Characteristics Analysis And Optimization Design Of Piezoelectric Actuator For Piezoelectric Fuel Injector

The vehicle diesel engine has become the main power source for a variety of cars with excellent power performance and reliability,especially with the application of high-pressure common rail technology which has made the advantages of diesel engine more prominent.The china has made great progress in the research,design,manufacture and test of high pressure common rail injector in recent years,but with the increasingly stringent energy saving and emission reduction requirements,the performance of solenoid injector has been unable to meet the needs of practical applications.With the continuous development of smart materials,piezoelectric stack-driven fuel injection technology is considered to be the future development direction of diesel engine technology.As the core component of piezoelectric fuel injector,piezoelectric actuators have the advantages of faster response,higher output force and higher repeatability than traditional solenoid actuators,which will lead the development trend of the future injector actuators.The piezoelectric injector works under the complex environment of high pressure and heavy load,and is easily coupled by various fields such as force field,electric field and temperature field.The coupling effect will have a great influence on the output characteristics of piezoelectric actuator,and then further make the performance of piezoelectric injector worse.At the same time,as a polycrystalline ferroelectric material,piezoelectric actuator has its own unique hysteresis nonlinear characteristics,which is greatly related to the external driving voltage,preload and temperature,and will bring great challenges to the improvement of control accuracy of piezoelectric actuators.Therefore,in this paper,the engineering problems caused by the optimization of the performance of piezoelectric injectors are analyzed,and the influence factors of piezoelectric injector piezoelectric actuator structure size,electro-mechanical coupling,and thermo-electro-mechanical coupling and hysteresis characteristics are discussed.The optimization of the output characteristics of piezoelectric actuator is studied in depth and the injection characteristics of the designed piezoelectric injector are tested,to provide experimental and theoretical reference for the optimization design of the piezoelectric injector.The main research content of this article is as follows:1 Study on the output characteristics of piezoelectric actuator by finite element methodThe output characteristics of the piezoelectric actuator are studied by the finite element method.The influence of the structure parameters and operating parameters of piezoelectric actuator on the output characteristics is obtained,and the modal analysis of the actuator is carried out.The finite element model of the piezoelectric actuator is established by using the parameterized modeling in the ANSYS finite element analysis software,and the accuracy of the model is verified by the experiment.Then,the thickness of the piezoelectric layer,the cross-sectional area,the cross-sectional shape,the number of piezoelectric stack and the driving voltage of piezoelectric actuator are simulated.The simulation results show that when the layers and the thickness is constant,changing the shape of cross section,it can be seen that the shape of cross section has no effect on the displacement output of the piezoelectric actuators,but with the increasing of cross-sectional area can improve the output force of piezoelectric actuator;when the thickness of a single piezoelectric layer is constant,with the increasing of the number of piezoelectric layers can increase the displacement of the piezoelectric actuator;when the number of the layers is constant,as the thickness of a single layer decreases,the displacement of the actuator also decreases,but the amount of reduction is negligible with respect to the thickness of the single layer;as the driving voltage increases,the displacement of the piezoelectric actuator also increases.Finally,the modal analysis of the piezoelectric actuator is carried out,and the natural frequencies of the actuator are obtained and it is analyzed.The results show that the actuator has good dynamic characteristics,and it will not affect the injection pulse width of the piezoelectric injector.2 Analysis of electro-mechanical coupling characteristics of piezoelectric actuatorsIn order to study the output characteristics of piezoelectric actuator under the condition of electromechanical coupling,the displacement output characteristics under the action of axial electromechanical coupling are tested with the piezoelectric actuator test system.Meanwhile,to facilitate the study of the dynamic characteristic of the actuator and the injection characteristic of the piezoelectric injector in the future,an energy recovery circuit is designed based on the equivalent capacitance model.The single injection and multiple injection control strategy for high pressure common rail diesel engine based on task scheduling are designed.The results show that within a certain range,with the increasing of preload,the displacement of piezoelectric actuator increases first and then decreases,and the position of the maximum displacement varies with the driving voltage.When the driving voltage is low(less than 30V),the displacement shows a linear change,the piezoelectric coupling coefficients obtained is almost a constant,when the voltage increases to a certain degree(above 30V),the displacement is nonlinear change,the same as the piezoelectric coefficient.Under the different driving voltages,as the frequency increases,the displacement will approximately a linear change when the frequency is low(less than 100Hz),when the frequency increases to a certain degree(more than 100Hz),the displacement increases as the frequency increases,and the greater the voltage,the greater the growth trend;under the different driving frequencies,the duty ratio of the pulse width is not correlated with the displacement;under the different voltages,the displacement