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Remember how much "fun" it used to be to keep fabric webs centered in calendaring operations? Especially if the width of the fabric varied a little one way or the other. Or if the roll tension was a bit different from one side to the other. That used to be a real blast. Especially explaining the high scrap rates to the boss. Loads of fun.

We used to dream about simple devices that would monitor how the web was tracking into the machine and make automatic adjustments. Some improvements were made with edge guides and the like. But these were often less reliable than we liked. Calibration problems as wen as dirt and dust often made them almost more trouble than they were worth.

Then someone would put in a roll that was narrower or wider than the previous one. Depending on the situation, the machine might sit idle for several hours while maintenance reset the guides and made sure the machine was ready for the new material.

Well, there's some new instrumentation out that is ready to take all this "fun" out of your operation and replace it with crisp, clean efficiency. Ifs caned Lasernet, produced by NAMCO Controls of Mentor, Ohio.

What is Lasernet?

As the name suggests, it is a laser system that is designed to scan over a target area and, through a shadowing technique, determine the location of an object in its field of view. Figure 1 illustrates this.

Within the sensor, a constant speed rotating mirror scans laser light over approximately a 90 degrees window. This is done 20 times per second. A special reflective background is placed behind the target area to reflect the laser light back to the source where a photo detector senses any returned light. If an object is placed between the light source and the reflective target, the reflected light in this area is disrupted. The sensor is able to locate where in the scan window this disruption occurs, thereby identifying where the object is. The system is able to determine both the size (width) of the object and its location.

In terms of location, the target will have both edge locations identified simultaneously. From this, the position of the center of the target can be calculated.

This information can be used in a wide variety of ways. For example, if the target is a web of fabric being fed into a calendar, the web can be automatically centered. Or it can be guided to index off one edge.

If materials are changed, the unit will automatically adjust for the new material. Depending on the application, it may be necessary to input new control parameters, but no adjustment to hardware will be necessary.

In addition, the unit can be set to determine any anomalies that occur, such as a broken web or a hole in the middle. Detecting of such a flaw can initiate some warning signal or shut the line down for corrective action.

One of the integral parts of the system is the retroreflector. This reflector has the unique property of returning light on the same path of its origin over a wide range of incident angles. Two types of reflectors are used. The better of the two uses "corner cube" or "microprism" reflectors. These are embedded into a plastic strip which is placed behind the target zone. The alternative strip uses glass beads instead of the microprisms and is less efficient.

How about accuracy?

The scan window is broken up into 15,360 elements or bits. Over a 90 degrees scan, each of these bits represents an angle of .00586 degrees. The linear dimension that this relates to depends on two things. First, the distance from the source to the target. Second, the relative distance from the center of the target.

The system has a resolution of one bit, or 0.00586 degrees. Referring to figure 1, if the distance from the source to the target, D, is 24 inches, the linear resolution can be expressed as

24" x tan(0.00586) = 0.00245".

Accuracy would be considered to be four times the resolution, or

[+ or -] 24" x tan(0.0234) = [+ or -] 0.00982".

At this distance, the maximum scan width is 48 inches. If that same distance D is 84 inches, resolution would be

84" x tan(0.00586) = 0.00859" and accuracy would be

[+ or -] 84 x tan(0.0234) = [+ or -] 0.03431".

At this distance, a web of up to 168 inches wide can be measured.

What about safety and use?

Safety is a major consideration for any tool using laser light. The U.S. Center for Devices and Radiological Health has segregated laser products into various classes depending on the potential hazard involved in their use.

This device has been included in Class II, which is reserved for laser sources with outputs below 1 milliwatt. Class II lasers are considered safe except for deliberate long term exposure by staring directly into the beam. In other words, there is a very low risk of injuring someone working with this product.

Before anything can be done with the system, it must be installed. Installation, however, is relatively simple. First, the mounting location for the source must be established so that it will cover the appropriate target area. The reflector must then be mounted behind the target. Exact location for the reflector can easily be determined by turning on the unit, observing where the visible red laser line is and mounting the reflector in that location. The only caution is to be sure that the reflector is in the same plane as the source. Angle of incidence of the laser light from the source to the reflector can be up to 300 off perpendicular without affecting the system performance.

The device includes its own software and is able to send out signals through various output ports on the unit. This includes RS-232 or RS-485 serial ports, 0-10 VDC analog signals and real time TTL signals. Other output options are also available.

Since this is a non-contact system, it causes no distortion of the target. More accurate information is therefore available than most contact sensors can provide.

Applications?

With the attributes described, the system has a wide range of potential applications. One of the principal uses in addition to web control is in the control of loops. In numerous operations, a web or strip is allowed to form a loop in the process line. This provides tension for the downline machinery. The amount of tension is controlled by the size of the loop.

Reactor arms are often used, but have the disadvantage of applying extra tension to the web and are more affected by the devices with no other interfaces.

Another application has been in tracking and controlling roll diameters in rewind operations. This can be particularly useful when the material being wound up would be sensitive to any foreign contamination. Also, the unit can be mounted out of the way and can be operated with very low maintenance cost.

Because of the width of the scan, it is possible to track multiple strips such as would occur in a slitting operation. Up to eight individual strips can be simultaneously monitored and guided.

Another novel application involves painting of irregularly shaped parts. In this application, a variety of parts of different sizes are passed through a paint line. Prior to entering the spray area, they are scanned by one of these units. The unit then automatically sets the size and dimensions of the part to be painted into the computer controlled spray booth where only the needed spray heads are activated. This eliminates overspray, reduces material losses and reduces the environmental impact from excess solvents being evaporated into the atmosphere.

What are the potential problems?

The unit is designed to be tolerant of most normal manufacturing environments. That is to say, it's not affected by dirt and dust in the atmosphere. And ifs designed to be knocked around somewhat.

However, there are precautions to take. If the surface being monitored is highly reflective, such as aluminum or a bright steel, some of the laser signal could be returned to the source, causing an erroneous reading. This can be avoided by placing the source in such a position that incident light from the source falls at an angle to the target surface. Incidental light reflected by the target is then reflected in some harmless direction.

In some web operations, flutter in the web can cause difficulty in determining exactly where the edges are. This will cause errors in the run.

Also, heavy vibration, where the source is subject to repeated and severe vibration can cause irregularity in readings. The source must be mounted in a solid, out-of-the-way location so that it isn't run into or damaged by other equipment in the area.

As used, the laser is actually looking and getting all its information from the source. As should be expected, regular cleaning of the reflector is necessary to ensure that the proper signal is returned to the source. This can be as simple as wiping at the beginning of each shift. However, it must be done. In severe environments, some companies have chosen to mount the reflector in a water bath or enclose it in plexiglass to protect it from damage and reduce the influence of oil sprays and drops occurring in the area.

Summary

One of the chief benefits of the system has not been mentioned. Reasonable cost. The main part of the system, the sensor, is priced at $2,400 for the 20 foot system and $3,350 for the 50 foot system. This dimension refers to the distance it can be placed from the target. Typically, use is at significantly closer ranges.

All in all, the Lasernet system appears reliable and cost effective. It certainly represents an improvement in our ability to measure and control our manufacturing operations. And it sure beats the old days.

(Figures Omitted)
COPYRIGHT 1990 Lippincott & Peto, Inc.
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1990, Gale Group. All rights reserved. Gale Group is a Thomson Corporation Company.

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Title Annotation:Lasernet, target-locating device for calendering operations
Author:Menough, Jon
Publication:Rubber World
Date:Nov 1, 1990
Words:1664
Previous Article:Developments in Rubber Technology, vol. 4.
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