History and development trend of electro-hydraulic proportional control technology
History and development trend of electro-hydraulic proportional control technology
1. Development history
Compared with the traditional electro-hydraulic servo technology, electro-hydraulic proportional control technology has the obvious characteristics of reliability, energy saving and low cost.
Electro hydraulic proportional control technology appeared in the late 1960s and early 1970s. It integrates the advantages of electrical and microelectronic technology in signal detection, amplification, processing and transmission. Combined with modern industrial computers, it realizes mechatronics and remote control, so that the controlled system can respond dynamically according to complex programs, It has become one of the basic technical components of modern control engineering. In the technological revolution of electromechanical hydraulic integration and computer control of engineering equipment, electro-hydraulic proportional control technology will obtain newer and faster development.
Electro hydraulic proportional control has gone through three major development stages.
The early proportional valve, produced in the late 1960s, only used the proportional electromagnet for the ordinary hydraulic control valve, while the principle of the control valve remained unchanged, so the performance was poor. The frequency response is 1 ~ 5Hz and the hysteresis is 4% ~ 7%. It is commonly used in open-loop control. In 1967, KL proportional compound valve produced by a Swiss company marked the formal application of proportional control technology in hydraulic system, mainly the application of proportional electro-mechanical converter (proportional electromagnet) to industrial hydraulic valve.
The improved proportional valve was produced in the early 1980s. It improves the design principle of the control valve and adopts various internal and external feedback, electrical correction and high-voltage resistant proportional electromagnets to greatly improve the characteristics of the electronic control device. The steady-state characteristics are close to the servo valve, with a frequency response of 5 ~ 30Hz, but there is a zero dead zone. It can be used for both open-loop and closed-loop control. By the 1990s, with the development of microelectronics technology, the proportional control technology has reached a relatively perfect level, which is mainly reflected in three aspects: ① the feedback and electrical correction means such as pressure, flow, displacement and dynamic pressure are adopted to improve the steady-state accuracy and dynamic response quality of the valve, marking that the design principle of proportional control has been improved; ② Proportional technology and cartridge valve have been combined to produce proportional cartridge technology; ③ The birth of proportional volume element represented by proportional control pump.
The servo proportional valve was produced in the mid-1990s. Its manufacturing accuracy and filtering accuracy have been improved. The first stage valve port is zero covered and there is no zero dead zone. The proportional electromagnet is used as the electro-mechanical converter. The main stage valve port of the second stage valve has a small differential pressure and a frequency response of 30 ~ 100Hz. It is generally used for closed-loop control.
2. Development trend
Because the electro-hydraulic proportional compound valve has good control characteristics, anti pollution, reliability and economy, it has become the development trend of hydraulic control technology and has broad market prospects. The hysteresis, repetition accuracy, resolution and nonlinearity of its steady-state performance are almost equivalent to those of general industrial use, but the dynamic response is slightly lower than that of servo valve and operates within a large parameter adjustment range, so the nonlinear factors in the control loop can not be ignored. The development trend of electro-hydraulic proportional control system mainly focuses on two aspects.
(1) Proportional valve.
1) Improve the performance of proportional valve and adapt to the development of electromechanical hydraulic integration host. Improve the performance of electro-hydraulic proportional valve and remote control multi-channel valve to adapt to field working conditions, and develop low-cost proportional valve. Its main parts are common with standard valve.
2) The proportional technology is combined with the two-way and three-way inserting technology to form the proportional inserting technology, which is characterized by simple structure, reliable performance, small flow resistance, large oil passing capacity and easy integration. In addition, proportional volume control appears, which provides a new way to save energy for medium and high-power control systems.
3) Due to the miniaturization of sensors and electronic devices, sensors, measuring amplifiers, control amplifiers and valves are integrated, which greatly improves the working bandwidth of proportional valve (electric feedback). Its main manifestations are: ① development of high frequency response, low power consumption proportional amplifier and high frequency response proportional electromagnet; ② The development of displacement sensor (200Hz) with integrated amplifier creates good conditions for the miniaturization and integration of electric feedback proportional valve; ③ The servo proportional valve (closed-loop proportional valve) is equipped with an amplifier, which has various characteristics of the servo valve, such as zero coverage, high precision and high-frequency response, but it has lower requirements for the cleanliness of the oil than the servo valve and has higher working reliability.
