In modern industrial automation system, electro-hydraulic servo control system is widely used in aerospace, robotics, precision machining and other fields because of its high responsiveness, high precision and high power output capability. As a key actuator in hydraulic servo system, servo valve’s control performance directly affects the dynamic response and control accuracy of the system. Sliding Mode Control, SMC), as a typical nonlinear control method, is widely used in servo valve control because of its good robustness and insensitivity to external disturbances and parameter uncertainties. However, in practical application, sliding mode controller often needs to switch modes according to the running state of the system or the change of external environment, so it is of great significance to study the switching strategy of sliding mode control of servo valve.
First, the basic principle of sliding mode control
Sliding mode control is a variable structure control method, the core idea of which is to design a sliding surface and make the system state approach and keep sliding on the sliding surface in a limited time. This control strategy can effectively overcome the uncertainty and external interference in the system, and realize fast response and high-precision tracking. In servo valve control, the sliding surface usually selects the function composed of system error and its derivative, and the controller realizes the system state approaching to the sliding surface by switching the function.
Second, the problems of sliding mode control
Although sliding mode control has good robustness and anti-interference ability, it also has some shortcomings in application. For example, due to the high frequency switching of control variables, it is easy to cause “chattering” phenomenon, which affects the life and control accuracy of actuators. In addition, sliding mode control is usually designed under a specific sliding mode surface. When the operating conditions of the system change significantly, a single sliding mode control strategy may not meet the performance requirements, so it is necessary to switch control modes.
Third, the switching strategy of sliding mode control
In order to improve the adaptability and flexibility of the servo valve control system, the switching strategy of sliding mode control can be mainly developed from the following aspects:
1. Switching based on system state
According to the running state of the system (such as error, speed change rate, etc.), the sliding mode control parameters or sliding mode surface form are automatically switched. For example, when the system error is large, the sliding mode control strategy with strong approaching ability is adopted; When the error is small, switch to the boundary layer method to weaken chattering or the improved reaching law method.
2. Multi-sliding surface switching strategy
Several different sliding surfaces are designed, and the appropriate sliding surfaces are selected according to the task requirements or system state to achieve different control objectives. For example, one sliding surface is used for fast response and the other is used for improving steady-state accuracy.
3. Mixed use of sliding mode control and other control methods.
In some specific working conditions, sliding mode control can be combined with PID control, fuzzy control or adaptive control to realize complementary advantages through switching mechanism. For example, in the start-up stage, sliding mode control is used to speed up the response speed, and in the stable stage, PID control is switched to reduce chattering.
4. Switching strategy based on intelligent algorithm
Intelligent algorithms such as neural network, genetic algorithm or fuzzy logic are used to identify and predict the state of the system, so as to realize the online adjustment of sliding mode controller parameters or the switching of control strategies and improve the adaptive ability of the system.
IV. Conclusion
To sum up, although the sliding mode control of servo valve is excellent in anti-interference and robustness, it still needs reasonable switching strategy to improve the control performance under complex and changeable working conditions. Future research directions should include the design of intelligent switching mechanism, multi-model sliding mode control fusion, and the cooperative application of sliding mode control and other advanced control methods to achieve more efficient, stable and adaptive servo system control.
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References:
1. Utkin, V. I. (1992). Sliding Modes in Control and Optimization. Springer.
2. Slotine, J. J. E., & Li, W. (1991). Applied Nonlinear Control. Prentice Hall.
3. Zhang Xiaohua. (2007). Sliding mode variable structure control theory and its application. Machinery Industry Press.
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