One problem often encountered in guiding and measuring non-woven material arises from determining the average position of material edges. Because the density and distribution of the fibers may vary, the usual camera-based vision system will have difficulty determining where the material ends. With carefully adjusted backlighting the position information works well until the density changes or the fiber color is altered.
At the same time, there are processes that require accurate measurement and positioning of non-woven material webs. One example is found in the manufacture of asphalt roofing material. Here a non-woven fiberglass substrate must be accurately positioned before coating the material with hot asphalt. A misguided web will result in a major cleanup process to remove the asphalt that missed the target. Not coating the entire strip evenly from side to side will cause quality problems that result in high scrap rates. With Scan-A-Line’s scanned LED technology, the substrate edge is clearly defined as the short term average edge position. This permits reliable position guiding to 0.050 inches or better. Color and density have little or no effect on the edge position measurements. These systems are performing very reliably in the roofing industry, where the environment is dirty, dusty, hot, and generally inhospitable to precision measurements.
Before Scan-A-Line, the webs were traditionally guided by paddles that actuated pneumatic valves to adjust the web position. This ‘bang/bang’ style of correction seldom guided to better than +/- ¼ inch. The abrasive nature of the fiberglass web eventually wore through the edge paddles requiring monthly replacements. When a paddle finally failed it often caused the line to crash resulting in production downtime.
As non-woven materials continue to be used for more applications every day, there are many applications where Scan-A-Line may offer significant process improvements.
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Continuous Extrusion Width Measurement
The measurement of extruded width in plastic and rubber products can have hidden benefits to process control. With most modern extruders the material is forced into the die with a constant displacement screw or pump. With a constant material feed into the die and proper die temperature control, there are several factors that can contribute to finished product thickness. The most significant factor is generally the takeaway tension pulling the product away from the output of the extruder. With greater takeaway tension the product will neck down on both the width and thickness of the extrusion. When the extrusion material feed is constant and the die temperature is tightly controlled, the thickness of the extrusion can be precisely controlled by maintaining a constant width. In many operations where the product dimension is much larger than the thickness, it is possible to control thickness to very tight tolerances by controlling the width using only take away tension as a control variable.
In the plastics industry, it is important to meet minimum thickness requirements. In many extruded sheet and film products a variation of a thousandth of an inch can be very significant. Considerable money must be invested to continuously measure the precise thickness of the extruded products using optical or radiation based measurement systems. This tiny deviation from the ideal can easily amount to 10% or more of the raw material used in the extrusion. With a continuous process, this can add up to many thousands of dollars in as little as one month.
Scan-A-Line, from Harris-Instrument Corporation, offers a very simple and cost effective solution to controlling extrusion thickness by measuring the extruded width. To determine the feasibility of this type of control we generally take a process capability survey for the extruder operation. A width measurement system is first installed to record any width variation in the extrusion. While observing the width readings we watch for maximums and minimums to occur. When we observe a maximum or minimum width reading, we mark the extrusion with a plus or minus sign. After the process is finished, we locate our marked locations and then carefully measure the thickness of the product at these locations. In nearly every case we can see a direct correlation between the maximum and minimum width and the maximum and minimum thickness.
By observing the period and magnitude of the variation in width, we can usually identify what is causing the width and thickness variation. Typical sources are a bad bearing or bent roller in the takeaway transportation system, or a lack of good synchronization between the drive systems and the extruder feed. The recorded strip chart of Scan-A-Line width measurements often serves to identify, help correct, and fine tune the line for optimum performance. This can easily shave several percentage points off of the required raw material while maintaining in-spec thickness in every case.
A sudden step change in extruded width should not always be used to adjust tension. This sort of sudden excess error may indicate foreign material caught in the die, a defective temperature control, or other problems that will affect the extrusion profile. By understanding when there is likely to be a problem, the extrusion can be checked and the problem cleared long before the product reaches the quality assurance checkpoint at the end of the process.
New Scan-A-Line Passline Independent Systems
In my recent venture out for a service call, I discovered that many of our faithful customers are unaware of recent developments in our measurement and control technology. A prime example is one of our original customers who is controlling his accumulator loop with one of our Loop Control Processing Units (LCPU) and a 10X-HD40 sensor. We had encountered a problem with a very long analog line back to the PLC that controlled the drive system for the line. The problem was quickly solved and then I began to ask some questions.
Why were they not guiding the line with Scan-A-Line? Had they ever tried to measure strip width on this line? Could there be any cost savings for using either of these applications here.?
As it turns out, the material they were pickling was a rather heavy gage material with very wavy edges. For any guide system to work, it needed to be independent of the strip pass line because of the constantly changing vertical edge positions. When I asked about using our PLI (Pass Line Independent) technology, they were totally unaware of this development.
