Design of a mechanical system for underactuation of hand prostheses

Abstract

The project has been developed working with the Biomechanics and Ergonomics research group in the framework of the UNIQUE-HAND project that has as final objective the creation of viable artificial hands valid as prostheses for individuals and also as robotic hands for industry, as well as the improvement of aesthetics of prior designs, allowing people to wear their prostheses with more confidence and trying the designs to go unnoticed. As mentioned above, the specific objective of this FDP is to design a mechanical underactuation system that improves weak points of previous developments and tries to reproduce as faithfully as possible the different hand grasps used by healthy people during activities of daily living. This mechanical system must be designed in order to be manufactured using the fused deposition modelling (FDM) 3D printing technique [2] and will be fitted into a completely new designed hand prototype which houses this underactuation system, the design of which is also reflected in this paper. The design of the prosthesis includes fingers, joints, returning system, flexion system, abduction and adduction of the thumb and palmar structure of the hand. This new system will be tested in the laboratory by carrying out two essays, one of them will be a grasping test and the other will be the water bottle test (WBT) that will be explained further in the paper and the other will be the Anthropomorphic Hand Assessment Protocol (AHAP) [3].

The presented design is based on differential mechanisms, such as pulleys and whiffletree mechanisms and will be explained in further detail later on. The design of the prosthetic hand has been based on the scan of a real human hand and then modified in order to adapt the different proportions and finger orientations, as well as the different mechanisms that allow the flexion and extension of the hand as mentioned above. The actuation of the different cables that allow the movement of the hand will be obtained by using a bowden cable together with a harness and triceps cuff system and the prosthesis will be located just below the tested subject hand, thus allowing a more natural behaviour when making grasps. This project contains the design of the underactuation mechanical system and its manufacture using a 3D printer. Furthermore, there are details about the assembly process and the different components of the design. Eventually, there is an explanation of the tests that will be carried out, which will determine the performance of the system, and the obtained results from these tests.