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that the user can make, limited by his trembling, and the sensory resolution which is
the smallest movement that he can perceive. Several authors have conducted studies
on the motor resolution of operators (Sälzer, 1972; Joyce & Rack, 1974; Stiles, 1976;
Hunter et al., 1993). The results show that the trembling is between a minimum value
during rest (control value) and a clearly higher value when the operator is tired (several
millimetres). The control value depends on the type of support used by the operator
and the force exerted: the trembling is lower when the operator has a support close to
the tool and as the force exerted is low. The compilation of this data helps to calculate
the control value during rest, according to the type of posture, and thus the type of
support used by the user (Gosselin, 2000). With a power grip, it is of the order of 12
m
µ
with the wrist resting, 58
m with the elbow resting and 109
m with the elbow not
µ
µ
resting, and with a precision grip it is of the order of 23
m with the wrist resting,
µ
69
m with the elbow not resting. In rotation, it is
of the order of 0.01 , irrespective of the type of grip.
As regards sensory resolution, one should distinguish between the direct displace-
ment threshold, which is the smallest movement that a person can detect, and the
differential threshold in positionwhich is the smallest difference in positionwith respect
to a reference position that the person can perceive. The study of these two param-
eters has led to numerous works (Tan et al., 1992; Tan et al., 1994; Barfield et al.,
1995; Jones, 1997). The direct displacement threshold is essentially linked to the tactile
sense, as the skin receptors are clearly more sensitive than the proprioceptive receptors
located in the muscles and tendons. It depends on the frequency and place of appli-
cation of the excitation signal (between 0.1
m with the elbow resting and 119
µ
µ
m and 11.2
m at the fingertips, 10 to
µ
µ
14
m in the palm of the hand). The differen-
tial threshold measured at very low speed involves the proprio-kinaesthetic sense, and
the distal articulations are more sensitive than the proximal ones.
It can be calculated using the results from the literature that are expressed in
degrees. It is of the order of 1 to 2mm at the level of the fingers, from 2.5 to 4.5mm
at the level of the wrist, from 11 to 13mm at the level of the elbow and from 8.6 to
9.5mm at the level of the shoulder according to the type of grip (power or precision).
m at the middle of the fingers, 15 to 24
µ
µ
8.3.1.5 Static capacity and force resolution
Force capacity : A person can apply a great amount of force for a short period but
then he feels tired quickly. This muscular fatigue is accompanied by a loss of force
sensitivity, which is dangerous and can cause errors during a series of power and
precision tasks (Wiker et al., 1990; Jones & Hunter, 1992). It is thus advisable to
restrict the range of feedback of forces for all the applications that do not require a
life-like force. In practice, it will be specified as a fraction of the maximum capacity of
the users according to the application duration of force.
As regards translations, the literature includes numerous data concerning the maxi-
mum force that an operator can exert (Daams, 1994; Jones, 1997). There is a noticeable
difference in these values depending on the authors because there is no reference defini-
tion of the maximum force, nor is there a common procedure to measure it. In addition,
it depends on the posture of the user when he exerts force and on the direction in which
it is applied. Taking into account the average values from the compilation of the avail-
able data, we obtain the maximum force of the order of 137 to 489 N, according to the
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