Game Development Reference
In-Depth Information
reference force that a person can detect. It is generally expressed in percentage of the
reference force and ranges between 5 and 10% of the reference force.
8.3.1.6 Dynamics, stiffness, inertia and bandwidth
The apparent mass of the force feedback interfaces must be minimised. Excess mass
will prevent them from being put in motion or stopped quickly. The increase of mass
leads to, for example, a deterioration of the static monitoring performance (“time
on target'') (Jones & Hunter, 1992). The perception of force is also reduced when
the apparent mass of the actuator increases (Paines, 1987). However, it is difficult to
specify the apparent mass acceptable to a user.
As regards the stiffness, it is theoretically necessary to obtain values more than
20,000N/m so that an unseeing user perceives a surface as being totally rigid (Tan
et al., 1994). However, this sensation of stiffness can be improved by combining vision
and force. As visual feedback is more dominant than haptic feedback, a user will
feel like he is touching a stiff surface if his avatar is blocked on contact even if his
hand penetrates it (slightly). Experience shows that stiffness of the order of 1500 to
3000N/m is enough to give a good sensation of stiffness to a virtual surface.
Finally, as regards bandwidth, we shall again distinguish between the motor band-
width, which is the frequency band in which the operator can generate force or position
signals, and the sensory bandwidth, which is the frequency band in which he can feel
a movement or force.
The motor bandwidth depends on the type of signal generated. A person can
follow random or periodic signals up to a frequency of 2Hz; 5Hz if the frequency
of the reference signal is constant. He can also generate signals or reproduce learnt
signals up to a frequency of 5Hz. The reflex actions show elements up to a frequency
of 25Hz if we consider the finger movements involved when one adopts a pen grip
and 20 to 30 Hz during jerky movements made with a handle grip (Paines, 1987;
Brooks, 1991; Ellis et al., 1996; Jones, 1997). As regards the abilities of the operator
to perceive his environment, we distinguish between the tactile sense and proprio-
kinaesthetic sense. The proprio-kinaesthetic bandwidth ranges between 20 and 30Hz
while the tactile bandwidth is a little more than 300Hz (an operator can feel vibrations
up to 10,000Hz but can distinguish between two signals only up to approx. 320Hz)
(Brooks, 1991; Ellis et al., 1996).
8.3.1.7 Report
For a force feedback interface to be efficient, it must be as transparent as possible in
free space and on contact. For this, it must be adapted to the abilities of the users.
Even if it is difficult to accurately quantify these abilities as they vary widely from
one person to another, it is useful to have an idea about them to design interfaces
that are suitable for the scheduled tasks. For example, tables 8.1 and 8.2 summarise
the average abilities of the users associated with two types of standard virtual reality
applications. The first is the verification of the mountability of average-sized units for
which the user can take a seated posture associated to a power grip. The second is the
fine handling of objects for which he will have to work with his elbow placed with a
precision grip.
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