Research On Control Strategy Of Piezoelectric Switch Type Pneumatic Control Valve

As the actuator of the closed-loop control part of the process,it is very important to realize the automatic control of the system.The valve positioner is the core device to realize the precise automatic adjustment of the valve.With the need of industrial automation and precision,its performance has attracted much attention.In recent years,the intelligent valve positioner based on piezoelectric has become a new trend.The device uses the piezoelectric effect of semiconductor materials to switch the piezoelectric valve.In the process of maintaining the valve position,the highpressure gas source is almost not consumed,so that the operating cost of the regulating valve based on this principle is much lower than that of the traditional nozzle flapper-type regulating valve.However,the domestic valve positioner technology based on piezoelectric technology started late,and there is a large gap in the quality of foreign products.So,Domestic valves need to be strengthened in terms of parameter adjustment,control accuracy,and environmental adaptability.On the basis of reading and analyzing a large number of domestic and foreign literatures,this thesis has carried out the following work:(1)Based on the analysis of the working principle of piezoelectric intelligent valve positioner,the mechanism modeling of its movement dynamic process is carried out.Firstly,the working principle of the piezoelectric intelligent valve positioner is deeply understood,and the components of the system are analyzed,and the key components of piezoelectric valve type I/P conversion unit is analyzed in detail.Then the detailed mechanism modeling of the pneumatic control valve is carried out.Finally,the experimental platform of the intelligent valve positioning control system is built for experimental testing.(2)Based on the working principle and modeling of piezoelectric intelligent valve positioner,the important factors affecting the control performance of the system are analyzed experimentally.Considering that the performance of the system is greatly affected by the operating parameters,this thesis conducts the simulation and experimental analysis under different working conditions according to the gas source pressure,pressure difference,actuator size and other factors.Finally,the control strategy is optimized.(3)Aiming at the shortcomings of traditional switch control algorithm,such as low control precision,difficult parameter adjustment and easy overshoot,this thesis proposes a five-step control algorithm based on speed.In this algorithm,the valve position speed is introduced into the control strategy.According to the real-time speed,increasing or decreasing PWM duty cycle,the speed threshold is set in the area near the dead zone.This method can control the valve stem to enter the dead zone smoothly at a small speed and avoid overshoot.The experimental platform shows its effectiveness.(4)In view of the situation that the control performance of pneumatic control valve is degraded or even unable to work due to leakage plugging,this thesis proposes a self-learning control strategy to improve the environmental adaptability of the control valve.For this reason,first of all,an experimental platform is built to simulate the environment of gas leakage and plugging.Then detect the air leakage and blockage.When the air leakage and gas blockage exceed a certain threshold,the alarm function will be triggered.If it does not exceed,the PWM control parameters will be increased or decreased according to the working conditions and updated to the closed-loop control algorithm,so as to realize the control of the valve position.Finally,the experimental results show that the controller can carry out effective control in the full range of travel.