TEMPERATURE SENSORS IN PROCESS CONTROL
Temperature is one of the most important parameters to be controlled in almost all industrial plants since it directly affects material properties and thus product quality. During the past few years, several temperature sensors have been developed for use in electrically or chemically hostile environments. Among these, the practical temperature sensors, which are now commercially available, are classified into two groups: (1) low-temperature sensors with a range of -100 to +400°C, using specific sensing materials such as phosphors, semiconductors, and liquid crystals; and (2) high-temperature sensors with a range of 500 to 2000°C based on blackbody radiation.
Semiconductor Absorption Sensors
Many of these sensors can be located up to 1500 m away from the optoelectronic instruments. The operation of semiconductor temperature sensors is based on the temperature-dependent absorption of semiconductor materials. Because the energy and gap of most semiconductors decrease almost linearly with increasing temperature T, the band-edge wavelength λg(T ) corresponding to the fundamental optical absorption shifts toward longer wavelengths at a rate of about 3 Å/°C
[for gallium arsenide (GaAs)] with T. As illustrated in Fig. 6.1, when a light-emitting diode with a radiation spectrum covering the wavelength λg(T ) is
used as a light source, the light intensity transmitted through a semiconductor decreases with T.
Figure 6.2 shows the reflection-type sensing element. A polished thin GaAs chip is attached to the fiber end and mounted in a stainless-steel capillary tube of 2-mm diameter. The front face of the GaAs is antireflection-coated, while the back face is gold-coated to return the light into the fiber.
The system configuration of the thermometer is illustrated in Fig. 6.3. In order to reduce the measuring errors caused by variations in parasitic losses, such as optical fiber loss and connector loss, this thermosensor employs two LED sources [one aluminum gallium arsenide (AlGaAs), the other indium gallium arsenide (InGaAs)] with different wavelengths. A pair of optical pulses with different wavelengths λs = 0.88 _m and λr = 1.3 µm are guided from the AlGaAs LED and the InGaAs LED to the sensing element along the fiber. The light of λs is intensity-modulated by temperature. On the other hand, GaAs is transparent for the light of λr, which is then utilized as a reference light. After detection by a germanium avalanche photodiode (GeAPD), the temperature-dependent signal λs is normalized by the reference signal λr in a microprocessor.
The performance of the thermometer is summarized in Table 6.1. An accuracy of better than ±2°C is obtained within a range of -2° to +150°C. The principle of operation for this temperature sensor is based on the temperature-dependent direct fluorescent emission from phosphors.
Semiconductor Temperature Detector Using Photoluminescence
The sensing element of this semiconductor photoluminescence sensor is a double heterostructure GaAs epitaxial layer surrounded by two AlxGa1-x As layers. When the GaAs absorbs the incoming exciting light, the electron-hole pairs are generated in the GaAs layer. The electron-hole pairs combine and reemit the photons with a wavelength determined by temperature. As illustrated in Fig. 6.4, the luminescent wavelength shifts monotonically toward longer wavelengths as the temperature T increases. This is a result of the decrease in the energy gap Eg with T. Therefore, analysis of the luminescent spectrum yields the required temperature information. The double heterostructure of the sensing element provides excellent quantum efficiency for the luminescence because the generated electron hole pairs are confined between the two potential barriers (Fig. 6.5).
The system is configured as shown in Fig. 6.6. The sensing element is attached to the end of the silica fiber (100-µm core diameter). The excitation light from an LED, with a peak wavelength of about 750 nm, is coupled into the fiber and guided to a special GRIN lens mounted to a block of glass. A first optical interference filter IF1, located between the GRIN lens and the glass block, reflects the excitation light, which 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 IF2 changes at about 900 nm. Because the peak wavelength of the luminescence shifts toward longer wavelength with temperature, the ratio between the transmitted and the reflected light intensifies if IF2 changes. However, the ratio is independent of any variation in the excitation light intensity and parasitic losses. The two lights separated by IF2 are detected by photodiodes 1 and 2. The detector module is kept at a constant temperature in order to eliminate any influence of the thermal drift of IF2.
The measuring temperature range is 0 to 200°C, and the accuracy is ±1°C. According to the manufacturer's report, good long-term stability, with a temperature drift of less than 1°C over a period of nine months, has been obtained.
Temperature Detector Using Point-Contact Sensors in Process Manufacturing Plant
Electrical sensors are sensitive to microwave radiation and corrosion. The needs 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 electrolytic processes.
The uses of microwaves include drying powder and wood; curing 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, and include plasma etching and stripping, ion implantation, plasma-assisted chemical vapor deposition, radio-frequency sputtering, and microwave-induced photoresist baking. These processes alter the temperature of the semiconductors being processed. However, the monitoring and controlling of temperature in such hostile environments is difficult with conventional electrical temperature sensors. These problems can be overcome by the contact-type optical-fiber thermometer.
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 radiation, which is illustrated in Fig. 6.7 for several different temperatures. According to the Stefan-Boltzmann law, the rate of the total radiated energy from a blackbody is proportional to the fourth power of absolute temperature and is expressed as:
Thus, the absolute temperature can be measured by analyzing the intensity of the spectrum of the radiated energy from a blackbody. A source of measurement error is the emissivity of the object, which depends on the material and its surface condition. Other causes of error are deviation from the required measurement distance and the presence of any absorbing medium between the object and the detector.
Use of optical fibers as signal transmission lines in pyrometers allows remote sensing over long distances, easy installation, and accurate determination of the position to be measured by observation of a focused beam of visible light from the fiber end to the object. The sensing head consists of a flexible bundle with a large number of single fibers and lens optics to pick up the radiated energy (Fig. 6.8).
The use of a single silica fiber instead of a bundle is advantageous for measuring small objects and longer distance transmission of the picked-up radiated light. The lowest measurable temperature is 500°C, because of the unavoidable optical loss in silica fibers at wavelengths longer than 2 _m. Air cooling of the sensing head is usually necessary when the temperature exceeds 1000°C.
Optical-fiber pyrometers are one of the most successful optical-fiber sensors in the field of process control in manufacturing. Typical applications include:
• Casting and rolling lines in steel and other metal plants
• Electric welding and annealing
• Furnaces in chemical and metal plants
• Fusion, epitaxial growth, and sputtering processes in the semiconductor industry
• Food processing, paper manufacturing, and plastic processing
Figure 6.9 is a block diagram of the typical application of optical-fiber pyrometers for casting lines in a steel plant, where the temperature distribution of the steel slab is measured. The sensing element consists of a linear array of fused-silica optical rods, thermally protected by air-purge cooling. Light radiated from the heated slabs is collected by the optical rods and coupled into a 15-m-long bundle of fibers, which transmits light to the optical processing unit. In this system, each fiber in the bundle carries the signal from a separate lens, which provides the temperature information at the designated spot of the slabs. An optical scanner in the processing unit scans the bundle and the selected light signal is analyzed in two wavelength bands by using two optical interference filters.
About the Author
Sabrie Soloman, Ph.D, is the Founder, Chairman and CEO of American SensoRx, Inc.
Excerpted from Sensors Handbook, 2nd Edition by Sabrie Soloman (McGraw-Hill; 2010) with permission by McGraw-Hill.
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