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Machine Vision in the Print Industry
by Nello Zuech, Contributing Editor - AIA Posted 03/14/2002
The newspaper you read this morning was most likely scanned by a machine vision system. Such systems are used to automatically maintain piece-to-piece color register, color-to-color register as well as print-to-cut register. The goal is consistent register and avoiding waste during make ready, speed changes, splices and restarts. Versions of these systems range from simple mark sensors to line scan-based to pattern recognition-based area camera. Versions can detect and correct for both circumferential and lateral register errors simultaneously. While some of these systems provide operator alerts to concerns, some today integrate into press control systems providing closed loop color-to-color register control. As you can appreciate, if this much attention is placed on the production of newspapers, you can imagine the value of such systems in the printing of magazines, packaging materials and other products.
Machine vision applications can be found throughout these industries but mostly in converters (those that add value to paper), packaging, and label printers. It is applied to both sheet-fed and continuous operations. Machine vision-based techniques, more so than other sensor based controls, are recognized as having the ability to build quality into the printing process. Statistical process control is a major factor that has increased the demand for machine vision-based systems because they can capture and tabulate data automatically.
Application-specific machine vision systems or video inspection systems developed for the printing industry, like all machine vision products, capture an image from either an area camera or a linear array. The images are digitized and manipulated in various ways depending on the specific niche application(s) a system is designed to address.
The driving force behind these systems is the 20 - 40% reductions in print waste. Some of the properties analyzed by these systems include:
- Monitor and control registration: color-to-color, print-to-die, paper-to-paper
- Monitor and control print defects: halo, plate squeeze, fill-in or spread, striation, roller marks, beading, ragged edges, chalking, pinholes, fisheyes
- Monitor and control traps on the fly
- Process color control and registration control: visual through the use of split screen features, or automatically through color monitoring software
- Monitor and control line and screen color consistency: roll-to-roll, run-to-run
- On-the-fly bar code inspection
- Inspect: laydown of clear varnish and ultraviolet coatings; adhesive laydown
- Monitor perforation and die-cut quality, print-to-die registration.
In addition to waste reduction, other advantages to the use of this technology in the printing industry includes the fact that such systems permit presses to run faster, make a substantial impact on quality and allow presses to be set up faster.
Two versions of these systems are cited: passive and active. Passive systems automate the entire inspection process or specific elements of it and, in some cases, remove the operator from the loop. Generally these are capable of 100% inspection of the web and print pattern using either high-resolution area camera or linear array camera arrangements. They can identify both random and repetitive print defects and can be used to sort good material from bad.
Active inspection systems require operator involvement. Most of these systems today do some machine vision-like processing, with the more intelligent ones requiring less operator involvement in the fundamental viewing and decision-making. At the very least in these cases images can be magnified by the system to make it easier for an operator to view the print pattern and displayed on split screens to compare sampled image to benchmark referenced image.
Many are able to make automatic comparisons to monitor color, evaluate bar code quality, monitor and correct registration, and check for print defects. These high-end systems alert the operator when errors are found and display the location of the defective condition on the web. In some cases these systems can initiate controls related to corrective actions on the part of the press automatically. This capability is expected to increase in the future.
Some of these ASMV systems for the print industry use traversing mechanisms to move the camera across a web where wide webs must be monitored. These have the ability to memorize a number of discrete images and to correlate to a specific location dependent print pattern (positional memory capability). Generally this is tied into zoom capability as well.
Color monitoring software automatically looks at specific positions on the web and compares sampled color to a previously stored standard color. Registration control software monitors color-to-color registration. Today this is being done by monitoring special marks printed on the edge of the web or within the pattern. Some systems can now perform this by viewing a piece of the print itself. Closed loop systems interface with press registration controls directly.
Several offer machine vision systems that can perform full inspections as the printed pattern is applied. There are various versions of these systems. Some can perform 100% inspection on a small area of the print on a sampling basis. Some can perform a 100% inspection on a small area on 100% of the web. Some can perform a 100% inspection on the entire web. Versions of these systems operate either on-line to remove random occurring defects before they affect downstream process or on a rewinder before sending the printed job to the customer to make sure of the quality he gets. Some are monochromatic-based and some color-based. Some use area cameras and some use line scans cameras.
