Servo valve is one of the core components in hydraulic control system, which is widely used in high-precision and high-performance automatic control systems, such as aviation, aerospace, metallurgy, machine tools and other fields. Among them, the Positive Overlap servo valve has obvious advantages in some specific applications because of its unique structure and working characteristics. This paper will introduce the basic structure, working principle and application characteristics of the positive cover servo valve.
First, the structural characteristics of the positive cover servo valve
Servo valve usually consists of electromagnetic part (torque motor or moving coil actuator) and hydraulic amplification stage (first and second slide valves). According to the communication of oil ports when the slide valve is in the middle position, the servo valve can be divided into three types: positive cover, zero cover and negative cover.
The so-called “positive cover” means that when the slide valve is in the neutral position, the control window is completely closed, and there is no passage between the oil inlet (P) and the control chambers (A and B) and between the oil return port (T) and the control chamber. That is to say, when the slide valve is in the middle position, it has a certain “shielding” effect on the oil circuit, thus preventing the hydraulic cylinder or hydraulic motor from drifting or misoperation when there is no command signal.
Second, the working principle of the positive cover servo valve
The working process of the positive cover servo valve can be divided into the following stages:
1. Neutral state: When there is no input signal, the slide valve is in the neutral position, at this time, the oil inlet and the working oil port are completely closed, and the hydraulic actuator remains in a static state. This design can prevent unexpected actions caused by external disturbance or leakage and improve the stability of the system.
2. After the control signal is input: when the electromagnetic part receives the control signal (current or voltage), the torque motor or moving coil mechanism generates corresponding deflection force, which drives the first-stage nozzle baffle mechanism to move and changes the gap between the nozzle and the baffle, thus forming a pressure difference to push the second-stage slide valve to move.
3. Slide valve deviation and flow output: After the slide valve moves, open the corresponding oil channel, so that high-pressure oil enters one cavity of the hydraulic cylinder and the other cavity returns oil, thus driving the actuator to move. The displacement of the slide valve is proportional to the input signal, so accurate control can be realized.
4. Feedback mechanism: Modern servo valves mostly adopt position or flow feedback mechanism to keep the actual position of the slide valve consistent with the input signal, realize closed-loop control, and improve response speed and control accuracy.
Third, the application characteristics of the positive cover servo valve
1. High stability: Because all oil ports are closed in the neutral position, the hydraulic actuator will not be displaced due to oil flow when there is no input signal, which is suitable for occasions with high requirements on positioning accuracy and stability.
2. Strong anti-interference ability: the positive cover structure can effectively prevent misoperation caused by external disturbance, and is suitable for systems with complex environment and large load fluctuation.
3. Slow response: Compared with the negative cover servo valve, the positive cover needs to overcome the initial “dead zone” at the beginning of the action, so the response speed is slightly slow, but it can be improved by optimizing the design.
4. Suitable for positioning control: it is often used in systems that need precise positioning, such as CNC machine tools, robots and flight control systems.
Fourth, summary
The positive cover servo valve has obvious advantages in ensuring system stability and anti-interference ability because of its unique neutral closing characteristics. Although its response speed is slightly lower than that of the negative cover type, the positive cover servo valve is still an indispensable and important component when high-precision positioning and reliable control are needed. With the development of hydraulic control technology, the design of positive cover servo valve is also improving, gradually making up for the lack of response speed and further expanding its application scope.
By deeply understanding the working mechanism and application characteristics of the positive cover servo valve, it is helpful to select and design the hydraulic control system more reasonably in practical engineering and improve the overall performance and reliability of the system.