| As a typical pneumatic actuator,the pneumatic cylinder has been widely used in the manufacturing field.The traditional cylinder is facing new challenges in pneumatic servo system.The compressibility of the gas makes it difficult to control and the performance goes down by the influence of friction when running at low speed.Since the gaseous medium does not have the self-lubricating properties,friction brings a lot of trouble to the pneumatic control system.In order to enhance the effect of the servo control,the researchers not only have to setup a study to control strategy,control valves or other aspects,but also have to carry out a depth study of friction.In addition to the difficulty to the pneumatic servo system,the friction also brings to the cylinder itself some problems,such as heating problem,noise,vibration,dust and so on.Thus,reducing the cylinder friction will undoubtedly bring more direct benefits.How to reduce the friction of the cylinder,developing the new-type low-friction cylinder or the cylinder without friction cylinder has become a new direction of development of cylinder.This paper presents a new-type low-friction cylinder based on the hydrostatic air bearing.The check valves placed on the two sides of the piston connect the inner chamber of the piston and the high-pressure cylinder chamber,and also play a role in stabilizing pressure of piston inner chamber during the commutation process which makes the air bearing more stable.A numeric solution of the pressure distribution in the film of the air bearing based on Matlab was constructed on the basis of the bearing's mathematical model.The flotation characteristics was studied,the leakage model was studied with the principle of capacitance micrometer.The commutating characteristics of low-friction cylinder were studied with a combination of simulation using Matlab/Simulink and test methods.A high precision pneumatic force supplying system was set up with the air-flow low-friction cylinder as actuator.A Fuzzy PID Control with steady state prediction was proposed to achieve the high precision steady pressure control goal.The experiment result shows that the steady pressure fluctuation was less than 50Pa and the dynamic pressure fluctuation was less than 150Pa when the piston was moving at a speed up to 1 OOOmm/s.A pneumatic cylinder load performance test bench based on the high precision pneumatic force supplying system was set up in line with China national standard.Finally,a new method for cylinder friction testing was proposed with which the friction can be measured easily when the piston is performing uniform motion.This doctoral dissertation consists of seven chapters.In chapter 1,the literature related to low-friction cylinder,gas lubrication and the development of some other related pneumatic techniques was reviewed,and the objective of this dissertation is illustrated.In chapter 2,the mechanical structure,working principle and technical difficulties of the low-friction cylinder were described in detail,a floating connection mechanism using between the piston and piston rod was proposed.The mathematical model of pressure distribution of the air in the bearing gap was established,and different solutions to this model was analyzed.The result shows that the traditional simplification based on one-dimensional flow assumption and the finite element simulation such as Fluent are not suited for the study of the properties of the bearing.In chapter 3,a finite element method based on Matlab was established.The detail of this method was described,including the formula derivation,the initialization of boundary conditions,the procedure of updating the boundary conditions,the mechanics of solving the outlet pressure of the orifice and the calculation method of the air consumption and the carrying capacity of the air bearing.Using this method,the influence of recess chamber and the recess groove to the air consumption and carrying capacity of the bearing were studied.The result shows that the recess chamber can improve the performance of the air bearing.The simulation based on Fluent was also carried out to validate the hypothesis that the pressure inside the recess chamber is a constant.In chapter 4,a new method of leakage modeling based on the principle of capacitance micrometer was constructed.Based on theoretical analysis and experiments,the eccentric cylindrical capacitor constituted by the inner wall of cylinder tube and outer surface of the piston was studied in purpose of analyzing the relationship between the capacitance,supply pressure and the air consumption.The experimental results indicate that the significant relationship between the capacitance and the eccentricity of the piston can be used as an indirect means of-study on the piston eccentricity,and also can be used as a working state indicator of the cylinder.Based on this view,a new off-line mathematic model of cylinder leakage was constructed on the basis of experimental results.The study on the working condition of the bearing during the commutation process of the air bearing based low-friction cylinder was carried out,the result shows that the piston inner chamber has an effect of keeping pressure during commutation process and the fast the commutation process the smaller the pressure fluctuation ratio.In chapter 5,a new type high-precision pneumatic force supplying system was established with a detailed description of the system structure,working principle and both the hardware and the software of the control system.The mathematical model of the system was constructed on the basis of the dynamic pressure model of a chamber with variable volume,the leakage model of low-friction cylinder and the flow model of the proportional directional valve.A Fuzzy PID Control with steady state prediction was proposed to achieve the high precision steady pressure control goal.The experimental result shows that the steady pressure fluctuation is less than 50Pa and the dynamic pressure fluctuation is less than 150Pa when the piston moves at a speed up to 1000mm/s.In chapter 6,a pneumatic cylinder load performance test bench based on the high precision pneumatic force supplying system was set up in line with China national standard.The load performance was tested according to procedure specified in the standard.The result shows that the metering-out control system suggested by the standard was not able to effectively control the speed of the test cylinder because of the impact during the test process.A promoted system based on an entrance throttle governing system was proposed.The motion characteristics under load of the test cylinder was analyzed according the velocity of the piston and the pressure of the two chamber measured during the test,the influence of different loads on the cylinder to motion stability was also studied experimentally.The result shows that the no crawling minimum speed of the cylinder decreases as the axial load is increased.Finally,a new method for cylinder friction testing was proposed with which the friction can be measured easily when the piston is performing uniform motion,providing a new way for the cylinder friction testing.In chapter 7,the conclusions of the thesis are presented.The main contributions of this research are summarized and the recommendations for future study are described. |
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