What is the difference between proportional valve and servo valve input control signal?

When there is a signal input, the main valve core drives the displacement sensor connected with it. When the feedback displacement signal is equal to the given signal, the main valve core stops moving and the proportional valve reaches a new balance position. There are two kinds of servo valves: mechanical feedback and electrical feedback. Generally, the frequency response of servo valves with electrical feedback is high, while that of mechanical feedback is slightly lower. The action process is basically the same as that of proportional valves.

Difference: generally, the input power of the proportional valve is large, which is basically in the range of hundreds of Ma to 1 ampere, while the input power of the servo valve is small, basically in tens of Ma; the control precision of the proportional valve is slightly lower, and the hysteresis loop is larger than that of the servo valve, and the control precision of the servo valve is high, but the requirements for the oil are also high.

In addition, the pressure of the valve core is not only controlled by the pressure of the valve core, but also by the pressure of the solenoid valve It is used for precise position and speed control, and also has a follow-up function. Therefore, the power steering system like your car is a servo system, which is difficult to achieve with proportional valve.

The up and down movement of the oil cylinder can be realized by switching the oil circuit through the valve in different positions. This kind of valve can be realized by using the ordinary reversing valve. The role of servo or proportional valve is not only to control the direction of the cylinder, but also to accurately control the opening of the valve, so as to accurately control the flow (the working state remains unchanged). The feedback of the valve can only determine whether the valve is accurate in place, but can not control the state of the system, because even if the opening of the valve is accurate, the flow to the actuator is also affected by many factors, such as pump pressure, pipeline consumption, load change and so on For the position and speed control, there is also a need for another loop to avoid the loop system closed loop. For the position and speed, there are formed speed and position control boards to achieve.

Some people say that "no matter how fine and accurate it is, it is only a problem of the valve itself, but it is not easy for the controlled object to do so". This is true, but the displacement of the actuator does not correspond to the displacement of the servo valve core, because after the valve core opens the oil port, even if it is not moving, the actuator will always keep moving, just like the faucet at home, when you turn on the water, the water will flow all the time. The proportional valve has no spool position closed loop of servo valve. It is a simplified design form by controlling the relatively accurate spool position by controlling the current supplied to the proportional valve core. The proportional valve does not need to be equipped with fixed actuator, so the system closed loop is needed to realize position and speed control. Whether proportional valve and servo valve need amplifier. Is the input control signal 4-20mA or 0-10V? Is there no - 10V - + 10V, that is to say, the action direction is realized by changing the oil circuit by two coils acting separately, rather than by its input control signal. Is that right?

The input control signals of proportional valve and servo valve are 4-20mA, 0-10V, - 10V - + 10V. Among them, the current input signal has strong anti-interference, but the corresponding price is expensive. The internal coil can be one or two. Many three position valves only use one coil, and the voltage is reversed to control the two sides. For three position valve, two coils are arranged on both sides of the three position valve, but the coil can be separate Control, can also be series or parallel control.

In fact, it is difficult to distinguish the proportional valve from the servo valve. The driving mode may be only different. The servo valve is the torque motor, the proportional valve is the proportional coil, the servo valve is the nozzle baffle, and the proportional valve is the slide valve. However, when it comes to the control accuracy, the proportional valve may still have a high point, especially in the two pole feedback valve, because the proportional valve feedback of the pilot pole is a potential displacement sensor The servo valve is a mechanical feedback rod, so in some parameters such as repeatability, hysteresis, linearity, proportional valve is better than servo valve, but the curve servo valve in Bode diagram is better.

If the input control signal of proportional valve and servo valve is 0-5V or - 5V - + 5V through potentiometer, the proportional gain of amplifier can be adjusted to 0-10V. The proportional valve and servo valve can be internal feedback or external feedback. In some cases where the control requirements are not very high, the internal feedback valve can be used and a differential pressure compensator is added under the valve to realize the independence of external pressure If the outer closed loop is used, then the proportional valve needs to be equipped with a PID control board, which is very expensive. On some die casting machines with large flow rate (4000L / min-12000l / min), the proportional servo valves are two pole inner closed-loop, because the outer loop is too late. The proportional valve has a large zero dead zone (about 20%), while the servo valve has no. From the control point of view, proportional valve is not suitable for position control.