What is Scan-A-Line’s PLI all about? As the name suggests, PLI enables edge position and width measurements that are independent of the vertical position of the strip. Our Measurement Processing Unit is using a formula from geometry to calculate the intersection of two lines of sight from two separate receivers. Because the positions for each of the receivers, and the positions along the Scan-A-Line LED array are both known, it is possible to find the view line intercepts to calculate the vertical and horizontal position of an edge. This enables a Scan-A-Line system to compensate for changes in pass line of an edge while accurately determining its “X” position across the line. This has implications for Edge and Center Guiding as well as Width Measurement of strips with poor edge shape or a constantly changing pass line.
Our original Pass Line Independent System required a special Scan-A-Line Emitter, with dual video processing circuitry. Although these systems worked very well, they were sometimes difficult to set up because of the LED balance as viewed from two different receivers. The receiver balance problem made them much more susceptible to troubles from misalignment and normal dirt buildup on the lenses.
Two years ago, we undertook a project to solve these problems and improve the PLI system performance. The project was called “The External BR” (binocular receiver) project. This led to two significant improvements to the Scan-A-Line PLI Systems, The first improvement was a method to permit a single video processing circuit to be used with both receivers in a time share arrangement. This makes it possible to use any standard Scan-A-Line Emitter for PLI service. The next improvement was in a program and circuit to permit separate storage of LED balance information for each of the receivers in the system. This improvement eliminated the problems with misalignment and stray lens contamination making the new External BR Systems as easy to install and operate as our original Scan-A-Line systems.
Rather than purchase a whole new sensor system to have PLI, it is now possible to have your existing 10XAS-series sensor upgraded to work as part of a PLI system. With the addition of a second receiver, cable, and a small external circuit to multiplex the receivers, we can turn your old Scan-A-Line sensor into the latest PLI system. The upgraded Emitter will still function as a single 10XAS-series sensor if ever needed as a spare.
Help! My Scan-A-Line system quit working
Help! My Scan-A-Line System has quit working
When a working Scan-A-Line system quits working there is a better than even chance that you have a damaged cable or connector. The sequence of tests that needs to be performed is important so that more damage to the system doesn’t result from your the attempts to repair it.
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- Look for proper power supply outputs. Most Scan-A-Line systems use LED indicators to show proper power supply operation. Generally there will be an indicator for +12V,-12V and on some systems a +5V indicator as well. These indicators are generally visible when the enclosure door is opened and line power is supplied. If you have a digital voltmeter you can verify the proper voltage levels. Grey wires are circuit ground. Measure from one of the Grey wires to a solid Red wire to measure the +12Volt buss. The -12Volt lines are solid Black and the +5Volt lines are Red & White striped. Actual voltages should be within 0.5Volts in most cases. If one of the Voltage indicators does not light or lights only very dimly, this indicates a possible short circuit in the system.
- To locate an external short circuit, try removing all of the cable connectors from the processing unit. When cables are not connected does the voltage indicator return to normal? If this restores proper voltages then try connecting one cable at a time until one of the indicators does not light or is again very dim. The offending cable or the module or sensor connected to it will have a short circuit.
- Try disconnecting the remote end of this cable from the Sensor or module it is connected to. If this doesn’t restore power there is a problem in the module or Sensor you just disconnected. It is best to return the defective Sensor or Module for factory repairs. Please call Harris Instrument for a Return Authorization.
- If the problem appears to be in the cable you can verify cable integrity with an Ohmmeter continuity test. Scan-A-Line cables are usually Male to Female. Pin(a) male should be connected to Pin(a) female and so forth. There should never be a cross connection between (a) and any other connection terminal and so forth. Examine your cable for a deformed cross section that could indicate that it has been crushed. This sort of damage may test OK until the cable is twisted or bent. Because damaged cables and cable connectors cause at least 50% of Scan-A-Line failures, it pays to have spare cables on standby to keep your system in operation.
- WARNING! Under no conditions should you substitute spare Receivers, Emitters or Processing units before these trouble shooting steps have been taken. Plugging a good module into a shorted cable usually causes damage to the spare module thus making it impossible to repair your system without help from field service.
- Other possible causes of system failure are improper receiver alignment or interference from an external light source, especially strobe lights. Pay special attention to any new lighting source that may be modulated at a high frequency. Poor grounding on line transportation drive and synchronizing controllers can also cause interference. Is there any part of the system that seems to cause the problem? Shut down systems one at a time to see which might be a problem. The important question to answer is “What could have changed since the system last worked properly?†Use this blog to describe your problem or give us a call at 740-369-3580.
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Many Thanks,
Dave Harris