full Paper - Nguyen Dang Binh

5.5. Human-like Workspace
In order to preserve the user arm dexterity, the workspace
available when wearing the device must match that of the
free human arm. The available workspace is the intersection
between the human arm workspace and the device
workspace. An anthropomorphic interface is in the best
conditions to maximize such intersection because the two
workspaces tend to coincide if the device’s joints ranges of
movement are matched to the human joints ranges of movement.
5.6. Mechanical Design Challenges
The mechanical design of an anthropomorphic interface
presents several challenges.
First of all, the kinematics of an anthropomorphic interface
is determined by anthropometric considerations. From
statistical data expressing the lengths of human upper limb
as percentiles, the links lengths have been computed to ensure,
on the 95 percentage of population, a maximum telerated
joint axes misalignment. Kinematic parameters are not
degrees of freedom in the design but rather constraints. As a
consequence, the stiffness and the inertial properties of the
interface cannot be improved by optimizing kinematics parameters.
Secondly, an anthropomorphic device is like a robot
which the human operator wears on his body (on his arm in
this specific case) and therefore it must be tailored on the operator
body. The elementary requirement of avoiding interferences
between the device and the human arm and trunk in
every configuration gives rise to big difficulties about "where
to place the links" with respect to the human body. The link
design is greatly affected by the problem of wearability too.
The links should be slender and close to the human limb
so that both the operator and the space around him is as
unencumbered as possible. For the same reason, the links
should have their surfaces free of protrusions, which could
hurt the operator coming in contact with his arm or trunk
during movements.
Finally, aligning the joint axes of the device with the approximated
anthropomorphic kinematics is particularly difficult
from the mechanical design point of view. In fact, it
is necessary to design joints whose axes intersect inside the
human arm at the level of the shoulder, elbow and wrist. The
case of the arm and forearm rotation joints is even more critical,
because they must have their rotation axes completely
inside the human arm and forearm (see Figure 5.1).
5.7. Precedent Work
At PERCRO, Scuola Superiore S. Anna of Pisa, Italy, a
Force Display Device, consisting of two exoskeleton systems
devoted to replicate forces at the level of the palm and
of the operator’s fingers, has been designed and realized. In
particular the two systems are:
Massimo Bergamasco / Haptic Interfaces
16
Figure 18: Arm and Hand Exoskeletons
¯ an Arm Exoskeleton, or External Force Feedback (EFF)
system, with 7 d.o.f. which are coincident with the principal
joint axes of the human arm. The EFF is attached to
the user arm at the level of the palm (see Figure 5.7);
¯ an Hand Exoskeleton of Hand Force Feedback (HFF)
system, consisting of 4 independent finger exoskeletons
wrapping up four fingers of the human hand (little finger
excluded) and each one possessing 3 actuated d.o.f. in correspondence
of the finger flexion axes, while fingers abduction
movements are just sensed. The HFF is connected
to a base plate located on the user’s metacarpus and corresponding
to the end -point of the Arm Exoskeleton (see
Figure 5.7).
The EFF and the HFF are both anthropomorphic haptic interfaces
in the sense specified in the precedent section. The
present work has its origins in the analysis and the experimental
work done on the EFF. Some limitations of the Arm
Exoskeleton mechanical design have been highlighed.
First of all, the workspace available to the operator is limited
by the reduced range of motion available at the shoulder
joints: the maximum shoulder adduction and extension are
respectively 73 Æ and 49 Æ . These limitations don’t allow to
take full advantage of its anthropomorphic design.
Secondly, the exoskeleton structure, wrapping completely
the human arm, forces the user to wear the system inserting
his arm through the shoulder hollow. This has proved
to be cumbersome to the point that unexperienced or impaired
users may require external help. Moreover a exoskeleton
around the arm, even if compact, is anyway an hindrance
to bringing the arm laterally close to the trunk, a position
which we have noticed to be the most natural for resting.
c­ The Eurographics Association 2005.