Page 339 - Sensors and Control Systems in Manufacturing

Advanced Sensors in Pr ecision Manufacturing
6.12 Visible Light Region 295
Applications at visible wavelengths with at least microwatts of opti-
cal power, in which the sensor is simply required to detect if light is
present, can use one of the least expensive and most rugged detectors
available: the photoconductive cell. Many standard devices are avail-
able as well as custom-designed options, including complete elec-
tronic assemblies.

6.13 Nanowatts Power
Silicon phototransistors and photodarlingtons—the second-lowest-
cost sensors—are also convenient and should be considered as the
next possible alternative. Only nanowatts of optical power within a
5-mm diameter spot are required at wavelengths between 350 and
1100 nm. The application must also tolerate some unit-to-unit sensi-
tivity variation.

6.14 Considering High-Performance Photodiodes
For UV to near IR-wavelengths, photodiodes offer the best overall
performance. They are only slightly more expensive than phototran-
sistors, but their spectral range is broader and they have lower noise,
more uniform sensitivity and reproducibility, a larger dynamic range,
better linearity, and more package options. Also, photodiodes can
routinely detect picowatts of optical power. Thus, if phototransistors
or photoconductive cells are not appropriate for an application, more
often than not a photodiode will afford the best alternative.

6.15 Hybrids of Photomultipliers Options
At least 90 percent of detector applications should be satisfied by one
of the sensors discussed earlier in this chapter. However, when light
levels are extremely low, or ambient electronic noise levels are high,
or space requirements are particularly limited, other alternatives,
such as optical ICs, hybrids, or photomultipliers should be seriously
investigated.

6.16 Design Considerations in Fuzzy Logic Color Sensors
The design of fuzzy logic color sensors aims to achieve maximum
color sensing ability, while maintaining the expected simplicity of
operation and durability of typical discrete industrial sensors. Sev-
eral other key goals must be considered in system design such as high
speed, small size, configurability, high repeatability, and long light
source life.