Recent Trends in Development of Active Magnetic Bearings

An active magnetic bearing which supports a non-contact floating rotor by adjusting the attractive force of an electromagnet has been used for various industrial rotating machinery from the mid 1970s. The active magnetic bearings are classified into several types depending on the number of control axes thereof. The most frequently-used type is a fivedegree-of-freedom-controlled active magnetic bearing which controls a total of five axes of four radial axes (X1, Y1, X2, and Y2) and the axial axis (Z). A turbo molecular pump which is used as a mechanical vacuum pump for a semiconductor manufacture equipment can be pointed out as the most successful applications of the active magnetic bearing. The active magnetic bearing, however, required a number of displacement sensors, complicated controllers, power amplifiers for driving the electromagnets or the like and thus had higher cost, consequently, still preventing itself from being used in other application fields. Conventionally, such application also has mainly used an analog controller which requires terribly long term of tuning process for the transfer function of the control circuit carried out by well-experienced rare expert engineers, which was another reason of higher cost. However, the introduction of the digital control by the application of the digital signal processor (hereinafter abbreviated as DSP) after 1990 has allowed the active magnetic bearings to be increasingly used for rotating machines in various industrial fields. This paper will describe the trends of technical development (mainly the digital control) and the international standardization of the active magnetic bearings.