Page 426 - Sensing, Intelligence, Motion : How Robots and Humans Move in an Unstructured World

SALIENT CHARACTERISTICS OF A SENSITIVE SKIN 401

capacitance, and ultrasound sensors are but a few examples—can be used for a
skin-like full coverage for robots.

Sensors Physical Shape, Dimensions, and other Physical Properties. The
diameter of a link of a typical robot manipulator ranges from a few centimeters to
20–25 cm. The link diameter of the NASA Shuttle Remote Manipulator System
(SRMS), likely the biggest robot arm built so far, is about 40 cm. Some arm
links are short, and some are very long. Proximity sensors that we choose to
cover the arm should satisfy some reasonable physical properties:

1. The sensing skin should not add significantly to the robot link diameter.
What is or is not “significant”? A skin that is 1–2 mm in thickness will likely be
acceptable for most arm manipulators. Many existing sensors and other necessary
electronic components fall into this range. Today’s surface mounting technology
allows one to put those components on the skin board with only a tiny addition
to the skin thickness. Future large-area electronics technology will allow printing
skin sheets in a manner we produce today newspaper or wallpaper sheets.
2. If the skin base is to be a continuous medium—which is highly desirable for
a high-density skin—it should be designed on a flexible carrier, so that the skin
can be wrapped around robot surfaces of various shapes. To make it scalable and
easier to install, the skin can be designed on separate more or less self-contained
circuit board modules. Each module can include, for example, n-by-n sensors
plus the related control electronics. The skin could then be extended functionally
and spatially by tiling the modules to cover large surfaces.
3. Look at your own arm. When you bend it, the skin on the elbow stretches.
When you stretch the arm, the elbow skin shrinks and forms wrinkles. Having
the stretchability property is as important for the robot sensitive skin as it is for
the human skin. In a skin built on unstretchable plastic material, every time a
robot joint makes the adjoining links bend (similar to the human elbow), a gap
will appear between the parts of the skin belonging to both links. The exposed
part of the robot body will then lose its sensing ability and become vulnerable
to the dangers of the surrounding unstructured world.
Note that having a stretchable sensing module implies stretchable wires in it,
which is quite a difficult technical problem in itself. No materials fitting the needs
of a stretchable sensitive skin exist today. The sensitive skin sample described later
in this chapter does not have the stretchability property: Less “natural” means, such
as parts of the skin that slide over each other as the robot links move, are used to
compensate for the unstretchable skin material. A new and very interesting area of
research in stretchable materials for sensitive skins belongs mostly to the disciplines
of material science and chemical engineering (see, e.g., Ref. 137).
4. Attaching a flexible skin board to some surface may require cutting off
pieces of the board. For example, if a part of the robot surface happen to be
of spherical shape, a planar skin board cannot be attached to it without cutting
off some portions. The board design should allow such cutting, at least to some
limited degree. One problem with this is that while sensors cover the whole