In contrast with traditional mechanical bearing, Active magnetic bearing (AMB) has no friction and lubrication, and its dynamic performance can be adjusted by active control. To isolate low frequency vibration of the rotating machinery under 50Hz, a novel design of cascade PID controller (CPC) with two control loops for AMB is proposed. The main loop is a position loop and the secondary loop is a transmission force loop. According to the theoretical derivations in this study, the CPC controls both the rotor position and the transmission force. Even when the control parameters maintain constant, the dynamic characteristic parameters, equivalent stiffness and equivalent damping, vary with frequency continuously and smoothly. Therefore, they can be adjusted in a wide range to achieve isolation of low frequency vibration when using proper control parameters. A simulation example shows that the transmission force with a CPC is lower in the 8–50Hz when the rotor displacement is almost same as with a single stage PID controller (SSPC). Experimental verification was carried out in an experimental bench of AMB under unbalanced rotor condition. Results show that a CPC can reduce the vibration acceleration at 15–50Hz especially near the peaks. Simulation and experimental results well demonstrate the effectiveness and guaranteed stability of the CPC in the present study.

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