Study On Energy-Saving Velocity Control Of Electrohydraulic System Under Time-Varying Negative Loads

Negative loads widely exist in hydraulic machinery.Since the system is required to guarantee outlet pressure higher enough to support external force,there is a difficult trade-off between energy conversation and motion smoothness.Taking account of the complexity of hardware configuration and energy consumption,electrohydraulic variable pump system with meter-out throttle(VPMOS)is chosen to be investigated.In state-of-the-art researches,advanced control technologies have been applied to electrohydraulic servo system,however,the system pressure is seldom involved in the control target.Introducing advanced control technology into VPMOS,this study combines system pressure and velocity requirement in the building of control target.As a result,solutions achieving energy conversation as well as regulating velocity are proposed.According to the demand for velocity,VPMOS has two working modes.One is named "Flow Rate Guiding Mode"(FRGM)in which pump is controlled in open-loop,determining the average velocity of the actuator.In this mode,meter-out orifice is regulated instantaneously to make the velocity match inlet flow rate and cut inlet pressure,which functions like a counterbalance valve.The other working mode is called "independent velocity and pressure control mode"(IVPM)where the meter-out orifice is assigned to control velocity and the variable pump reacts to the pressure feedback similar to a load sensing pump.The inlet pressure target of this research is 3bar,much less than the inlet pressure loss of a load sensing system or pilot pressure of any a counterbalance valve(12?20bar).This thesis consists of 6 chapters,the outlines are as below.In Chapter 1,firstly,the hydraulic system related to negative load in applications are classified and main drawbacks are pointed out.From the view of energy conversation,three methods to cut energy consumption is pronounced:using counterbalance valve(CBV),Independent Metering control technique and inlet flow matching via variable pump.The researches on these technologies are respectively introduced with main characteristic addressed,including working principle,control strategies and energy conversation.Afterwards,an overview of the development of control technology on electrohydraulic systems is presented.the research contents of this thesis are subsequently given.Chapter 2 investigates the basic properties of VPMOS.The relationship between control inputs and states is articulated under stationary external loads.According to the quantitative comparison between velocity and external loads,the external load is divided into three categories:resistive load,negative load and insufficient negative load(INL).Stationary control law is given and turns out to be unable to effectively achieve energy conversation because of the tiny model errors.Therefore,component properties as well as dynamic model of actuator and chambers are built,with the uncertainties distribution analyzed.The requirements for controller are addressed,specifically,the velocity target is either tracking reference signal or matching inlet flow rate.Then The experimental rig for this research is displayed afterwards,including motion control part and loading part.Afterwards,the property of meter-out orifice is obtained.Organization of dataset and query rules are proposed,appear to be effective by experimental test.Acquiring the varying rate of chamber pressure at different cylinder positions,the chamber volumes and bulk modulus are figured out.Chapter 3 proposes a control strategy for FRGM.The concept "Flow Rate Follower"(FRF)is proposed,as linear combination of nominal velocity error and inlet pressure error.It can be proved velocity error and pressure error converge towards zero if FRF does so.A disturbance observer is constructed,providing second order filter of real external load.Regarding FRF as system output,output linearization feedback method is adopted to build second order differential equation of FRF.As a result,the proposed control law consists of model compensation items,load compensation components and robust parts.The experiments show that estimated error of inlet flow rate could be transferred into inlet pressure offset,leaving velocity match the inlet flow rate.Moreover,the proposed controller can reduce the impact from time-varying external load,keeping inlet pressure slightly fluctuating around preset low pressure,cutting energy consumption significantly.Chapter 4 proposes a control solution for IVPM.Based on chamber model,inlet pressure controller employs feedforward as velocity compensation,utilizing PI feedback to deal with flow rate error of variable pump.Cavitation may occur when the velocity varies too fast.In the period of cavitation,taking advantage of the fact that the total supply flow volume equals to the expanded volume,the control law is revised.As a result,the cavitation time is shortened and the inlet pressure successfully returns back to normal range smoothly.Velocity controller is proposed via back-stepping method.An integral is appended in the design of virtual control law for velocity to eliminate steady state error.In the control design of velocity,the inlet pressure dynamic estimation by model is invalid due to quite a little flow error.In order to overcome this difficulty,the inlet pressure is regarded as a part of external force and a related load observer is built.Verified by experiments,the proposed control solution can succeed in velocity tracking,load impact inhibition and cutting energy consumption.Chapter 5 focuses on the situation with INL.An index function involved anti-saturation ability and energy consumption is constructed to determine appropriate inlet pressure under INL,and the solution of extremum condition is also performed.Furthermore,making use of load provided by load observer,revise the control law for inlet pressure in IPVM.the revised control law appears to succeed in anti-saturation,verified by experiments.At last,anti-saturation in FRGM turns out to be not feasible theoretically and experimentally.Chapter 6 summarizes the work of this study,comparing two proposed solution.Major innovations of the thesis are emphasized.The future work is also prospected.