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1.4 Sensors 13

segmentation, etc.). Some sensors, including the proprioceptors needed for
servo-level feedback control, are integral parts of their host devices, and so
processing of sensor data and use of the data occurs within that device; then,
the sensor data is incorporated at the servocontrol level or Machine
Coordination level. Other sensors, often vision systems, rival the robot
manipulator in sophistication and are coordinated by a Job Coordinator,
which treats them as valuable shared resources whose use is assigned to jobs
that need them by some priority assignment (e.g. dispatching) scheme. An
interesting coordination problem is posed by so-called active sensing, where,
e.g., a robot may hold a scanning camera, and the camera effectively takes
charge of the motion coordination problem, directing the robot where to
move to effect the maximum reduction in entropy (increase in information)
with subsequent images.

Types of Sensors
This section summarizes sensors from an operational point of view. More
information on functional and physical principles can be found in [Fraden
1993], [Fu et al. 1987], [Snyder 1985].

Tactile Sensors. Tactile sensors rely on physical contact with external objects.
Digital sensors such as limit switches, microswitches, and vaccuum devices
give binary information on whether contact occurs or not. Sensors are
available to detect the onset of slippage. Analog sensors such as spring-loaded
rods give more information. Tactile sensors based on rubberlike carbon- or
silicon-based elastomers with embedded electrical or mechanical components
can provide very detailed information about part geometry, location, and
more. Elastomers can contain resistive or capacitive elements whose electrical
properties change as the elastomer conmpresses. Designs based on LSI
technology can produce tactile grid pads with, e.g., 64×64 ‘forcel’ points on
a single pad. Such sensors produce ‘tactile images’ that have properties akin
to digital images from a camera and require similar data processing.
Additional tactile sensors fall under the classification of ‘force sensors’
discussed subsequently.

Proximity and Distance Sensors. The noncontact proximity sensors include
devices based on the Hall effect or inductive devices based on the
electromagnetic effect that can detect ferrous materials within about 5 mm.
Such sensors are often digital, yielding binary information about whether or
not an object is near. Capacitance-based sensors detect any nearby solid or
liquid with ranges of about 5mm. Optical and ultrasound sensors have longer
ranges.

Copyright © 2004 by Marcel Dekker, Inc.