Proceedings of zyxwvutsrqpo the 2004 IEEE Conference on Cybernetics zyxwvutsrq and Intelligent Systems Singapore, 1-3 December, 2004 Fuzzy Logic Based Control of Rotor Motion in Active Magnetic Bearings zy Maki K. Habib School of Engineering Monash University Malaysia Sclangor, Malaysia. zyxwvutsrqp niciki @ieee.org Abstract zyxwvutsrqpo - Active magnetic bearings (AMB) are increasingly being used as an alternative to rolling element and fluid-film bearings in rotating machinery application. Stablc opcration of AMB can only be achievcd via feedback control of which the most widcly used controllers are of thc linear PID type. Undcr extreme conditions, however, the dynamic responsc of the rotor in AMB bccomes highly nonlinear. As a result, the linear controllers are no longer capable of suppressing or controlling the bifurcations of the rotor response. In order to suppress the non-linearity in AMB, a nonlinear control strategy is required. One of such strategies is the use of fuzzy logic based control approach. This paper aims to investigate and analyze the dynamical response of rotors in active magnetic bearings in AMB and presents the dcvclopmcnt and implemcntation of a fuzzy logic control strategy for suppressing the nonsynchronous response in AMB. This control strategy is expected to stabilize the rotor-bearing system or to delay its onset of instability. Through modcling and simulation of the non-linear rotor response. it is found that for certain operating parameters, the rotor in AMB exhibits non-synchronous response (quasi-periodic or chaotic). However, the use of fuzzy logic control has been able to eliminate the undesirable non-synchronous response and improve the rotor stability pcrformance. 1. INTRODUCTION The application of active magnetic bearings in rotating machinery has increased in recent years. They are being used as an alternative to rolling-element and fluid-film bearings. Their dimension may vary from 25.4 mm diameter corresponding to 150 N load capacity to 1300 mm diameter corresponding to a load capacity of 250 kN [l]. The availability of these bearings in a wide range of dimension and load capacity have resulted in their utilization in a wide class zyxwvu of rotating machinery ranging from turbo-molecular pumps to industrial steam turbines. The main advantage of the active magnetic bearings over the conventional bearings is their higher mechanical efficicncy due to reduced friction losses since no contact occurs between the rotor and the bearing stator during operation of the machine. The fact that the operation of the magnetic bearings is contactless allows them to achieve a higher operating speed compared to conventional bearings; the achievable speed is essentially limited by the strength of the rotor material. These bearings do not require lubrication and therefore their installation are cleaner Jawaid I. Inayat-Hussain School of Engineering Monash University Malaysia Selangor, Malaysia. compared to the rolling-element and fluid-film bearing types. Furthermore the ancillary cquipment that are associated with the lubrication system of conventionat bearings. are no longer required for magnetic bearing installation resulting in lower cost and less requirement for space. The rotor dynamic characteristics of a rotor-magnetic bearing systcm can be varied in a considerably .wide rangc by changing the parametcrs of its controller. without having to modify the dimensions of the rotor or the bearing. This in turn allows active control of the rotor vibration and automatic balancing of thc rotor, which is not easily implemented for rotor supported by conventional bearing types. The main disadvantage of the active magnetic bearings is their lower bearing .pressure capacity compared to fluid-film bearings of the same physical dimension. In general active magnetic bearings havc il bearing pressure of 200 kPa as opposed to fluid-film bearings that have a bearing pressure of 500 kPa. The stiffness of magnetic bearings is thereforc less than their equivalent fluid-film bearings. Another disadvantage of the magnetic bearings is their highly non-linear characteristics, which is the concern of this work. The sources of non- linearity in rotor-magnetic bearing system are discussed below. There are several sources of non-linearity in an active magnetic bearing system, of which the most prominent is the relationship between the forces generated in the electromagnetic actuator and the coil current and the air gap between the rotor and the stator. The force is proportional to the current squared and inversely proportional to the gap squared. Crosscoupling between the electromagnetic forces acting in two orthogonal directions is also a source of non- linearity in a magnetic bearing system. There are three causes of cross coupling, i.e., geometry of the actuators, eddycurrent effect and gyroscopic effect. The air gap at a point on a magnetic pole is actually not constant over the entire pole area due to the geometrical curvature of the pole. This results in a norma1 force, which is perpendicular to the principal force, which in turn causes geometric coupling between these forces. Eddy currents are induced in a solid (non-laminated) magnetic core when the shaft rotates. Their effective lift is reduced and their drag cause coupling between the two orthogonal directions. Gyroscopic effect, which is more pronounced in rotors with large or overhung discs, causes the motion in the two orthogonal directions to be coupled. Non-linearity in a magnetic bearing system may also arise due to hysteresis of the magnetic core material. Therefore a definite rclationship 0-7803-8643-4104/$20.00 0 2004 IEEE 1219