Linescannning and thermography essentially extend the concept of point radiation thermometry to one-dimensional profiles or two-dimensional pictures of non-contact temperature data.
One-dimensional linescanners have a wide range of application. They are used in the production of fiberboard, carpets, vinyl flooring, paper, packaging material, pressure sensitive tapes, laminates, float glass, safety glass, and nearly any other web-type product for which temperature control is critical. Linescanners also monitor hot rolling mills, cement and lime kilns, and other rotary thermal processing equipment.
Thermographic "cameras" find use in the maintenance function in manufacturing plants, especially in the asset-driven capital intensive industries in which temperature is an active concern and diagnostic tool. Typical targets for infrared inspection include electrical equipment, frictional effects of power transmission equipment, and thermal processing steps in a production line.
The typical sensor unit for linescan thermography uses a single detector that, by itself, is limited to measuring the temperature at only one point. However, a rotating mirror assembly focuses a single, constantly changing, narrow slice of real-world view on the detector surface (Figure 6-1).
|Figure 6-1: Linescanner Operation
Although adequate thermal measurement resolution may require only a few dozen scans per second, contemporary units offer up to 500 scans per second. The electronic circuitry behind the detector element chops the thermal data for each linear pass into several hundred to several thousand individual measurement points. High speed data collection circuitry then quantifies, digitizes, and captures the temperature of the object at the measurement points along each scan. Additional circuitry then analyzes and manipulates the digital data to produce a real-time display of the temperature of the view presented to the detector.
By itself, a linear temperature scan constitutes a rather myopic view of a stationary object. However, an object moving past a stationary linescanner provides a data-rich measuring environment. Mounted several feet above and focused on the moving web of product in a production line makes the linescanner an element of a real-time process feedback and closed-loop control scheme.
The resolution of a linescanner is a function of the speed of the moving web, the scan rate, the number of measurements per scan, and the width of the scanned line. The accuracy and response of the linescanner depends on a clean optical path between the target and the overhead detector, which can be problematic in a typical manufacturing environment. Linescanners are available with air purge system to keep the optics clean. Water cooling of the sensor assembly may be necessary to maintain reliable operation in hot environments. Sealed housings and bearings protect delicate components against moisture and dust.
The digital electronic output from the linescanner typically feeds a personal computer running software that converts the stream of data into a real-time moving image of the web passing the detector assembly. Because the human retina is not sensitive to infrared radiation, the screen image is necessarily rendered in false color the hue of which corresponds to the temperature of the specific location.
|Figure 6-2: 1-D Scans Composited Into a 2-D Image
The linescan output converts the traditional plot of temperature versus time into a three-axis measurement of temperature as a function of time and location across the web of material (Figure 6-2). The scanner "maps" the "thermal terrain" moving below the detector assembly. As is typical of false color thermographic output, the electronics assigns the color of a given screen pixel on the basis of the temperature implied by the measured infrared radiation.
Linescanning technology offers industry an opportunity to optimize thermally-based processes. For example, plastic thermoforming fabrication requires heating a plastic sheet using an array of heaters. The temperature is critical if the vacuum molding process is to successfully form pleats, deeply drawn recesses, and sharp corners in the completed piece part. A high-resolution linescanner, used in a closed-loop configuration to control the output of the individual heaters, helps to maximize the productivity of the press.
Another application for linescanning in the plastic industry is in blowmolding plastic film. The process involves extruding a polymer melt through a circular die and drawing the formed tube upward. At some point as the plastic rises it solidifies at what is called the frost line. Additional cooling must occur so that the plastic sheeting achieves the proper temperature as it enters the overhead takeup rolls.
Maintaining a proper temperature profile of the moving plastic is critical to eliminating functional and aesthetic defects in the sheeting. Mounting a scanner to monitor the rising plastic gives the operators instant information about the precise location of the frost line and other temperature-related information in real-time.
A similar approach is used in the glass industry. Glass sheets are heat treated to give them the required strength. As the glass moves on a conveyor belt, electric heaters raise its temperature as uniformly as possible. After a suitable holding time in the oven, the glass sheet is cooled uniformly with compressed air. In this process are all the elements
of a linescanner application
for a moving web of temperature sensitive material.