<p>Shape memory alloy (SMA) wires have been used to actuate prosthetic arms and robotic grippers, among other interesting applications. SMA wires contract by 4-5% of their lengths when being Joule heated, as the material transforms from the Martensite to the Austenite phase. Accordingly, spooling systems in the prosthesis forearm have been used to enable enough length of SMA wires to actuate the fingers when the wires are activated. In most previous designs, SMA wires needed to stay at elevated temperatures to keep the fingers deformed. Pulse-Width-Modulation (PWM) technique was utilized to maintain the temperature of SMA wires without supplying continuous electric current, thus saving energy. This work focuses on designing a compact system that amplifies the stroke of SMA wire actuators in the prosthesis’ forearm and locks them after actuation to save energy. The proposed system, called the “Clever-Lever-and-Locking” (CLL) system, is located in the forearm space close to the elbow. It includes an array of levers with built-in ratchets that are locked in place using two spring-supported pawls. Solenoids are used to release the levers when an unlocking signal is received, allowing for the finger restoring forces to act as bias forces for the SMA actuators. CLL system highly reduces the amount of SMA wires required to fully deform the fingers, thus enables reducing the overall size and weight of the prosthesis. The locking aspect of the system avoids supplying current to the SMA wires once the final finger deformation is reached, thus saves energy even more than the PWM technique, and prevents the SMA wires from being at high temperature for a long time. The need to operate a cooling system every time the user wants to relax the fingers is even reduced to only the cases when the user wants to perform frequent gripping. A computer application combining the dynamic model of the SMA wires in the CLL with that of the prosthetic finger deformation, was created to aid in the design of the proposed system. CLL system was manufactured and tested to prove the efficiency and effectiveness of the proposed design.</p>

Design of a Lightweight Shape Memory Alloy Stroke-Amplification and Locking System in a Transradial Prosthetic Arm

Paper Proceedings: https://asmedigitalcollection.asme.org/SMASIS/SMASIS2021/volume/85499

ASME 2021 Smart Materials, Adaptive Structures and Intelligent Systems (SMASIS2021)

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