Magnetic bearings allow contact-free levitation. This offers a number of interesting advantages. Magnetic bearings do not require lubrication, they allow high circumferential speeds at high loads, they do not suffer friction nor wear, and therefore they offer a virtually unlimited lifetime while no maintenance is needed.
Furthermore, the bearing force can be modulated, either for compensating unbalance forces, or for deliberately exciting vibrations. Because of these advantages, they are used in an increasing number of commercial high-performance applications in the domain of rotating machinery. These include ultra-high vacuum pumps, canned pipeline compressors and expanders, high-speed milling and grinding spindles, flywheels for energy storage, gyroscopes for space navigation, spinning spindles, and others.
The research carried out at the International Center of Magnetic Bearing (ICMB) focusses on basic research and new applications areas. Current projects at the ICMB are outlined below. A spin-off company of our institute, MECOS Traxler AG, is taking care of industrial applications of magnetic bearings.
Active Magnetic Bearings (AMBs) with their control system inherently offer the possibility of continuously recording bearing forces and rotor displacements. This allows on-line monitoring of critical process parameters and early detection of incipient faults, such that reliability is increased. Furthermore, AMBs can actively influence a machine's behaviour. Thereby, operation can be adjusted and optimized according to process changes. A research project supported by the EU aims at exploiting this potential for using magnetic bearings as components for smart machines.
AMB systems are often used to control structural resonance frequencies coming from the rotor or from elastic supports. The resonance frequencies may vary significantly with the rotational speed. Controller design for AMB systems is therefore important for the system performance. Controller design requires a plant model. Identification, i.e. modelling based on dynamic measurements, is a fast way for obtaining such a model. Both controller design and identification are topics of current research.
AMBs could be attractive for gas turbines in airplanes and power production. Problems to be investigated include design, materials, sensors, control, backup bearings, and other topics relevant in the context of high-temperature operation. We participate in a research project of the EU addressing these new applications.
AMBs are ideally suited for excitation of rotating rotors, since excitation forces can be applied to the rotor without contact and can be measured exactly. Magnetic bearing exciters tailored to the needs of rotordynamic experiments have been developed. Rotor-stator interaction is one of the topics investigated using AMB excitation. Rotor-stator contact, for example, can occur at the seals of turbomachinery with all types of bearings. It can cause the complete destruction of a machine. On an experimental test stand, rotor-stator contact is deliberately brought about using an AMB exciter. The resulting behaviour of the rotor is then compared to theory and simulation, and constructive counter-measures are investigated.
The self-sensing (sensorless) magnetic bearing is a special kind of magnetic bearing, which needs no external position sensors. The position information is deduced from the air gap dependent properties of the electromagnets. The main advantage is the reduction of the manufacturing costs. Furthermore, self-sensing bearings have a number of features that make them interesting for solving technical problems. The absence of the position sensor simplifies the construction, the assembly, and the maintenance of the magnetic bearing system. Additionally, it allows a more compact design of the rotor, which increases its natural frequencies. Two different concepts for self-sensing bearings have been developed and realized at the ICMB.
Alex Bartha, Philipp Bühler, Florian Lösch (email@example.com), Letian Wang, Longxiang Xu (firstname.lastname@example.org)
Institute of Robotics Homepage
Jul 1999, M. Honegger