Research On Characteristic Analyzing And Control Of Flexible Actuators Based On Shape Memory Alloy

As a new kind of smart material,Shape Memory Alloys(SMA)can produce deformation and recovery tension by causing the phase changes through the changes of material temperature,and thus the SMA can achieve the conversion of thermal energy and mechanical energy.The special actuated characteristics of the SMA makes the SMA actuators have excellent characteristics such as high power density ratio and high resilience,so it has gradually been applied in the fields of bionic robots and aerospace.However,there are strong saturation hysteresis nonlinear characteristics between the input and output of the SMA actuators due to the special properties inside the material,Thus characteristic will result in the decline of actuators output accuracy,oscillation and even instability,which limits the improvement of actuators performance.In this thesis,the SMA wire is used to realize the bending motion of the single-degree-of-freedom like the human joint,and the antagonistic SMA flexible actuator platform is designed.Considering the hysteresis effect inside the SMA wire,the composite control scheme of feedforward control and feedback control is proposed.In the feedforward control,the hysteresis inverse model is constructed based on the Duhem model to compensate the hysteresis.The adaptive dynamic surface control method is designed in the feedback control to ensure the closed-loop stability of the system.Finally,the accurate tracking of the output of the SMA flexible actuator component is realized.The main works of this thesis are as follows:1)In order to realize the output performance of artificial muscle flexible actuator,an antagonistic SMA flexible actuator platform is designed in this thesis.The platform has achieved bidirectional complementary actuating force by alternating heating and cooling of the two sets of SMA wires.By torsion spring,the actuating force can be rapidly restored,so that the SMA actuator has the advantages of large stroke,fast recovery,and high actuating positioning accuracy.In addition,this designed overcomes the problems of slow response speed and poor repeatability in the design of the traditional SMA wire actuators structure effectively.2)In order to reduce the influence of hysteresis characteristics on the actuating precision of SMA wire,a feedforward control scheme based on Duhem inverse model is designed.For the Duhem model with differential equation structure,the Gaussian probability density function is chosen to define the slope function in the Duhem model.Therefore,the accurate description of the SMA saturation hysteresis effect is realized.The effectiveness of the feedforward compensation control strategy is verified by the multi-input signal experimental3)On the basis of increasing the hysteresis feedforward controller,this thesis adopts the scheme consist of the feedforward control and the feedback control which adopts the adaptive dynamic surface control method.The application of the adaptive dynamic surface control strategy can overcome the compensation error caused by the feedforward hysteresis compensation part.The design method based on the Lyapunov stability ensures the stability of the closed-loop system,and considering the influence of external unknown interference,the output performance of SMA flexible actuators components has improved effectively.The results have proved the effectiveness of the proposed control strategy both in simulation and experimental.In this thesis,the goal is to realize human-like muscle flexible actuate output with SMA material,and to improve the performance of flexible actuators in structural design and control strategies.The research results provide technical support for the subsequent SMA flexible actuators components to achieve complex motion and operational requirements in bionic robots.