Page 513 - The Mechatronics Handbook

0066_frame_C19 Page 135 Wednesday, January 9, 2002 5:32 PM

FIGURE 19.111 Three solid state cameras are shown. The nearest is an inexpensive single board camera with a
CMOS sensor and 4-mm lens. The middle one is a miniature CCIR interline frame-transfer CCD camera, dwarfed
by the 9.5- to 75-mm C-mount zoom lens. The rear camera is a high quality Cohu 4712 monochrome frame-transfer
CCD camera fitted with a 16-mm C-mount lens and 20-mm extension tube.
Fig. 19.111, including sensing array, associated electronics, output signal format, and lens. Depending
on the application the camera could be RS-170/CCIR monochrome, NTSC/PAL color, progressive scan,
variable scan, or line scan. Five major system parameters which govern the choice of camera are field of
view, resolution, working distance, depth of field, and image data acquisition rate. Color may also be
important to the application, but otherwise monochrome images are preferred because they require less
memory and process faster. As a rule of thumb, for size measurement applications, the sensor should
have a number of pixels at least equal to twice the ratio of the largest to smallest object sizes of interest.
Lighting should be arranged to illuminate the objects of interest so that the best possible images can be
acquired. Lighting might be ambient, high-frequency fluorescent, LED, incandescent, or quartz halogen.
A frame grabber or video capture card, usually in the form of a plug-in board which is installed in
the computer, is often required to interface the camera to a host computer. Camera suppliers can
recommend compatible frame grabbers. The frame grabber will store the image data from the camera
in on-board, or system memory, sampling and digitizing analog data as necessary. In some cases the
camera may output digital data, which is compatible with a standard computer interface like USB 2.0 or
IEE-1394 Fire Wire, so a separate frame grabber may not be needed. The computer is often a PC or
Macintosh and should be as fast as possible to keep the time needed to process each image as short as
possible, or to allow more processing to be done in the time available. Machine vision software is needed
to create the program which processes the image data. This may come in many forms, including C libraries
of device drivers and functions, ActiveX controls, and point and click programming environments which
allow easy assembly of image processing operations. When an image has been analyzed the system must
be able to communicate the result to control the process or to pass information to a database. This
requires a digital I/O interface or network connection. The human eye and brain can identify objects
and interpret scenes under a wide variety of conditions. Machine vision systems are far less versatile so
the creation of a successful system requires careful consideration of all elements of the system and precise
identification of the goals to be accomplished, which should be kept as simple as possible.

References

1. Wilson, J. and Hawkes, J. F. B., Optoelectronics: An Introduction, Prentice-Hall International,
London, 1983.
2. Jenkins, F. A. and White, H. E., Fundamentals of Physical Optics, 4th ed., McGraw-Hill, New York,
1981.

©2002 CRC Press LLC