Design and Analysis of Artificial Muscle Robotic Elbow Joint Using Shape Memory Alloy Actuator

Artificial muscle is one of the more prominent topics in modern robotics as it can be applied to robotic arms, electric vehicles and wearable robots (Shahinpoor et al. in Smart Mater Struct 7:15-30, 1998; Jani et al. in Mater Des 56:1078-1113, 2014). The advantages of Shape Memory Alloy (SMA) artificial muscle are lightness and high energy density. The high energy density allows the actuator to make powerful motions. Meanwhile, SMA wire contracts 6% of its length, which means that the required displacement cannot be achieved by a simple connection. To resolve these disadvantages, the SMA wires are coiled in a diamond-shaped structure. If the electric current is given by contracting wires in the longitudinal direction, the actuator can exert force and displacement in the diagonal direction. As the crossed tendon finds its minimal length when actuated, the rotation angle converges to 90 degrees. Parameters related with the rotating motion were selected, such as SMA wires' diameter and length, distance between the crossed part and elbow part, size of the diamond-shaped structure, friction, etc. To determine the maximum force of the actuator, a graphical method was used, which is similar to the yield strength determination (0.2% offset). Because the robotic elbow joint is connected by the tendon, the connections between links are flexible, and without motor it does not generate any sound or noise during operation. The robotic elbow joint using the SMA actuator is designed and analyzed, which can rotate 86.7 degrees and generates maximum 56.3 N force.