1 Research status of AC magnetic bearing drive mode and control method
1.1 Driving mode of AC magnetic bearing
At present, the main driving methods of AC magnetic bearings are three-phase inverter driving and matrix converter driving.
1.1.1 Three-phase inverter drive
At present, scholars who are focusing on the study of three-pole AC active magnetic bearings driven by three-phase inverters are from Chemnitz University of Technology, Germany.
HofmannW ; domestic Zhejiang University and Jiangsu University are also researching inverter-driven magnetic bearings [5, 8]. HofmannW conducted a steady-state analysis on the three-pole radial active magnetic bearing (the structure is shown in Figure 2), and designed the control system of the single-pole active magnetic bearing considering the mutual coupling relationship between electromagnetic and mechanical variables . Under different bias currents, the control system achieves a good decoupling effect. Figure 7 shows the main circuit topology of the inverter-driven three-pole radial active magnetic bearing . The AC magnetic bearing structure and control block diagram studied by Zhejiang University in China are similar to those studied by HofmannW, as shown in Figure 8.
For the three-pole radial active magnetic bearing, Zhejiang University established its linear analytical mathematical model, and used the finite element software Ansoft/Maxwell2D/3D to verify its principle. A three-phase power control circuit of hysteresis current tracking type is designed, in dSPACE
Preliminary tests were carried out. And the voltage feedback circuit, voltage hysteresis comparison circuit, pulse distribution circuit, fault output protection circuit, intelligent power module IPM and corresponding interface circuit in the power inverter circuit are designed .
Jiangsu University has studied three-pole AC active magnetic bearings and hybrid magnetic bearings [6-8, 16]. The structure of the active magnetic bearing is shown in Figure 2 [7, 16], and the structure of the hybrid magnetic bearing is shown in Figure 4 [6, 8] ], according to the two structures, the basic structure and working principle of the three-phase inverter-driven AC magnetic bearing are introduced, the magnetic circuit of the magnetic bearing is calculated by the equivalent magnetic circuit method, and the mathematical model of its levitation force is obtained. ;According to the requirements of the suspended magnetic force of the test prototype, the material selection, parameter design and calculation process of the test prototype are given; the magnetic field and rotor force of the magnetic bearing of the structure are simulated and checked by the finite element Ansoft software, and the magnetic The nonlinearity of the bearing suspension force and the coupling between the two radial degrees of freedom near the equilibrium position are calculated and analyzed, and a detailed control block diagram is given, as shown in Figure 1. The research results show that the mechanical and magnetic circuit structures of the two magnetic bearings are reasonable, the levitation force meets the design requirements, and has good linearity and symmetry near the equilibrium position, and there is almost no motion coupling between the two radial degrees of freedom.
1.1.2 Matrix Converter Drive
The matrix converter is an “all-silicon” power converter with advanced topology. It allows single-stage frequency conversion without large-capacity energy storage elements, and the input current and output voltage are sinusoidal, and the input power factor can reach 0.99. The above can be freely adjusted, and the energy can flow in both directions.
This driving method was proposed by Javier Vadillo of Spain at the 11th International Magnetic Bearing Conference held in Japan in 2008 . At that time, this driving method was proposed based on the direct space vector modulation strategy and compared with the traditional voltage source inverter driving method, which confirmed the rationality of this new driving method. For the matrix converter, the fast Fourier analysis proves that the design of the input filter is simpler, more effective and less expensive. The relationship of the entire system to the grid is significantly improved. Compared with the voltage source inverter, this important feature of the matrix converter, as well as the advantages of strong firmness and superior flexibility, will make it a new bright spot for future power supply applications. The research of foreign scholars provides a valuable reference basis.
1.2 Control method of AC magnetic bearing
1.2.1 Redundant Coordinate Method
ParkSH of Korea proposed to use redundant coordinate method to decouple the AC radial active magnetic bearing system based on PD control, and analyzed the controllability of the mathematical model of the system according to the derived system formula and the proposed decoupling control strategy. . The results show that this kind of controller can make the rotor achieve stable suspension with a speed of 5000r/min, and has good dynamic performance and static performance .
