Study On Oil Bulk Modulus And Temperature And Pressure Control In Hydraulic Source With High Flow

Hydraulic transmission and control system has been widely used in various fields for its great load stiffness and high power-to-weight ratio.With the development of higher power and faster response of hydraulic system,the improvement of system performance is always on the agenda of research community.In all performance parameters,the frequency width and the stability of the hydraulic system are two essential characteristics.They are both limited by hydraulic natural frequencyω_h and damping ratioξ_h.The hydraulic natural frequencyω_h is determined mainly by bulk modulus of oil,and the damping ratioξ_h is mostly affected by viscosity of oil.The bulk modulus and the viscosity of oil are difficult to determine,as they are varying with the temperature and pressure obviously.With the increase of hydraulic natural frequencyω_h,the bulk modulus of oil should be improved and tested.In order to get a steady damping ratioξ_h the temperature of oil must be kept constantly.The techniques of oil bulk modulus monitoring on-line and oil temperature precision control are studied in detail in this thesis.For instance,the researches have been done on the motion simulator for a docking mechanism comprehensive test-rig which has an electro-hydraulic servo system required high flow and high performance.There are several ways for oil bulk modulus to be monitored on-line,among them the method of definition is most direct and reliable.To improve the test accuracy,an installation was invented and discussed according to the definition of oil bulk modulus,in which information of pressure and displacement was directly extracted from test chamber.The effects of test chamber deformation,leakage and temperature on measurement accuracy were analyzed in this thesis.The measurement accuracy reached as high as 3%because the approaches were effective.Using this measurement device,experiments were carried out on bulk modulus of hydraulic oil and a set of values of oil bulk modulus with different air contents were made for the first time.On account of characters of time delay and time-varying parameters in oil temperature control,a set of control method was proposed. The proportional water valve was used to control the flow rate of cooling water and the parameter self-tuning fuzzy PID control algorithm was used to control the opening of the valve.The simulation and experiment results indicated oil temperature maintaining in 45±1℃.The study of steady-state and dynamic properties of large-bore pressure control cartridge valve identified the rules of performance changing with structure parameters.The result provides reference to improvement of pressure performance in high flow hydraulic systems. The main contents of each chapter are summarized as follows:In chapter 1,the history and current research status of the rendezvous and docking technology and the simulator test rig are reviewed based on a lot of publications and conference papers.The research statuses of oil bulk modulus measuring and oil temperature control are also presented.Accordingly,the research and the goal of the project are brought forward.In chapter 2,the construction and composition of docking mechanism comprehensive test-rig is briefly introduced.The hydraulic system of motion simulator is designed, including composition,principle and main parameters of power mechanism and hydraulic resource.Several key technologies in hydraulic resource design,such as oil temperature control,vacuum degassing,oil bulk modulus measuring and oil supply pressure adjustment, are stated and the principles are explained.In chapter 3,based on the theoretical model of oil bulk modulus,the influence factors are considered.The accuracy of oil bulk modulus measurement device is analyzed in detail. Experiment researches on the effects of entrained air on oil bulk modulus,the changes of oil bulk modulus with pressure and the effects of temperature on oil bulk modulus are made. The effects of pressure fluctuation in test system,leakage and air solubility on measurement results are analyzed with experiments.In chapter 4,based on the mathematical model of temperature control object,the difficulties of oil temperature control are analyzed.A kind of oil temperature control strategy based on parameter self-tuning fuzzy PID control method is put forward.The parameter self-tuning fuzzy PID controller is designed.Oil temperature control experiment is made on facilities of hydraulic resource of motion simulator.The simulation and experiment results show oil temperature maintaining in 45±1℃.In chapter 5,the simulation model of pressure control cartridge valve is established with AMESim.The relevant parameters with steady characteristics are analyzed.The effects of spring in main spool,volume ahead main spool,spring in pilot valve and damping on dynamic characteristics are observed.Pressure control experiment is made on facilities of hydraulic resource of motion simulator.In chapter 6,some conclusions are given and some new views are put forward in the future.