full Paper - Nguyen Dang Binh

Figure 20: Spatial placement of idlers
7.1. Tendon transmission
We have designed a tendon based driving system which permits
to place the motors away from the joints. Our goals have
been to achieve reduced link weight (especially for the distal
joints), increased joint compactness, reduced encumbrance
in the workspace of the moving parts of the device. Moreover,
such solution allows the usage of grounded actuators
with high peak torque and consequent reduction or elimination
of gearboxes (with the associated friction and torque
transmission problems).
The main drawbacks of tendon transmissions are associated
to the tendon elasticity and routing. The elasticity between
the motor and the link, which is inevitably introduced
by tendons, tends to lower the stiffness and the mechanical
bandwidth of the device. Therefore these values have been
kept under control during the design as described in Section
10.
The routing of the tendons from the motor to the joint becomes
complex in multi d.o.f. systems. If the tendons are
guided by sheaths the routing is simplified but severe problems
arise in force control due to dry friction. In our design,
the tendons are routed over idlers mounted on ball bearings.
Such a method has been used in several other robotic structures.
Usually such type of transmission is planar, that is to
say, all the idlers lay in a common plane. A variant to this
approach has been proposed in [19] and it allows for a small
skew angle between an idler and the following. In other designs,
such as the WAM from MIT and the Arm Exoskeleton
from PERCRO, the axes of two successive idlers can be
perpendicular in order to route the tendon between two orthogonal
planes.
The innovative design solution introduced inthe anthropomorpic
haptic interface is that the idlers of tendon transmissions
are arbitrarily placed in the space; just the constitutive
condition that two successive idlers share a common tangent
line, as shown in Figure 20, is respected. This is the most
general type of tendon transmission guided over pulleys and,
to our knowledge, it has never been adopted in other designs.
In this case, it has been a key technology which has allowed
to route the tendons around the human arm and forearm.
Massimo Bergamasco / Haptic Interfaces
18
Figure 21: Anthropomorphic kinematics
7.2. Lateral wearing
In the aim of achieving a comfortable wearability, ‘open’
links for the arm and forearm have been designed. This
means that the links do not wrap completely the user limb
like in an exoskeleton, but instead just adhere to the external
part of the arm. This solution allows easiness of wearing,
since the user arm enters laterally into the device (while
an arm exoskeleton has to be worn just like a sleeve) and
it adapts to a broader range of user arm circumferences.
Fast lateral wearing/unwearing gives also a greater intrinsic
safety and a broader acceptance among users. Moreover
such solution allows the user to bring his arm very close laterally
to his trunk.
The design of open links around the user’s arm and forearm,
while meeting the constraint of coincident anthropomorphic
kinematics, has been possible thanks to an on purposely
developed a partial (semicircular) rolling ball bearing.
Such a mechanical component, depicted in Figure 7.2,
has the same performance in terms of stiffness, weight and
friction of a precision ball bearing of the same diameter.
The lateral wearing is only partially allowed by the
present design, in fact only the arm link is open (while the
forearm link wraps the user forearm as can be seen in Figure
19). This is only due to time/budget constraints.
8. Design tools
The mechanical design of the anthropomorphic haptic interface
for the human arm has been possible only thanks to a
set of software tools, which have given to the mechanical
designers a better control of their work.
First of all, a key element has been the adoption of a fully
associative, 3D, parametric CAD environment, integrated
with a structural analysis tool. This has allowed to draw and
c­ The Eurographics Association 2005.