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

Abstract

Abstract Shape memory alloy (SMA) wires have been used to actuate prosthetic arms and robotic grippers, among other interesting applications. This work presents a compact system that amplifies the stroke of SMA wire actuators in a prosthesis’ forearm and locks them after actuation to save energy. The proposed system, called the “Clever Lever & 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. Then, the CLL system amplifies the finger restoring forces, and applies them as bias forces on the SMA actuators. The 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, and prevents the wires from staying at high temperature for a long time. The need to operate a cooling system during finger relaxation is also reduced. The CLL system was manufactured and tested to prove the efficiency and effectiveness of the proposed design.