Design Of Flexible Actuator Based On Shape Memory Alloy

The commonly used actuators in mechanical systems such as servo motors,hydraulic drives are complex in structures and the transmission system.Also there are wear and impact between various components.These actuators have been unable to meet the requirements of size and precision in the micro-nano and robot field.That's why new magnetic,electrostatic,and thermal actuators are gradually developed.Shape memory alloy(SMA)actuators are typical one.The advantages of SMA actuators stem from shape memory alloys(SMA).Shape memory alloys are widely used in actuator design because of their shape memory effect,high energy density,low driving voltage and large restoring force.However,SMA actuators also have shortcomings such as short strokes,unstable limit positions and non-uniformity of useful force during the stroke.In order to solve the above problems,based on the elastic compensation method,this paper designs a flexible actuator by using a full-flexible bistable mechanism.The actuator has the characteristics of large stroke,fast response and two stable positions.The main accomplishments are the following three aspects:(1)In view of the shortcomings of SMA actuators such as short stroke,unstable limit position,and inconsistent useful force during the stroke,the existing methods for improving the performance of SMA actuators are studied.For instance,the displacement amplification mechanism is used to amplify the stroke of the SMA actuator;Special mechanical structures are designed to prevent the actuator's over-range movement and increase the stability of the SMA actuator's extreme position;To resolve the conflict between useful force and effective displacement in the SMA actuator stroke,an elastic compensation mechanism is used to uniform the useful force in the SMA actuator stroke.However,none of the above methods can simultaneously improve shortcomings such as short travel of the SMA actuator,unstable limit position,uneven use of force during travel.(2)Based on the study of the compensation method,the full-flexible bistable mechanism are selected as a compensation mechanism for the biased SMA actuator.The full-flexible bistable mechanism can amplify the effective stroke of the biased SMA actuator without sacrificing the output force because of its negative stiffness characteristics.By optimizing the geometric parameters of the full-flexible bistable mechanism,a force-displacement curve with a large negative stiffness range and a large first turning force is obtained.Based on this,an SMA flexible actuator with an effective stroke magnification of 2 times is designed.(3)Comparing the processing materials and processing methods of the full-flexible bistable mechanism,a polylactic acid material and a 3D printing processing method are selected to make a prototype.Test the force-displacement characteristics of the prototype and the results are basically consistent with the theory.Install the SMA flexible actuator,build an experimental platform,conduct experiments and analyze the experimental results.The results show that the stroke of the SMA flexible actuator is 16 mm,which is 2 times of the stroke of the SMA actuator.The maximum output force is 2.5N.Under the maximum output force,the output displacement can still maintain 16 mm,which resolves the contradiction between output force and output displacement of the biased SMA actuator.The response speed from energization to the end of the operation is 3 times that of the biased SMA actuator.And it is stable in the limit position when there is no energy input.The experimental results show that the design achieves the initial design goal-the travel is amplified,the response is fast,and the extreme position is stable.