the bearing size.