(2) Proportional control system. The electro-hydraulic proportional control system is essentially nonlinear and uncertain, such as the pressure flow characteristics of electro-hydraulic servo valve, the friction characteristics and dead time characteristics of hydraulic power mechanism, load characteristics and so on; The uncertain factors include external interference force, temperature change, oil source pressure and flow pulsation. Therefore, the improvement of proportional control performance also depends on many new control technologies.
1) PID control. PID control method is the representative of classical control theory. It determines the control quantity based on the linear combination of realistic factors, past factors and future factors of system error. It has the characteristics of simple structure and easy implementation. It is widely used in electro-hydraulic servo system. However, the traditional PID controller adopts the linear combination method, which is difficult to coordinate the contradiction between rapidity and stability characteristics. Its robustness is not good enough in the case of parameter change and external interference. However, the parameters of the electro-hydraulic proportional control system change with time and the parameters change nonlinearly. Therefore, in a considerable number of cases, PID can not achieve satisfactory results, In recent years, it has absorbed the basic idea of intelligent control and made use of the advantages of computer to form variant controllers such as fuzzy PID, adaptive PID and nonlinear PID.
2) State feedback control. In addition to the feedback method of the hydraulic actuator, the feedback method of the hydraulic actuator is also used to control the position of the acceleration system ξ Generally low, the damping of the system can be greatly improved through acceleration (pressure) feedback, so as to significantly improve the response of the system.
3) Adaptive control. For the nonlinearity and uncertainty of electro-hydraulic proportional control system, adaptive control is widely used, because the adaptive control algorithm can automatically identify the time-varying system parameters, change the control function accordingly, and make the performance of the system reach the optimal or suboptimal. At present, there are two kinds of most mature applications: ① self-tuning control (STC); ② Model reference adaptive control (MRAC). STC - is generally applicable to the regulation of slow time-varying objects, but the electro-hydraulic proportional control system with sudden parameter mutation and sudden external load interference can not meet it. Therefore, the adaptive control applied in hydraulic system is mostly MRAC or its deformation. Although adaptive control greatly improves the system performance, it also brings some problems in the use process. For example, for STC, it is difficult to control the system with fast response in real time due to a large number of identification calculations; For MRAC, the main difficulty is to select an appropriate reference model and design the adaptive law according to Lyapunov stability theory or Popov hyperstability theory. Therefore, absorbing the advantages of other control methods and studying the adaptive law with simple algorithm and strong robustness is the development direction in recent years, such as adaptive feedforward control, robust adaptive control, nonlinear adaptive control and so on.
4) Variable structure control (VSC). Variable structure control is a control method that changes the structure of the controller according to the degree that the system state deviates from the sliding mode to make the system operate according to the law specified by the sliding mode. It is widely used in electro-hydraulic control system is sliding mode control. Compared with the traditional control system, VSC system has simple control law and can coordinate the contradiction between dynamic and steady-state performance. In particular, its sliding mode (SM) is completely invariant to the change of system parameters and external interference. Its main disadvantage is that there is serious jitter due to frequent switching. In addition, it is not suitable for the control system with long sampling period. In recent years, discrete variable structure control realized by fuzzy control and neural network control has emerged.
5) Fuzzy logic control (FLC). The introduction of FLC mainly takes into account that there is no need to establish a mathematical model, but rely on fuzzy reasoning or other prior knowledge to adjust the controller. Fuzzy control is suitable for the situation that there is no accurate representation of the controlled parameters and there is no accurate relationship between the parameters of the controlled object. In this case, FLC is superior to precise control, and electro-hydraulic proportional control system belongs to this kind of situation (such as hydraulic natural frequency affecting the dynamic quality of the system) ω And damping ratio ξ And so on, which is related to the soft quantity of the system and is difficult to calculate accurately). The application of FLC in electro-hydraulic proportional system mainly has two forms: ① fuzzy controller directly drives the object; ② It is used to determine the feedback gain of the state feedback controller.
6) Neural network control (NNC). NNC works by imitating the human sense and the working principle of brain cells. The hardware in the system imitates the neural cell network, while the software imitates the working mode of neural cells, that is, each neural unit receives signals according to "multiply and add after weight", the output signal is determined according to the size of "threshold", and the "weight" and "threshold" are determined through known input The output relationship and appropriate algorithm make the deviation between the output actual value and the required value as small as possible.
