Created on:2022-03-07 10:01

Classification and introduction of electro-hydraulic servo valve

Classification and introduction of electro-hydraulic servo valve

The output flow or pressure of electro-hydraulic servo valve is controlled by the input electric signal. It is mainly used in high-speed closed-loop hydraulic system to realize the control of position, speed and force; The proportional valve is mostly used in the open-loop control system with relatively low response speed. Servo valve has the advantages of high precision and fast response, but its price is also high and the requirements for filtration accuracy are also high. At present, servo valves are widely used in automatic control equipment with high-precision control.

Electro hydraulic servo valves are mostly two-stage valves, including pressure servo valves and flow servo valves. Most servo valves are flow servo valves. In the flow type servo valve, the displacement XP of the main valve core is required to be directly proportional to its input current signal I. in order to ensure the positioning control of the main valve core, negative position feedback is set between the main valve and the pilot valve. The forms of position feedback mainly include direct position feedback and position force feedback.

1. Direct position feedback electro-hydraulic servo valve

The main valve core and pilot valve core of direct position feedback electro-hydraulic servo valve form direct position comparison and feedback, and its working principle is shown in Figure 168. The pilot valve is driven by the coil of the moving coil force motor. The displacement x center of the pilot valve core is proportional to the input current I, and its movement direction is consistent with the direction of the current. The diameter of the pilot valve core is small, which can not control the large flow in the system; The resistance of the main valve core is very large, and the thrust fork of the force motor is not enough to drive the main valve core. The solution is to first force the motor to drive the pilot valve core with small diameter, and then use the direct position feedback (position follow-up) method to make the main valve core follow the pilot valve by the same amount, so as to achieve the purpose of controlling the large flow in the system with small signal.

168

The cavities at both ends of the main valve core can be regarded as a symmetrical double acting hydraulic cylinder driving the main valve core, which is supplied with oil by the pilot valve to control the up and down movement of the main valve core. The diameter of the pilot valve core is small. In order to reduce the processing difficulty, two fixed orifices are usually used to replace the oil inlet valve ports on the pilot valve to control the upper and lower chambers of the main valve core. In order to realize direct position feedback, the main valve core, the driving hydraulic cylinder and the pilot valve sleeve are integrated. Therefore, the displacement XP (controlled displacement) of the main valve core is fed back to the pilot valve and is equal to the displacement x sleeve of the pilot valve sleeve. When the pilot valve core moves upward under the drive of the force motor and displaces x center, the opening amount between the pilot valve core and the valve sleeve is x center-x sleeve. At this time, the oil return port of the upper cavity of the main valve core is opened, the pressure difference drives the main valve core to move from bottom to top, and the pilot valve port is gradually reduced under the action of feedback. When the pilot valve port is completely closed, the main valve stops moving and the displacement of the main valve core XP = x sleeve = x center. The same is true for reverse motion. In this kind of feedback, the main valve core moves with the pilot valve by the same amount, so it is called direct position feedback.

Figure 169 (a) shows the structure of Dy series direct position feedback electro-hydraulic servo valve. The upper part is a moving coil force motor and the lower part is a two-stage slide valve device. The hydraulic oil enters from port P, ports a and B are connected to the actuator, and port t is connected to the return oil. The pilot valve 6 driven by the coil (moving coil) 7 is matched with the inner hole of the hollow main slide valve 4. The moving coil is fixedly connected with the pilot valve, and is positioned and centered with two springs 8 and 9. The two control sides on the pilot valve and the two transverse holes on the main slide valve form two variable throttling ports 12 and 12. In addition to flowing through the main control oil circuit, the hydraulic oil from port P also flows through the fixed throttle ports 3 and 5 and variable throttle ports 12, the annular groove of the pilot valve and the oil circuit from the transverse hole in the middle of the main slide valve to the oil return port to form the oil circuit of the pre hydraulic amplifier [see Fig. 169 (b)]. Obviously, the pre stage hydraulic amplifier is composed of two variable throttles 11 and 12 and fixed throttles 3 and 5. The connecting nodes a and B of the fixed throttle port and the variable throttle port in the oil circuit are respectively connected with the end faces of the upper and lower shoulders of the main slide valve, and the main slide valve can move under the action of the liquid flow pressure at the node; When the balance position is reached, the pressure of nodes a and B is the same, and the main spool valve remains stationary. If the pilot valve moves upward under the action of the coil, orifice 11 increases and orifice 12 decreases, the pressure at point a decreases and the pressure at point B increases, and the main spool valve moves upward. Since the main slide valve is also used as the valve sleeve of the pilot valve (position feedback), when the upward movement distance of the main slide valve is the same as that of the pilot valve, it stops moving. Similarly, when the pilot valve moves downward, the main spool valve moves downward by the same distance. So it is called direct position feedback system.

