Design of Shape Memory Alloy Coil Spring Actuator for Improving Performance in Cyclic Actuation

Area of Science:

• Materials Science
• Mechanical Engineering
• Robotics

Background:

• The performance of shape memory alloy (SMA) coil spring actuators, used as artificial muscles, is heavily influenced by their coil geometry.
• Optimizing SMA actuators for cyclic actuation requires improving actuation frequency and efficiency.

Purpose of the Study:

• To propose a practical design method for enhancing the frequency and efficiency of SMA coil spring actuators.
• To develop a design strategy that improves cooling characteristics and enables larger deformations.

Main Methods:

• A Large Index and Pitch (LIP) design approach was developed, focusing on coil diameter, wire diameter, and pitch angle.
• The design process utilizes a two-state static model of SMA coil springs (austenite and martensite phases), incorporating stress-induced detwinning for maximum shear strain.
• Design parameters were determined to meet specific stroke and force requirements.

Main Results:

• The LIP design significantly improves cooling, leading to higher actuation frequencies.
• The method allows for achieving large deformations while optimizing SMA mass, power density, and efficiency.
• Experimental results showed the LIP design doubled the maximum actuation frequency and reduced actuator mass by one-sixth compared to non-LIP designs.

Conclusions:

• The proposed LIP design method offers a practical approach to significantly enhance SMA coil spring actuator performance.
• This design strategy improves both the speed and efficiency of SMA actuators, making them more viable for artificial muscle applications.
• The LIP design demonstrates a substantial improvement in power density and a reduction in material usage.