Active magnetic bearings of high reliability

Active magnetic bearings (AMB) offer a new way in bearing technology for supporting structures as, for example, platforms or rotors in rotating machinery. Compared with conventional journal bearings or rolling element bearings AMBs possess substantial advantages: No lubricant and no friction due to contact less suspension; no maintenance required, bearing parameters as stiffness or damping may easily be adapted to various operating states, for example, when passing a rotor through its critical speeds; instantaneous engine start-up without any warm-up of bearings; etc. However, the most significant drawback of AMB-technology is, when a single component fails then the bearing load capacity breaks down. This may cause an unintentional machine standstill and is one of the major reasons that the industrial level of acceptance of AMBs is quite low at the moment. A typical example is textile and paper mill industry. Even a very short interruption causes extremely high costs due to complicated startup procedures and synchronization requirements along the whole production line. Some concepts to realize higher operational reliability of AMB-systems may be found in the literature. In general, they are based, more or less, on a multiple redundancy of single AMB-components, which has some significant drawbacks: high costs, higher weight and higher space requirements. In spite of these drawbacks there is still a remaining risk of failure, for example, due to a missing concept for safe decoupling of defect modules or coupling of working ones. In this research project an alternative concept is developed to substantially increase the overall reliability of AMBs. This goal shall be reached by the following measures: - Decrease of costs, weight, and space requirements by intentionally waiving the concept of a complete redundancy; - Modular redundancy only for the switching amplifiers which usually are the most endangered components in industrial applications; - Automatic and decentralized recognition of a module failure by a so-called "local voter"; - Automatic and reliable decoupling of a defect module; - Replacement of defect modules of the switching amplifier during operation (hot-swap); - Increasing the noise immunity and the signal qualities by - spatial and electrical isolation of digital controllers and signal processing units from the power modules by using light guides; - a fully digitized actuator control; - using a highly integrated DSP-controller. The switching amplifier will be designed in module technique including signal coupling, fault detection and decoupling circuits and will be simulated first on the computer by a mathematical simulation model. With this model the system response for various failure scenarios will be investigated in view of a reliable and interruption- free control of the bearing electromagnets. Based on the simulation results a real switching amplifier will be designed and built. Its characteristics will be tested on a simple AMB-test rig. Finally, a complete prototype of an AMB-system with electrically isolated fully digital control will be built. The experimental results will be used for optimizing the amplifier prototype and updating of the simulation model parameters.

Active magnetic bearings (AMBs) allow a lubricant and friction free support of structures, e.g. rotors. Important advantages of this ecologically friendly technology are that there is no system-atical requirement for maintenance as well as the possibility to adjust bearing parameters to vari-ous operating conditions. The most significant drawback of AMB-technology is that even a single failure within one sub-assembly (digital controller, magnetic actuator, amplifier, position sensors and signal converters) causes a break down of the bearing load capacity. The risk of an unintentional machine standstill combined with the resulting costs is one major reason that the broad industrial acceptance of active magnetic bearings is quite low at the time. Known concepts from the literature for increas-ing the reliability of active magnetic bearings are mainly based on a more or less multiple redun-dancy of sub-systems. In spite of this redundancy there is still a remaining risk of failure, for ex-ample the safe decoupling of defect modules or coupling of working ones. In addition, they have some significant drawbacks, e.g. high costs, higher weight and higher space requirements. Within this research project an alternative concept considering economical aspects has been developed, built and tested to increase the overall reliability of active magnetic bearings. The main goals, which have been achieved, are: Modular redundancy only for the switching amplifiers which usually are the most endan-gered components in industrial applications, automatic and decentralized recognition of a module failure by a so-called "local voter", automatic and reliable decoupling of a defect module, replacement of defect modules during operation (hot-swap), high noise immunity and signal quality by spatial and electrical isolation of the controller from electromagnetically stray fields with a highly integrated and fully digitized actuator control. First, a simulation model of the module-based switching amplifier has been developed including the required signal coupling, error detection and decoupling circuits. With this simulation model effects of component failures have been analyzed with regard to a reliable and uninterruptible control of the magnetic actuator. Measurements of an AMB test rig equipped with this new tech-nology show no reduction in bearing load capacity despite of manually induced component er-rors. These errors would cause a complete break down of the bearing, if conventional switching amplifiers were used. Also the hot-swap procedure of a redundant module has no influence on the bearing load capacity. A reliability analysis of the switching amplifier shows, that using this new concept the reliability could be increased by a factor of 20.