Structure, Classification and Characteristics of Hydraulic Servo System

   Servo system, also known as servo system or tracking system, is the 1 kind of automatic control system. In this system, the actuator can automatically act according to the change law of the input signal with a certain degree of accuracy. Hydraulic servo system is a servo system composed of hydraulic components.

   Composition of hydraulic servo system
The hydraulic servo system consists of the following basic components:
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Input element: also known as the command element, it gives the input signal (command signal) added to the input of the system, is mechanical, electrical, pneumatic, etc. Such as the model, instruction potentiometer or computer.
Feedback measurement element: measures the output of the system and converts it into a feedback signal. Such elements are also in various forms. Various sensors are often used as feedback measuring elements.
Comparison element: The feedback signal is compared with the input signal to give a deviation signal.
Amplification conversion element: the deviation signal is large, converted into a hydraulic signal (flow or pressure). Such as servo amplifier, machine-hydraulic servo valve, electro-hydraulic servo valve, etc.
Executive element: generate adjustment action and add it to the control object to realize the adjustment task. Such as hydraulic cylinders and hydraulic motors.
Control object: the controlled machine equipment or object, that is, the load.
Other: various correction devices, and hydraulic power devices not included in the control circuit.

Classification of Hydraulic Servo System
Hydraulic servo system according to different principles have a variety of classification methods:

1. According to the input signal change law classification:

Fixed value control system: The input signal of the system is a fixed value, which is called a fixed value control system. Its basic task is to improve the anti-interference ability of the system;

Program control system: the input signal of the system changes according to a predetermined law;

Servo system: also known as the servo system, its input signal is an unknown function of time, the output can accurately and quickly reproduce the change of the input.
2. According to the controlled physical quantity of different classification

Divided into position control system, speed control system, force (pressure) control system, other controlled output control system. The first three are the most commonly used in engineering.

3. According to the transmission signal (refers to the input and deviation signal) component different classification

Electro-hydraulic servo system: the components that transmit signals are electrical components;
Machine-hydraulic servo system: the component that transmits the signal is a mechanical device;
Pneumatic servo system: The components that transmit signals are pneumatic components.
4. According to the different classification of hydraulic control components
Valve control system: the use of throttling principle, by hydraulic servo valve to control the flow into the implementation of components of the system;
Pump control system: the use of servo variable pump to change the pump displacement method to control the flow into the implementation of components of the system.

The working principle of hydraulic servo system
The working principle of the hydraulic servo system has been in what is the servo hydraulic station? What are the advantages compared with the ordinary hydraulic station? Briefly introduced, today we explain with examples.


    
    The key component of the servo hydraulic system is the servo valve. The following figure shows the structure diagram of the electro-hydraulic servo valve. Its working principle is:
    The torque motor of the electro-hydraulic servo valve uses a permanent magnet torque motor, which generates polarized magnetic flux from two permanent magnets, and the two ends of the armature extend into the air gap of the magnetic flux circuit. In addition to the elastic support of the armature-baffle, the spring tube also acts as a seal between the electromagnetic part and the hydraulic part of the valve. The baffle of the second 1 stage hydraulic amplifier is inserted between the two nozzles, forming two variable orifices. The feedback rod extends from the baffle, and its small ball is inserted into the small groove in the middle position of the spool of the second-stage four-way spool valve. The hydraulic oil continuously passes through the internal filter and two fixed orifices from the oil supply chamber P, then flows through the variable orifice formed by the nozzle baffle, and then flows back to the oil return chamber R through the oil return orifice.


   When a control current is input to the coil of the torque motor, due to the interaction of the control flux and the polarized flux, a torque is generated on the armature. The torque causes the armature assembly to rotate about the center of rotation of the spring tube, thereby moving the baffle. It causes the nozzle-baffle variable throttle area on one side to decrease and the variable throttle area on the other side to increase, resulting in a pressure difference in the nozzle cavity and pushing the spool to move. This displacement continues until the feedback torque generated by the bending of the feedback rod is balanced with the torque generated by the control current. At this time, the baffle is approximately in the neutral position. Since the torque of the torque motor is basically proportional to the control current that is lost to the valve, the feedback torque is proportional to the displacement of the spool. In this way, when the forces are balanced, a valve core displacement proportional to the control current is obtained, that is, when the valve pressure drop is constant and the valve sleeve adopts a rectangular hole or ring groove throttling edge, the output flow is proportional to the input control current.



Thus, as long as the servo valve to a regular input signal, the implementation of components will automatically and accurately follow the movement according to this 1 law.





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Characteristics of hydraulic servo system
In addition to a series of advantages inherent in the hydraulic transmission system, the hydraulic servo system also has the following advantages:

High control accuracy, fast dynamic response, easy operation;

Compact internal structure, small size and light weight;

Stable working performance, high reliability, long service life, small internal leakage, low power consumption;

Low hysteresis, high resolution, high repeatability;

Good linearity, large driving force and small zero drift.


However, the hydraulic servo system has a complex structure and high processing accuracy, so the price is more expensive; it is more sensitive to oil pollution, so reliability is affected; in a low-power system, hydraulic servo control is not as flexible as electrical control.

With the development of science and technology, the shortcomings of the hydraulic servo system will continue to be overcome. In the field of automation technology, hydraulic servo control has a wide range of applications.