Page 429 - Sensing, Intelligence, Motion : How Robots and Humans Move in an Unstructured World

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

shape, formed by a tiny lens on the top of the LED (Figure 8.2a). The beam cones
of neighboring LEDs must overlap, forming a continuous detection cushion in
the space around the robot.
To increase the skin reliability, it is desirable to decrease the amount of wiring
running within sensor modules, between modules, and especially between mod-
ules attached to different robot links (because these wires will have to run over
robot joints). This requirement is in conflict with a desire to control every sen-
sor independently. The latter requires parallel addressing of sensors, hence many
wires, whereas a serial addressing scheme allows one to minimize the number
of interconnecting wires. Another advantage of a parallel scheme is that sens-
ing information it produces in each cycle is known to correspond to the same
time moment, hence the same position of all robot links. With the serial polling
scheme, the sensing information obtained from polling sensors corresponds to
the robot links being in slightly different positions. The motion within one serial
polling cycle is usually insignificant: The actual uncertainty depends on the serial
scheme implementation and the robot speed.
A fully parallel scheme with n sensors requires roughly log wires. In a fully
n
serial addressing scheme, only one wire will be sufficient to do the job. In the
system described here, this conflict is resolved via a compromise parallel–serial
system: The system is divided into modules that are run in parallel, whereas
sensors in each module are divided into rows and columns and addressed serially.

Sensor Interface. The purpose of the sensor interface circuit (Figure 8.1) is to
realize computer access to the skin sensor. The circuit’s two major components
are an analog-to-digital converter and a number of one-shots that control sensor
addressing. In each sensor module, sensors are addressed in a serial fashion.
The entire skin is reset regularly, synchronizing address counters of the sensor
modules. (More information on a version of this unit appears in Ref. 134.)

Sensor Circuit Module. A sensor circuit module contains a group of sensors
that, from the standpoint of control and mechanical design, are handled as a unit.
A number of sensor modules makes the whole skin. The skin system described
in Ref. 134 and shown in Figure 8.6 included three sensor modules, each with a
different geometric shape and with an unequal number of sensors, totaling about
500 sensors. A later system described in Ref. 135 and shown in Figure 8.7 fea-
tured smaller standardized modules, each about 23 by 23 cm in size and with 8
by 8 sensors, with the whole system totaling over 1200 sensors. Each module is
wrapped around and fastened to the robot arm. Neighboring modules are con-
nected physically, using appropriate fasteners—such as Velcro fasteners—and
electrically, through appropriate connectors.
Besides sensors, each module contains all necessary control electronics. The
latter can be divided into two parts. The first part is a sensor addressing circuit,

of the pair Y. This scenario suggests an interesting hardware and processing schemes that would
be checking for various combinatorial possibilities, to determine which object actually triggered the
signal. No such attempts have been done so far, to my knowledge.