Chapter 2. Prehension

238 THE PHASES OF PREHENSION
Figure 6.12 Two soft fingers in contact with object. The wrench
system from finger 1 has four wrenches (wl,w3,w4,w8) consisting
of three forces and one moment relative to the contact normal.
The wrench system for finger 2 (w2,w5,w6,w7) is similar. Wrench
wl and w2 are normal to the surface and are maintained positive in
order to assure contact with the object and to allow friction to be
active. The object has a coordinate frame centered at its center of
mass (from Salisbury, 1985; adapted by permission).
can resist moments around y. Finally, wrenches w7 and Wg can resist
moments around x. A wrench matrix W can be formed such that W =
[wl, w2, w3, w4, w5, w6, w7, WS], Or
1
1-1 0 0 0 0 0 0
1
0 0 1 0 1 0 0 0
0 0 0 1 0 1 0 0
W=/
(3)
0 0 0 0 0 01-1
L 1
0 0 0 1 0 - 1 0 0
0 0-101 0 0 0
When two wrench systems act on an object, some wrenches may
intersect, and the effect of a wrench in one system can be