Aiming to suppress the influence of uncertain disturbances in the drive control of permanent magnet synchronous machines (PMSM), such as the parameter uncertainties and load disturbance, a robust anti-interference control for the angular position tracking control of a PMSM servo system has been proposed in this paper. During the position tracking, uncertain system disturbances being regarded as a lumped unknown term will be online observed by a nonlinear disturbance observer (NDOB), of which the influence will consequently be counteracted by a robust backstepping compensator (RBC). The asymptotical stability of proposed control scheme is analyzed and designed according to the Lyapunov stability criterion, and its convergence against the system uncertain disturbance is verified on a prototype PMSM servo platform and shows good performance in rotor angular position tracking and anti-interference.

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Due to the nonlinearities of the voltage-source inverter (VSI) in a permanent magnet synchronous machine (PMSM) drive system, there is always an error between the reference voltage and the actual output voltage. To compensate the voltage error, many schemes have been proposed based on the phase current polarity. However, due to factors such as current clamping, measurement noises, and control system delay, the accuracy of the detected current polarity is relatively low, especially when the current is around zero, which would therefore affect the compensation performance. To solve this issue, a deadbeat prediction-based current zero-crossing detection method (DP-CZD) is proposed in this paper. With the proposed method, the measured three-phase currents are replaced by the predicted three-phase currents in terms of the polarity determination, when the absolute value of the phase current is within the threshold range. Compared with the conventional phase current polarity detecting methods, the proposed method can greatly improve the accuracy of detected current polarity due to its smooth transient waveform, and consequently, contributes to the much higher accuracy and lower total harmonic distortion (THD) in the compensation of VSI nonlinearity, which is verified through a prototype surface-mounted PMSM.

Autonomous underwater vehicle (AUV) is a nonlinear system with characteristics of strong coupling, parameter perturbations, and non-modeling disturbance. In this paper, a nonlinear observer is designed based on extended state observer (ESO) principle for the problem of AUV mathematical model parameter perturbations caused by hydrodynamic effect, and the impact of outside disturbance by ocean currents, etc. The observer has strong ability of anti-interference, it can estimate external disturbance on systems and attribute uncertain disturbance in the system model and external disturbance to the disturbance of total system, at the same time make the real-time estimation and effective compensation to the system. The result of simulation shows that the nonlinear observer designed by this method can estimate and compensate the interference and unknown uncertainties on AUV. It has good anti-disturbance performance, and can effectively estimate the appropriate state for observer design.

Abstract-To improve the high frequency compensation performance, LCL-filter is used instead of L-filter in three-phase four-wire active power filter (APF). In this paper, the characteristic of LCL-filter is discussed. The three-loop control strategy with active damping using capacitor current feedback is introduced. Then the system stability is analyzed by bode and zero-pole diagram, especially considering the time delay of the control system. The results of simulation and experiment prove the effectiveness and stability of the proposed control strategy.Index Terms-Active Power Filter (APF), active damping, delay, lcl-filter, stability. I. INTRODUCTIONAs the power quality problem becomes more serious, active power filter (APF) is considered that has better compensation effect in compensating the harmonics and reactive power current since it doesn't depend on the parameters of the grid [1].Generally, APF is connected to the grid with single L-filter. But it has worse high frequency proformance. LCL-filter has been proposed to replace L-filter because it can suppress the switching harmonics and has better result with lower inductance.As a third-order resonance system, LCL-filter may cause the system disability. Passive damping is widely used by adding a resistance in series with the capacitor. However, it will cause power consumption and decrease the attenuation ability. Active damping can reduce the power consumption and make the system stable by changing the structure of the controller. Besides, the time delay of control system also has a big influence on the stability. In [2], [3], the filter characteristic of LCL is given but it is used in active power rectifier.[4], [5] have proved that passive and active damping are effective in APF, but it is just for three-phase three-wire APF and the influence of time delay is not put into consideration.In this paper, a three-phase four-wire APF with LCL-filter is introduced. The system structure and the control strategy with active damping method is proposed and the stability is analyzed, with the consideration of system delay. Finally, simulation and experiment results of a 75kVA APF are provided to verify the effectiveness and stability of the proposed control strategy. Manuscript received October 11, 2012; revised November 21, 2012. Longfei Li and Xinjian Jiang are with the Department of Electrical Engineering, Tsinghua University, Beijing 100084, China (e-mail: lilf11@ mails.tsinghua.edu.cn, jiangxj@ mail.tsinghua.edu.cn). II. SYSTEM MODEL ANALYZE A. APF PrincipleThe structure of APF with LCL-filter is shown in Fig. 1, where L, L g and C f form the three-order filter. APF detects the harmonic current to generate the reference current and provides the opposite harmonic current into the grid so that the grid current is sinusoidal. Meanwhile, it can also compensate the zero-sequence current by the control of the fourth bridge arm. The output filter should suppress the current ripple and satisfy the demand of current compensation. LCL-filter is adopted for its better pe...