Kinetostatic analysis and design optimization of an n-finger underactuated hand exoskeleton

• A kinetostatic model of an underactuated mechanism for a hand exoskeleton is shown.
• Two alternative solutions for the palm differential mechanism are analyzed.
• An optimization process based on the Global Performance Index is described.
• A case study of a 2-finger exoskeleton for Extra-Vehicular Activity is proposed.
• The solution based on the seesaw differential element is ideal for the EVA case.

The concept of underactuation, applied to wearable exoskeletons to empower human users, is very promising, as it permits the designer to limit the number of actuators and consequently the bulk and weight of the device. This paper deals with a scheme of underactuated hand exoskeleton counting generically n fingers. The proposed architecture realizes such a principle at two stages: an intra-finger level and an inter-finger one. Both of them are here analyzed from a kinetostatic point of view; in particular, two alternative transmission mechanisms, relative to the inter-finger stage, are solved, in order to set a comparison. At this point, a design optimization can be performed: in fact, given an objective force profile on the finger phalanges, a number of geometric parameters can be investigated, to let the transmitted forces match the reference ones. Each configuration is numerically evaluated for what concerns the transmitted forces through a properly defined Global Performance Index, while the investigation is conducted through a stochastic algorithm. Finally, an example is provided: a 2-finger exoskeleton is presented and optimized, conceived to help astronauts during Extra Vehicular Activity, an operation that causes hand fatigue and therefore is an ideal field of application for empowering exoskeletons.