Conditions these systems can detect include: color variations, misregistration, streaks, splashes, hazing, misprints, spots, etc. In the case of offset presses flaws detected can include: scrumming, slur, fill-in, doubling, picking, hickeys, spots, fill-ins, dirty print, misprint, doctor blade streaks, color density, variations in color registration, front-to-back registration, ink/water imbalance problems, etc. In the case of bar code verification inspection includes: edge determination, minimum reflectance, symbol contrast, minimum edge contrast, modulation, defects, decode, decodability, etc.
Many segments of the printing industry are adopting machine vision: Packaging, labels for consumer products (pharmaceutical, cosmetic, beverage, cigarette, etc.), bank notes/currency, pharmaceutical, 6,8, 10 color printing machines, floor and wall coverings, bar code printing, etc. Systems exist that can handle any type of media: paper, foils, flexible translucent and opaque films.
To gain insights into what is taking place in the printing industry a series of questions was developed and forwarded to known suppliers of machine vision systems targeted at the printing industry. What follows is a summary of responses.
- How would you characterize the various applications addressed by machine vision in the printing industry? Or What are the specific applications of machine vision found in the printing industry?
Jim Doerr of Trucolor Video Systems provided a good summary: 'Basic or automatic inspection and monitoring or register, control, bar code print quality, general printing defects, and control over part of the process i.e. automatic register control.' Reflecting the concerns of the pharmaceutical industry especially, Roman Malicki of VRP Web Technology observes inspection of a pharmaceutical label is critical 'where missing or defective print could have serious consequences, i.e. a zero missing from '100 mg' or a small dot imprinted in a location that turns '100 mg' into '10 mg'.'
- What are some of the application challenges of the systems that you offer?
Jim Doerr suggested that 'press or machine real-estate for installing machine vision system can be challenging.' Nat Stern of Bobst Registron adds, 'Consistent results for various substrates and inks including very low contrasts and certain print degradations.' Roman Malicki observes: 'Common challenges include machine dynamics such as vibration, poor web tension and position control. An interesting psychological effect also takes place - 'shoot the messenger.' A customer may be dazzled by the ability of a system to spot tiny defects and is prepared to use that information to improve the printing process. Unfortunately, print consistency is far worse than the customer realizes. The result is that a system with tight tolerance settings may flag many more defects than the customer is willing to accept. The usual comments then occur: 'The system flags false defects,' 'it's too picky' or 'too difficult to set up'. What they really mean is that it's not easy to select inspection parameters to distinguish between visually acceptable 'defects' and visually unacceptable 'defects'. An additional small dot in an area filled with dots may be irrelevant, in an area where dots convey numerical information they matter. To a computer a defect is a defect unless it's told otherwise.'
- Of the applications cited, which is the one most widely adopted by the printing industry?
By far the most widespread application in printing is register control. This would seem to be followed by video inspection typically based on split screens for color and copy control. The up and coming application is 100% inspection of the entire printed product. As suggested by Koby Shtaierman of Advanced Vision Technology, systems with inspection capabilities 'can detect faults before the human eye sees them and before they become waste. More than just a web viewing system, the automatic defect detection system incorporates the 'brains behind the eyes' to analyze and classify data by type and alert the press operator when the printing results exceeds the job specified quality thresholds.'
- What are the drivers for adoption of machine vision? What is different today than say five years ago when the technology also held much promise?
As might be expected the answers suggested the main drivers relate to improving productivity and quality and reducing waste. The main reasons why today the market is more ready for machine vision include: Nat Stern - 'Lower prices (of the machine vision systems) and a better understanding of inspection systems by the average printer.' And Roman Malicki - 'The technology just wasn't there to keep up with machine speeds. The 'infancy' stage is slowly passing.'
- What are the barriers to the adoption of machine vision, specifically for each of the generic applications you have cited?
Jim Doerr suggests a barrier common to all machine vision systems 'Operator acceptance, basically due to system usability or ease of use.' Nat Stern sites another common concern associated with the adoption of machine vision in general - 'Very small tolerance in the industry for false alarms - unwanted detection.'
- What are some factors that can be used to justify the purchase of a system?
