full Paper - Nguyen Dang Binh

G. Cini, A. Frisoli, S. Marcheschi, F. Salsedo, M. BergamascoPERCRO Scuola Superiore S. Anna, Italy / Future directions
3. Design guidelines
In order to allow the user to support the weight of the device
on its finger and to avoid interference of the device with the
rest of hand, reduction of bulk and weight was addressed in
the design of the device.
4. Kinematic analysis
4.1. Kinematics
The kinematic structure should be able to position the final
plate in a predefined workspace around the finger, such that
in any position, the final plate could be oriented with respect
to the finger in a given range of angles. These requirements
can be satisfied if the structure has at least 5 DOF. Three
possible kinematic solutions have been considered:
Fully serial mechanism In this case a suitable kinematic
chain can be based on an anthropomorphic manipulator
for the positioning task, supporting a spherical joint. It
should be mentioned that a fully serial solution is suitable
for obtaining large translations and rotation of the
end-effector, while in our application only large rotations
are requested. Major drawbacks of this solution are due to
the transmission system. In fact, a cable transmission can
reach the actuated joint n in two possible ways:
• through all the n−1 joints placed before joint n. This is
possible if an idle pulley is placed at each joint preceding
the n one; requirements of simplicity and reduced
encumbrance lead to reject this solution;
• by sheathed cables directly connected to the actuated
joint. In this configuration, the load generated by the
sheath bending would act directly on the moving parts
of the mechanism, thus introducing a force disturbance;
moreover, the relative orientation of the endsections
of the sheathed cables would depend on the
mechanism configuration, and consequently the resulting
friction between sheath and cable too.
Fully parallel mechanism Parallel kinematic mechanisms
are stiffer than serial ones and also allow to easily
locate motors in a remote position, i.e. at the base frame.
However a 5-dof fully parallel solution would involve
high complexity in the design. Moreover in fully parallel
mechanisms the range of reachable orientation of the
platform is usually limited by the kinematic constraints.
Hybrid mechanism Hybrid solutions are generally composed
by two or more subsystems, with serial or parallel
kinematics, that are put in succession. In our application,
we considered to use two subsystems, one to generate the
translation displacements and the other one the orientation
rotations of the end-effector.
4.2. Translational stage
In [AF00a] and [A.F00b] all possible parallel kinematics
mechanisms that allow only translational movements of a
c○ The Eurographics Association 2005.
47
mobile platform have been found. From these ones, the best
kinematics fitting with the requirements of our problem, was
found to be the one shown in Figure 3. It is composed of
three legs with two universal joints and one rotational pair
on the elbow joint (equivalent to a prismatic pair, see Figure
4), that is supposed to be actuated for each leg. The axes of
the two universal joints are parallel to each other, as shown
in Figure 3. To achieve the best isotropic kinematic performance
the legs were symmetrically placed around a central
axis at 120 degrees.
Figure 3: The kinematics of the translational stage
Figure 4: The Leg Actuation Axis for each leg
When the rotational pair on the elbow joint is actuated, an
actuation force F is generated directed through the centers
of the two universal joints (Leg Actuation Axis, see Figure
4). The constraint moment M generated by each leg is perpendicular
to the plane of two rotational pairs composing
the universal joints. It is possible to align the Leg Actuation
Axis with the force applied by each motor to the leg through
the transmission system, in order to increase the mechanical
stiffness of the device and make the force transmission ratio
independent of the configuration. This has been accomplished
by implementing the actuation system for each leg
as shown in Figure 5. A cable connected to the motor and a
compression spring are mounted aligned with the Leg Actuation
Axis: clearly since the tension cable should be always
positive, the compression spring works in a opposition with
the motor, so that a minimum pre-load is always guaranteed
on the cable. The constraint moment M for each leg (balancing
the external moment applied on the upper platform) is
transmitted to the base only through the links 1 and 2, while