Interests in the application of sliding mode control technique in variable speed drives have increased in recent years. It is well known that a distinguished property of a sliding mode control technique is its insensitivity to system uncertainties and external disturbances. Compared to the conventional PI controller, the system is sensitive to the parameter variations and inadequate rejection of external disturbances or load variations. Furthermore in order to design PI controller, the challenge faced by the researchers due to multi loop system structure and trial and error design approach which make the control design time consuming and expensive. This has lead to the development of the sliding modes control technique, which is very attractive for its excellent performance, easy to implement with simple control algorithm. It is desirable to achieve robust performance against external disturbances especially sudden or step load applications. In this thesis, a control system of DC motor for speed and torque control based on variable structure systems with sliding mode control approach is discussed. The choice of switching functions for different control goals using a method of the switching function estimation based on control error and armature current information is presented. The current limiter mechanism to limit the current during startup, acceleration and deceleration for speed control loop is proposed and the external torque observer is constructed. The simulations of the performance comparisons between sliding mode control and PI control show that variable structure system with sliding mode control approach is less sensitive to parameter variations, produce faster dynamic response, eliminates overshoot and performs better in rejecting disturbance. The excellent features of the sliding mode control based on variable structure system are mainly due to the high gain effect, which suppresses influence of disturbances and uncertainties in system behavior.
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