Page 428 - Sensing, Intelligence, Motion : How Robots and Humans Move in an Unstructured World

SKIN DESIGN 403

sensors produce analog electric signals. Before those signals are passed to the
robot computer and used by motion planning algorithms, they have to be cleaned
of noise, perhaps brought to some standard form, and turned into the digital
form using an analog-to-digital transformation. This is done by the skin control
electronics. Ideally, this could be done by an appropriate tiny control unit built
into each sensor.
Today an electronic control unit will likely handle a group of sensors, say an
n-by-n sensor subarray, thereby allowing an easy scaling up of the skin device.
The unit also takes care of polling the whole subarray, identifying sensors that
sense something in front of them, collecting information about their physical
coordinates on the robot body, and passing this information to the robot “brain”
for making decisions on collision-free motion. How often the polling is done
depends on the robot joint motors sampling rate: 20 to 50 times per second are
typical polling frequencies for large arm manipulators. Larger groups of sensors
and control components are united under the control of local computer micro-
processors, forming a hierarchical control system. Such architecture frees the
“brain” computer for more intelligent work, and it allows scaling up the system
to practically any number of sensors on the skin.
We now turn to an example of implementation of the sensitive skin concept.
Space shortage will not allow us to cover all the questions that an electronics
professional may have. Appropriate references will be given. The intent here is
to give an idea of how the sensing skin hardware can be approached.

8.3 SKIN DESIGN

The large-area skin versions built so far are all based on optical (infrared, IR)
sensors; other sensors are still waiting for their implementation in a sensitive
skin. The main reason for choosing infrared sensors is the best resolution one
can get with them compared to other sensors. This advantage may overweigh the
drawbacks of IR sensors, such as their mechanical brittleness or their inability
to measure distances at a short range. Other than this similarity, the projects
carried out so far have differed in the specifications of sensors and other elec-
tronic components, in overall physical and electrical architecture of skin sections,
implementation of the control scheme and robot intelligence, the mechanical
installation of components on the skin (such as direct soldering or surface mount-
ing), and so on. (For details, see references in Section 8.1 and citations therein.)
As mentioned above, an infrared sensor is an active sensing device. Each sen-
sor presents a pair consisting of a light-emitting diode (LED) and a light detector.
When initiated, the LED sends in space in front of it a beam of directed infrared
light. The associated light detector detects the reflected light. If a noticeable
amount of reflected light has been detected, the system assumes it was reflected
5
from an object located in front of the sensor. The LED light beam is of a conical

5 In principle, a signal detected by the detector in the sensor pair X can be the light sent by an LED
of some other sensor pair Y and reflected “in a wrong direction” by an object positioned in front