Research On Key Technologies Of Pneumatic Suspension System For Zero-gravity Environment Simulation

To simulate the zero-gravity environment for dynamic testing of low frequency space structures,a high precision pneumatic suspension system(PSS) is developed and some pneumatic key technological problems in design of the suspension system are solved.First of all, a type of air-suspending frictionless cylinders is designed based on inner gas pressure supporting by orifice restrictors,and a multi-objective optimization design method of the cylinders using nondominated sorting genetic algorithm is proposed to optimize the structural parameters.Then a high precision pneumatic proportional pressure valve is developed by adopting plunger-type structure with two-stage poppet,and the steady-state precision of pressure control is no less than 0.25KPa.A nonlinear mathematical modle of the PSS is built,and the effects of main system parameters on static/dynamic characteristics,control performance and plunge suspension frequency of the PSS are analyzed.After that a constant pressure robust control method based on a high precision pressure sensor and proportional valves is proposed to achieve high precision of pressure control.Experimental results show that the total friction force of the PSS is less than 0.0098N,and the steady-state pressure flunctuation is less than 25Pa,which meet the demand of no friction and high precision,and verify the feasibility and validity of the system scheme.The main contents of this dissertation are as follows:In chapter 1,structural style,main charateristics and range of application of the zero-gravity environment simulation system on the ground are introduced.The current stage in domestic and foreign research of suspension system is summarized.Some pneumatic key technologies in development of the PSS and related research progress are explained.Finally,necessity and main contents of this project are illustrated briefly.In chapter 2,using the theory of aerostatic lubrication,a type of air-suspending frictionless cylinders is designed based on inner gas pressure supporting by orifice restrictors.The mathematical models of gas pressure distribution in the clearance between the cylinder and the piston,gas leakage and radial load capacity are established and the influences of gas pressure, clearance and structural parameters on cylinder performances are analyzed by simulation.A multi-objective optimization design method of the cylinders using nondominated sorting genetic algorithm is proposed to achieve structural parameters matching optimization.At last correctness of the modles and validity of the optimization method are verified by experiments.In chapter 3,a high precision pneumatic proportional pressure valve is designed.The valve adopts plunger-type structure with two-stage poppet.A proportional electromagnetic actuator is used as control component and the control strategy is electric closed-loop feedback.The output pressure ranges from 0 to 0.5MPa and the steady-state precision is no less than 0.25KPa.A nonlinear dynamic model is developed to analyze the effects of main physical and geometrical parameters on the valve's dynamic behavior and control.A self-adaptive fuzzy proportional plus integral controller with adjusting output scale factor is designed for high precision and quickly control of different pressure target values,and carried out based on single chip-microcomputer ATmega16.Finally,some performance tests of the propotional valve are carried out.In chapter 4,a completely mathematical model of the PSS controlled by proportional pressure valve is established,and the influences of key physical parameters on dynamic behavior and control performance of the system are analyzed by simulation method.The key parameters include the pressure and the temperature of air source,the volume of air tank,the length and the diameter of connecting tubes and so on.Finally,theoretical modle of vertical plunge frequency is deduced and the effects of system parameters on the vertical plunge frequency are analyzed.In chapter 5,a high precision pressure control method of the PSS is proposed.High precision proportional valves are used as control component;meanwhile a high precision pressure sensor is adopted to detect the cylinder pressure for close-loop feedback control.Then two high performance robust control strategies are proposed and the steady-state pressure fluctuation is less than 26Pa which realizes high precision constant pressure control while the system has the parametric changes of the model and external disturbances.In the first control scheme,intelligent control techniques are introduced to controller design.An intelligent hybrid control algorithm using proportional pressure valve is put forward and the tuning parameters of the controller are optimized based on real code genetic algorithm.In the following control scheme a proportional flow valve is utilized.The second-order norm form model is achieved by using input-output linearization approach and a fuzzy sliding mode controller is developed combined with equivalent control and observer of the valve orifice area.At last,dynamic,static characteristics and robustness of the two controllers are analyzed and verified by simulation.In chapter 6,a pneumatic suspension system test platform is developed based on the two frictionless pneumatic cylinder actuators.Pneumatic suspension device,hardware and software of the control system are designed in detail.Various constant pressure control experiments including pressure step response,sudden disturbances,air source change and alternate motion of the piston,are made using the intelligent hybrid controller and the fuzzy sliding mode controller respectively.The performance of the controllers and the effects of control parameters on system are analyzed.Finally,the total friction force of the PSS is measured.In chapter 7,main research work,conclusions and innovation points of the dissertation are summarized and the future research proposals are suggested.