Game Development Reference
Figure 8.13 Virtuose 6D Medical. (Illustration: CEA-LIST, with permission)
This defect can be limited by getting rid of the helical linkage, which guarantees
a fixed distance between the ends of the two branches. This solution has been used
on the Twin Pantograph Haptic Interface developed towards the end of the 90's by
the University of British Columbia in Canada (Stocco & Salcudean, 1996). This inter-
face only has 5 degrees of freedom, since rotation around the axis of the pen is not
possible. It was industrialised and marketed by the Quanser company under the name
5DOFHaptic Wand. This interface only has 5 degrees of freedom with force feedback.
To obtain 6 degrees of freedom, it is sufficient to motorise the axis of rotation of the pen
itself. This is the solution used by the CEA-LIST on Virtuose 6DMedical (Figure 8.13)
(Gosselin et al., 2005b). On this interface, the pen is not directly placed between the
ends of the two substructures but is slightly off-centre. This helps to reduce the distance
between them and thus limit the couplings between the translations and rotations that
are relatively significant and can be obtained at every point of the useful work space.
This off-centering also helps to maintain the centre of the grip equidistant from the
two ends, which favours force efficiency.
In addition, additional passive articulations have been introduced to maintain
the configuration of the wrist constant, independent of the translation movements,
which helps to get rid of all the singularities of this structure that are outside its work
space. The placing of 6 of the 7 motors in the base and the use of capstan reducers
help to obtain very good force transparency on 6 degrees of freedom. The use of a
parallel mixed structure also helps to obtain a force feedback in rotation (Table 8.6)
that is higher than the one on the serial structure interfaces with equivalent dimen-
sions (PHANToM 1.5 or Freedom 6S). In fact, only one motor must be loaded on
the structure, which without excessively increasing the apparent mass allows the use
of a more powerful robot actuator. Finally, this robot is statically balanced using a
counterweight, which helps to maintain the balancing, irrespective of the orientation
of the base, which can be optimised to increase the ergonomics of the interface.