Chapter 2. Prehension

Load force, N . .
Grip force, N
30
Position, mm
0
Grip force
Load force
A B
la: b c d j if; ;
. .
i f
. . : : .
. .
i
C
a
............
_..'. a
'TJh (D
T
a h
0 -1 10 11 10 12 0 2 4
Figure 6.18
Time. s
The principal phases of a lifting trial.
Load force. N
Note the time scale is interrupted. A. Load
P
force, grip force, vertical position of object and ratio between grip and load force as a function of
time for a sample trial (weight 400 g, surface structure sandpaper). The periods indicated are: a -
preload phase (increase of grip force from initial contact until object is securely grasped); b -
loading phase (parallel isometric increase of the forces); c - transitional phase (while the object is
elevated); d - static phase (the object is held stationary in space); e - replacement phase; f - delay;
g- unloading phase (parallel isometric decrease of the forces). B. Slip ratio measurements at the
end of a separate trial subsequent to the trial in A by asking the subject to hold the object in air
and decrease the grip force until slips occurred. The vertical dashed line in B indicates the start of a
slow voluntary grip force decrease. Horizontal dashed lines in A and B indicate the slip ratio
obtained, and the shaded areas indicate the safety margin used to avoid slip. C. Grip force as a
function of load force for a lift similar to A. Phases are as in A. The dotted line indicates the slip
ratio (from Johansson & Westling, 1990; reprinted by permission).
I
b
2
s
ff
-b