Siemens Reaches for Sensorless Active-Magnetic Bearings

You might have heard of magnetic-levitation (maglev) transportation vehicles that use magnetic fields to suspend a vehicle and in some cases propel it. In collaboration with the Technical University of Vienna, researchers from the Corporate Technology division of Siemens demonstrated the basis for a future sensorless magnetic bearing that uses active electromagnets to suspend and keep in place a rotating shaft. Magnetic-type bearings offer advantages over conventional bearings. Because they operate without friction, they do not wear or deteriorate from physical contact. As a result active-magnetic bearings can increase bearing life, have a long service life, require little maintenance, and operate at more than 200,000 rpm. The lack of lubricant could make this type of bearing well suited for vacuum-pump shafts, where leaking oil could contaminate an environment, such as that in a clean room.

The demonstrator comprised a ball held suspended by magnetic fields without any sensors. To me, the lack of sensors makes a big difference. Traditionally, active magnetic levitation requires sensors to measure fields so a controller can adjust the fields in real time to keep a body properly suspended. An active-magnetic bearing requires a similar type of real-time control to keep a shaft centered and suspended in a surrounding magnetic field. The controller relies on sensors that measure the shaft’s position. These sensors require extra space and increase bearing costs, so today’s active-magnetic bearings find limited use.

So, how did the researchers at Siemens and the Technical University of Vienna measure a shaft’s position without sensors? They took advantage of how the shaft alters the magnetic field used to suspend the shaft. The movement of the shaft changes the inductance of the electromagnetic coils, which in turn alter the current and voltage at the electromagnets. The measurements–made at the electromagnet power supply–require no extra wires or connections with the electromagnets themselves. As a result, magnetic bearings would become smaller and less expensive. The researchers developed a technique that uses the voltage and current measurements to determines the position of the shaft quickly and accurately and control the surrounding magnetic fields to keep the shaft in position. That technique should give sensorless active-magnetic bearings a good future. –Jon Titus