1 Application of pneumatic technology and the importance of research and development With the development of science and technology, automatic control technology has been widely used in industrial and agricultural production and national defense construction. There are two main technical means for achieving automation: electrical (electronic) control and fluid power control. There are three types of fluid power control: (1) Hydraulic control, and the working fluid is mainly mineral oil. (2) Air pressure control, the working medium is mainly compressed air, as well as gas and steam. (3) Jet technology, where the working medium contains gas and liquid, this technology has been applied in the production process of some multi-channels [1]. 2 Development of electro-pneumatic servo control The pneumatic servo control system can be divided into an electro-pneumatic proportional servo system and an electro-pneumatic switch servo system according to the different electro-pneumatic conversion elements used. An electro-pneumatic proportional servo system analog signal controlled proportional valve or servo valve is used as an electro-pneumatic signal conversion element. This type of system has high control accuracy and quick response, but the servo valve or proportional valve is expensive to produce, resulting in high system cost and strict requirements on the working environment. Fig. 1 PWM control principle Figure 2 Typical Pneumatic PWM Circuit PWM control was originally an electro-hydraulic servo system used by Stephen of the United States for servo valves. The PWM-controlled servo system solves the phenomenon of temperature drift and clamping, improves stability and immunity to medium contamination, relaxes manufacturing tolerances, and facilitates digital control directly with a computer interface. At the same time, with the rapid development of the switch valve, Goldstein proposed replacing the expensive servo valve with a fast switching solenoid valve. The first to introduce the PWM switch servo control into the system is Japan's Toshio Shojiro [28] who successfully applied a PWM electro-pneumatic switch/servo system to a robot [29]. His positioning accuracy was ±0.06 mm. Xiao Ji et al. used two on-off valves to implement generalized PWM control and achieved high accuracy of ±0.02 mm. In the United States, Jing-Yih Lai [30] and others used the five-freedom robot arm as the control object to carry out PWM aerodynamic control theoretical analysis and experimental research. Domestically, the research group headed by Prof. Liu Qinghe of Harbin Institute of Technology has also conducted research on pneumatic PWM control and obtained a position accuracy of the air motor rotation angle of ±0.09° [31]. Professor Wu Pei Rong also studied the PWM control pneumatic system [32] and achieved certain results. In addition, Yang Shuxing and Yao Xiaoguang of Beijing Institute of Technology conducted research on the theory and experiment of PWM control systems [33]. Fig. 3 Pneumatic PCM control principle The PCM control method is very suitable for computer direct digital control. Its control principle and process are: In each sampling period, the computer compares the set value of the control quantity with the detected control quantity, and judges according to the designed control rate. , Calculation, issued a set of binary code control n switch valve, get a different comprehensive opening area, thereby controlling the gas flow of the cylinder, so that the cylinder movement according to requirements. Japan's Nakamura first used PCM control technology for hydraulic control. Hirohisa Tanaka also studied the PCM hydraulic control system and proposed to use software to overcome the flow fluctuation caused by the opening and closing time of the on-off valve [34]. Later, Shoujiro was the first to use PCM control technology for pneumatic systems, and successfully used PCM to control the three-degree-of-freedom robot of the British Pendar Corporation. His positioning accuracy was about ±0.25mm. China's research on pneumatic PCM control began in the early 1990s. The main researchers were task forces led by Prof. Liu Qinghe of HIT [35][36][37] and Ning Shu [38] of Shandong Institute of Light Industry. Dr. Zheng Xueming studied the position of the Fuzzy-PI control pneumatic PCM position system and obtained a positioning accuracy of ±0.25mm [37]. Dr. Wang Xuanyin proposed variable-gain PCM control for the first time, and used a self-tuning, self-learning control algorithm to obtain a positioning accuracy of ±0.18 mm [46]. 3 Necessity of gas-liquid linkage and development overview However, due to the large compressibility, low viscosity, and low stiffness of the gas, the unsteady aerodynamic motion and inaccuracy of the aerodynamic positioning (ie, the positioning accuracy is not high) [39]. In order to improve its positioning accuracy and smoothness of movement. The advantages of aerodynamics are fully utilized. Therefore, considering the introduction of incompressible oil (as viewed under low pressure) as a medium, gas-liquid interlock control is used to improve the performance of the system. Figure 4 Oil Damping Cylinder Figure 5 Gas-liquid boost circuit Figure 6 A gas-liquid converter circuit Figure 7 Two gas-liquid converter circuits In addition, there is a loop for the gas-liquid converter, such as the one-way throttle speed control circuit shown in FIG. 6 and the bi-directional throttle speed control circuit shown in FIG. 7 . The gas-liquid converter is a component that converts air pressure to oil pressure (pressure ratio 1:1) and can be used as an accessory to enter the gas-liquid circuit. Use it to eliminate the crawling and instability of low-speed motion in the general pneumatic circuit, and it can be used in combination with various pneumatic components. Gas-liquid converter features: (1) many specifications and strong adaptability. For example, the gas-liquid converter produced by Daktronics of Japan has 48 types of specifications, the effective volume of the output pressure oil is 40 to 27000 cm3, and the common working pressure is 0.3 to 0.7 MPa. As long as the pneumatic pressure reducing valve is adjusted, the corresponding pressure change can be obtained. . The reaction speed is fast, and it can meet the requirements of different users; (2) Because the working oil temperature is stable, the air is not mixed into the oil and the efficiency is high, so the stable movement can be obtained; (3) Compared with the hydraulic pressure, it does not need to be complicated and bulky. The pump station and cooling system are inexpensive. Because of the pulsation caused by no pump, the oil temperature is stable. It can be used for precision cutting and precise and stable feeding. (4) Compared with the hydraulic damping cylinder, the gas-liquid converter and the oil cylinder can be separated and can be placed at any position, and the operation is convenient. Gao [44].
