Turbomolecular pumps on active conical magnetic bearings

A field in which the magnetic suspensions technology is finding wide application with an increasing rate is the vacuum industry and, in particular, the market of turbomolecular pumps. Nowadays, most used solutions adopted in particular for small to medium pumping capacity are based on hybrid architectures where passive magnetic bearings are combined to ceramic ball bearings. By converse, fully active magnetic suspensions with cylindrical configuration represent the standard for medium to high pumping rate machines. Although simple, the cylindrical configuration is prone to drawbacks related in particular to the straines growing in the disc of the axial actuator that motivate the investigation of alternative architectures. As shown in previous literature, the use of conical configuration, besides compacting driving electronics, seems to be promising considering that the control of the radial and axial degree of freedom is performed simultaneously by the same devices in two actuation planes. This paper describes the main design steps and experimental characterization of a turbomolecular pump supported by conical active magnetic bearings. The control design is based on a SISO decentralized technique with position and control embedded loops. A rotor centering technique based on the characterization of current loops is exposed while the external position loop is tuned measuring relevant transfer functions to refine the controller allowing critical speed crossing. The power actuation of the eight electromagnets is performed with a three-phase configuration drive technique instead of standard H-bridges to minimize the number of power switches. Experimental results along with numerical computations obtained with simulation models are reported in order to prove the validity of modeling approach and the effectiveness of the conical actuation system.