FIG. 1A depicts an existing line-scanning thermal imaging system for thermoforming applications. In FIG. 1A, an infrared line-scanner 10 forms images of a plastic sheet 12 being registered out of a thermoforming machine (not shown). An output signal from the infrared line-scanner 10 is coupled to a first input of a computer 14 via a signal conversion device. Software, executed by the computer 14, generates thermal images of the plastic sheet which can be displayed on a computer monitor screen and also generates output signals, to be described more fully below, which can be supplied in either analog or digital form.
FIG. 1B is a more detailed view of the infrared line-scanner 10. In FIG. 1B, a conveyor mechanism 20 moves plastic sheets 12 output from the thermoforming machine under the field of view of the infrared line-scanner 10. The infrared line-scanner measures a line of 256 points using a rotating mirror that scans a 90° field-of-view up to 48 times per second. Thus, the IR image is formed by rasters of 256 measurement points where each raster line represents a temperature profile for a given raster scan. A two-dimensional temperature distribution or thermal image is formed line by line when the sheet passes across the line-scanner's field of view. The scanning of a sheet can be initiated by the measured temperature, or by an external “trigger” signal. As the heated sheet traverses the field-of-view, a two-dimensional thermal image or “thermogram” is formed. Thermal images are displayed each time the scanned sheet indexes out of the heating section or oven of the thermoforming machine.
An example of an image formed by the data output by the infrared image forming system depicted in FIGS. 1A and B is depicted in FIG. 2. Each scan of the infrared line-scanner forms a raster of image points which are displayed contiguously to form the thermal image depicted in FIG. 2.
Measuring the temperature distribution of a sheet as it exits a thermoforming oven allows thermoformers to adjust the oven heating zone temperatures to achieve the desired degree of sheet temperature uniformity. Data from an infrared line-scanner provides a 2-dimensional thermal image of each sheet exiting the oven.
Thermoforming machines include an oven having heating elements for heating the plastic sheet. In this example, thermoforming software, manufactured by the assignee of the present application, subdivides the measured thermal image (or “snapshot”) into a grid of rectangular zones corresponding to the oven's heating zones to allow manual or closed-loop control of each heating element or “zone”. This software allows configuration of only rectangular zones because thermoforming ovens also have rectangular heating zones (or clusters of heating elements grouped in rectangular arrays). The software calculates the average temperature of each zone, which is displayed in the corner of each rectangle in the grid. Zones can be tailored to each application depending on heater size and location. Temperatures for each zone are displayed as average, maximum, or minimum values. The serial or available analog outputs, described above, can be configured to provide outputs proportional to each zone's temperature.
FIG. 3 depicts a zone configuration screen 30 displayed by the software to allow creation of the zone grid. A drawing window is provided that is superimposed over the thermal image to allow a user to employ a mouse, or other input device, to create rectangular zones corresponding to, for example, the location of heater elements with respect to a plastic sheet when it is being heated in a thermoforming device.
It is known that an accurate registration of the scanned image to the actual locations of the heating device can only be accomplished if the velocity of the plastic sheet (and hence the conveyer mechanism) is constant. As described above, the infrared line-scanner 10 is static and it is the movement of the sheet that allows the scanned rasters to form a two-dimensional thermal image. However, if, for example, there were acceleration along the axis of motion of the conveyer mechanism, a particular zone would appear reduced in length and would no longer conform accurately to the location of the heating elements.
The distortion of thermal images from an infrared line-scanner on rotary thermoforming machines is well known, but understanding the nature and causes of the distortion have not been previously described. It is not possible to “linearize” or “flatten-out” distorted thermal images, because to do so requires transferring or interpolating “pixels” (one of the 256 measured raster-scanned points) from one area of the thermal image to another and would not result in reconstructing an accurate undistorted thermal image.
In an in-line thermoforming machine, when a plastic sheet indexes out of the oven after being heated, the sheet is transported at variable speed (i.e., the sheet accelerates and decelerates) as it passes through the infrared line-scanner's field-of-view. This non-uniform movement causes the apparent shape of the resulting thermal image to distort thereby preventing the use of the actual size/shape/location of the heating zones to subdivide the thermal image into the corresponding oven heating zone segments.
Thus, with non-linear sheet movement, it is not obvious how to subdivide the resulting thermal image into zones of the correct size/shape. “In-line” or “continuous” thermoforming machines transport the sheet in a straight-line, but at varying speed. Accordingly, zones distort only in the direction of sheet movement (the “machine direction”), but not at right angles to sheet movement (the “cross-machine direction”) assuming the line-scanner raster scans exactly in the “cross-machine” direction. For example, sheet acceleration reduces the apparent sheet length on the resulting thermal image (and hence zone length) in the direction of travel (relative to that of a sheet traveling at constant speed), but does not affect the apparent width of the sheet. Likewise, if a sheet decelerates while passing through the line-scanner's field-of-view, the apparent length of the thermal image appears enlarged.
Besides distortion resulting from non-uniform sheet movement, additional distortion results on rotary thermoforming machines because the sheet traverses a curved path resulting in zones of non-rectangular shape and irregular dimension.
In principle, it might be possible to dynamically adjust or correct, in real-time, the resulting thermal image for instantaneous sheet speed changes, but to do so also requires knowledge of the line-scanner/sheet geometry, the location of the line-scanner's scanned line on the sheet and the lateral position and dimensions of the sheet. While theoretically possible, implementing such a method would be complex and costly.