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Figure 16.7 Special effects for point-based interactions
which allows a progressive evolution of the haptic rendering (Kim et al., 2004). Certain
purely discrete models (voxels) have also been proposed (McNeely et al., 1999).
Besides, in the rendering calculations that we discussed, we focused only on the
“normal'' component of the force. This component prevents the penetration in the
obstacles of the environment and constitutes the main part of force rendering. A real-
istic interaction must also consider other phenomena like dry friction or roughness
of the contacted surfaces. More specifically, it involves calculating the tangent forces
as well as the states of the object's surface. In other words, we will then be trying to
calculate tactile rendering as well. As regards friction, some special models have been
proposed. They often use F Tangential =
F ( F Normal ) type ad-hoc models, where F reflects
the nature and model of the friction in question.
Restitution of the state of surface depends on the type of control circuit. In case of
impedancemode, the forces sent are adjusted by changing the normal, using a technique
similar to “bump-mapping'' used in image synthesis. In an admittance type of circuit,
the raised patterns are reproduced on the surface using height maps for instance (see
figure 16.7(b)). In this case, the models of the object should have a haptic texture
representing the state of the surface (Mark et al., 1996). Taking haptic textures into
account, however, creates several problems. Owing to the frequencies of the associated
signals, the interfaces are often specialised in force or tactile rendering, which makes
it necessary to separately process various components of the forces to be returned.
16.3 FREQUENCY ADAPTATION
We have seen at the beginning of this chapter that a stable force feedback in a rigid
interaction requires updating a set point to a frequency of approximately 1 kHz. How-
ever, it is rare that the simulation can actually effectively calculate forces or positions
at this frequency. Generally, the simulation loop has a frequency close to that of image
generation (25 to 30Hz, sometimes less). Thus there is a significant difference in fre-
quency, which explains why the simple interfacing of a haptic interface in a simulation
software package leads to frequent instabilities in the position of the device.
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