Design of an Efficient Non-backdrivable Mechanism with Wrap Spring for Hand Prosthesis

The aim of this article is to create a system that enables power transmission non-backdrivability in a hand prosthesis with a single actuator. This system allows the motor to be stopped while maintaining the gripping force to prevent the held object from being dropped. This non-backdrivability allows users, for example, to release muscle contractions while still keeping a tight grip on an object, as well as completely turning off the prosthesis to avoid unintentional commands that could lead to loosening the object. Beyond the functional aspect of non-backdrivability, the physical non-backdrivability of the transmission enables the full power of the motors to be utilized without exceeding their thermal limits. To be effectively used, the non-backdrivable system must be energy efficient. A state-of-the-art analysis of different non-backdrivable mechanisms is conducted, evaluating their functioning and maximum efficiency. A novel system is developed based on an existing principle but with a focus on simplicity of manufacturing and fewer components compared to existing systems. An analysis is conducted to understand the effect of each mechanism parameter, and a dimensioning procedure is derived. A prototype is developed to compare theoretical values with measured values. The obtained results are analyzed and discussed.