Page 422 - Sensing, Intelligence, Motion : How Robots and Humans Move in an Unstructured World

SALIENT CHARACTERISTICS OF A SENSITIVE SKIN 397

Imagine you walk into a room. It is an evening. Suddenly the lights go out. It is
pitch dark. You stop for a moment, reconsider your plans, and then resume your
motion—except now your movement pattern changes dramatically: You move
much slower, keeping your knees and the whole body slightly bent; perhaps
you outstretch your hands forward and slightly to the sides. Your motion is now
guided by your tactile sensors. Your slow speed is a clear demonstration that the
efficiency of our tactile sensors is not as good as of our vision.
But our tactile sensors play a much more important role in our lives than
this example suggests. We use them almost continuously, sometimes in paral-
lel with our vision and sometimes as a sole source of information. We touch
objects; we keep turning in the chair depending on what the tactile sensors in
the back tell us; we measure the comfort of our shoes by what our foot tactile
sensors feel; we take pleasure at stroking a child’s head or a fur coat. Our tac-
tile sensing is an important component of that pleasure. We often use our tactile
sensors in situations where vision would be of little help, as in the example
with shoes.
In fact, while we all know that people can live without vision—we see blind
people having productive lives—science tells us that humans cannot live without
at least some capacity for tactile sensing. A person with no tactile sensing cannot
even stand: Tactile sensing is actively used in maintaining the standing balance.
Diabetes patients—who often lose partially their tactile sensing—are warned by
their doctors that they should be extremely careful in their interaction with the
environment.
Turning again to the moving-in-the-dark example, the reason you moved so
slowly in the dark has to do with an important side effect of tactile sensing: You
cannot know of an impending collision up until the moment the collision takes
place. Once your hand bumps into some object on your way, you will stop, think
it over, and modify the direction of your movement as you see fit.
But your body has a mass—you cannot stop instantaneously. Regardless of
how slowly you are moving at the moment of collision, the “stop” will still cause
a sharp deceleration of your body and forces at the point of collision. For a tiny
fraction of time, your body will continue moving in the direction of your prior
motion. This residual motion will be absorbed by the soft tissue of your hand,
and so the collision will cause no serious harm to your body. In fact, the speed
you have chosen for moving in the dark was “calculated” by an algorithm that
has been refined by many bumps and pain in your childhood: Experience teaches
us how slowly we should move under the guidance of tactile sensors in order to
prevent a serious harm from possible collision.
Today’s robot arm manipulators have no similar soft tissue to absorb forces.
Their bodies are made of steel or aluminum, sometimes of hard plastic. If the
robot body were to move under the guidance of tactile sensors, bumping into
a suddenly discovered obstacle would spell a disaster. Once the arm collided
with an obstacle, it would be too late to carry out an avoiding maneuver: Infi-
nite accelerations would develop, and an accident would ensue. A theoretical