Mechanisms for linkage-driven underactuated hand exoskeletons: conceptual design including anatomical and mechanical specifications

The design of a successful exoskeleton device starts with the definition of a set of requirements according to the intended task and with the selection of the appropriate kinematic structure. Despite this, however, most authors barely report details relative to these activities and the decisions behind them. This article explores the conceptual design for linkage-driven underactuated hand exoskeletons to provide clear specifications for the synthesis of such mechanisms. These specifications are classified as functional requirements, structural requirements and design constraints. The first two are obtained based on anatomical and mechanical considerations (for two and three degrees of freedom). The latter are specifically related to underactuation and link redundancy. We use the: (1) functional requirements to define a basic kinematic chain containing the functioning links and their characteristics (functioning kinematic chain), and (2) structural requirements to generate admissible kinematic chains via the number synthesis of closed-loop mechanisms. Mapping the functioning kinematic chain to admissible kinematic chains leads to the compatible chains. These chains are evaluated based on the criteria from the design constraints yielding, ultimately, the feasible mechanisms. In addition to illustrating the conceptual design process in a methodical way, we provide examples and tables summarizing the specifications for the design of underactuated hand exoskeletons according to the aforementioned classification. We also provide the enumeration resulting from the number synthesis as well as the different admissible chains for the given design problem. With the above, the designer of these devices will be able to conceptualize mechanisms in a more systematic manner.