full Paper - Nguyen Dang Binh

Thirdly, although the EFF is fully anthropomorphic, it
does not exploit the possibility, discussed in Subsection 5.4,
to replicate forces also on the users arm and forearm link.
The actuation system of the EFF is dimensioned to apply
a20N force on the user palm positioned still at the center
of the workspace with the HFF (weighting around 14 kilos)
mounted on the seventh link. Such performance is realized
using small DC servo motors, designed for linear torque output,
gearboxes with reduction ratio of 66, and a cable transmission
allowing for a further reduction ratio of around 3.
The presence of gearboxes causes friction which affect negatively
joints backdrivability. The reduction of friction by
means of closed loop joint torque control has proved successful
but it has been implemented only on the wrist joints,
where torque sensors are available.
The joint position sensing obtained with optical encoders
on the motor axes gives a satisfactory resolution, also because
of the reduction rate, but it doesn’t allow to derive a
clean velocity signal by numerical differentiation. The need
of a noise free velocity signal, especially for the implementation
of interactions with virtual rigid surfaces, has experimentally
demonstrated by Prisco.
The EFF control strategy is based on driving the joints
with a 7-dimensional torque vector τcontrol which compensates
the weight of the device and balancea the wrench Freact
desired on the user’s hand. The torque vector τcontrol is computed
according to the relation:
τcontrol ˆG J T q Freact
where ˆG indicates an estimate of the gravity effects at the
joints and J q is the jacobian matrix.
6. Objectives of the Research
The scope of the present research is to proceed towards the
realization of a satisfactory implementation of the concept of
anthropomorphic haptic interface, as it has been outlined in
Section 5.1, by designing a device which exploits the advantages
of the anthropomorphic approach and at the same time
overcomes some of the limitations of the Arm Exoskeleton
described in Section 5.7.
Costs and development time have played no role in the
system specifications; reliability has been considered of secondary
importance at this stage of the project. In fact, our
effort has aimed at demonstrating the system feasibility and
its usefulness; we rely on technology advance and industrial
re-engineering to cut on costs and realization times and to
increase system reliability.
Apart from the requirements associated to the anthropomorphism
of our device, our design has been influenced by
two other functional requirements:
¯ adjustability. It is particularly important for an anthropomorphic
haptic interface to be adjustable to accommodate
c­ The Eurographics Association 2005.
Massimo Bergamasco / Haptic Interfaces
17
Figure 19: Anthropomorphic haptic device for the upper limb in
reference posture
to different sizes of the user arm (different arm and forearm
lengths and circumferences);
¯ wearability. From our experience with the Arm Exoskeleton,
easiness of wearing is a key issue for the acceptance
of a device among users which are not specifically trained.
6.1. Dynamic Requirements
The dynamic properties, which we have chosen as design requirements
for our device, are the exertable peak force (transient
and continuous value) and the force resolution (both
specified at the palm). The maximum velocity and acceleration
have been specified defining a set of reference arm
trajectories, as discussed in Section 8.1.
The backdrivability, i.e. the impedance of the controlled
device measured at the palm, and the force bandwidth have
been judged very important requirements too. It is very difficult
to translate into design indications such requirements
since they depend also on the joint torque control laws.
Therefore the open loop joint torque bandwidth has been
used as reference criteria for the dynamic properties of the
final controlled system.
7. Design Innovations
The main innovative design solutions which we have introduced
in our system are related to the design of the tendon
transmission and to the mechanical solution which permits
lateral wearing of the device.