Shape Memory Alloy Robotic Truss

Abstract

The development, design, and analysis of a Shape Memory Alloy Robotic Truss (SMART) actuator is presented in this research paper. SMART is a three-degree of freedom actuator capable of large rotary and bending displacements using shape memory alloy (SMA) wires as the mechanism for actuation. Using SMA actuator wires instead of conventional hydraulic actuators simplifies the overall complexity of design by reducing the number of working parts. SMA actuator wires, because of vibrational dampening in the material itself, have a natural advantage over hydraulic actuators because they are not susceptible to large parasitic vibrations and long settling times inherent in hydraulic systems. With that said, the most radical development in the actuator design of SMART is that the entire structure acts as an actuator instead of actuation occurring at only a few synthetic joints. That is to say that the amount of actuation is dependent on the length of SMART and corresponds directly to the structural stiffness of the truss. For this reason, the truss backbone of the SMART actuator was designed to be structurally weak in torsion, strong in tension, and weak in axial bending to allow for the twisting and bending actuations. The actuation force is provided by the contraction of SMA wires which are attached in a specific pattern, to be described in further detail later, to wire guides at nodes along the truss. The force of the SMA wire’s contraction is distributed to the truss through the nodes at which the SMA is attached. That is to say that the nodes connected to the SMA wire become closer, and as a result, the SMA’s contraction actuates the entire truss. The ability of the SMAs to contract is a unique material property of their crystalline structure to be trained at high heat to remember a desired length. When cold, SMAs can be mechanically stretched easily; however, they immediately return to the remembered length when a heat stimulus is applied. Exploiting this material property, electricity was supplied to one or multiple SMA wires in a simple circuit in which the SMA wires acted as the resistors. The resistance produced heat in the SMA wires which then contracted in approximately a second to their remembered length. The result is that the entire truss actuated in a specified mode depending on which wires were heated. Results of SMART from ground based testing and reduced gravity testing aboard the NASA C-9 aircraft, while undergoing parabolic trajectories to simulate reduced gravity, demonstrated the feasibility of SMART as an actuator truss capable of large actuations and functionality in a reduced gravity environment such as space.