Game Development Reference
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position of the hand and the direction of force, for a power grip and 104N to 127N
for a precision grip. In rotation, the torques that a user can exert strongly depends
on the type of grip and above all on the diameter of the interface. The torques also
noticeably differ according to the axis around which they are exerted (Daams, 1994;
Snell-Massie et al., 1997). Taking into account the average values from the compilation
of the available data, for a handle grip with a diameter ranging between 30 to 40mm,
we obtain a force capacity of the order of 2.4 to 4.5N
m around the axis of the han-
dle and from 12.5 to 17.9N
m at its right angles. For a pen grip with a diameter of
approx. 20mm, we obtain torques of the order of 0.37 to 0.45N
m around the axis
of the pen and from 2.2 to 4.2N
m around the other axes.
This force can be applied for a short period. It is useful to design interfaces which
give a life-like force feedback. However, for most applications, this feel can be given
at lower values without it affecting the efficiency of the interface. Generally, the force
is also limited for safety. To study fatigue according to the level of force, we use the
concepts of endurance (period for which an operator can exert a given force) and
comfort of use (period for which an operator can exert a given force without feeling
pain or requiring to change his posture) (Daams, 1994). The existing data shows
that these concepts depend very little on a muscle or muscle group. They are mainly
determined by the level of force exerted. If we consider that the operator must be able
to apply force for a few minutes without stopping, the level of force must be 10 to
15% of the maximum force. The maximum instantaneous static force can be distinctly
higher without compromising the comfort of use. Taking into account the maximum
force that a user can exert, we obtain a useful force capacity of the order of 40N and
m for a power grip (grip using the palm of the hand) and 10N and 0.25N
m for
a precision grip (grip involving only fingers).
When the user takes the robot gripper between his fingers and also rests it on the
side of his hand, he uses a type of intermediate grip between the previous grips, and
the level of force will be selected accordingly. The figures given previously are given
as a rough guide. They correspond to a compromise between the capacities that vary
widely according to the individuals, the posture adopted while exerting force and the
direction of force.
Force resolution : The hand being a bidirectional organ, we will distinguish between
the motor resolution, which is the resolution with which a user can apply a force and
which is restricted by his trembling, and the sensory resolution, which is the minimum
perceptible force threshold, for the force as well as movements. Studies on power grips
as well as precision grips have shown that force motor resolution is proportional to
the applied force (Sutton & Sykes, 1967; Albers et al., 1973; Tan et al., 1994). It seems
identical for a precision grip (between 0.71 and 1.46%) and for a power grip (between
0.60 and 1.56%). We shall take an average value of 1%, which corresponds to a
maximum resolution of 0.4N for a power grip and 0.1N for a precision grip. Please
note that it is always possible to reduce this trembling to improve the performance of
the user. As for the sensory resolution, we shall distinguish between direct threshold
and differential threshold in force (Paines, 1987; Tan et al., 1992; Hunter et al.,
1993; Tan et al., 1994; Barfield et al., 1995; Jones, 1997). The direct threshold is
the smallest force that an operator can perceive. It is 0.06N if we consider only the
proprio-kinaesthetic sense and 0.0006N only if we take into account the tactile sense.
The differential threshold in force is the smallest difference in force with respect to a
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