Nonlinear Control Type Magnetic Bearing for Adding Damping Force to Existing Rotor

This paper proposes a nonlinear control method of magnetic bearings for adding the desired damping force to an existing rotor supported by passive magnetic bearings (PMBs) or gas bearings. PMBs composed only of permanent magnets do not require a control circuit. Therefore, PMBs can be economically manufactured. However, the vibrations cannot be suppressed because PMBs cannot generate damping force. Therefore, studies have been conducted on hybridized magnetic bearings. However, hybridized magnetic bearings have a disadvantage in that they are complicated and expensive. Gas bearings can generate excellent support force and damping force during high-speed rotation. However, it is difficult to dampen certain vibrations such as whirl vibration. Therefore, a method combining gas bearings and active magnetic bearings (AMBs) was considered in order to dampen such vibrations. Because the support rigidity of the AMBs are applied, the dynamics is significantly changed as compared with rotors supported only by the gas bearings. Moreover, because sufficient stiffness can be obtained from the gas bearings, it is useless to add stiffness through the magnetic bearings. To solve the above problems, we propose the use of the AMBs only for the generation of damping force. When using a general linear control, it is necessary to supply the bias current to the AMBs. However, it adversely affects the support stiffness of the rotor because a negative stiffness is generated by the bias current. We have previously proposed a control method with unnecessary bias current through nonlinear control of the AMB, and applied this method to actual equipment. However, this method involved the generation of both support force and damping force. Here, we focus on generating only damping force. This method is verified through the simulation.