Research On Multilayer Structure And Parameter Optimization Of SAW Torque Sensor For Automotive EPS

With the increasingly serious energy crisis,the new energy vehicle industry and related technologies have been developed by leaps and bounds.The parts industry supporting new energy vehicles also faces the problems of independent innovation,technology upgrading,energy conservation and emission reduction.Electric power steering（EPS）is now called the most widely used steering system because it conforms to the development direction of low-carbon,lightweight and intelligent vehicles,and has the advantages of good handling stability,easy modular design and installation,and support for personalized power assistance mode.The surface acoustic wave（SAW）sensor has the characteristics of passive and wireless operation.It can work stably in such harsh enviro N·ments as vacuum,confined space,high and low temperature,and strong radiation.It has unique advantages in torque measurement,and can avoid the inconvenience of energy supply and signal transmission in the traditional torque measurement process.Therefore,many domestic and foreign scientific researchers have been attracted to carry out research on SAW torque measurement.After nearly 30 years of development,the SAW torque measurement system using single crystal quartz as the piezoelectric substrate material has made great progress,and a large number of scientific research achievements have emerged in the single crystal substrate SAW torque sensing mechanism and theoretical model,sensor structure,echo signal acquisition and frequency estimation.However,due to the limitations of the characteristics of single crystal materials,the development of SAW torque sensors in high frequency,high performance,miniaturization and marketization has become slow,which seriously hinders the development and practical process of SAW torque sensors.This paper proposes an EPS-SAW torque sensor and torque measurement system based on IDT/Li Nb O₃/diamond/Si multilayer piezoelectric thin film structure for Automotive EPS system,uses the methods of literature collation,theoretical analysis,mathematical modeling and experimental verification.Several key technical problems of SAW torque sensor with multilayer piezoelectric thin film structure are deeply studied:（1）The characteristics of SAW propagation in multilayer piezoelectric thin films are studied theoretically.The wave equation of elastic wave is deduced,and the wave equation of SAW in isotropic,anisotropic and piezoelectric media is established.Then,based on the wave equation in the piezoelectric medium,taking the multilayer piezoelectric thin film structure as the characteristic and the SAW propagation characteristic equation as the objective,the wave equation of the particles in the electrode layer,the piezoelectric thin film layer and the acceleration layer is established,and the complex mechanical vibration behavior of the particles of each layer in the multilayer piezoelectric thin film structure is revealed.Finally,the structure derived theoretically is verified and studied by numerical calculation and simulation analysis,the research results show that the multi-layer structure with diamond film layer has a very significant effect on improving the wave velocity of SAW devices.Under the same conditions of other structural parameters,the fundamental wave and first-order wave velocity excited by the multi-layer structure with diamond film layer have increased by 33%and 49%respectively.（2）The preferred orientation of Li Nb O₃ piezoelectric thin films is studied.During the theoretical study,it was found that the elastic constant,piezoelectric constant and dielectric constant of the piezoelectric thin film material had a great influence on the calculation results,and these parameters were directly affected by the orientation of the thin film.Therefore,in this paper,COMSOL multi physical field coupling is used to build the SAW device model of multilayer piezoelectric thin film structure,and the relationship between film orientation and device center frequency and electromechanical coupling coefficient is established by using the function of multi parameter scanning.After comprehensive optimization,the orientation of Li Nb O₃ piezoelectric thin film in multilayer piezoelectric thin film structure is finally determined to be（0°,0°,90°）.（3）The layout design of interdigital transducer（IDT）of SAW device with multilayer piezoelectric thin film structure is studied.As the core component of SAW excitation and reception,the structure and position parameters of IDT directly determine the performance of SAW devices.Therefore,this paper proposes three kinds of IDT layout structures（flat layer,staggered layer and cross layer）,and makes a comprehensive comparative analysis of the three IDT structures with COMSOL.The results show that the flat layout is more suitable for the EPS system in terms of overall performance,but the other two layout structures have obvious promotion on further reducing the structural size of SAW devices.（4）The IDT analysis model of SAW device with multilayer piezoelectric thin film structure is optimized.SAW devices with multilayer piezoelectric thin film structure usually have a large number of second-order effects in operation.The influence of these second-order effects on device performance is more obvious with the decrease of device structure size and the increase of center frequency.The existing IDT analysis models have the problems of large error and low accuracy when facing high-frequency devices.Therefore,based on the traditional COM model,this paper derives a more comprehensive second-order COM model by introducing parameters such as electrode reflection coefficient,acoustic electric conversion coefficient,film electrode resistance and distributed capacitance.（5）EPS-SAW torque sensor with multilayer piezoelectric thin film structure is designed and studied.A new type of EPS-SAW torque sensor is proposed,which is composed of a constraint seat,a support plane,and two SAW resonators with multilayer piezoelectric thin film structure in differential arrangement,and the force analysis of EPS-SAW torque sensor is carried out by COMSOL.（6）The torque testing system of EPS-SAW torque sensor is built in stages.The first stage is the performance test of SAW torque sensor with single crystal quartz piezoelectric substrate structure.In the second stage,the steady-state shaft performance was tested on the self built test bench,and the linearity and repeatability data of the EPS SAW torque sensor were obtained.The torque sensitivity of the multi-layer piezoelectric film structure SAW torque sensor was obtained through linear fitting.In the third stage,the EPS SAW torque sensor performance test was conducted on the vehicle fixed bench,and the linearity,repeatability and torque sensitivity data were also obtained.