Game Development Reference
In-Depth Information
level of friction. These frictions can be compensated for by using a measure of the force
exerted by the user, but this solution is more complex.
Transparency on contact: In order to ensure that the interactions with the remote
or virtual world are credible, the force feedback needs to be as real as possible. This
requires the force that the interface can generate to be adequate so that the operator
clearly feels the presence of the objects. In addition, the bandwidth needs to be sufficient
to enable clear transitions between free space and contact as well as to ensure a realistic
rendering of the surface of the objects. Finally, there should be adequate stiffness so
that the user can distinguish the flexible objects from the rigid surfaces.
The performance criteria of the haptic interfaces are thus limited in number. It
refers to work space, capacity and force sensitivity, stiffness, inertia and bandwidth
(which is generally improperly considered to be roughly equal to the resonant frequency
of the interface considered as a simple spring-mass-system), to which is to be added
the position resolution that provides smooth control of the movements of the objects,
whether in free space or on contact.
8.3.1.2 Necessity of specif ications
Most of the existing haptic interfaces have been designed while trying to satisfy the pre-
viously stated criteria in the best possible way. However, these criteria conflict, as for
example, to obtain a great amount of force it is necessary to use powerful robot actu-
ators that introduce a significant level of inertia. Similarly, high mechanical stiffness
is obtained by using voluminous segments whose mass is equally significant. There is
thus no universal interface which makes it possible to optimise all these criteria simul-
taneously, and it is necessary to compromise. For this, the respective weight of various
criteria must be adjusted depending on the targeted application. Other types of criteria
can also be used such as the ease of integration or use, overall dimensions, safety or even
the cost of the interfaces. In the following part of this chapter, we will only consider
the performance criteria of interfaces, as the other criteria are much more subjective.
8.3.1.3 Posture and type of grip
For a haptic interface to be efficient, it is necessary that it be made as appropriate as
possible for the abilities of the user. Now, these abilities strongly depend on the strategy
adopted to carry out a given task, in particular, the posture that he adopts and the type
of grip that he uses.
Posture : A user will adopt a posture that makes it possible for him to work as
comfortably as possible according to the task to be accomplished. He will stand if he
has to move objects over a long distance, so that he can move. He will sit if he needs to
handle these objects in a smaller volume, which would be more comfortable. Finally,
he will lean on his desk (at the wrist or elbow level) if he needs to move objects in a
very precise manner.
Type of grip : The hand is an unbelievably complex and versatile organ that makes
it possible for a human being to use numerous and varied types of grips. On the whole,
we distinguish between two types of grips (Burdea & Zhuang, 1991; Jones, 1997):
Power grips: These grips are used when a considerable amount of force is to be
applied or for considerations of stability and safety. The object is grasped between
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