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FLEXO Magazine : August 2009
TECHNOLOGIES & TECHNIQUES PRESS BUYER’S GUIDE Servos, Servos, Servos! The Days of Buying a New Gear-Driven Narrow/Mid Web Press Are Over By Steve Leibin Completely shaftless servo-driven presses are the present and future of fl exo printing. Photo courtesy OMET/Matik North America. Completely shaftless servo-driven presses are the present and futu he manufacturing emphasis of all narrow and mid web press manufacturers is now servo drive press technology. It is a well-recognized fact that servo-driven presses offer enhanced effi ciencies and better performance for converters. T So, the question is not why use servo technology but what is the performance and benefi ts of different servo-drive press designs? DEFINITIONS Although servo is an often-used term, what we are really describing is the motion control system in the press. In older, geared presses there was a mechanical system incorporating large DC drive motors connected to lineshafts and gears. Now, electronic components can be combined to eliminate some or all of the mechanical interference in the press, depending on the sophistication of the design, and deliver different levels of performance. For this discussion, a servomechanism, or servo, is an electric motor that uses self-correcting feedback (closed loop feedback) to correct the position/performance of the electric motor. The feedback, or error-correction signals, help control the mechanical position or other parameters (velocity, speed) of the electric motor. The basic architecture of an electronic motion control system can have several components depending on design: • A motion controller generates set points (to control the position/velocity of the shaft) and close a position and/or velocity feedback loop. • The motor drive or amplifi er transforms the signal from the motion controller into an electrical current that is sent • Intelligent drives close the position and velocity loops internally. • A complete motion control system contains matched components--motors, drives, encoders, touchscreens and software. • Benefi ts of design: precise web transport, exceptional print registration, and enhanced repeatability. 56 FLEXO AUGUST to the actuator (servo motor). New more sophisticated systems utilize “intelligent” drives that can close the position and velocity loops internally, providing much more precise control. • An actuator, such as an electric motor servo, outputs motion. • One or more feedback sensors, such as optical encoders or resolvers, close the position and/or velocity control loops. • Sometimes mechanical components are used (oftentimes this depends on the sophistication of the system) to transform the motion of the actuator/servo into the desired motion, such as gears, lineshafts or shafts. Servos typically operate on the principle of negative feedback. The calculated electronic position is compared to the actual position of the shaft, measured by some sort of encoder on the motor. Any difference between the actual and desired values generates an error signal, which is sent to drive the system and corrects the motor in the direction necessary to reduce or eliminate the error. This provides for a very precise control of the web and shafts in the press. Control theory is the engineering science that focuses solely on this technology. A complete motion control system contains matched components such as motion controllers, motor drives, motors, encoders, touchscreens and software. The press manufacturer determines the design and components of the system which can be customized for specifi c applications. The number of axes/shafts, the motor power and torque required for various applications (printing versus diecutting, for example), controller interface and networking options need to be considered in entirety when designing the motion control system. MOTION CONTROL DESIGNS A simple and early press motion control design had the infeed and outfeed nips driven by servo motors. This provided more precise web transport and control through the press. Although this provides benefi ts over a traditional geared press, it still has many of the mechanical limitations of a geared press design (such as lineshafts and gears) and offers limited effi ciency improvement to the production process. 2009 www.flexography.org