The increasing demand for more stringent quality requirements, greater production yield, conservation of energy and lower operating costs are putting pressure on conventional thermal monitoring systems to affect in-process and ultimately, end term quality. What happens if the requirements exceed the capabilities of the thermocouple or similar temperature measuring devices that are today's workhorses of industry.
To date temperature measurement and control of industrial processes are being performed mainly by thermocouples. The advantages of the initial low cost and simplicity of these devices are normally considered in their selection over their major disadvantages - namely their need to physically contact the test object, slow response, inaccuracy due to pick-up interference from extraneous electrical signals or adjacent temperature effects, and susceptibility to damage.
The widespread application of infrared temperature measuring is of particular interest when processes deal with extremes of heat, voltage, radiation, or corrosion have made accurate temperature measurements difficult or impossible by contact thermocouple methods.
View Around Corners
All matter is made up of a large number of molecules which are in constant motion creating thermal agitation, causing in the process the radiation of electromagnetic energy. Basically, the total radiated energy is in direct proportion to the absolute temperature of an object raised to the fourth power, and the peak emission shifts towards shorter wavelengths with increasing temperature. As a result of this phenomenon, optoelectronic equipment (radiometers) using an infrared detector as a sensing element can remotely and without physical contact, measure the temperature of an object.
Modern infrared temperature detectors use basically the same type line-of-sight optics that were used in the initially developed systems. As a result of this design feature, these systems have certain limitations which restricts their use when a direct optical path to the test object does not exist: 1. When fumes or other normally signal absorbing or sometimes signal producing phenomena, would erroneously affect the amount of energy collected by the optics. Also, when the presence of strong electrical fields, or other false signal generators exist or hostile process conditions prevent the proper location of the optical system to view the test object. These types of application related problems have now been solved by a radiometer in which the conventional line of sight procedure has been replaced by optical fiber optics.
Any optical fiber basically acts like a waveguide or a sheltered optical path that enables the temperature of an object to be monitored and if desired, controlled at a proximity that simply cannot be achieved by conventional radiometers. Consequently, temperature measurement accuracy and long term stability can be assured because the fiber optic probe enables the sensitive electronic infrared detector assembly to be remotely located from environmental hostilities. Typically optical fiber is usually constructed of a silicon (glass) material. Every optical fiber consists of a core with a high refractive index and a cladding with a low refractive index. IR energy entering one end is guided along the core by total internal reflection at the core/cladding boundary. The cladding also serves as a protection of the core finish. All fibers used in infrared instrumentation are made of glasses especially chosen for their ability to transmit the radiation comprised in the chosen spectral region. Generally fiber optic light guides display a strong absorption in the OH bands. By using special water-free glass, the otherwise quite normal strong absorption in the OH band at 1.4 microns is avoided.
All rays entering the front surface of the polished fiber bundle that acquire an inclination smaller than the critical angle are totally reflected inside the fiber core, and keep propagating in this fashion until they reach the opposite end or are totally absorbed. The value of the critical angle is a function of the ratio between the refractive indexes of the glass of which the core is made and of the medium surrounding it. By controlling the ratio an increase or decrease can be controlled, obtaining special performance characteristics.
For most IR monitoring applications, optical fibers are assembled into fiber bundles consisting of many hundreds of individual fibers contained within a flexible or rigid sheathing of either a metallic or nonmetallic material. Each end of the bundle is held in place using a high temperature epoxy. The end surface is then highly polished to assure a clearly defined angle of acceptance diminishing the reflectance losses due to irregular surfaces. Using such a large number of fibers in a bundle allows the gathering of IR energy to the detector while retaining mechanical flexibility. Typically the outside diameter of a fiber is 20 thousands of an inch. In the majority of applications where optical fibers are used with infrared detectors, the lengths are 1 to 2 meters long. On occasion fibers will be made up to 10 meters in length. The determining factors in using fiber bundles to transmit IR, are minimum measured temperature (MMT), target distance and spot size. The higher the temperature the longer the fiber, conversely low temperatures require a shorter fiber due to the glass attenuation of IR energy. Fibers can be used unfocused with a viewing field of view or angle of acceptance of 60o. Unfocused fibers are used when the target area is large and it is desirable to measure its average temperature. Focused fibers (those with a viewing lens assembly attached to the front end) are used to measure targets as small as 0.010" from as far away as 4 feet. Back lighting with bifurcated or trifurcated fibers give a visual outline defining the spot size.
