Page 417 - Sensing, Intelligence, Motion : How Robots and Humans Move in an Unstructured World

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

collision. How close is close enough depends on a number of variables, such as
the robot mass, velocity, and the agility of the robot sensors. The term “contact”
will thus cover a range of distances from a physical contact (a zero distance) to
distant sensing, and it can therefore include tactile as well as various proximal
sensors—infrared, ultrasound, capacitance, vision, and so on. Infrared sensing
has been used in the system described in Sections 8.3 and 8.4.
The full coverage requirement makes one think of a sensing hardware that
wouldbeakintoa sensitive skin, enveloping the whole robot body the way a
human skin envelops our bodies. Technically, this may be a real skin or a “virtual
skin”—that is, a set of sensors that together provide information about the space
around the robot’s whole body.
Besides the full coverage and locality identification properties of robot sensing,
the designer of a sensitive skin system has to worry about its other characteristics,
such as these:

• Reliability
• Accuracy
• Resolution
• Tactile or proximal sensing?
• Ability to measure distances
• Physical principle of action: force, vision, infrared, capacitance, ultrasound,
etc
• Sensors’ physical shape and dimensions
• Control electronics

In the following sections we will consider details and implications of these char-
acteristics for the robot-sensitive skin system, followed by a brief description of
one such system developed and installed on a large arm manipulator, along with
examples of its operation.

8.2 SALIENT CHARACTERISTICS OF A SENSITIVE SKIN

Reliability. Robot sensors must be reliable: We do not want our robot to bump
into an object that its sensors “did not notice.” To provide the full coverage,
many sensors will likely be needed, thousands or even millions of them. (Later
we will address the question, Why is having many sensors on the skin better than
having fewer sensors, even if fewer sensors could do the coverage?)
Each sensor is a single device. The common wisdom says that the more
devices, the bigger the chance that some of them will misbehave or die. Notice
that the latter does not necessarily mean worse reliability. If sensors on the skin
do their work in parallel, and if more than one sensor can functionally cover
every point of the robot body (thus providing a system redundancy), then more
misbehaving elements does not necessarily mean a less reliable system.