full Paper - Nguyen Dang Binh

Figure 27: Differential tendon tension sensor
placed near the joint (see Figure 26). In such cases, the fact
that the tendons driving a joint are routed over the preceeding
joints, causes a coupling in the relationship between motor
variables (angular displacements and torques) and joint
variables. Multi strands, steel cables, with a diameter of 2
mm, are used as tendons; the tendons are routed over pulleys
mounted on ball bearings. Pretensioning is obtained with a
regulation mechanism located at the driven pulleys.
9.4. Sensors
Resolvers, mounted on the motor shafts, are used in order
to have high resolution in the position measurement and in
order to have a direct velocity measurement available to implement
damping control.
On purposely developed, differential tendon tension sensors
are employed, in order to measure joint torques (see Figure
27). Such sensors use strain gauge full bridges as sensing
elements. They are equipped with local electronics for signal
conditioning in order to improve signal to noise ratio.
The performance of the sensor module is summarized in Table
4. The sensing resolution is adapted to the requirement
of each joint, by varying the incidence angle of the cables on
the sensor frame.
10. Design analysis
An analysis of the global results of the design has been performed
iteratively during the design process, in order to evaluate
the impact of some solutions on the most critical global
performance indexes. Three sets of global properties of the
haptic interface have been kept under control:
¯ the masses and inertia tensors of the links;
c­ The Eurographics Association 2005.
Massimo Bergamasco / Haptic Interfaces
21
¯ the worst-case end point stiffness of the device;
¯ the worst-case mechanical bandwidths of the joints.
10.1. Link inertial properties
The estimation of the mass and inertia tensors of the links
has been obtained directly by the 3D CAD design environment.
In this way, every single mechanical part, up to the
screws, has been taken into consideration; we have also been
able to evaluate the gain in mass which is realizable by using
special materials, such as composite material or carbon fiber,
instead of aluminium, for the structural parts.
The results of the analysis have shown that the greatest
impact on link masses is due to the actuator modules, each
integrating an unhoused pancake rotor/stator, a resolver and
a mechanical frame. In fact, from Table 5, it is noticeable the
high masses of links 0 and 4, which house respectively four
and three actuator modules. A second significant contribution
to the link masses is represented by the steel parts, such
as torque sensor frames, ball bearing, idlers and pulleys.
10.2. Stiffness
The estimation of the worst-case end point stiffness of the
device has required a special care, because of the complexity
of the mechanism. From a methodological point of view, the
correct computation of the end-point stiffness has required to
take into account the contribution of both the link structural
parts and the tendons. The worst case has been identified as
occurring when the human arm is completely stretched in
the front (the joint angles values in such case are reported
in Table 6). In Figure 28 and 29, the diagrams representing
the displacement magnitudes, under a general load condition,
are reported. The elastic constant along z, which is by
far the worst direction, is 18Nmm.
References
[1]. Lederman S.J., Klatzky R.L., “Haptic Exploration and
Object Representation,” in M.A. Goodale (Ed.), Vision and
Transversal force Max 200 N
Min -200 N
Transversal torque Max 2 Nm
Min -2 Nm
Overload factor 1.5
Force resolution 0.1 N
Force accuracy 2.0 N
Table 4: Performance of the torque sensor module