Research On Electro-thermo-mechanical Behavior Of A Vertical-suspended Shape Memory Alloy Wire Actuator Under Constant Load

With excellent thermo-mechanical properties,shape memory alloys(SMAs)have appeared more and more frequently in actuator designs.Since the Ni Ti SMA wire has a high resistivity,the actuator can be activated by Joule heating at low current,achieving a short-term response.SMA wire can be activated by Joule heating at low voltage due to its high electrical resistivity to achieve a fast response.When the actuator is activated,the high temperature induces the martensite reverse transformation,which is a macroscopic shortening of the length of the SMA wire,and the internal electric resistance also changes.Therefore,the characteristic research of SMA wire actuators needs to cover the macroscopic deformation,response time,recovery time and internal resistance.However,the existing electro-thermo-mechanical research has shortcomings in the deformation response test,and the measurement of the transient temperature of the SMA wire is difficult.Moreover,the number of parameters to be measured in the phase transformation kinetics empirical model is so large that it is difficult to apply to practical engineering applications.Therefore,it is necessary to conduct a comprehensive and systematic study on the electro-thermal-mechanical characteristics of SMA wire actuators.This thesis takes Ni Ti SMA wire as the research object,and establish a phase transformation kinetics model based on Logistic function.In order to verify the effectiveness of the proposed model,a comprehensive performance test device for SMA wire actuator is designed and built based on the influencing factors of the electricalthermo-mechanical characteristics of SMA wire.On the one hand,the influence of voltage,stress,wire diameter,and convection form on the deformation response of SMA wire is explored through the controlled variable method.On the other hand,the feasibility of using resistance to detect phase transformation is verified.Finally,based on the experimental results,the design parameter guidance,phase transformation detection method and power consumption analysis suitable for SMA wire actuators are proposed.The main research contents of the thesis are as follows:1.The kinetic equations based on the Logistic function is improved,and the difference among the former and the exponential function model,Brinson model and Gaussian probability density function model under phase transition is theoretically analyzed.The results show that based on the Logistic function form,reasonable adjustment of internal parameters can change the curve of phase transformation,and has trend adjustability.Based on the improved phase transformation kinetics equation,this thesis establishes a one-dimensional constitutive model,phase change kinetic equation,thermodynamic equation and kinetic model for describing the electricalthermo-mechanical characteristics of the suspended constant load SMA wire actuator.Moreover,based on the research of the SMA wire,the thermo-mechanical behavior of the SMA spring is also analyzed.2.An experimental device for pretreatment and performance testing of SMA wire actuators is designed and developed.The device activates the SMA wire with Joule heat generated by a constant voltage source and adjusts parameters including stress,strain,voltage/current,wire diameter,and convection form.Its modular design meets the requirements of two experimental environments: constant load thermal cycle and fixedlength variable load,and realizes the functional requirements of the time response,and resistance characteristics of the SMA wire actuator.3.On the basis of the developed experimental device,the dynamic response characteristics of the SMA wire under constant load thermal cycle are studied.Furthmore,to prevent the SMA wire from overheating,an empirical model for calculating the temperature of the SMA wire with a smaller cross-sectional diameter is proposed.4.According to the resistance behavior of the SMA wire,the phase transition is detected.The first law of thermodynamics is used to calculate the temperature of the SMA wire during the heating process,and the transition temperatures determined by the resistance is compared with the transition temperatures measured by the differential scanning calorimeter.The experimental results indicated that the phase transformation detection method(RSF)by the resistance captures the start and end points of the phase transformation effectively,which can be applied to the self-sensing drive of SMA wire actuators without external sensors.In addition,this thesis proposes an excitation method to reduce the power consumption of the actuator based on the phase transformation detection method by the resistance.Compared with the pulse width modulation signal,this detection method effectively reduces the power consumption of the actuator and realizes the lightweight design.This thesis proposes the method for the multi-factor coupling analysis and the resistance phase transition detection of electrical-thermo-mechanical characteristics of SMA wire actuators,and solves the optimization problem of actuator design parameters and driving strategies.The research purpose of time response characteristic and resistance behavior characteristic of the SMA wire actuator is realized.It has important theoretical significance and application value in real-time phase transition detection,overheating protection,power consumption reduction and self-sensing drive of actuators.