Research On Hydraulic-Mechanical Feedback Principle And Its Application In Large Rated Flow Control Valve

Fluid power system has been widely applied in various mechtronic machines,where hydraulic valves are used to control the pressure,flow and direction of the fluid.Hydraulic valves have sig?nificant effect on the performance,reliability and cost of fluid power systems.Flow control valve is one of the three types of hydraulic valves.It's used to adjust the flow and accordingly the veloc-ity of machine by regulating the inner metering orifice.For larger rated flow control,multi-stage:flow control valves are commonly required.The displacement of the main spool is controlled by pilot spool based on various feedback methods.There are four types of common feedback meth-ods,which are hydraulic feedback,mechanical feedback,hydraulic follow principle and electronic feedback.As the requirements of reliability,leakage,dynamic response and load adaptivity in flow control valves are becoming higher,it is necessary to investigate multi-feedback methods in order to combine the advantages of different feedback methods and overcome the disadvantages such as bad reliability,poor spool control accuracy or slow dynamic response of single feedback method.The aim of the thesis is to propose a novel double feedback method and study its application in flow control valves.The thesis may provide valuable reference for the research of this kind of valve and there is great application potential of the proposed method.In existing single feedback method,the displacement of main spool is converted into the pres-sure rise in control chamber or the force rise worked on the pilot spool.These characteristics are applied to realize the proportional control of the main spool displacement.The method based on control chamber pressure rise has good reliability and stability.In comparison,the other method based on force rise has good accuracy and dynamic response.The hydraulic-mechanical combined feedback method presented in this thesis converts the main spool displacement into the pressure rise in control chamber and the force rise worked on the pilot spool simultaneously.The two feedback methods work together,inherit the advantages of both methods and give the valve further advan-tages such as simpler structure,fewer sealings and wider control stroke.The hydraulic-mechanical feedback method can be used in flow control valves such as single/double direction proportional throttle valve,proportional flow control valve and load control valve.In addition,design of damping tail with holes on the main spool is proposed in the thesis.It can compensate the flow force in the direction of opening the spool by the crashing effect of the fluid and therefore change the direction of flow force and lower the amplitudes of the flow forces in different spool openings.It is found by both simulation and experiment tests,that due to the effect of the damping tail,the direction of flow force worked on the main spool changes from opening to closing,and the amplitude of maximum flow force decreases 88.3%and 87.5%respectively.As a result,the stability of the main spool is better and the flow force disturbance on spool control accuracy has been almost eliminated.Furthermore,a novel design of the fast close orifice is also designed in the thesis.The orifice opens when the pilot spool displacement is smaller than a certain opening.Accordingly,the load port and the control chamber is connected directly,which accelerates the pressure rise of the control chamber.As a result,the closing forces worked on both the pilot and the main spools are increased and the closing time of both spools are shortened.The closing time of the main spool has been shortened by 22%by applying the fast close orifice.Based on above work,a load control valve employing hydraulic-mechanical feedback prin-ciple has been designed and manufactured.The prototypes are tested both on a test rig and on the booming system of mobile cranes.Test results show that compared with existing widely used products,the novel valve has a 11%wider control stroke,a 60%smaller hysteresis of high load pressure,a 58%shorter opening time and a 17%shorter closing time.Chapters are briefed as follows.Chapter 1 firstly introduced the feedback methods in flow control valves and analyzed the advantages and disadvantages of each method.Secondly,the features and state of art of load control valves,which were a special type of flow control valve,were given.At last,the methodology and contents of the research were brought up.In chapter 2,double feedback principle was firstly introduced and its features was analyzed.In addition,the advantages and disadvantages of double feedback and single feedback methods were given.Secondly,the features that could be added to the double feedback principle,such as load pressure compensation and fast close,were introduced.At last,the application of the double feed-back principle in single/double way proportional throttle valves,proportional flow control valves and load control valves was presented.In chapter 3,the reason of the spool displacement control and load pressure compensation in double feedback principle was analyzed.It was also conducted how the parameters affected the features of double feedback valve.The design logic was concluded of the spool displacement control and pressure compensation features.A static feature simulation model,which was directly based on the parameters of irregular shaped orifices,was built up in Matlab/Simulink software.The features of double feedback valve were investigated based on the simulation model.At last,a static and dynamic performance test rig of the spools was designed and set up.Tests conducted on the test rig validated the analysis and simulations above.In chapter 4,the definition and research tools of flow reaction forces in hydraulic valves were introduced first.Secondly,CFD simulations of the flow reaction forces on a main spool without a damping tail were conducted.In addition,CFD simulations of how the diameter and length of the damping tail affected the flow reaction forces were given.The parameter pair which resulted in largest flow reaction forces in closing direction was found.Based on the parameter pair,the effect on the amplitude of the flow reaction forces of drilling holes on the damping tail was investigated by CFD simulations.The diameter of the holes which led to smallest amplitude of the flow reaction forces was achieved.Tests were conducted on a flow reaction force test rig and the results validated former simulations.In chapter 5,the dynamic equations of the motion parts,the dynamic chambers and the orifices were firstly built.Secondly,linearizations of the dynamic equations were conducted and the state space model of double feedback valve was achieved.Based on the state space model,the stability and dynamic response were analyzed on different work points.The effect of the key parameters on the stability and dynamic response were investigated by the method of root locus.In addition,a dynamic simulation model of the valve were built based on Matlab S function and the opening and closing features of the valve were analyzed by the dynamic model.At last,the dynamic response of the spools were tested on a test rig and the results validated former conclusions.In chapter 6,the applications and requirements of load control valves were firstly introduced.The suitability of double feedback principle to load control valves was analyzed and the structure of the double feedback principle load control valve(DFPLCV)was given.The structure and parameter design of main spool-main seat,pilot spool-pilot seat were presented in detail.The strength of the structures were also checked by finite element methods.In addition,the design of the pilot piston,the end cover and the sealing structures were also introduced.At last,the strength checks of the valve bodies and the connections were given.In chapter 7,prototypes were manufactured according to the conclusions of chapters 3-5 and the designs of chapter 6.A test rig for the test of two-stage load control valve were set up.The test results on the rig validated the check valve feature,the proportional flow control feature,the load pressure compensation feature and the fast opening/closing feature of DFPLCV.The DFPLCV prototype was also tested on mobile crane booming system,whose results were compared with similar products.The comparison gave evidence of the advantage and potential applications of double feedback principle.In chapter 8,the conclusions of the research were summarized,the innovations of the research were given and the discussion of the future work was presented.