For more than 20 years Erhardt+Leimer GmbH has developed CCD array cameras and has been intensively involved in the surface inspection of webs.
ELSIS inspection systems manufactured by Erhardt+Leimer continuously monitor the moving web in a paper machine and prepare reports and evaluations that provide comprehensive information on the quality of the production.
Flaws of varying size and type are reliably detected over the entire width of the paper at more than 2000 m/min and then classified into corresponding classes.
Depending on the number of cameras used over the width of the web, flaws smaller than 0.5 mm can be detected. During this process the high line rate of 52 kHz makes possible, e.g., a resolution of 1 mm at a web speed of 2000 m/min. Holes, creases, soiling, included particles and cracks are detected, classified and saved in the defect report.
The report is synchronized using dedicated length sensors (incremental encoders) and a code sprayed on the edge of the paper. As a result it is possible to accurately locate flaws on reels more than 100 km long.
The reports produced at the various inspection points are made available to the downstream processes via a central server.
The defect report contains an exact list of all flaws detected with a detailed description of the size, their characteristics, the position on the paper web and a digital photograph of the defect in the form of a gray-scale image.
The inspection camera
The array cameras used are so-called “intelligent cameras” that undertake independently all the image processing tasks and that communicate via Ethernet connections.
A special feature of “intelligent cameras” is their high level of integration that reduces the effort for wiring and electronics to a minimum.
Large control cabinets for external computer units are unnecessary, as all important, high speed image processing tasks are undertaken “on-board”. It is not necessary to transmit massive amounts of data over large distances.
The inspection cameras in the OL series are specially optimized for use in surface inspection; consistent quality is therefore guaranteed for many years to come as is the availability of spare parts!
The cameras are developed considering industrial usage and have extremely robust, chemically nickel-plated aluminum housing features ingenious heat management and provides the best possible protection against a very wide range of electromagnetic interference.
The cameras are equipped with CAN and Ethernet interfaces that, due to their high reliability, have in the meantime become standard in industry; they also permit distances of more than 100mbetween the individual components.
Worthy of mention is the patented, high precision, motorized camera chip adjustment, which in combination with the also patented calibration template makes possible very quick, automatic alignment and calibration of entire lines of cameras.
In the case of large web widths, a large number of individual cameras form a continuous inspection line to provide the resolution required.
On a paper machine with a web of 10 meters wide and a camera resolution of half a millimeter, 10 cameras with 2000 pixels each are arranged in a line.
The cameras in the inspection line are exactly aligned over the entire width of the web and calibrated in relation to each other in just a few minutes by the fully automatic chip adjustment. Overlapping areas, noise and the resolutions of the individual cameras in the line are taken into account, as are brightness control and sharpness.
Precisely aligned camera arrays combined with high power light sources are indispensable if defects of widely varying types and shapes need to be reliably detected and classified on a fast moving paper web.
During commissioning or the replacement of a camera in the event of service, only a few minutes are needed until the complete line is calibrated and available again.
Light transmitter
LED-based light sources are increasingly used to illuminate the paper webs to be inspected. LED light transmitters feature extremely high intensity, very long service life, extreme robustness and excellent homogeneity over the width of the web.
Colors from blue to infrared are available and are used to suit the type of material and characteristics on the flaws to be detected.
Important factors related to the quality of the LED light source are very ingenious heat management and the choice of the optical components that bundle and homogeneously combine the light emitted by the light emitting diodes.
The more homogeneous the light intensity over the complete working width, the better the quality of the images and as a result the inspection system can operate more sensitively and classify the flaws detected as appropriate.
The selection of optimal lighting fittings and appropriate know how in the related application are crucial for the quality of the inspection system.
The choice of the right lighting technology is the key to successful, high performance inspection technology!
Length coding
A further key aspect of the inspection system for paper machines is a robust length coding system. Without a reliable marking process, it will be impossible to locate a flaw to the centimeter on a reel more than 60 km long.
Erhardt+Leimer uses special nozzles for length coding; these nozzles spray a code at a fixed distance from the edge of the paper and at adjustable length intervals.
