Page 425 - Sensing, Intelligence, Motion : How Robots and Humans Move in an Unstructured World

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

power and communication lines to the end effector, or by some other objects is
hard to avoid when sensing the robot surface from a distance. Robot links often
have nonconvex concavities, indentations, holes, bolts, or other pieces sticking
out of them. Seeing behind every nook and cranny at all times is simply not
practical. Sooner or later, some object will be hidden from all those cameras.
Short of unreasonable, no number of cameras on the surrounding walls or on the
robot body will do the job.
Another consideration is that for a single-step motion planning decision (and
there will be 20 to 50 such steps per second) the robot needs information on all
nearby obstacles simultaneously. Doing vision processing simultaneously for a
significant number of cameras is too computationally expensive. We must con-
clude that, powerful as it is, vision is not a right solution for protecting the robot
body at short distances.
Nature, of course, “noticed” this fact long ago. While supplying us with the
powerful stereo vision, evolution has also supplied us with other sensors to help
protect our bodies when moving in space. It gave us, in particular, the tactile
sensing of our skin. The nature “concluded,” in other words, that vision is not a
good sensor to protect one’s own body at short distances. In combination with
vision and with the effect of soft tissue force absorption discussed above, tactile
skin provides a rather universal protective sensor.
If so, one may ask indignantly, why hasn’t the evolution been gracious enough
to supply us with something better than tactile sensors—covering our skin, for
example, with some proximal sensors? Then our life would be so much safer,
and we would be able to move so much faster in the dark than we do now with
our tactile sensing.
Unfortunately, proximal sensors that we find in nature do not fit our purpose.
A bat’s sonar is one example: Acting as a substitute for vision, at distances
much larger than the bat’s body, sonar does not protect the bat’s body at very
small distances. For this purpose, bats have sensitive skin. Cat’s whiskers are
another example: While whiskers work on a physical contact, they supply the cat
with input information far enough from its body to allow for motion planning
decisions typical of a proximity sensor performance. (And again, cats still need
their tactile sensitive skin.) We humans have proximal sensing as well: Besides
vision, we have hearing, smelling, and temperature sensing. Of these, temperature
sensing is the only type of proximal sensing that appears in one’s whole body
and hence satisfies our requirement of full coverage. It also operates at a range
of temperatures and distances: We sense a hot cup at a few centimeters’ distance,
and we can sense volcano lava from a distance of many meters. Unfortunately,
the range of temperatures in the world around us makes temperature sensing of
a limited use.
The list of sensors provided to us by technology is much bigger. Engineering
progress moves in ways very different from nature. The proverbial inability of the
evolution to invent a wheel does not stop there: Engineers have a whole panoply
of proximity sensors that are not available in nature. Many of these—infrared,