full Paper - Nguyen Dang Binh

Figure 2: Double haptic contact concept
Haptic has proven to be useful during the interaction of
users within virtual environment. Within these environment
the use of the haptic sense can be employed for enhancing
the sense of presence and realism, improve object manipulation,
create physical effects, teach handling procedure and
much more. The most frequent integration of the haptic feedback
is usually done with visual or other haptic feedback. In
the £rst case the visual feedback completes the force information
given by the device for a more integrated environment;
in the second one the supplementary force information
is used to have more dexterous environments.
Some researchers have tested the potentialities of multihaptic
interfaces when addresses in a sightless environment
(Fig. 2).
Jansson [Jan01,Jan98] and Klatzky [Kl99,Kl90] conducted
some experiments to evaluate the perceptive aspects
and the performances when varying the number of contact
points and other factors. In [BLSH95] Hirzinger experimented
a novel software architecture for improving the sense
of presence when teleoperation systems where addressed. A
local VE created on the copy of the remote environment, will
provide to the user the required collocation and coherence
(especially in terms of time delays).
In the present paper a novel algorithm to achieve the mixing
of multiple HI in the same VE is considered. The algorithm
has been implemented on a novel haptic device and the
resulting performances compared to those of existing solutions.
3. Coherence and collocation issues in multiple haptic
systems
Even if the principles of collocation and coherence seem to
be radicated in the way the interaction is driven, they are not
Massimo Bergamasco / Haptic Rendering: Control Strategy
60
limited to it. The design of the interacting device strongly
affects the resulting quality. Even when the interface seems
to be well suited for a collocated interaction, sometimes the
complexity of the mechanical design, such as in the case of
the case of the CyberGrasp [TGTC98], makes the quality of
the force feedback very poor and unrealistic the exploration
of features. Conversely, when the devices are not thought
for being integrated, as in the case of using 2 independent
PHANToM devices [HDD99], some drawbacks are present:
the available workspace is extremely limited and the resulting
mobility is furthermore reduced by device interference
problems. Even considering a double “PHANToMed” system,
even when subjected to a proper calibration, as did by
Heyde [PHB99], the results in terms of position accuracy are
very poor. Device internal kinematic provides error as larger
as 0.033 mm/mm. Which means that with these system we
have a 1 mm error each 30 mm of displacement.
A set of design constraints should be matched from such
devices when these issues are taken into consideration:
Workspace: the mobility of the device should be large
enough to match the mobility of the human arm, any reduction
in this design criteria will force the user to move
according to speci£c strategies and therefore diminish the
naturalism of the interaction.
Interference: when multiple haptic devices are placed in
the same environment, interference between mechanical
structures limits the mobility of the user. The interference
is mainly related to the kinematics adopted for the device.
Encumbrance of £nger attachments: the type and the encumbrance
of the £nger attachments can further reduce
the quality of the interaction. The recognition of small
details in multi £nger mode requires that the detail size
should be greater than the size of the attachment.
Force: “force workspace” of the device represents the
minmax condition of replicable force all over the dexterous
workspace. The human hand/£nger as multisensorial
receptor is a very complex structure that can perceive and
exert force over more than 3 decades (from grams to tenth
of kilos). Conversely current design limitation rarely overcomes
the 2 decades in terms of maximum force/force resolution.
Dynamics of the device, structure large enough to cover
wide workspace may have serious drawbacks on the rigidity
of the structure itself, reducing in this way the bandwidth
of the effective controllable forces. The adequate
design of large-workspace HI requires that the £rst oscillating
mode of the structure is higher enough to not impact
the interaction performances.
Stiffness of the mechanical structure: the stiffness of the
mechanical structure is twice important when considering
multi-HI environments. Beside the classical stability
issues [CH88,Col93], the de¤ection of the interface may
affect the relative position accuracy.
Isotropy: non isotropic devices may generate spurious
c○ The Eurographics Association 2005.