Page 421 - Sensing, Intelligence, Motion : How Robots and Humans Move in an Unstructured World

396 SENSITIVE SKIN—DESIGNING AN ALL-SENSITIVE ROBOT ARM MANIPULATOR

P 1
B C
P 3
A l 2
P 2

O O′
1 l

Figure 8.3 A higher density of sensors on the robot arm body translates into the better
dexterity of the arm’s motion.

roughly 10 sensors—perhaps a little more due to specifics of the robot shape.
This is a great savings in cost and design simplicity compared to the infrared sen-
sitive skin—but the resolution of this skin is much poorer, only 20 cm compared
with the 2 cm resolution in the infrared skin. 2
Consider the planar two-link arm manipulator shown in Figure 8.3. The body
3
of this arm is covered with proximity sensors. In its current position A (shown
by solid lines), link l 2 is positioned near the obstacle O (a solid black circle).
Assume that, following its motion planning algorithm, the arm intends to slide
along the obstacle, with link l 1 rotating clockwise as indicated by the arrow. If
an arm’s sensor tells it (correctly) that the point on its body closest to obstacle
O is P 1 , then the next position of link l 2 would be position B, with the link l 1
position calculated accordingly (dotted lines in Figure 8.3). If, however, because
of its sensors’ limited resolution the arm concludes that the point of its body
closest to obstacle O is somewhere at point P 3 , safety considerations will make
the arm move its link l 2 in the position C instead of B, which is equivalent to
its reacting to a much bigger obstacle O . Increasing the number of sensors and

placing them, for example, in each of the points P 1 , P 2 ,and P 3 would improve
the skin resolution and be effectively equivalent to making the arm workspace
less crowded.

Tactile or Proximal Sensing? When moving around, humans and legged ani-
mals prefer to use proximal sensing, usually vision. (More rarely we also use
auditory or olfactory (smell) sensing information to guide our motion planning.)

2 In theory, the resolution can be improved with extra processing of signals obtained from a few neigh-
boring sensors. This would likely require additional assumptions about the shapes and orientations
of obstacles, along with a more complex data processing scheme.
3 In Figure 8.3 sensors are distributed along the robot’s one-dimensional contour; in a real three-
dimensional arm, sensors are of course distributed along the robot body’s two-dimensional surfaces.