Principle of servo valve

Generally speaking, the main valve of servo valve has the same slide valve structure as the directional valve, except that the directional control of valve core is not driven by electromagnet, but by the hydraulic pressure output from the front stage valve, which is similar to the electro-hydraulic directional valve, except that the front stage valve of the electro-hydraulic directional valve is an electromagnetic directional valve, The front stage valve of servo valve is nozzle flapper valve or jet pipe valve with better dynamic characteristics.

In other words, the main valve of the servo valve is controlled by the output pressure of the front stage valve, and the pressure of the front stage valve comes from the inlet P of the servo valve. If the pressure of the P port is insufficient, the front stage valve can not output enough pressure to push the main spool action.

As we know, when the load is zero, if the four-way slide valve is fully opened, port P pressure = port t pressure + port pressure loss (ignoring other pressure losses on the oil circuit), if the port pressure loss is very small and port t pressure is zero, then the port P pressure is not enough to supply the pre stage valve to push the main valve core, and the whole servo valve will fail. Therefore, the port of servo valve is too small, even when the port is fully open, there must be a certain pressure loss to maintain the normal operation of the front stage valve.

In fact, servo valve has many disadvantages: large energy consumption, easy to failure, poor anti pollution ability, expensive and so on. There is only one advantage: its dynamic performance is the highest among all hydraulic valves. With this advantage, servo valve has to be used in many occasions with high dynamic characteristics, such as the control of aircraft and rocket steering gear, turbine speed regulation and so on. The lower dynamic requirements are basically proportional valves.

Generally speaking, it seems that servo system is closed-loop control, and proportion is mostly used for open-loop control; Secondly, there are many types of proportional valves, such as proportional pressure and flow control valves, so the control is more flexible than servo. From their internal structure, servo valve is mostly zero cover, proportional valve has a certain dead zone, low control accuracy, slow response. But from the perspective of development trend, especially in the aspect of proportional directional flow control valve and servo valve, the performance difference between them is gradually narrowing. In addition, the cost of proportional valve is much lower than servo valve, and the anti pollution ability is also strong!