Abstract: To address the motor model variations caused by time-varying parameters and external disturbances in DC servo systems, a sliding mode variable structure control strategy is designed to control the DC servo motor, and the system stability is analyzed. Furthermore, a comparative simulation of the designed sliding mode variable structure control system and PID control is conducted; the results show that for systems with large-range perturbations in motor parameters, the sliding mode variable structure control system exhibits strong robustness and speed.
Keywords : sliding mode variable structure; PID controller; DC servo motor
1 Introduction
Sliding mode variable structure control system is a special type of variable structure control system. Its special feature is that the control of the system not only has switching, but the switching characteristic can force the state of the system to make small-amplitude, high-frequency approaching motion-sliding mode motion along a predetermined trajectory on the switching surface. This sliding mode can be designed to be independent of the system parameters and disturbances. In this way, sliding mode variable structure control has good robustness without the need for online identification of the system. In recent years, sliding mode variable structure control has been widely used to deal with some complex linear systems, nonlinear systems, time-varying, multivariable coupling deterministic and uncertain systems, which can enable the system to obtain good dynamic quality[1]. For high-performance DC servo systems, high precision, no overshoot, fast response speed and good robustness are generally required. Conventional PID, PI and other controllers designed according to the automatic adjustment principle have simple structure, mature design method and are easy to implement, but the nonlinearity, time-varying parameters and external disturbances in DC servo systems affect the control performance of the system and it is difficult to meet the requirements of high precision servo systems. The sliding mode of sliding mode variable structure control is fully adaptive to the disturbances applied to the system and various perturbations of the system, so it can fully meet the control requirements of high-precision servo systems [2,4].
2. Sliding Mode Variable Structure Controller Design
The structural block diagram of the DC servo system is shown in Figure 1. Although the parameters of the motor are often regarded as constants in most cases, in reality, when the speed changes over a wide range, the parameters of the motor are not constant. Sliding mode control is not very sensitive to changes in parameters, so using this control method to control the motor has great advantages.
Figure 1 Block diagram of DC servo system
2.1 DC Servo System and its Mathematical Model
2.2 Fundamental Problems of Slip Mode Variable Structure Control
2.3 Selection of the switching function s
2.4 Determination of control variable u
2.5 System Stability Analysis
3. Simulation and Conclusion
The simulation results show that when there is a large perturbation of motor parameters, the sliding mode variable structure control system can adapt to the changes in system parameters, with a fast response time, quickly reaching steady state without overshoot; while the PID controller produces a large overshoot, which, for the DC servo motor control system, is too large and seriously affects the machining process.
5. Conclusion
This paper analyzes a DC servo system and designs a sliding mode variable structure control strategy for it. The design method is simple and easy to implement. Simulation analysis shows that sliding mode variable structure control is superior to PID control, exhibiting strong suppression of parameter perturbations and strong adaptability. Furthermore, with reasonable selection of switching functions and control law parameters, the sliding mode variable structure control system possesses excellent dynamic and static performance with good speed, no overshoot, and no steady-state error, and demonstrates good robustness to system parameter perturbations.