1.2.2 Sliding mode control method
Spanish scholar BeizamaAM compared the performance of three-pole radial magnetic bearings under sliding mode control (SMC) and pole configuration control (PPC) in order to analyze the performance of external disturbances on the rotor  (Fig. 9-Fig. 10). . Figures 9 to 10 show that the experimental results of the pole configuration control do not agree well with the simulation results; however, the two results of the sliding mode control method are very similar, proving that the sliding mode control method has high reliability and is almost immune to parameter changes and Superior performance for model dynamic performance effects. Taiwan scholar Chen Shyhleh designed and completed three controllers for the current-controlled three-pole magnetic bearing rotor system : linear state feedback controller, linear state controller based on feedback linearization, and sliding mode controller. After comparing the three controllers, it can be proved that the sliding mode controller can make the system produce the best transient and steady state performance .
1.2.3 Sensorless control method based on INFORM method
The sensorless control method based on the INFORM method was proposed by HoferM of the Vienna University of Technology in Austria, and used as a control method for permanent magnet biased three-phase hybrid magnetic bearings . The INFORM method is based on the voltage injection method for rotor position measurement. Through experiments, the sensorless INFORM signal is compared with the sensor signal to prove the feasibility of the INFORM method. The measured contrast signal graph is shown in Figure 11. The hybrid magnetic bearing model can be optimized by changing parameters such as bias flux, number of coil turns, core length, and rotor type, thereby improving the control performance of INFORM.
1.2.4 Displacement self-detection control method based on Kalman filter
The displacement self-detection control of Kalman filter was proposed by MatsudaK of Kyushu University in Japan in 2006 . This method addresses the viewing angle bias problem by introducing a Kalman filter. To demonstrate the effectiveness of this method, a Kalman filter was developed for estimating radial displacement and rotational speed of a three-pole magnetic bearing. Driven by a linear power amplifier circuit that causes current to flow into the coil. The coil end voltage is passed through an analog first-order low-pass filter with a cutoff frequency of 100 Hz. The designed Kalman filter uses the filtered coil voltage and the controller input as the input for the calculation evaluation; the non-view angle deviation is used as the unknown state. The Kalman filter is digitally simulated by the measured input and output data. The results showed that the bias did overlap with the predicted measurements.
1.2.5 Traditional PID control method
The traditional PID control method is the most widely used magnetic bearing control method at present. The domestic Jiangsu University has used the traditional PID control method to realize the stable suspension of several AC magnetic bearing test benches . The more representative ones are the AC three-DOF hybrid magnetic bearing test bench (Figure 12) and the AC two-DOF hybrid magnetic bearing test bench.
2 Development Trend of AC Magnetic Bearings
Magnetic bearing technology currently has some applications in the machinery industry and aerospace and other fields. The development trend of AC magnetic bearings is mainly concentrated in the following aspects.
(1) Since the AC magnetic bearing adopts a three-stage structure, there is a coupling between the displacement control of its radial two degrees of freedom. Therefore, it is necessary to adopt the nonlinear multivariable decoupling control strategy to study and realize the high precision multivariable decoupling control of the AC magnetic bearing.
(2) Research on sensorless technology of AC magnetic bearing. By identifying the current or voltage change in the magnetic bearing coil, the displacement change can be obtained indirectly without the need for a displacement sensor. Therefore, it can reduce the cost, reduce the volume and improve the reliability of the system, and has broad application prospects in the industry.
(3) The intelligent and integrated development of the control system. In order to meet the high performance, high reliability and low cost of magnetic bearing applications, it is an inevitable trend for the magnetic bearing control system to develop towards digitization, intelligence and integration, especially the miniaturization trend of electronic components and mechanical and electronic devices.
(4) Industrial application of AC magnetic bearings. The ultimate purpose of researching magnetic bearings is industrial application. Due to its various advantages, the application industry of magnetic bearings is rapidly breaking through from traditional turbomachinery, high-speed machine tools and other industries to new industries, such as artificial heart blood pumps, dryers, and precision position platforms. And computer hard disk and other application fields.
AC magnetic bearings have the characteristics of small overall volume, light weight, high efficiency and low cost, and have broad application prospects in various suspension supports. This paper makes a detailed classification and comparison of AC magnetic bearings from different perspectives such as system composition, working principle, classification and characteristics, driving mode and control method, analyzes and discusses several typical AC magnetic bearings, summarizes and summarizes The current control methods and driving methods are presented, and the research and development direction of AC magnetic bearings is pointed out.
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