Fiber assembly varieties
The wide selection of Vanzetti fibers and lens configurations allows for a satisfying and endless number of applications. Following are some of the many components that make up a fiber optic system and allow for such versatility.
Sheathing
Lenses
Replaceable Tips
Specials
Applications
Since virtually every manufactured product from automobiles to safety pins - requires the application of heat treatment in some form, the use for non-contact temperature monitoring and control is virtually limitless.
Induction Heating
Because of the strong RF inductive energy field needed to heat the metal parts being treated, conventional measuring devices are of little value since they will be heated directly by the induction coil. A typical application of fiber optic systems used to monitor and control induction treatment of metal objects either stationary in, or moving through induction furnaces. Precision control of the temperature needed for perfect heat treatment of metal parts is essential to produce the crystal structure that will ensure meeting the mechanical characteristics specifications.
This control function is achieved either by on/off or high speed proportional control incorporated in the fiber optic and Thermal Monitoring System. Using fiber optics vs. the conventional method of direct line of sight infrared detection systems allows the placing of the viewing end of the fiber optic in close proximity of the target. The tip of the fiber in many cases may be positioned between the induction coils to view the processed material. To eliminate the adverse affects of the RF field a ceramic replaceable tip is utilized. In those instances where the design of the system won't allow room for the fibers a lens systems will then be provided to view and monitor targets from a distance.
Metal Forging, Hot Stamping, Pipe Bending
Forging of metal parts includes both rough shape as well as precision forging which requires less material removal and waste. Pipe bending and shaping is also included in this application. These operations are carried out by heating the parts to be worked upon to the optimum temperature with any of the several means available (ovens, flame, induction field, etc.). If the part temperature is below the optimum, cracks and internal tensions will develop, while if it is above the optimum, drooping will take place. The precise temperature control afforded by the use of infrared fiber optic controllers will:
Metal Die Casting
The die temperature is of critical importance in die casting of metals. Thermal cycling of aluminum products, with reference to die temperatures, has been successfully implemented with the help of optical fibers. This accomplished by inserting a fiber optic probe through the mold frame and into the corner of the runner plate, in contact with the aluminum flowing through it.
The major advantages offered by this solution are:
Direct indication of the die and furnace pot temperature. Low level and blocked water lines are easily indicated several shots before the casting can display conditions visibly.
Control of Metal-Working Laser
Lasers, generally high-power CO2 lasers, are used for welding, surface treating and finishing metals of various types. The conventional approach is to sample periodically the beam to keep its power at the desired level. This approach, however cannot take automatically into account the emissivity variations, in turn, affect the amount of laser power absorbed by the target, and consequently the target's temperature, which is of paramount importance for good operating performance.
This difficulty is overcome by the use of a two wavelength emissivity-independent infrared fiber optics system (EITM) aimed at the spot of laser beam impact. The infrared system is made blind to the laser wavelength, and in this way it measures precisely the target temperature at the same spot, and, via a feedback loop, it controls the laser power to ensure that the operation is carried out at the optimum temperature.
Among the advantages offered by the fiber optics infrared approach are the following:
The above are but a few of the many and varied uses of fiber optics. The range of applications for these systems is only limited by one's imagination. The technology is expanding exponentially. Fiber optics are no longer an extra option for IR temperature sensing. Fiber optics are here to gain the necessary temperature control and to provide accurate temperature characterization of any manufacturing process.
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