A prosthetic hand actuation system inspired by the FDS and FDP flexor muscle behaviors

This dissertation presents a novel prosthetic hand actuation system and the accompanying mechanical design of a two-finger prototype. The presented actuation scheme is based on a unique perspective of the finger's flexor muscle's strength space when executing a task. The actuation structure developed by this research is a consequence of a more in depth evaluation of the Flexor Digitorum Superficialis (FDS) and the Flexor Digitorum Profundus (FDP) muscles embedded in the forearm which are most responsible for the finger's flexing behavior.Nearly all robotic/prosthetic hands implement the FDS and FDP muscle function. Robotic/prosthetic hands with their actuation structures embedded in the hand itself primarily focus on the degrees of freedom for which the muscles act and how those degrees of freedom can be fully actuated or underactuated to achieve desired tasks. Each actuator is responsible for the full strength space of the desired tasks for its respective joint(s) and as a result their shortcomings with respect to size, strength, efficiency, and/or noise are inherited throughout the entire execution of a task.Robotic/prosthetic hands with their actuation structure existing external to the hand (within a robotic forearm or an external large unit with multiple actuators) generally implement one actuator to replace the FDS muscle and another to replace the FDP muscle. These actuators are often connected to the robotic finger components in a very similar way to that of the human hand. The result is a system with accurate FDS and FDP muscle behavior but whose size restraints prevent such a system from being embedded in the confines of the hand.The actuation scheme developed by this research separates the strength space of the FDS and FDP muscles into two regions. The first region encompasses the lower strength requirements for the more active actions of simple task approach, finer manipulation tasks, and light grasps. The second region encompasses the less active robust tasks with higher strength requirements. The varying behaviors of the two regions drove the selection of two different types of actuators and actuation structures that have the ability to act independently or in parallel to accommodate the associated requirements.Based on a detailed comparison of actuators and actuation structures the small, quick, efficient, and somewhat weak dc motors were implemented in a nearly fully actuated actuation structure to achieve the first region of the FDS/FDP strength space. The light, strong, inefficient, and somewhat slow shape memory alloy actuators achieve the more robust strength movements of the second region of the FDS/FDP strength space. The parallel actuation system has been implemented in the mechanical design of a finger-thumb prototype with 20 degrees of freedom, anthropomorphic dimensions similar to that of the human hand, and weighing only 535g.Validation of the design was performed by targeting three tasks identified in hand movement theory literature to embody the three levels of hand tasks. These included the pinch grasp (used in precision tasks), the lateral grasp (used in intermediate tasks), and the power grasp (used in robust tasks). The geometric ability to perform these grasps was validated via the CAD software and the hand's kinematic model generated in MATLAB(TM). The prototype's structural adequacy during the execution of these three tasks for the maximum applied loadings observed in the FDS/FDP strength space was validated using SolidWorks COSMOS finite element analysis software.A dynamic model was also generated for the hand. However, the dynamic model is only a supplement to this research because the FDS/FDP strength space behavior of primary interest to this research occurs during the actual grasp action, which has insignificant dynamic influences, and is sufficiently validated via a generated static model.Physical testing and simulation using the prototype's static model validated the effectiveness of the developed strength space distribution actuation strategy. During the experiment the finger and thumb were positioned in orientations representing the three grasps above and promises to achieve a good approximation of the full capabilities associated with the human hand without compromising strength, dexterity, appearance, or weight which are common issues associated with prosthetic hands.