Machine-vision systems target ink-jet inspection

Having an exclusive license from Xerox, Spectra has developed multiple generations of proprietary drop-on-demand ink-jet print heads capable of producing digital images in a variety of printing and fluid-jetting applications.

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Having an exclusive license from Xerox (Stamford, CT;, Spectra (Denton, TX;, a developer and manufacturer of piezoelectric ink-jet print heads and related consumables, has developed multiple generations of proprietary drop-on-demand ink-jet print heads capable of producing digital images in a variety of printing and fluid-jetting applications. These print heads are used by manufacturers such as Vutek (Meredith, NH; in the design of the Vutek 2360, a wide-format, 6.7-ft-wide, 720-dpi resolution printer that is capable of imaging areas of up to 1600 sq ft/hour.

According to Linda Creagh, director of business development at Spectra, the development and production of the print heads used in such applications must meet strict manufacturing criteria. "Once designs are developed," she says, "it is necessary to test the print heads under operational conditions. This requires monitoring the flow of the ink jets at speeds of between 8 and 12 m/s." By doing so, Spectra can both improve the design of next-generation print heads and test different fluids to optimize image quality.

Spectra contracted imaging Technology international (iTi; Boulder, CO; to perform this analysis. With the loan of a Spectra print-head evaluation kit, Spectra asked iTi to design and develop an automated vision system for the Spectra ink-jet print head. The resultant PC-based system uses a combination of off-the-shelf illumination, cameras, frame grabbers, data-acquisition hardware, and image-processing software, together with iTi-developed custom software and mounting hardware. Now available, the completed system—iTi Drop Watcher—magnifies images from the print head approximately 140X and allows ink drops to be captured and analyzed as they are formed and emitted from the print-head at 8- to 12-m/s rates (see figure).

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iTi Drop Watcher high-speed print-inspection system measures ink-jet particles as they are formed and travel at rates as fast as 8 to 12 m/s. Built using LabView and IMAQ Vision software, the system can accurately measure drop formation, flight time, velocity, straightness, and uniformity.
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In the design of the Drop Watcher, iTi mounted the piezoelectric print head on a 370-RC positioning stage from Newport Corp (Irvine, CA; This allowed the print head to be precisely positioned in x, y, and z axes, as well as turned on two rotary axes. By incorporating this positioning system, the print head can be precisely aligned to the imaging system.

To image the print head, a VZM450 zoom lens from Edmund Industrial Optics (Barrington, NJ; is mounted to an XC-75 0.5-in. CCD miniature camera from Sony (Park Ridge, NJ; Images from the CCD camera are captured into PC memory with a NI-1407 PCI-based frame grabber from National Instruments (NI; Austin, TX;

"A major design criterion in the development of the system," says Aimee Lutz, electrical engineer with iTi, "was to synchronize the firing of the piezoelectric print head with the LED module." Because of the high firing rate of the piezoelectric print head, it was necessary to freeze the motion of the print head with a Varioflash LED strobe lamp from The Imaging Source (Charlotte, NC; To accomplish this, Lutz used a NI PCI-6601 data-acquisition card to synchronize the firing of the print head with the strobe unit.

"By using virtual instruments in both LabView and IMAQ Vision," says Lutz, "5-V TTL signals from the output of the data-acquisition board simultaneously triggered the strobe and the firing of the print head." In this way, image droplets can be visualized as they form and move across the system's field of view.

"Building the system in a modular fashion using NI Virtual Instruments, a number of different parameters can be accurately measured," says Lutz. And, by building the graphical user interface using the same tools, the user can quickly configure print-head set-up, calibration, measurement, and data recording. Because this software is modular, data can be automatically recorded to a Microsoft Excel spreadsheet.

The use of NI IMAQ Vision and LabView software reduced development time to less than two months. The modular nature of the system also allows the system to be tailored for other controlled high-speed imaging applications.

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