of actuator increases with the increasing of the driving voltage.To reveal the influence of the electro-mechanical coupling effect on the displacement output characteristics of piezoelectric actuator,a mathematical model based on the linear piezoelectric effect and the non-180° domain switching effect is constructed.Considering the influence of the volume fraction of the material domain,the domain wall motion,the form of the electric domain,the coercive field,the preload and the energy conversion between the force and the electricity,the theoretical analysis of the non-180° domain motion in the crystal is performed under the action of electro-mechanical coupling,and the internal deformation of the crystal is analyzed and calculated.The displacement characteristics under different preload are measured through the test method,and the strain under different preload condition is studied.The simulation model fits quite well with the experimental results at medium and high preload.However at the lower preload,the prediction of the model is not good.The mathematical model can describe the strain generated by the non-180°domain induction under the force field in single-crystal and polycrystalline piezoelectric materials,which provides a theoretical basis for the study the electromechanical coupling characteristics of the piezoelectric actuator.On the whole,the electro-mechanical coupling under axial stress has a great influence on the displacement of the piezoelectric actuator.3 Non-linear mathematical model for the displacement of the piezoelectric actuatorOn the basis of the electromechanical coupling,in order to further understand the influence of temperature on the output characteristics of piezoelectric actuators,and based on the law of thermodynamics,the heat generation and heat release models of piezoelectric actuators are established.The effect of the amplitude,working frequency and the duty ratio of driving pulse width on the temperature change of the piezoelectric actuator is studied in this model.Then,based on the experimental results and the constitutive equation of piezoelectric material,the displacement relation criterion of piezoelectric actuator is fitted under the condition of force thermos-electro-mechanical coupling,and a nonlinear mathematical model of displacement is constructed.The self-heating of the actuator is studied under different piezoelectric injector operating conditions.The results show that:when the driving voltage amplitude and operating frequency increase,the temperature also increases;when the duty ratio of control pulse width increases and the temperature also rises.Thus decreasing the duty cycle can improve the functionality of the actuator and lead to their expanded use in high-power applications.With the help of numerical method,the influence degree of assumptions and minor factors is analyzed,and the error of correlation obtained by the finite element method is analyzed to determine the accuracy and practicality of correlation calculation results.The results show that the nonlinear mathematical model can effectively predict the displacement of piezoelectric actuators under different temperatures and preloads,and it can provide theoretical basis for the design of piezoelectric actuators under the condition of thermo-electro-mechanical coupling.4 Modeling and control of hysteresis characteristics of piezoelectric actuatorsTo further improve the precise control of the fuel injection of the piezoelectric injector,it is necessary to control the hysteresis nonlinearity of the piezoelectric actuators accurately.Based on the duhem model,a hysteresis nonlinear model of piezoelectric actuator is built.According to first approximation law of Weierstrass,polynomial fitting method is applied to approximate the functions in the model,and the parameters of the hysteresis model are identified by recursive least square method.With the help of the relationship between piezoelectric coefficient and temperature of piezoelectric actuators,the hysteresis model at different temperatures is established and used to fit the hysteresis model.It is the basis for special study on hysteresis characteristics of piezoelectric actuators at different temperatures.In order to reduce the hysteresis of piezoelectric actuator,a duhem hysteresis inverse model is established on the basis of duhem model of piezoelectric actuators.The hysteresis inverse model is applied to feedforward open-loop control for piezoelectric actuator.The simulation results show that the nonlinearity of the hysteresis system of piezoelectric actuator has been greatly improved and achieves the better control effect.To further provide control accuracy,the fuzzy adaptive PID control is added on the basis of feedforward control to form a combined control of inverse model feedforward and fuzzy adaptive PID feedback.The simulation results show that the maximum output error of hysteresis system is further reduced,and the composite control can further improve the control precision of the piezoelectric actuator.5 Design and experimental study of piezoelectric actuatorsBased on the study of the characteristics of the piezoelectric actuator,the design scheme of actuator is proposed.Because of the limitations of the technology and the level of knowledge,only the existing piezoelectric actuators can be chosen.Therefore,on the basis of the design scheme,the actuator is selected based on the structural characteristics of piezoelectric injector.In order to facilitate the study of the output characteristics of the selected piezoelectric actuators,the hysteresis,stiffness and displacement characteristics of piezoelectric actuator are tested on the designed actuator characteristics test platform,to provide support for the reliability of the performance of the piezoelectric actuators.The piezoelectric actuator and injector body were integrated to test the injection characteristics of the designed piezoelectric injector.The results show that the fuel injection law of the designed piezoelectric injector is equivalent to the level of the reference injector,which can better meet the design requirements,...