Scrap reduction and improved quality were the common response. Jim also suggested the opportunity machine vision systems offer to increase machine speeds. He also suggested bottom line results - increased profits. Nat also suggested that preventive maintenance could add to the justification, while Koby suggested the ultimate justification is improved customer satisfaction.
- In which of the applications is color-based processing essential? How important is color-based processing in the printing industry? What are the advantages of color-based processing? What specifically constitutes color-based processing? Absolute color measurements? Relative measurements of color? Color segmentation?
Different observations were made. Andre Beaudoin of Omega Systems/Burton group suggested, 'Color has everything to do with it - corporate colors/corporate logos are critical.' Jim observes 'Approximately 90%+ are interested in at least monitoring color visually; probably less than 20% require automatic color monitoring. Today's automatic color monitoring in the most part is relative color monitoring (trending) based on an operator selectable tolerance or target.'
Nat suggests, 'automatic recognition of colors for register controls allows for automatic start-ups. For quality control online absolute measurements are the ideal - no one has really done it. Relative measurements are useful for process control. Color processing also improves signal-to-noise ratio compared to monochrome systems.'
- For each of the applications cited, what exactly does the printing industry want from a machine vision solution?
Nat suggests 'Extreme ease of use with a meaningful alarm system with feedback to the process.' And Roman suggests, 'Simplicity of use and versatility are the most popular. The ability to find defects of a particular type or size as well as maximum machine speed is important of course.'
- What are some of the technical challenges in applying machine vision in the printing industry?
Roman suggests, 'Machine dynamics and material stability are challenging.' And Nat indicates 'Precise alignment in process color regions, detection of defects within the natural variation of the process, classification in operators' terms and feedback to the process.' A challenge cited by Andre applies to all machine vision applications - 'Keeping up with the demands of the end user (the ultimate customer of the printed material), who is driving the need for vision inspection.'
- What does a customer need to succeed in applying machine vision in the printing industry? What makes for a successful installation? What does the buying company have to do to assure the installation will be successful? What does the vendor have to do? What should be avoided to avoid failure?
Andre felt that machine-specific training is essential. The vendor has to be prepared to provide quality training and the customer has to be prepared to invest in the time it takes to become trained so that the line operator takes ownership. Jim makes the following observations: Customer must have a 'Willingness to accept machine vision as a production enhancing tool and be prepared to rely on it - simply use it. Installation on most systems is very simple and straightforward; the recommendation we often make is to insure some forethought is given as to the location of the display monitor, system controls in relationship to the press or machine controls. The closer the viewing monitor is to the actual location where corrections or press adjustments are made the better.'
Nat adds, 'The customer must rain and support the product from top management down to the plant floor. The vendor must provide a trouble free system requiring little or not training in regard to its use. The major training should be applied to using the imaging as a tool to control the process.'
And Roman cites 'Both parties must define the operating parameters and requirement. Machine vision systems are too complicated and sophisticated to be installed and used like some add-on afterthought. The simpler systems offer simplicity but sacrifice sensitivity or performance. Correct specifications and requirements are essential.'
- What does the future have in store: What are the technical trends in the print industry that will impact machine vision requirements and performance requirements? What are technical trends that will impact future machine vision products?
From the machine vision technology perspective, Roman offers 'Speed of operation, ease of use and flexibility are areas which will be improved upon in the near future. These advancements will be made mostly through software development. Costs should also come down through increased competition.'
From the printing industry's perspective Jim suggests 'Faster press/machine speeds, more color options (10 color machines), shorter production runs and quick change over machines mean the machine vision system must be easily and quickly setup. Sophisticated packaging design with smaller and smaller print with more complicated design. Increased demand to eliminate all defects from printed materials.'
And Nat puts it succinctly The printing industry is confronted with higher quality demand, higher speeds and at the same time less and less skilled operators. The challenge is to replace humans with imaging robots = total automation.'
The following table depicts suppliers and their offerings for the printing industry. The key is that some specialize on one or another type of press - offset vs. gravure, flexo or litho. Also, some are better at narrow web than wide web or vice versa.
Many thanks for the help from the following for input to this article:
Koby Schaierman, Advanced Vision Technology
Nat Stern, Bobst/Registron
Andre Beaudoin, Burton Group/Omega Systems
Jim Doerr, Trucolor Vision System
Roman Malicki, VRP Web Technology
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