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On-line scanning of extrusion and tires.

Quality and production control standards are getting tighter and tighter. Quality control methods often require material or completed objects to be removed from the production stream. Additional handling for measurements and note taking for records takes time and money. There are several advantages to on-line inspection. Material is inspected as it is produced so the production flow is not interrupted. Records are maintained electronically to eliminate paperwork and human error (perhaps eliminating the human altogether). Tolerance problems are diagnosed fast and quickly fixed so material use is optimized, not wasted. This article will describe a new approach to full inspection by non-contact measurement on the production line.

How does laser scanning work?

Figure 1 is a schematic of single-point laser scanning geometry. Note how the video signal changes position in the sensor as the height of the object under measurement changes. This algorithm provides a single height measurement as the sensor is read. In order to compute a two-dimensional shape many readings must be taken. An encoder is often used to provide the second axis to connect these readings.

[Figure 1 ILLUSTRATION OMITTED]

The line laser measurement approach is shown in figure 2. Instead of many single point measurements, a single line laser snapshot is exposed in a charge coupled device (CCD) camera array. The laser/camera assembly does not have to be moved to compute a two-dimensional shape.

[Figure 2 ILLUSTRATION OMITTED]

Figure 3 is an example of a line laser system with three scanners. It is shown scanning a short extruded piece. A complete profile is obtained instantaneously. There are no moving parts.

[Figure 3 ILLUSTRATION OMITTED]

How does a line laser scanner work?

The camera view in figure 4 shows the brightness of the line laser on a piece of extruded rubber. The CCD camera is exposed to provide a view window. Notice the laser line that runs completely across the sample. Using a visible laser line and showing a camera view provides a useful diagnostic for the operator to see just how the system is functioning. As one can imagine, adjustment and calibration are simplified with a picture instead of a list of obscure values. The CCD camera exposure time is adjusted to optimize the view of the laser line which will give the measurement profile. The exact position of the laser on the object is lost if the laser line is so bright that it covers several pixels with maximum brightness (saturated white). The proprietary software locates the laser line in the camera image and calculates the number of pixels that are too bright or saturated. Setup parameters in the software allow a certain percentage of saturated pixels. After each picture is taken, the exposure of the camera is adjusted to maximize the positional accuracy of the laser line.

[Figure 4 ILLUSTRATION OMITTED]

Please note that in figure 4 and most of the computer screen examples the informational heading has been left out. This heading includes screen status, scanner status, machine status, job status, the current measurements at each electronic probe and the commands: archive, calib, exit, freeze, graph, help, laser, param, QC, setup and view.

After the laser line is located in the field of view of the camera, the laser coordinates are transformed from the camera view into an X/Y view. Measurement information is available along the entire laser line which provides a profile of the object. The high resolution camera has hundreds of pixels and a measurement can be calculated for each pixel.

However, not all dimensions of the object are of interest. In order to focus on critical dimensions, the system comes with software probes which are not unlike calipers. Each probe can be configured around a critical dimension. We have designed a variety that includes averaging X and Y dimensions, peaks and valleys, and left and right edge. These probes are displayed on the graphical X/Y representation of the object.

The software probes are designed to display more than just a dimensional analysis. Two limits are programmed into the probes. One provides a warning (probe goes from green to yellow) when the critical dimension goes slightly out of tolerance. The other provides an error (probe goes from yellow to red) when the dimension goes dangerously out of tolerance.

Perhaps a more interesting view of the probe is provided with a QC chart of the extrusion over time. The bottom display of figure 5 is a running standard deviation that measures production consistency. Here the probe has wandered into the yellow warning range. Note how measurement variation is reflected in the S bar or standard deviation chart. An individual QC chart is available for each probe.

[Figure 5 ILLUSTRATION OMITTED]

How do you calibrate the line laser scanner?

When an extrusion run is satisfactorily configured, the configuration is saved in a computer file. This provides a quick turnaround when confronted with a changeover to a different product or run. In order to adjust the system to material that varies in size and shape, the system must be flexible and quickly recalibrated.

