The storage of rotating shafts is sometimes a challenge in a wide variety of applications. Increasing efficiency by minimizing friction and wear losses is becoming ever more important, with roller and ball bearings increasingly no longer meeting the increasing requirements. Magnetic bearings are usually the best choice for such requirements. The Vienna University of Technology is now presenting a sensorless magnetic bearing.

With active magnetic bearings (AMB - Active Magnetic Bearing), forces are generated with the aid of electromagnets, in which, in contrast to conventional bearings, mechanical properties such as stiffness and damping can be specifically influenced. However, active magnetic bearings are unstable systems that require stabilizing control. Many useful applications of magnetic bearings are not realized only because hitherto the use of a position sensor to stabilize the magnetic bearing system is considered necessary.

A 3-phase structure is used to minimize the hardware effort. This structure can be used with standard frequency convertern be operated from the drive sector. In contrast to most active magnetic bearings, which require a frequency converter for each degree of freedom, a 3-phase structure uses only 2 frequency converters, which leads to a significant reduction in hardware costs.

To stabilize the system, the controller is nowadays always implemented in a digital signal processor (DSP), which normally also performs other tasks. This digital loop must provide sufficient stability and robustness. The usual position sensors have unavoidable analogue paths which cause additional noise of the measuring signal and thus vibrations of the rotor to be stored. If the mounting of position sensors is not desired or not possible, the position information must be calculated from the physical properties of the system.

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Position sensors for special effects in the big blockbuster cinema

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A method for the sensorless determination of the rotor position is the "Inform" method developed by Prof. Schrödl. The Inform process has been used successfully for many years to identify the rotor angle of electric machines and has been further developed for the field of active magnetic bearings. It is based on the change of the magnetic properties as a function of the rotor position. By evaluating test pulses lasting only a few millionths of a second, the rotor position can be determined.