Composition of Hydraulic Servo System_Pros and Cons of Hydraulic Servo System

The hydraulic servo system makes the output of the system, such as displacement, speed or force, change automatically and quickly and accurately following the change of the input quantity. At the same time, the output power is greatly amplified. The working principle of the hydraulic servo system can be illustrated by Fig. 1 that the hydraulic servo system has been widely used in industrial control due to its unique advantages such as fast response speed, large load stiffness, and large control power. Hydraulic servo control is a complex hydraulic control method. Hydraulic servo system is a closed-loop hydraulic control system.

This article first introduced the working principle and composition of the hydraulic servo system, followed by the introduction of the hydraulic servo system classification and work characteristics, and finally introduced the advantages and disadvantages of the hydraulic servo system, the specific follow Xiaobian together to find out.

Hydraulic servo system has been widely used in industrial control due to its unique advantages such as fast response speed, large load stiffness and large control power.

The electro-hydraulic servo system converts low-power electrical signals into high-power hydraulic power through the use of electro-hydraulic servo valves, thus realizing the servo control of some heavy-duty mechanical devices.

The hydraulic servo system makes the output of the system, such as displacement, speed or force, change automatically and quickly and accurately following the change of the input quantity. At the same time, the output power is greatly amplified. The working principle of hydraulic servo system can be illustrated by Fig. 1.

Figure 1 shows an electro-hydraulic servo system with continuous control of pipe flow. In the large-diameter fluid pipe 1, the daily change of the rotation angle of the valve plate 2 generates a throttling effect and plays a role in adjusting the flow qT. The rotation of the valve plate is achieved by the hydraulic cylinder driving the gears and the rack. The input volume of this system is the given value Xio of the potentiometer 5 corresponding to the given value X, there is a certain voltage is output to the amplifier 7, the amplifier converts the voltage signal into a current signal to the electromagnetic coil of the servo valve, so that the valve core corresponding The ground produces a certain amount of opening X. Valve opening x. Bring hydraulic oil into the upper chamber of the cylinder and push the cylinder down. The fluid in the lower chamber of the hydraulic cylinder flows back to the tank via the servo valve. The downward movement of the hydraulic cylinder causes the gears and racks to deflect the valve plate. At the same time, the hydraulic cylinder piston rod also brings the contact of the potentiometer 6 down to Xpo when x. When the voltage corresponding to x corresponds to the voltage, the difference between the two voltages is zero. At this time, the output current of the amplifier is also zero, the servo valve is closed, and the valve plate driven by the hydraulic cylinder is stopped at the corresponding qT position.

Fig. 1 Electro-hydraulic servo system of pipeline flow (or static pressure)

1 a fluid pipe; 2 a valve plate; 3 a gear, rack; 4 a hydraulic cylinder; 5 a given potentiometer; 6 a flow sensor potentiometer; 7 an amplifier; 8 an electro-hydraulic servo valve

In the control system, the output signal of the controlled object is fed back to the input of the system and compared with the set value to form a deviation signal to generate the control effect on the controlled object. This type of control is called feedback control. . The feedback signal is opposite to the given signal symbol, ie it always forms a difference. This feedback is called negative feedback. Adjusting the deviation signal generated by negative feedback is the basic feature of feedback control. In the example shown in FIG. 1, the potentiometer 6 is a feedback device, and the deviation signal is the Δu generated at the amplifier input terminal for the given signal voltage and feedback signal voltage.

FIG. 2 shows a block diagram corresponding to the example of FIG. The common block diagram of the control system represents the links between the components of the system. In the above figure, the elements are shown by text in the box, and the expression of the block diagram using mathematical formulas will be described later.

As can be seen from the above example, the hydraulic servo system consists of the following basic components:

The input element is the one that adds the given value to the input of the system. The elements may be mechanical, electrical, hydraulic or other combinations. Feedback Measurement Element - A device that measures the output of the system and converts it into a feedback signal. Various types of sensors are often used as feedback measuring elements.

The comparison element compares the input signal with the feedback signal to derive the error signal component.

