Page 428 - Sensors and Control Systems in Manufacturing

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the destination will be measured in relative terms, not absolute. Plac-
ing the sensor on the end effector allows absolute measurement to be
taken, reducing considerably the need for calibration of mechanical
position and for imaging linearity. Image sensory feedback in this
situation can be reduced to the simplicity of range finding in some
applications.
Extensive research and development activities were conducted
recently to find ways to integrate various sensors close to the gripper
jaws of robots. The promise of solid-state arrays for this particular
application has not entirely materialized, primarily because of diver-
sion of effort resulting from the commercial incentives associated
with the television industry. It might be accurate to predict that, over
the next decade, imaging devices manufactured primarily for the
television market will be both small and affordable enough to be use-
ful for robotics applications. However, at present, array cameras are
expensive and, though smaller than most thermionic tube cameras,
they are still far too large to be installed in the region of a gripper.
Most of the early prototype arrays of modest resolution (developed
during the mid-1970s) have been abandoned.
Some researchers have attacked the problem of size reduction by
using coherent fiber optics to retrieve the image from the gripper
array, which imposes a cost penalty on the total system. This approach
can, however, exploit a fundamental property of optical fiber in that
a bundle of coherent fibers can be subdivided to allow a single high-
resolution imaging device to be used to retrieve and combine a num-
ber of lower-resolution images from various paths of the work area
including the gripper with each subdivided bundle associated with
its own optical arrangement.
Linear arrays have been used for parts moving on a conveyer in
such a way that mechanical motion is used to generate one axis of a
two-dimensional image. The same technique can be applied to a robot
manipulator by suing the motion of the end effector to generate a
two-dimensional image.
Tactile sensing is required in situations involving placement. Both
active and passive compliant sensors have been successfully applied
in the field. This is not the case for tactile array sensors because they
are essentially discrete in design, are inevitably cumbersome, and
have very low resolution.
Acoustic sensors, optical sensors, and laser sensors are well devel-
oped for effective use in manufacturing applications. Although laser
range-finding sensors are well developed, they are significantly
underused in FMS, especially in robotic applications. Laser probes
placed at the end effector of an industrial robot will form a natural
automated inspection system in manufacturing.
Sensing for robot applications does not depend on a relentless
pursuit for devices with higher resolution; rather, the fundamental