Figure 6 is a graphical view of the calibration fixture. Note how 13 different probes are used to fill the viewing area with known dimensions. After a probe is positioned in the viewing area, the system is issued a command to calibrate. Parameters tell the software the actual dimension of the calibration fixture and it runs through a calibration algorithm to fix the various X and Y coordinates.

[Figure 6 ILLUSTRATION OMITTED]

Some systems require accuracy over such a large viewing area that one laser/camera scanner is not enough. The capability for multiple scanners has been built into the software. Configuration parameters tell the system how many scanners are in the system. The system shown consists of two scanners. The yellow line is information from one laser/camera scanner; the white one is information from the other. The software marries both sets of data to provide one continuous measurement profile.

What kind of objects can be measured?

Examples of extruded products show how the system adapts to each. A piece of extrusion molding under one camera with the laser line may capture all points of interest. Objects with flat or rounded edges are ideal for a line laser measurement system. If there are sharp edges or pockets in the piece, the laser line can be lost from camera view. However, it is still possible to measure difficult areas by adjusting one scanner to illuminate inside any cracks. The system is designed to allow quick and flexible motion of any or all scanners to "see" many different angles.

Figures 7 and 8 show how the scanner adjusts its exposure time depending upon the color and reflectivity of the material. Notice the sharp angles in which the laser line has disappeared. In order to see, and thus measure, these areas, the scanner angle must be able to be adjusted. Figure 7 is a camera view of the off-white rubber extrusion. This is in contrast to the black matte of the rubber sample in figure 4. The ProScan adjusts for any color or finish.

[Figures 7-8 ILLUSTRATION OMITTED]

The exposure time displayed at the top of the main screen shortens markedly from that on the black rubber. By autoexposing, the system adjusts for the color and finish of the material under measurement, as well as for variable lighting conditions on the factory floor.

A tire tread is not fiat. Fortunately, line laser profiling does not care if the object under measurement is fiat or round. A system has been specifically designed to measure tires and tire tread. Multiple scanners would give a bead-to-bead profile of a tire.

How do you measure a tire?

For a tire, the scan head is simply suspended above the tire. Each profile measures the tread profile at the instant the laser is fired and the camera is exposed.

As a snapshot is taken, a profile is calculated and added to the array of previous profiles. In this fashion, a complete 3D image is created. If more measurements are required, the tire is simply spun slower. With up to 100 profiles/second the tire scanner measurements can be as dense as desired.

The data from a partial tire scan is compiled and displayed. A 3D color coded display combines all the profiles which were taken as the tire was spun. An incredibly detailed tread analysis is possible.

Summary

The ProScan line of inspection equipment consists of CCD cameras and line generating lasers which measure cross sectional profiles. A single camera and laser pair are referred to as a scanner. Multiple scanners are aligned both to cover the field of view and to meet measurement specifications. Computer control of the scanners allows scan rates in excess of 100 profiles per second.

As material passes in front of each scanner, images are collected and processed by a personal computer. The image is processed to convert the laser line in camera space into a profile in X/Y space. The computer collects all the profiles and combines the data into a 3D map of the material.

Multiple scanner configurations of the ProScan provide the ability to measure a variety of rubber extrusion products. The use of flexible software measurement probes to monitor critical dimensions allows a simplified visual production display. For measuring tires, the mechanical configuration is easily adjusted to build a three dimensional tire tread measurement system.

Conclusion

With little capital investment, ProScans will help produce products with closer tolerances, fewer rejects, less setup time, reduced paperwork, and, most of all, increased monetary return.
COPYRIGHT 1998 Lippincott & Peto, Inc.
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1998, Gale Group. All rights reserved. Gale Group is a Thomson Corporation Company.

Article Details
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Author:McCarthy, Earl T.
Publication:Rubber World
Date:Jul 1, 1998
Words:1596
Previous Article:Understanding the influence of polymer and compounding variations on EPDM extrusions.
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