Page 380 - Sensors and Control Systems in Manufacturing

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Se v e n
Cha p te r

light that is guided to the sensing element along the fiber. However,
this optical filter is transparent to the returned photoluminescent light.
The reflectivity of the second interference filter, IF , changes at about
2
900 nm. Because the peak wavelength of the luminescence shifts toward
longer wavelengths with temperature, the ratio between the trans-
mitted and reflected light intensifies if IF changes. However, the ratio
2
is independent of any variation in the excitation light intensity and
parasitic losses. The two lights separated by IF are detected by pho-
2
todiodes 1 and 2. The detector module is kept at a constant tempera-
ture in order to eliminate any influence of the thermal drift of IF .
2
The measuring temperature range is 0 to 200°C, and the accuracy
is ±1°C. According to the manufacturer’s report, good long-term sta-
bility, with a temperature drift of less than 1°C over a period of nine
months, has been obtained.
7.3.3 Temperature Detectors Using Point-Contact Sensors
in Process Manufacturing Plants
Electrical sensors are sensitive to microwave radiation and corrosion.
The need for contact-type temperature sensors have lead to the
development of point-contact sensors that are immune to microwave
radiation, for use in: (1) electric power plants using transformers,
generators, surge arresters, cables, and bus bars; (2) industrial plants
utilizing microwave processes; and (3) chemical plants utilizing elec-
trolytic processes.
The uses of microwaves include drying powder and wood; cur-
ing glues, resins, and plastics; heating processes for food, rubber, and
oil; device fabrication in semiconductor manufacturing; and joint
welding of plastic packages, for example.
Semiconductor device fabrication is currently receiving strong
attention. Most semiconductor device fabrication processes are now
performed in vacuum chambers. They include plasma etching and
stripping, ion implantation, plasma-assisted chemical vapor deposi-
tion, radio-frequency sputtering, and microwave-induced photoresist
baking. These processes alter the temperature of the semiconductors
being processed. However, the monitoring and controlling of tem-
perature in such hostile environments is difficult with conventional
electrical temperature sensors. These problems can be overcome by
the contact-type optical-fiber thermometer.

7.3.4 Noncontact Sensors—Pyrometers
Because they are noncontact sensors, pyrometers do not affect the
temperature of the object they are measuring. The operation of the
pyrometer is based on the spectral distribution of blackbody radia-
tion, which is illustrated in Fig. 7.7 for several different temperatures.
According to the Stefan-Boltzmann law, the rate of the total radiated