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FLEXO Magazine : August 2009
TECHNOLOGIES & TECHNIQUES FIGURE 1. A geared press confi guration. All rollers are tied together with gears driven by a single motor. causes the roller and cylinder to rapidly accelerate and decelerate as the gear teeth bounce back and forth against each other. The result sometimes is the appearance of alternating light and dark horizontal lines on the printed product. “Gearless” printing presses (Figure 2) refer to those machines that eliminated gearing together off all of the axes running from a single motor. Instead, each anilox, plate, and impression cylinder has an independent servo applied. However, a gearbox is installed between the load and the feedback device, which adds inaccuracies. The result is that positioning accuracy in a “gearless” system typically is between +/- 1 arc minute and +/- 10 arc minutes. The acceleration and decelerations in a geared system are also limited by the gear-train backlash. Increasing acceleration past the safe level will lead to instability or gear damage. Some printing companies address these problems by frequently adjusting the antibacklash control system on the press, sometimes as often as weekly. This can result in a substantial amount of downtime without solving the underlying problem. ELIMINATING THE MECHANICAL TRANSMISSION Advancements in control and motor technology over the past decade now make it possible for the motion of the anilox and plate cylinders to be electronically synchronized by a closed loop control system to a much higher level of accuracy by eliminating the mechanical transmission system and creating a totally directdrive confi guration (Figure 3). The basic idea is that the anilox roller and plate cylinder are each driven independently by separate, direct-drive servo motors. A feedback device such as a high resolution sine encoder provides the servo motors with far more accurate position and velocity information that the controller compares to its programmed motion profi le and based on this signal sends velocity command signals to the amplifi er that drives the servo motor. A motion profi le defi nes the operation of each servo motor in terms of time position and velocity. In practice, the anilox roller and plate cylinder are synchronized in both speed and phase, ensuring that every point around the surface of the anilox roller is synchronized with the plate cylinder. The latest generation of servo motor controllers provides resolution feedback up to 27 bits, with 64-bit positioning resolution, and 125msec position loops, 62.5msec velocity loops and 0.670msec current control loops. So DDR systems can deliver much greater accuracy than the best mechanical transmission systems even immediately after adjustment of the antibacklash control system. As a general rule, DDR systems offer accuracy of about +/- 25 arc seconds system accuracy, which can be up to 20 times higher accuracy than conventional geared servo systems. The result is substantial improvements in print quality. IMPROVING THROUGHPUT FIGURE 2. 50 FLEXO AUGUST A gearless press confi guration. Each roller is driven independently by a motor/gearbox combination. When the load is directly coupled, the settling time is no longer limited by the transmission, so the servo loop gain can be increased. This provides the necessary servo stiffness to achieve excellent speed regulation and phase control between the anilox, plate, and central impression cylinders. Press speeds using direct-drive technology can be increased in many applications because the accuracy of the mechanical transmission system is often the limiting factor. Switching to direct-drive further improves press throughput by reducing setup and maintenance time. A typical fl exo press servo system equipped with gearboxes requires periodic tuning adjustments of the antibacklash control system to compensate for gear wear. DDR systems, on the other hand, since they are directly coupled to the load, require no periodic tuning. There is complete elimination of backlash and the need for antibacklash controls. Years later, the tuning settings are typically the same as the day the machine was installed. With a direct-drive press, the parts count on a typical Bill of Material (BOM) is reduced by up to 10 parts per color print deck. This mechanical simplifi cation translates into faster assembly, less maintenance, and less overhead to purchase parts. Table 1 shows a comparison of a typical geared solution with a direct-drive system. When 2009 www.flexography.org