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1.4 Sensors 17

If signal processing does need to be addressed, it is often very useful to use
finite state machine (FSM) design. A typical signal from an incremental optical
encoder is shown in Figure 1.4.2(a); a FSM for decoding this into the angular
position is given in Figure 1.4.2(b). FSM are very easy to convert directly to
hardware in terms of logical gates. A FSM for sequencing a sonar is given in
Figure 1.4.3(a); the sonar driver hardware derived from this FSM is shown
in Figure 1.4.3(b).
A particular problem is obtaining angular velocity from angular position
measurements. All too often the position measurements are simply differenced
using a small sample period to compute velocity. This is guaranteed to lead
to problems if there is any noise in the signal. It is almost always necessary to
employ a low-pass-filtered derivative where velocity samples v k are computed
from position measurement samples p k using, e.g.,

where T is the sample period and is a small filtering coefficient. A similar
approach is needed to compute acceleration.

Vision Systems, Cameras, and Illumination. Typical commercially available
vision systems conform to the RS-170 standard of the 1950’s, so that frames
are acquired through a framegrabber board at a rate of 30 frames/sec. Images
are scanned; in a popular US standard, each complete scan or frame consists
of 525 lines of which 480 contain image information. This sample rate and
image resolutions of this order are adequate for most applications with the
exception of vision-based robot arm servoing. Robot vision system cameras
are usually TV cameras—either the solid-state charge-coupled device (CCD),
which is responsive to wavelengths of light from below 350nm (ultraviolet)
to 1100nm (near infrared) and has peak response at approximately 800nm,
or the charge injection device (CID), which offers a similar spectral response
and has a peak response at approximately 650nm. Both line-scan CCD
cameras, having resolutions ranging between 256 and 2048 elements, and
area-scan CCD cameras are available. Medium-resolution area-scan cameras
yield images of 256×256, though high-resolution devices of 1024×1024 are
by now available. Line-scan cameras are suitable for applications where parts
move past the camera, e.g., on conveyor belts. Framegrabbers often support
multiple cameras, with a common number being four, and may support black-
and-white or color images.
If left to chance, illumination of the robotic workcell will probably result
in severe problems in operations. Common problems include low-contrast
images, specular reflections, shadows, and extraneuos details. Such prob-
lems can be corrected by overly sophisticated image processing, but all of

Copyright © 2004 by Marcel Dekker, Inc.