The Study Of Discrete Variable Structure Control And Its Application In UPS Control

Variable structure control (VSC) is gradually gotten extensive attention for its very strong robustness on perturbation and external disturbances. VSC has been widely used in many domains, such as robot, power system and power electronics. In recent years, with the development of microcomputer technology and demand of industrial automation, the control algorithm can be realized by computer. However, while realizing the VSC, because of the limits of sampling, the ideal sliding mode does not exist. The state trajectory of system moves along the sliding plane in the form of chattering, which restricts its practical applications. Under the circumstance of discrete-time systme, properties of the sliding mode, for example, the reaching condition and the existence condition, have been changed, so it is of important significance to study discrete-time VSC (DVSC).This thesis focuses on DVSC and the application of it in the control of the output voltage of uninterruptible power supply(UPS). The main contents in this thesis are as follows:1.This thesis introduces the elementary concepts, elementary definitions, elementary properties, and elementary principles. Then it discusses the design methods of continuous variable structure control in detail.2.Starting from the elementary concepts of continuous variable structure control (CVSC), in the consideration of the new features of VSC after discretization, this thesis elaborates the reaching condition, the ideal sliding mode, the nonideal sliding mode and the quasi-sliding mode of DVSC.3. Based on the discrete asymptotic law reaching condition and inequality reaching condition of DVSC, the corresponding DVSC asymptotic law designing method and DVSC selective gain designing method are studied.The investigation of DVSC asymptotic law designing method includes:(1) Three basic types of discrete asymptotic law derivate from asymptotic law of CVSC.(2) Theoretic and simulative analysis on the switching region and chattering region of three basic discrete asymptotic law.(3) Discrete index asymptotic law and discrete variable speed asymptotic law are combined to form the smooth switching asymptotic law, and the theoretic analysis is validated by simulation.(4) Based on the disturbance dynamic compensation, the discrete asymptotic law is presented. The simulative results manifest that this control law adapts to discrete-time systems with small rate of disturbance change.The investigation of DVSC selective gain designing method includes:(1) Two fundamental concepts of DVSC are expatiated, that is equivalent control and equivalent control balance line.(2) Based on inequality reaching condition, selective gain designing method is analyzed and the theoretic analysis is validated by simulation.(3) DVSC selective gain designing method is presented, which can restrain the disturbance, and likewise, the theoretic analysis is validated by simulation.4. A discrete sliding-mode control scheme with linear feedforward control for the closed-loop regulation of the pulse-width modulation (PWM) inverter applied in an uninterruptible power supply (UPS). Simulation validates that the feedforward controller can effectively improve the UPS output voltage tracking performance and the sliding-mode controller can enhance the robustness of PWM inverter to the disturbance.