The code comprises a single line and an ascending number sequence that, in combination with an incremental encoder, ensures the position of a defect on a reel can be detected and located again to within a few cm.
The print head is precisely guided along the edge of the web by a support system and is therefore always a constant distance from the edge of the web. The sensors in the downstream production steps have a similar adjustment feature and can detect the code very reliably.
Immediately after the print nozzle an optical sensor checks the sprayed length information for legibility and in this way significantly increases the process reliability.
The minimum opening period for the nozzle is 3 ms. This time corresponds to a line length of 90 mmat a machine speed of 1800 m/min.
The code for a 12-digit number has a length of only a few meters even at this high speed of 30 m/s. At an interval of 300 m (10s) between the individual length markings, the consumption of ink is kept within reasonable limits without the need for significant compromises in the positioning accuracy.
If, in the process chain, the defect report is passed from the paper machine to the calender, the length marking ensures the defects detected are positioned exactly on the calender. A combined defect report is produced that accurately reflects all events all the way to the last element in the chain, the reel cutter.
Manufacturing process
What is then the concept for a comprehensive inspection system in a paper mill that ensures the quality of the paper web produced all the way to the end user?
The first ELSIS inspection line is in the outfeed from the paper machine. In the reel report ELSIS lists all flaws that are produced in the paper machine.
Each defect is entered in the report with all its features (type of defect, size of defect, position in direction of web travel, position transverse to the web, gray-scale image) and linked to the length marks sprayed on the edge of the paper.
The reel changing signal brings the current run of material to an end and closes the report in the inspection computer, once the operator has marked the finished reel with a barcode label and the information has been read using a handheld scanner.
Barcode and length mark are imperative for subsequent processing, as the report from the paper machine must be allocated to the correct reel in the downstreammanufacturing steps.
From the paper machine the report is transferred to the database on the central server and from there made available to the subsequent processes.
If, after the paper machine, the reel is run through the calender, the ELSIS computer there obtains the related data record from the server using the barcode information and synchronizes it with the length marking, once it has been detected at the edge of the web by the sensor. This extremely precise method enters the flaws found on the calender in the paper machine report, accurate to the centimeter. A new report is produced that combines the information from both work steps.
The paper maker is provided with extensive information on the complete process chain. Defects that were produced on the paper machine may disappear again due to the calender process or change significantly.
The reel cutter is the next stop in the process chain in a paper mill. It is also equipped with a reader for the length code and barcode and using this information loads the final report into its memory.
The ELSIS targeted stop system moves accurately and without any lengthy stop phase to each serious defect presented on the reel cutter, thereby providing a massive increase in productivity. The machine operator does not need to spend a long time at creep speed searching for flaws.
CONCLUSION:
The integrated inspection solution ELSIS from Erhardt+Leimer is a complete system and provides the customer with a flexible inspection solution for the entire manufacturing process in a paper mill.
ELSIS is indispensable for consistently high quality in production. ELSIS supports the paper maker in his day-to-day work with flexible evaluations and reports that provide maximum transparency over the actual state of production at any time. The following aspects make a significant contribution to this transparency:
• Highly sensitive inspection cameras and ingenious lighting fittings guarantee very high detection rates for a very wide range of defects.
• The comprehensive workflow solution provides a high level of process control.
• The graphic user interface provides all parameters and data of relevance for production clearly laid out on the screen.
• Flexible filters make it possible to sort different flaws into meaningful classes.
• For more complex defects, e.g. repetitive defects, special algorithms and functions are available.
• The report manager makes it possible to very flexible design a wide range of reports and evaluations.
• A special custom reel report can be supplied with each finished, cut reel.
• Production-related inspection data at the paper factory are saved in a central database and can be viewed at any time for monitoring production.
With the aid of ELSIS surface inspection, the printing industry is supplied with the highest quality reels of paper that make a crucial contribution to waste reduction on the printing presses. Paper tears caused by flaws in the paper web can be almost completely excluded.
