Research On Pneumatic Servo Control Based On High-speed On-off Valve Using Time Interlaced Modulation

Pneumatic servo control technology which is an important technology in aviation,aerospace,and automation uses compressed air as the power source,and then drives the pneumatic actuators through various control elements.The traditional high-speed on-off valve usually uses Pulse Width Modulation(PWM)as the main modulation technique.Although this kind of control system has the advantage of simple structure,it still remains the problem of dead zone and low accuracy.To solve the shortcomings of the PWM method,this paper proposes a new modulation technique called Time Interlaced Modulation(TIM).TIM adjusts the pressure precisely by making a precise interlaced time of two valves.In order to discuss the control performance and feasibility of TIM,this paper applies TIM in pressure,force,and position servo control systems.Then,modeling,control,simulation,and experimental research are also conducted on the pneumatic servo control system.(1)A pneumatic servo system model is established.Firstly,four subsystems of the high-speed on-off valve are mathematically modeled.These subsystems are called electrical subsystem,magnetic subsystem,mechanical subsystem,and fluid subsystem.The model consisting of four subsystems can accurately present the movement of the spool during the switching process.Secondly,the air chamber model is developed based on energy conservation law and continuity equation.The model can show the pressure and mass flow changes accurately.Then the cylinder model is established with Newton’s laws of motion.This mathematical model can precisely present the changes in the pressure,mass flow,output force,and position.Finally,integrated models are established for the pressure,force,and position servo system,and simulations are performed.The simulations show that the total opening time of the valve is 6 ms,and the closing time is 2 ms.The switching time is consistent with the real valve,which indicates that the model established is feasible and effective.(2)Research on a pressure servo control method based on high-speed on-off valve using TIM is carried out.From the perspective of optimizing the modulation strategy of the high-speed on-off valve,a new modulation technique called TIM is proposed.This technique optimizes the combination of switching time of the high-pressure side and the low-pressure side in the pressure servo system.TIM adjusts the pressure precisely by making a precise interlaced time on the basis of the minimum opening time of the valve.The time schedule of the TIM module for the pressure servo control is designed.It aims to maximize the switching ability of the valves and improve the speed and accuracy.In order to solve the problems of time delay and nonlinearity in the pneumatic servo system,the sliding mode control is studied to implement TIM.Simulations are carried out,and an air chamber is produced for the experimental platform.A control software is written based on Labview.In the steady-state and dynamic pressure tests,the experimental results show that the steady-state errors are lower than 0.18%.The overshoot is 1.2% and the settling time is 0.225 s in the step response test.These results are better than the results using the PID controller with PWM.(3)Research on a force servo control method based on high-speed on-off valve using TIM is carried out.In order to improve the applicability of TIM in the force servo control system and solve the problem of the system nonlinearity,tests and analyses of the volume flow feature of the high-speed on-off valve are carried out.The feature can show the relationship between the duty cycle of PWM,pressure difference,and the volume flow of the high-speed on-off valve.The calculation accuracy of the interlaced time and the synchronized time are both improved by using the volume flow feature,and a fuzzy logic controller is designed to implement the proposed scheme.Finally,an experimental platform is built,and several steady-state and dynamic force tests are performed.The results show that the force steady-state errors are less than 3.51%.In the step response test,only1.41% of the force overshoot and 0.304 s of the settling time are observed.(4)Research on a position servo control method based on high-speed on-off valve using TIM is carried out.To solve the problem of the traditional PWM strategy for four valves and the mode switching strategy,an intelligent phase-change PWM strategy is proposed.This strategy is a combination of the old strategies and uses three air pressure conditions to classify and summarize all possible modes.The reasonable switching process ensures the dynamic balance of the pressure during the control process.Then a sliding mode controller and a new method for calculating the duty cycle of PWM are designed based on the strategy,and the response performance is improved.In order to improve the control performance with variable payloads,the method of the fuzzy nonlinear gain is proposed.Then,the TIM technique is combined with the fuzzy nonlinear gain and the intelligent phase-change PWM strategy to further improve the response performance.Through several simulations,the feasibility of the above methods is verified.By setting up an experimental platform the steady-state and dynamic tests are performed.The test results show that the average error of the steady-state position controlled by the TIM method is 0.77%.In the step response from 0 m to 0.03 m,the position overshoot controlled by the TIM method is 0.33%,the steady-state error is 0.1 mm,and the settling time is 0.22 s,which are all better than the other methods.In addition,the TIM method is able to control the position with the maximum payloads of 6.25 kg.The servo control algorithms proposed in this paper are all based on TIM.This technique optimizes the combination of switching time of the high-pressure side and the low-pressure side in the pressure servo system.TIM adjusts the pressure precisely by using the differential mass flow between the inlet and outlet valve.In the force servo system,the TIM method is improved by using the volume flow feature of the high-speed on-off valve.Then,TIM is combined with the fuzzy logic controller to further improve the force control performance.In the position servo system,an intelligent phase-change PWM strategy is proposed instead of the traditional PWM strategy for four valves and the mode switching strategy.Fuzzy nonlinear gain and TIM is combined with the new strategy to improve the accuracy of the control system and the control performance with variable payloads.