The extrinsic digit muscles naturally couple wrist action and grip force
in prehensile tasks. We explored the effects of wrist position on the
steady-state grip force and grip-force change during imposed changes in the grip
aperture (apparent stiffness). Subjects held an instrumented handle steady using
a prismatic five-digit grip. The grip aperture was changed slowly, while the
subjects were instructed not to react voluntarily to these changes. An increase
in the aperture resulted in an increase in grip force and its contraction
resulted in a proportional drop in grip force. The apparent stiffness values
(between 4 and 6 N/cm) were consistent across a wide range of wrist positions.
These values were larger when the subjects performed the task with eyes open as
compared to eyes-closed trials. They were also larger for trials that started
from a larger initial aperture. After a sequence of aperture increase and
decrease to the initial width, grip force dropped by about 25% without
the subjects being aware of this. We interpret the findings within the referent
configuration hypothesis of grip force production. The results support the idea
of back-coupling between the referent and actual digit coordinates. According to
this idea, the central nervous system defines referent coordinates for the digit
tips, and the difference between the referent and actual coordinates leads to
force production. If actual coordinates are not allowed to move to referent
ones, referent coordinates show a relatively slow drift towards the actual
ones.
To this day, despite the increasing motor capability of robotic devices, elaborating efficient control strategies is still a key challenge in the field of humanoid robotic arms. In particular, providing a human “pilot” with efficient ways to drive such a robotic arm requires thorough testing prior to integration into a finished system. Additionally, when it is needed to preserve anatomical consistency between pilot and robot, such testing requires to employ devices showing human-like features. To fulfill this need for a biomimetic test platform, we present Reachy, a human-like life-scale robotic arm with seven joints from shoulder to wrist. Although Reachy does not include a poly-articulated hand and is therefore more suitable for studying reaching than manipulation, a robotic hand prototype from available third-party projects could be integrated to it. Its 3D-printed structure and off-the-shelf actuators make it inexpensive relatively to the price of an industrial-grade robot. Using an open-source architecture, its design makes it broadly connectable and customizable, so it can be integrated into many applications. To illustrate how Reachy can connect to external devices, this paper presents several proofs of concept where it is operated with various control strategies, such as tele-operation or gaze-driven control. In this way, Reachy can help researchers to explore, develop and test innovative control strategies and interfaces on a human-like robot.
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