Amplification, energy conversion element An element that amplifies an error signal and converts various forms of signals into high-powered hydraulic energy. Electrical servo amplifiers and electro-hydraulic servo valves belong to such components;

The actuator element will generate the hydraulic energy that regulates the action on the control object, such as a hydraulic cylinder or a hydraulic motor. Various types of production equipment for control objects, such as machine tables, tool holders, etc.

1, according to the system input signal changes in the classification. Fixed value control system, program control system, servo control system

2. Classify by the name of the controlled physical quantity. Position servo control system, speed servo control system, force control system and other physical quantity control system

3, according to the hydraulic power element control method or hydraulic control element form. Throttling control system (valve control type) and volumetric control (variable pump control and variable motor control) systems

4, according to the form of signal transmission media. Mechanical Hydraulic Servo System, Electrical Hydraulic Servo System and Pneumatic Hydraulic Servo System

(1) There is a feedback connection between the system's output and input, which forms a closed-loop control system. The feedback medium can be mechanical, electrical, pneumatic, hydraulic or a combination thereof.

(2) The main feedback of the system is negative feedback, that is, the feedback signal is opposite to the input signal, and the two are compared to the deviation signal to control the hydraulic energy. The energy input to the hydraulic element makes it move in the direction of reducing the deviation, both in deviation. To reduce the deviation.

(3) The input signal power of the system is very small, and the output power of the system can be very large. Therefore, it is a power amplifying device, and the energy required for power amplification is supplied by hydraulic energy. The control of the supplied energy is automatically performed according to the size of the servo system deviation.

The hydraulic servo system began to appear in 1950 and has achieved great development for decades. At present, hydraulic control is widely used in various technical fields.

advantage:

(1) The hydraulic actuator performs quickly and rapidly. As far as the flow-speed transfer function is concerned, it is basically an oscillation frequency with a large natural frequency, and the natural frequency can be increased to the natural frequency of the electro-hydraulic servo valve as the flow rate increases and the parameter is optimally matched. compared to. The natural frequency of electro-hydraulic servo valve is generally above 100HZ, so the frequency response of the hydraulic actuator is very fast, and it is easy to start, brake and commutate at high speed. Natural frequencies are usually higher than those of electromechanical system actuators.

(2) The volume and weight of hydraulic actuators are much smaller than those of electromechanical actuators with the same power. Because with the increase of power, the increase in the volume and weight of hydraulic actuators (such as valves, hydraulic cylinders, or motors) is much slower than that of electromechanical actuators, because the former mainly increases the power by increasing the liquid flow and pressure, though The power mechanism will also increase in volume and weight, but it can use high strength and light metal materials to reduce the volume and weight.

(3) The hydraulic actuator has stable transmission and strong anti-interference ability, especially good low-speed performance, but the transmission stability of the electromechanical system is poor, and it is susceptible to various external disturbances such as electromagnetic waves.

(4) The hydraulic actuator has a wide speed range and high power gain.

Disadvantages:

(1) The hydraulic signal transmission speed is not easy to be corrected, and the electrical signal is transmitted according to the speed of light, and it is easy to integrate and correct. However, since the electro-hydraulic servo system uses electric signals in the power stage before, it does not have such a disadvantage, and in a sense, this type of system has the advantages of electric and liquid servos.

(2) The hydraulic servo system has a complicated structure and high machining accuracy, which results in high costs.

(3) The volumetric elastic modulus of the liquid varies with the temperature and the air content mixed in the oil. When temperature changes have a significant impact on system performance. In contrast, the temperature has little effect on the volumetric elastic modulus of the gas and therefore has little effect on the performance of the pneumatic control system. The temperature has a great influence on the viscosity of the liquid. At low temperatures, the friction loss increases. At high temperatures, the leakage increases and cavitation occurs easily.

(4) Oil leakage is the weak point of the hydraulic system. It not only pollutes the environment, but also easily causes fire. Hydraulic oil is susceptible to contamination and can cause blockage in the actuator.