What are the composition and characteristics of electro hydraulic servo valves

To add some content, the electro-hydraulic servo valve is composed of two parts: the pilot stage and the power stage, the function of the pilot stage is to "realize the conversion and amplification of electric and hydraulic signals" on the first floor, and the power stage contains the spool valve sleeve, which directly "controls the hydraulic actuator".

The advantages are divided into advantages and disadvantages, and the advantages are the "fast dynamic response, high control accuracy, and long service life" mentioned on the first floor

The disadvantage is that the cost is high and it is easy to pollute (NAS6 level is required for oil cleanliness).

The following is the copied information, very detailed:

A device that converts and amplifies electrical and hydraulic signals and controls hydraulic actuators.

Electro-hydraulic servo valve is a key component in electro-hydraulic servo control, it is a kind of hydraulic control valve that receives analog electrical signals and outputs modulated flow and pressure accordingly. The electro-hydraulic servo valve has the advantages of fast dynamic response, high control accuracy and long service life, and has been widely used in electro-hydraulic servo control systems in aviation, aerospace, ships, metallurgy, chemical industry and other fields. ”

<> product features. 1. Adopt torque motor and two-stage hydraulic amplifier structure;

2. The pre-stage is a double-nozzle baffle valve;

3. The driving force of the valve core is large;

4. High dynamic response performance;

5. High resolution and low hysteresis;

6. Compact structure and long service life.

At present, the servo valve structure is mainly divided into nozzle baffle type, jet tube type and direct-acting type. Since only one ** can be inserted, the following is a simple single-stage direct-acting structure.

The single-stage control valve is basically composed of a torque motor (1) and a longitudinal slide valve (3) with a four-way structure.

A torque motor is an electromechanical converter that converts an electrical signal into a linear motion at the end of a bolt (4), which is hermetically isolated from the hydraulic part. The armature (5), the flexible pipe (6) and the bolt (4) are connected to each other without gaps. The end of the bolt (4) that extends outward on the torque motor is connected with the control valve core (2) through the connecting rod (7), and the rigidity of the flexible pipe (6) competes with the control force of the torque motor when the bolt (4) is deflected, and plays a centering role.

The amount of movement of the valve core (2) is controlled so that the flow rate of the valve is proportional to the input electrical signal.

The hydraulic zero point can be adjusted by a bolt (8), which adjusts the relative position of the valve sleeve (10) moving axially in the housing (9) and the control valve core.

A typical servo valve consists of a permanent magnet torque motor, nozzles, baffles, spools, valve sleeves, and control chambers (see figure). When the current is introduced into the input coil, the baffle moves to the right, so that the throttling effect of the right nozzle is strengthened, the flow rate is reduced, and the back pressure on the right side rises; At the same time, the throttling effect of the left nozzle is reduced, the flow rate increases, and the left back pressure decreases. The forces at both ends of the spool are unbalanced, and the spool moves to the left.

The high-pressure oil flows from S to C2 and is sent to the load. The load return oil flows through the return port through C1 and into the oil tank. The displacement of the spool is proportional to the input current of the torque motor, and the hydraulic force acting on the spool is balanced by the spring force, so the differential current of the torque motor in the equilibrium state is proportional to the displacement of the spool.

If the input current is reversed, then the flow rate is also reversed. The table shows the classification of servo valves.

Servo valves are mainly used as actuators in electro-hydraulic servo systems (see Hydraulic Servo Systems). In the servo system, compared with the electric and pneumatic actuators, the hydraulic actuator has the characteristics of good speed, large output power per unit weight, stable transmission and strong anti-interference ability. On the other hand, electrical components are often used to transmit signals and correct characteristics in servo systems.

Therefore, modern high-performance servo systems also use electro-hydraulic methods, and servo valves are the necessary components of such systems.

The structure of the servo valve is relatively complex, the cost is high, and the quality and cleanliness of the oil are required. New servo valves are trying to overcome these shortcomings, such as servo valves with electrostrictive elements, which greatly simplify the structure. Another direction is the development of special working oils (e.g. electrical viscous oils).

This working oil can change the viscosity coefficient under the action of electromagnetism. This property allows the oil flow to be directly controlled by means of electrical signals.

The working principle of the electro-hydraulic servo valve.

The electro-hydraulic servo valve is mainly composed of a torque motor, a pre-amplification stage, and a power amplification stage.

Torque motor working principle:

The magnet magnetizes the conductive magnet into N and S poles, forming a magnetic field. The armature and baffle are connected and are supported by springs in the middle of the magnet. The ball head at the lower end of the baffle is embedded in the middle groove of the slide valve; When there is no current in the coil, the torque motor has no torque output, and the baffle is in the middle of the two nozzles; When the input current passes through the coil, the left end of the armature 3 is magnetized into an N pole, and the right end is an S pole, and the armature is deflected counterclockwise.

The bending of the spring tube creates a reaction moment that turns the armature at an angle. The larger the current, the greater the angle, and the torque motor converts the input electrical signal into a torque signal output.

How the pre-amplification stage works:

Pressure oil flows to the oil chamber and two nozzle chambers at the left and right ends of the slide valve through the oil filter and the throttle hole, is sprayed out by the nozzle, flows back to the oil tank through the middle part of the valve 9 when the torque motor has no output signal, the baffle does not move, and the pressure at both ends of the slide valve is equal. When the torque motor has signal output, the baffle is deflected, and the gap between the two nozzles and the baffle plate is unequal, resulting in the pressure at both ends of the slide valve being unequal, and the valve core is pushed to move.

How the power amplification stage works:

When the differential pressure signal of the front amplification stage makes the spool of the spool move, the main oil circuit is connected. The opening of the spool valve after displacement is proportional to the input current of the torque motor, so the output flow rate of the valve is proportional to the input current; When the input current is reversed, the output flow is also reversed. At the same time, the ball at the lower end of the baffle also moves with it, so that the spring plate of the baffle produces an elastic reaction force to prevent the slide valve from continuing to move; The deformation of the baffle reduces its displacement between the two nozzles, enabling feedback.

When the hydraulic force on the spool valve and the elastic reaction force of the baffle are balanced, the spool valve will remain at this opening and will not move.

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