Study On Position Sensorless And Control Of Bearingless Permanent Magnet Synchronous Motor

Bearingless motors not only have the functions of rotation and rotor self suspension, but also have the characteristics of no lubrication, no wear, no mechanical noise, high speed and high precision. With these advantages, bearingless motors have broad application prospect in high speed precision numerical control machine tools, aerospace, flywheel energy storage system and other high-tech fields. Compared with other type of bearingless motors, the bearingless permanent magnet synchronous motor, due to its advantages of simple structure, high power density, high efficiency, reliable operation, has become one of the hot spots in the research field of bearingless motors presently. The installation and use of the traditional mechanical sensors has maken the motor volume increase, cost increase, at the same time, it has seriously limited the excellent performance of high speed to the bearingless permanent magnet synchronous motor. Compared with ordinary permanent magnet synchronous motor, the rotor is self-suspension, when the bearingless permanent magnet synchronous motor work at steady state, rotor position, speed and radial displacement are more prone to vibrate and mutate because of the external disturbance, parameter perturbation and other factors. The stable operation without position sensor and high performance control will meet the requirement of ultra high speed, low cost and practical application of the bearingless permanent magnet synchronous motor. Supported by the National High Technology Research and Development Plan of China (2007AA04Z213) and the National Natural Science Foundation of China under grant(60974053), in order to improve the performance of bearingless permanent magnet synchronous motor, several key theoretical and technological problems, such as the mathematical model, rotor magnetic field orientated control, adaptive nonsingular terminal sliding mode control, position sensorless operation in full speed, and digital control system of bearingless permanent magnet synchronous motor and experimental operation are comprehensively studied. The main researches and the corresponding achievement are as follows:1. A profound analysis on the generation of magnetic levitation forces for a surface-mounted bearingless permanent magnet synchronous motor with two degree of freedom is presented. From Maxwell tensor method, levitation force analytical model is deduced considering the rotor eccentricity. The mathematical model of electromagnetic torque is derived from the electromechanical energy conversion relationship considering rotor eccentricity and suspension winding current, motion equations of the rotor and systems are established.2. An adaptive nonsingular terminal sliding mode control is put forward. In order to obtain faster convergence of state variables during the whole process in nonsingular terminal sliding mode control, an adaptive variable-rated exponential reaching law is presented where the L1norm of state variables is introduced. Exponential and constant approach speed can adaptively adjust according to the state variables’ distance to the equilibrium position, which can short the reaching time and weak system chattering. The method is applied to the bearingless permanent magnet synchronous motor system, speed and radial displacement controller of adaptive nonsingular terminal sliding mode control are designed, simulation results show that, the proposed adaptive nonsingular terminal sliding mode controller could rapidly track the given values, system overshoot and static error are small, higher robustness can be obtained.3. Aiming at the defects of low robustness to model parameter variation, slow convergence, and undesirable tracking ability to abrupt state changes in square root unscented Kalman filter algorithm, an improved square root unscented Kalman filter is proposed. Combined with the strong tracking filtering principle, by introducing the time-varying fading factor and the diminishing factor to adjust gain matrices and the state-forecast covariance square root matrix, in order to realize the orthogonality of the residual sequences and force the square root unscented Kalman filter to track the real state rapidly. In the process of filtering, Cholesky and QR decomposition are used. Covariance square root matrix is used instead of covariance in iterative computation, which can effectively avoid the filter divergence and improve the algorithm convergence speed and stability. The vector control system for bearingless permanent magnet synchronous motor without a speed sensor is set up based on this approach. Rotor position and speed estimators designed by this method, can improve the estimate accuracy at stationary state, speed mutation status and the external disturbance.4. Because of one method cannot obtain precise estimation of rotor speed and position in full speed, a compound method using high frequency signal injection and improved square root unscented Kalman filter algorithm is proposed to estimate the rotor speed and position for a surface-mounted bearingless permanent magnet synchronous motor. When the motor work at state of zero and ultra-low speed, fluctuating high frequency signal injection is used, which does not depend on the motor parameter, only depend on motor space-saliency effect, accurate estimation will be achieved; at the high speed, the improved square root unscented Kalman filter is used. The improved square root unscented Kalman filter has strong robustness of the model mismatch and unique strong tracking ability, which can effectively improve the tracking performance to steady state and mutation status. By designing speed switch, smooth switching is achieved. The proposed method is capable of precisely estimating rotor speed and position in full speed. 5. Aiming at the defects of low precision, long delay and complicated process in the signal processing of traditional rotating high frequency signal injection method, an improved rotation high frequency signal injection method based on finite impulse response filter is proposed. Equiripple and optimal approximating finite impulse response filter is introduced to extract the high frequency current, which can realize minimum extraction error of the high frequency current signal. Using heterodyning processing to the high frequency current, the rotor position error signal can be extracted. This method can eliminate the synchronous shaft filter and reduce the complexity of the system.In order to achieve minimum delay of rotor speed and position estimation, linear phase compensation is used in the finite impulse response filter. The method has the advantages of simple structure, high accuracy, easy realization and debugging, accurate estimation of rotor position and speed can be obtained.6. The digital control system of bearingless permanent magnet synchronous motor is designed, the corresponding hardware systems and software systems are designed. The space voltage vector pulse width modulation is used to realize the rotor magnetic field orientated control strategy. Stable suspension operation in speed adjustment and speed step are achieved in the platform. Detailed experiment scheme for position sensorless control of bearingless permanent magnet synchronous motor based on adaptive nonsingular terminal sliding mode control is proposed, which establishes a solid foundation for developing the high performance position sensorless control of the bearingless permanent magnet synchronous motor further.