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Pneumatic servo control uses a gas as a working medium to realize energy transfer, conversion, distribution, and control. Pneumatic systems have been rapidly developed because of their advantages of energy saving, non-pollution, simple structure, low cost, high speed, high efficiency, reliable operation, long service life, wide temperature range, and working media that are flame-proof, explosion-proof, and electromagnetic interference-proof. The development [2]. Numerous reports indicate that pneumatic technology is a key technology for modern transmission and control. Its level and speed of development directly affect the quantity and level of electromechanical products. The degree to which pneumatic technology has been used has become an important indicator of a country's performance [3].
According to the data, the current pneumatic control device occupies an important position in the automation, has been widely used in various industries, are summarized as follows: (1) The vast majority of production departments with pipeline production processes often use air pressure control. Such as: petroleum processing, gas processing, chemicals, fertilizers, non-ferrous metal smelting and food industry. (2) In the light industry, the electrical control and pneumatic control devices are roughly equal. In China, it has been widely used in textile machinery, papermaking and leather making and other light industries. (3) In transportation, the brakes of trains, the packing and loading and unloading of goods, the management of warehouses, and the opening and closing of vehicle doors and windows [3][4]. (4) It has also been widely used in the aviation industry. Since electronic devices are difficult to work under a high temperature of 300° C. to 500° C. without a cooling device, pneumatic devices are widely used in modern aircraft. At the same time, pneumatic devices are widely used in rockets and missiles. (5) Automatic devices for torpedoes are mostly pneumatic because they use compressed air as a power source, they are small and lightweight, and even smaller and lighter than batteries with the same power. (6) It is also widely used in bioengineering, medical treatment, and atomic energy. (7) It has also been widely used in the mechanical industry.
From the characteristics and applications of aerodynamics, it is known that the research and development of pneumatic technology has very important theoretical and practical significance. Pneumatic technology has developed rapidly in the United States, France, Japan, Germany and other major industrial countries. China began to pay attention to and organize research on pneumatic technology in the early 1970s. Regardless of the product specifications, types, quantities, sales volume, application scope, or research level and number of researchers, China is lagging behind most major industrial countries in the world. In order to develop our country's pneumatic industry, improve our country's aerodynamic technology level, shorten the gap with developed countries, it is necessary to develop and strengthen the research of pneumatic technology.
As early as 1956, Shearer et al. successfully used high-pressure, high-temperature gas as the working medium for pneumatic servos in attitude and flight stability control of spacecraft and missiles [6][7][8][9]. Due to the large air compressibility, low viscosity, and low stiffness, it is difficult to achieve precise servo control with classical control methods and analog regulators for low-voltage systems. Therefore, pneumatic servo control has long stayed in the theoretical and experimental stages. In 1979, the first pneumatic servo valve developed by Prof. W. Backe' of Aachen RW Institute of Technology in Germany greatly advanced the development of pneumatic servo control [10]. Since then, industrialized countries such as Germany, Japan, and the United States have invested a large amount of funds and manpower to successfully develop proportional valves and servo valves of various specifications, as well as high-performance cylinders and air motors [11][12][13]. With the rapid development of high-performance electro-pneumatic control elements and actuators, research on pneumatic servo control technology has also achieved certain results [14][15]. Dr. Zhou Hong and Dr. Chen Dajun in China have studied the electro-pneumatic ratio/servo system and its control strategy. In addition, Prof. Xu Yaoming of Harbin Institute of Technology undertook the development of "Electrical-Gas Servo System and Electro-pneumatic Servo Device" in the main topic of the intelligent robot in the field of automation of the national "863" plan and achieved certain results[16] .
The electro-pneumatic switch/servo system uses a digital signal-controlled on-off valve as an electro-pneumatic signal conversion element. This type of system has low cost, low requirements for the working environment, and easy computer control; however, it is difficult to obtain a wide frequency band and high accuracy. The switch/servo control first appeared in the hydraulic system [17], and Burrows first used the switch/servo control in the pneumatic servo mechanism [18][19]. In the early 1980s, T.Eun et al. designed a new pneumatic switch servomechanism and studied in detail the stability and accuracy of the mechanism [20]. The above-mentioned switching servo mechanism is to feed back the cylinder position through a simple logic judgment, and can only achieve point-to-point (PTP) control, and the accuracy is very low. During this period, G.Belforate et al. introduced the brake technology of locomotives into the pneumatic mechanism and designed a cylinder with a brake to locate the target at the target position. This kind of pneumatic switch servo mechanism is greatly influenced by the load and other disturbances, but the rigidity after positioning is large, and its positioning accuracy is about ±0.3 mm.
