The U.S. Department of the Treasury’s Bureau of Engraving and Printing (BEP) is responsible for producing all of the currency for the world’s largest economy. From the paper on which the currency is printed to its sheer output of approximately 37 million notes each day, nearly every aspect of the BEP’s design and printing process is distinctive. Many of the BEP’s quality assurance needs are unique as well.
Like all printers, the BEP must verify that its printed materials are clear and accurate.
However, the BEP also needs to ensure that various security measures are in place to help deter counterfeiting and to allow the government to account for all bank notes. To help meet these demanding requirements, the BEP converted several of its inspection stations from human operators spot checking currency to automated visual inspection of each note.
The BEP selected the Sherlock machine vision software application for the development of three machine vision systems: automated inspection systems for printing plate quality assurance; automation of measuring finished notes for print registration consistency; and on-line measurement of a quality and security feature on newly printed bills.
Assuring plate quality
In order to reduce or eliminate inconsistencies associated with physical note size and the location of engraved images on the chrome intaglio printing plates on which the currency is printed, the BEP’s Production Engineering group developed a Plate Measurement Device (PMD). The PMD uses state-of-the-art positioning technology and machine vision to automatically measure the layout pattern of the security features on these plates. The engraved artwork and registration marks are gauged before production to verify plate accuracy—which is essential for precise print registration—and afterwards, so that any distortion caused by prolonged intaglio printing is identified.
The team developed the PMD’s machine vision inspection system using a variety of advanced, off-the-shelf opto-electronic components. A frame grabber integrated within the host personal computer captures images from a 1K x 1K pixel monochrome camera. Using a precision lens with a 0.5-inch field of view, the camera achieves a measurement tolerance of +/- 0.001 inch. In addition to capturing images for analysis, the frame grabber provides the digital Input/Output (I/O) that is used to monitor manual switch settings and safety sensors on the PMD. The operator controls the vision system using a customized graphical interface via keyboard and mouse, and a 21-inch color monitor displays a live image of the currency plate being inspected.
Because the polished chrome plates are shaped to follow a four-plate printing cylinder radius, obtaining a quality, high-resolution image was one of the major challenges that team members faced. They solved the problem by using a light emitting diode (LED)-based, dark field illuminator in conjunction with a megapixel camera, making the engraving appear clear and well-defined for the machine vision software’s calipers to perform measurement functions with minimal pre-processing of the image. After the PMD captures all pertinent plate data, analysis is performed using machine vision software and an object oriented program language, and then all data is automatically saved to a network database.
All finished banknotes are targeted to be the same size and are printed 32 per sheet. Although each sheet is electronically inspected on the printing press and again before banknote numbering, the BEP did not have a reliable way of identifying finished banknote feature registration discrepancies that occur as a result of accumulated subtle variations with the many print phases and trim operations associated with currency production. Without this information, the BEP was unable to quantify its process capabilities or to measure improvements in banknote quality.
Vision challenges
Therefore, the Production Engineering group developed a vision-based note measurement system that automatically measures and records 27 note registration features (137 data points) on each cut bill. These include substrate size, intaglio print size and position (on both sides) and positions of the seals and serial numbers. Measuring banknotes in reflected light is quite challenging, as the intaglio printing process is somewhat variable by nature, and finding print edges consistently on the fine engraved artwork is very difficult.
An image-based inspection system proved to be a successful approach and provided enough flexibility to accommodate multiple banknote designs. Each note measurement station consists of a vacuum fixture to hold and position the banknote, three digital cameras, a microprocessor-controlled light source, a Continuously Diffuse Illuminator (CDI), a PC equipped with two frame grabbers and two machine vision software packages that co-exist for image analysis. After each note is precisely positioned beneath the camera, images of the front and back of each note are taken and transferred to the frame grabber. Each captured image is then analyzed by both machine vision applications for optical character recognition (OCR) and caliper measurement.
The software’s tools are also used to account for variations in trim and print and to compensate for notes incorrectly oriented on the vacuum table. Using grayscale edge detection, the machine vision software application is able to accurately measure pixels and sub-pixels for precision within +/- 0.2 mm. This was a crucial aspect of the solution’s success, as many of the fine line features printed on banknotes are smaller than the camera’s pixel resolution.
With ongoing quality considerations and the increasing need for security features to help deter counterfeiting, the BEP’s Production Engineering team also developed an automated inspection system to examine a semi-covert, machine-readable feature on each banknote. This feature is created by applying inks in denomination-specific patterns in precise locations throughout each printed sheet.
The BEP had used human inspectors for this application, but with 8,000 to 10,000 sheets of 32 banknotes coming off the presses each hour, only a very small percentage of sheets were inspected, and with less reliability than the BEP knew it could get from an online, automated system.
The Production Engineering group’s inspection system starts with an 8-bit monochrome tethered head camera that provides increased sensitivity and low light imaging capabilities. In addition to capturing images from the camera, a frame grabber provides digital I/O capabilities that activate a light bar and voice module. These indicators provide operators with feedback about each sheet (both printed and substrate areas) so that any problems can be remedied quickly.
System benefits
In each of the three applications for which it has implemented machine vision software solutions, the BEP has benefited from improved quality assurance, better information about its processes—which allows any needed action to be taken quickly—and reduced human inspection costs, along with increased inspection accuracy.
The Production Engineering team is currently making improvements to the existing systems to gain even greater reductions in print variability and to inspect even more sophisticated security features. The group continually makes improvements to its processes for quality assurance and security purposes, and machine vision plays an important role in this regard. The data received from these inspection systems also will be instrumental in implementing future security or design changes.
