Abstract Failure of single lower actuator of magnetic bearing (MB) can easily lead to collision between suspended rotor and auxiliary bearing, which affects the operation stability, even leads to the instability of the suspension system and endangers the safe operation. Existing fault-tolerant control methods for actuators generally require adding magnetic poles and power amplifiers or changing the topology of power amplifiers. Adding hardware increases both the design cost of MB and the volume of the entire system. In order to overcome the above defects, a fault-tolerant control strategy based on Bang-Bang + feedforward is proposed in this paper, in which radial MB for horizontal rotating equipment is studied. The fault-tolerant control of single lower actuator failure is realized without changing the hardware architecture of MB. Firstly, a two-degree-of-freedom (DOF) rotor dynamics model is established and proved to be a normal system in general, and the time optimal control to a normal system is Bang-Bang control. Then, the phase plane analysis method is used to fit the rotor motion curve and obtain the equation for calculating the Bang-Bang control switching point. After switching back to PID control, current feed-forward control is further used to make the rotor stable quickly. The feed-forward value is calculated from the rotor dynamic equation. Meanwhile, the method of actuator fault diagnosis is analyzed in this paper. The fluctuation in the sum of upper and lower actuator current under normal conditions can be decomposed into two parts, the ripple caused by the pulse width modulation and the current variation deviation of coils during rotor motion, thus the diagnosis threshold is determined. Finally, the effectiveness of the fault-tolerant control strategy is verified by simulation and tests. The Bang-Bang control can suppress the maximum displacement amplitude of the rotor with lower actuator failure, but it will lead to high frequency oscillation. In order to avoid the oscillation, the control method should be switched back to PID control after the Bang-Bang control, while PID control can only change the value of steady-state current by displacement integration, which means rotor cannot restore stability quickly. Therefore, feedforward control is needed to change the value quickly. Experiments show when the lower actuator failure occurs without fault-tolerant control, the rotor and auxiliary bearing will collide. The maximum displacement of the rotor will reach 0.13 mm, the stator acceleration peak reaches 10 m/s2 and continues for about 0.3s before returning to stability. After adopting the proposed fault-tolerant controller, the maximum displacement amplitude in fault freedom exceed the normal vibration amplitude lees than 0.04 mm and the rotor can recover to steady suspension in 0.2 s. Because there is no collision, the stator acceleration does not change significantly. The following conclusions can be drawn through simulation analysis and experiments: ①Bang-Bang control can effectively suppress the maximum displacement amplitude of rotor and feedforward control can accelerate rotor stability after a single lower actuator failure. ② When the fault DOF reaches stable, the other three DOFs also reach steady state, so the rotor stability can be judged by the state of fault DOF. ③ After the failure of the lower actuator, the fault-tolerant controller can effectively prevent the collision between the rotor and auxiliary bearing in full speed range. Improving the stability of the rotor and the service life of the auxiliary bearing, the maximum displacement amplitude of the fault side exceed the normal vibration amplitude lees than 0.04mm and the rotor can recover to the steady suspension in 0.2s. the stator acceleration changes little and recover to stability within 0.03s.
Liu Qi,Su Zhenzhong,Jiang Hao等. Fault Tolerant Control of Magnetic Bearing Actuator Failure Based on Bang-Bang + Feedforward Strategy[J]. Transactions of China Electrotechnical Society, 2023, 38(1): 177-189.
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