169

2. Nozzle baffle force feedback electro-hydraulic servo valve

It consists of two parts: Hydraulic baffle and electromagnetic nozzle. The electromagnetic part is a moving iron torque motor. The hydraulic part is of two stages; The first stage is the double nozzle flapper valve, which is called the front stage (pilot stage); The second stage is the four side slide valve, which is called the power amplification stage (main valve).

The front stage composed of double nozzle baffle valve is shown in Figure 170. It is composed of two fixed orifices, two nozzles and one baffle. Two symmetrically configured nozzles share a baffle, and a variable orifice is formed between the baffle and the nozzle. The baffle is generally supported by a torsion shaft or spring and can deflect around the support point. The rotation of the baffle is driven by a torque motor. When there is no input signal on the baffle, the baffle is in the middle position (i.e. zero position), and the distance between the baffle and the two nozzles is x0. At this time, the pressure P1 and P2 in the control chamber of the two nozzles are equal. When the baffle rotates, the pressure in the two control chambers increases on one side and decreases on the other, and the load pressure pl (PL = P1-P2) is output. The double nozzle flapper valve has 4 channels (1 oil supply port, 1 oil return port and 2 load ports) and 4 throttles. It is a full bridge structure.

170

The working principle of force feedback nozzle baffle electro-hydraulic servo valve is shown in Figure 171. The cavities at both ends of the main valve core can be regarded as a symmetrical hydraulic cylinder driving the main slide valve, which is controlled by the pilot stage double nozzle baffle valve. The lower part of the baffle plate 5 extends a feedback spring rod 11, which is connected with the main valve core 9 through a steel ball. The displacement of the main valve is transformed into elastic deformation force through the feedback spring rod, which acts on the baffle and balances with the electromagnetic torque. When no current passes through the coil 13, the torque motor has no torque output, and the baffle plate 5 is in the middle of the two nozzles. When the coil is energized with current, the deflection angle of armature 3 is day due to the action of electromagnetic torque. Since the armature is fixed on the spring tube 12, the baffle on the spring tube also deflects the day angle accordingly, so that the gap between the baffle and the two nozzles changes. If the gap on the right increases, the pressure in the left nozzle cavity increases and the pressure in the right cavity decreases, Main spool 9 moves to the right under the action of this pressure difference. Since the lower end of the baffle is the feedback spring rod 11, the lower end of the feedback spring rod is the ball head, which is embedded in the groove of the main valve core 9. While the main valve core moves, the ball head drives the upper baffle to move to the right through the feedback spring rod, so as to gradually reduce the gap between the right nozzle and the baffle. When the electromagnetic torque acting on the armature baffle assembly is balanced with the deformation torque (feedback force) of the feedback spring rod acting on the lower end of the baffle due to the movement of the ball head, the slide valve will no longer move and keep its valve port at this opening. The valve feeds back the displacement of the main valve core to the armature baffle assembly through the deformation of the feedback spring rod, and compares it with the electromagnetic torque to form feedback, so it is called force feedback electro-hydraulic servo valve.

The greater the control current through the coil, the greater the torque of armature deflection, the deflection deformation of the baffle, the pressure difference at both ends of the slide valve and the displacement of the slide valve, the greater the flow output of the servo valve.

 

 

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