full Paper - Nguyen Dang Binh

Figure 4: The Dual point HI: GRAB device
to be actuated (by three DC motors and tendon transmissions)
in order to exert forces of arbitrary orientation at the
fingertip, while the last DOF’s can be passive because it is
not required to exert moments. This solution allows a very
high degree of isotropy of the device w.r.t. other kinematics
solutions, like that used for the PHANToM. A high degree of
isotropy is important in order to achieve a uniform use of the
actuators in the workspace of the device and to have a uniform
reflected inertia. The realized system has the following
characteristics:
¯ A wide dual point Workspace (600x400x400mm);
¯ Large Peak and continuous force: 10-15N peak, 4-6N continuous;
¯ High stiffness: 3-7N/mm;
¯ Low Mechanical friction: 20mN with active friction compensation
and 200mN without compensation;
¯ Zero backlash kinematic;
¯ Reflected inertia: 0.4kg.
3. Wearable devices
Wearable haptic interfaces have a kinematics similar to the
human body. The design of these components is very complex
due to both kinematics constraints as well as size and
weight limitations. Two types of wearable devices will be
presented here, one for the arn, the latter for the hand.
3.1. L-Exos
The L-Exos (Light EXOSkeleton) is an innovative Haptic
interface wearable on the arm that allows the user to experience
a direct interaction with a Virtual Reality System. L-
Exos has a versatile design with two interchangeable configurations.
In the first one it can exert a force on the hand’s
palm through a handle; in the second one it can be coupled
with another device (Hand Exoskeleton) that is able to exert
forces independently on two fingertips (typically thumb’s
and index finger’s).
Massimo Bergamasco / Haptic Interfaces, Design considerations
27
Figure 5: The L-exos first configuration
L-Exos is a serial robotic structure, with 4 primary sensorized
and actuated dofs to which may be added respectively
1 sensorized dof in one configuration (figure 5), or
3 sensorized and actuated dofs and 3 idle dofs in the coupled
configuration (figure 6). All the primary DOFs are rotational
joints connecting in series 4 movable links to a fixed
one (Link 0), which can be rigidly connected to a fixed or
movable support. Its particular open structure (thanks to an
open circular bearing on third joint) ensures the safety of the
user that, in any emergency case, can immediately undoff
the system. The first three rotational joints emulate the kinematics
of a spherical joint (human shoulder joint) while the
fourth joint of the robotic device coincides with the human
elbow joint. From a structural point of view a high stiffness
with reduced mass (i.e. weight and inertia) is obtained by using
composite materials (carbon fibers) in combination with
a hollow shell link’s morphology, containing within themselves
the power transmission system. This one is constituted
by a series of steel tendons, two for each joint, transmitting
the bi-directional torque to the joint from the remote
located actuators. All the actuators are in fact situated in the
fixed link (link 0) in order to reduce again the movable mass.
Each transmission carries out an overall gear ration between
the torque output of the motor an the torque transmitted
through the actuated joint, by means both of motor/driven
pulleys radii ratio and of joint-integrated epicycloidal gearheads.
Mechanical custom subsystem, like the open- structure
circular guide [15], the joint-integrated reduction-gear
[16] and the motor-group, have been designed and developed
by PERCRO. In the configuration with the handle the
L-Exos can exert a continuous force of 50 N and a peak force
of almost 100 N to the hand palm in every direction of the
space. The L-Exos has a mass of 11 kg, of which approximately
6 kg distributed on the link 0, i.e. the fixed part, and
mostly due to the mass of the 4 motor-groups. This means
that in this configuration the robotic interface can achieve
the desirable very low value of weight /payload ratio of al-
c­ The Eurographics Association 2005.