Discrete-time Sliding Mode Control For Sensorless Induction Motors

Because of its cost and maintenance advantages,induction motors are widely used nowadays.With the continuous development of digital chip technology,the control system of induction motor increasingly applies microprocessor based hardware system.Therefore,the research of the digital discrete control system becomes particularly important.At the same time,in t he motor speed control system,the sensorless vector control method has attracted the attention of researchers widely because of its good dynamic performance and no need for the sensor of detecting speed.Combined with the above situation,further research on vector control based on digital discrete system to improve its performance is of great significance to the modern induction motor speed control system.In this paper,a discrete-time sliding mode control method is proposed for discrete-time systems.Based on the investigation of the reaching phase and quasi-sliding mode,the novel discrete-time sliding mode control strategies are developed to make the systems to satisfy the existence of the sliding mode.The system can move towards the quasi-sliding mode band in finite steps and stay within that band thereafter.The discrete-time sliding mode control can increase the reaching speed to the quasi-sliding mode band and guarantee the robustness to both internal parameter variations and external disturbances.The control strategy is applied for the speed control of induction motors and achieve faster error convergence and higher steady-state accuracy.The specific contents are as follows: By analyzing the discrete Lyapunov stability theorem,the existence condition of discrete sliding mode is obtained.Thereafter,discrete sliding mode control law with variable gain is further developed and its control performance is analyzed.According to the mathematical model of three-phase induction motor,the vector control system of induction motor is designed based on the model after the decoupling.Then,based on the proposed discrete sliding mode control method,a discrete sliding mode speed controller and a current controller are designed through the error system s.Finally,the speed observation is realized by combining the model reference adaptive system and the sliding mode control to realize the motor speed control system without the load speed sensor.