Research On The Design And Control Of A Large-strain SMA Actuator

SMA has been widely used in engineering due to its advantages of simple structure,large power density ratio,and reliable operation.NiTi thin wire is widely used,which can generate one-dimensional contraction motion.However,it has an obvious disadvantage.Its recoverable strain is only about 4%,which leads to a limited travel.The strain of the spring made of shape memory material is large,but its force output capacity is significantly reduced.In order to solve the problem of small strain of NiTi wire,a new actuator is designed in this paper.The actuator is made by twining the NiTi wire on a specific fixture,heat-treating it,and binding together with a high-temperature wire.The deformation principle is the change of the size in the length direction caused by the deformation of NiTi wire between curved and straight.The strain of the actuator is significantly improved compared with the recoverable strain of NiTi wire,and the force output capacity is almost the same as that of NiTi wire.Based on the knowledge of material mechanics,the quasi-static mathematical model of the actuator is established,the functional relationship between displacement and force is constructed,and the stiffness curve of the actuator is obtained,which provides a reference for the design process.The control of SMA is very challenging.This paper studies the control method of the actuator on a single-joint experimental platform.It is found that PI control has a good effect in position step response of the actuator designed in this paper.However,the adaptability of PI control is poor.When the displacement curve changes relatively little near the initial value,PI control cannot follow the curve well.This project constructs an s-type resistance feedback function,which is in parallel with PI controller.This function can judge the state of SMA according to the resistance value,and adjust the control input,so that the system can heat SMA at the fastest speed.The controller with resistance feedback shows good performance in the experiment of slope and sinusoidal curve.In order to enable the joints to reciprocate,the joint structure of SMA-SMA was constructed in this paper.Air cooling device was added to accelerate the cooling speed,so that the joints could move back and forth quickly.At the same time,in order to solve the problem that the stroke of the driver drops under the condition of load,the displacement amplification mechanism is connected in series at the end of the driver in this paper,so that the stroke is amplified by 3 times.In order to speed up the response speed in the use of SMA driver,the heating power is often increased.And sometimes the movement instruction is larger than the drive stroke,which will cause SMA overheating and reduce the performance and fatigue life.In this paper,a temperature control switch is added on the basis of the controller to keep the temperature near the end of austenitic phase transition by using resistance feedback.Simulation results show that the control logic makes the controller have a frequent switching action after the temperature is raised,which avoids the excessive temperature.