Later, Japan's Hirohide Hideo and Harada Masaharu used the on-off valves and throttle valves in series and in parallel to realize the zonal control of the cylinder and achieved positioning accuracy of ±0.4 mm. Italy's G.Belforate et al. also studied this system. He used the non-sealed cylinder and FESTO's on-off valve, one-way throttle valve and FPC606 microprocessor and other components. Theoretically, this control can achieve ± 0.0314mm positioning accuracy, the actual system is affected by the gap, to obtain positioning accuracy of about ± 0.35mm [21]. Dr. Mo Songfeng of Beijing University of Aeronautics and Astronautics used three on-off valves to form a new pneumatic position switch control system [22]. The experimental results show that the system has the advantages of simple implementation, convenience, low cost, and good performance.
The above pneumatic switch control system, although using a displacement sensor, but the displacement signal is only used as a logic judgment, there is no use to adjust the size of the control signal, and its essence is still open-loop control, or quasi-closed-loop control. Therefore, this system is characterized by low cost and simple control; however, the accuracy is further limited. With the increase of control indexes, the development of pneumatic switch control to pulse modulation switching/servo control has been developed. Pulse modulation methods include pulse width modulation (PWM), pulse code modulation (PCM), pulse number modulation (PNM) and pulse frequency modulation (PFM), etc. [23][24].
The PWM control principle is to drive the on-off valve with a pulse signal of a certain period Ts (see FIG. 1), and control the pulse width DiTs (i=1, 2,..., N) with the control signal, that is, the closing time of the on-off valve. Therefore, controlling the size of Di is macroscopically equivalent to controlling the flow of media through the valve [25][26][27]. A typical pneumatic PWM control loop is shown in Figure 2. 
Pulse coding (PCM) control is to encode control signals as n-bit binary signals to control the opening and closing of n open and close valves. The effective cross-sectional area of ​​the n switching valves (for ease of description, the effective cross-sectional area of ​​the switching valve referred to herein refers to the integrated throttle area of ​​the switching valve and the series connection throttle valve): S0: S1: S2:...:Sn-1=1:2:4:...:2n-1. The number of combinations of switching states of n valves is 2n, and 2n-level different cross-sectional areas can be obtained. The PCM control principle and process will be described with the pneumatic system shown in FIGS. 1 to 3 . 
With respect to gas-liquid linkage, the first to apply is a gas-liquid damping cylinder, also called an oil damping cylinder. It is actually a double-acting double-piston cylinder with a piston rod connecting the two pistons in series. The output force of the piston is the difference between the thrust (or pull) of the cylinder and the resistance of the cylinder. The cylinder is driven by the cylinder behind the cylinder, and its speed is adjusted by adjusting the opening of the throttle [40], as shown in Figure 4. 
In addition, there is an auxiliary circuit for gas-liquid boosting, as shown in Figure 5. The gas-liquid supercharging device has been widely used in the production practice, especially in the hydraulic fixture of the machine tool, and is well known to people. In practical work, pressures of more than 1 MPa are sometimes applied. At this time, a typical small-sized air compressor is incapable and it is often unnecessary to purchase a high-pressure air compressor. This problem is often solved with a gas-liquid supercharger.
In short, pneumatic transmission is generally used for rapid transmission, is not afraid of impact, and occasions where speed is not strict; hydraulic transmission is generally used for smooth transmission and must be controlled and adjusted. Compressed air is the power source of the gas-liquid transmission system. Instead of the oil pump of the power unit of the hydraulic drive system, the oil pump pushes the oil and pushes the feed cylinder for cutting, and can also be used for the direct drive of other cylinders. Under conditions of low and medium pressure, the fluid is considered incompressible, allowing the feed cylinder to achieve a smooth motion. The disadvantage of this system is that the oil filtering and sealing requirements are strict, and an oil cup or charge pump is required to compensate for the leak. Moreover, when the load changes, the pressure fluctuates, the response of the pump is poor, and the application range is narrow, and it is not suitable for occasions in which large quantities of continuous oil supply or pressure fluctuation requirements are strict. In addition, irrespective of whether the oil damping cylinder or the circuit using the gas-liquid converter combination, the feed rate is often unstable, and the cause of instability is due to the presence of gas in the oil. On the one hand, it is caused by gas-liquid leakage. On the other hand, due to the improper position of the gas cylinder or the exhaust hole, the gas in the cylinder cannot be discharged when the cylinder is refueled. Due to the compressibility of gas and the interference of turbulence of gas and oil, the damping cylinder works to produce crawling, impact, and pause, which reduces the machining accuracy of the machine tool. In order to solve this problem, when designing and manufacturing gas-liquid cylinders, the tolerances of the groove at the sealing ring and the matching accuracy between the piston and the inner wall of the cylinder barrel and between the piston rod and the cylinder head should be guaranteed, so as to change the performance of the gas-liquid system.
Overview of Development of Pneumatic Servo and Gas-liquid Coupling Control