History and development trend of electro-hydraulic servo control technology
History and development trend of electro-hydraulic servo control technology
1. Development history
The development process of electro-hydraulic servo control is mainly divided into three stages: early stage, during World War II and after World War II.
(1) Early. From 247 to 285 BC, ktesbios, an ancient Egyptian living in Alexandria, invented many hydraulic servo mechanisms. One of the most outstanding is the water clock. The water clock designed by him can show the accurate time of up to one month. The principle is to form a closed-loop feedback system between the liquid level displayed by the buoy and the container through the orifice. In a sense, this kind of buoy has the rudiment of modern hydraulic servo valve. Around 1750, the level control valve used to control the water level of feed water system and steam boiler appeared in England. With the development of industrial revolution and the continuous improvement of control strategy, it affects the development of hydraulic technology.
In 1795, Joseph Brahman applied Pascal principle to make the hydraulic press. In 1796, mozley designed the sealing device of the hydraulic press pump - leather cup seal in order to make the hydraulic press work better. It is the rudiment of modern sealing technology. By the end of the 18th century, accumulators appeared in Britain. In the early 19th century, oil began to replace water as the medium of hydraulic system, and the directional control valve was driven by electric signal.
(2) During World War II. On the eve of World War II, the application of aerodynamics requires a device that can realize the conversion of mechanical signal and gas signal. Askania controller and Askania Werke invented the jet pipe valve according to the jet principle and applied for a patent. According to the same principle, Foxboro applied for a patent for the double nozzle flapper valve. German Siemens invented the permanent magnet torque motor, which can accept the mechanical signal input through the spring and the electrical signal generated by the moving coil, and is creatively used in the aviation field. At the end of World War II, the servo valve was a structure in which the spool valve core moved in the valve sleeve. The movement of the valve core is the result of the joint action of the electromagnetic force generated by the DC solenoid and the pressure generated by the spring. Therefore, the servo valve at this time is only a single-stage open-loop control valve.
(3) After World War II. After the Second World War, due to the stimulation of military, the research of automatic control theory, especially the control system of weapons and aircraft, has been further developed, which greatly stimulated the research and innovation of hydraulic servo valve from another aspect. In 1946, Tinsley of the UK invented the two-stage hydraulic valve. Raytheon and bell aircraft company obtained the patent of the two-stage servo valve with feedback. MIT used the torque motor with better linearity and more energy saving to replace the solenoid as the driving device of the slide valve.
In 1950, Moog invented a two-stage servo valve with nozzle orifice as the front stage. On this basis, from 1953 to 1955, Carson invented the mechanical feedback two-stage servo valve, Moog improved the double nozzle orifice structure, and volping changed the wet electromagnet to dry, eliminating the reliability problem of the torque motor immersed in oil caused by oil pollution. In 1957, achilly invented the two-stage electro-hydraulic servo valve with jet pipe valve as the front stage, and successfully developed the Z-stage electric signal feedback servo valve in 1959.
At this time, the development of electro-hydraulic servo valve has entered a period of rapid development, and many structural designs further improve the performance of electro-hydraulic servo valve. In particular, the design of electro-hydraulic servo valve in 1960 shows more the characteristics of modern servo valve. For example, a closed-loop feedback control is formed between the two stages, the torque motor is lighter and the moving distance is smaller. The pressure difference between the front stage and the power stage can usually reach more than 50%. The front stage has no friction and is independent of the working oil. The mechanical symmetrical structure of the front stage reduces the influence of temperature and pressure changes on the zero position.
In more than 20 years, the electro-hydraulic servo valve has completed the transformation from the early single-stage open-loop control valve to the two-stage closed-loop control servo valve. As you can see. At that time, the development of electro-hydraulic servo valve was more due to the needs of military applications. Therefore, its development was regardless of cost, which also resulted in the superior performance but expensive price of electro-hydraulic servo valve at that time.
Subsequently, some companies began to develop industrial applications of electro-hydraulic servo valves. Moog company developed 73 series electro-hydraulic servo valves in 1963, which can meet the cleanliness requirements of industrial oil. Since then, in order to meet the requirements of modern industry, the structural design and development of servo valve based on servo valve in 1960 is still continuing. If the volume of the valve becomes larger (compared with the aviation valve), the material is no longer forged steel; The pilot stage is independent to facilitate maintenance and commissioning; The allowable pressure range of the valve is reduced to 10MPa to 20MPa instead of the original 30MPa; Start standardized production to reduce costs and meet general requirements.
2. Development trend
At present, the research of electro-hydraulic servo valve mainly focuses on the improvement of structure, the use of materials and the change of test methods.
(1) Structure. In the aspect of structural improvement, the common faults of servo valve are analyzed, and the improvement scheme is put forward. The structural optimization is carried out by means of computer-aided design (CAD) and computer-aided manufacturing (CAM) to meet the performance requirements of valve, such as the emergence of direct drive servo valve and the emergence of piezoelectric ceramic servo valve. In addition, the mechanism of the servo valve is reformed by using the three margin technology, and the torque motor, feedback element and slide valve pair of the servo valve are made into multiple sets. If a key component fails, another spare sleeve can be switched at any time, so as to ensure the normal operation of the valve and improve the reliability and service life of the system.
(2) Materials. In terms of material replacement, materials with better mechanical properties such as strength, elasticity and hardness can be used for specific parts according to the performance requirements of electro-hydraulic servo valve. Replacing the material of the sealing ring can improve the high-pressure and corrosion resistance of the servo valve. The nozzle baffle made of ruby material can prevent the damage of baffle and nozzle caused by air feed, reduce the dynamic and static performance and shorten the working life. In addition, the material of electromagnet in permanent magnet torque motor adopts giant magnetostrictive material, which can improve the dynamic response characteristics of electro-hydraulic servo valve.
(3) Testing. When testing the dynamic and static characteristics of the servo valve, the influence of the measuring instrument itself and the interference of external electromagnetic signals will affect the test results. In serious cases, it can not correctly reflect the performance of the servo valve.
In addition, improving the measurement accuracy of the measuring instrument as much as possible can more accurately reflect the characteristics of the servo valve product itself, and help to ensure the stability, rapidity and economy of the whole hydraulic system. Therefore, many individuals and units have made in-depth research on testing instruments and testing technology.