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Answers by Tim Exton-McGuinness of Scienta. Question: How does the FoilControl system differ from the competition? Simple Answer This system is single tower format² capable of operating both as single web on air chuck shafts or as multiple web on differential shafts. The most significant difference is that the system does not use dancers¹. Dancers have become a measure of how well the system is working and a block to further development, solution, remove them completely. Also offering the system in a packaged form allows integrators to use their internal engineering resource, bringing the application of this technology to a much broader audience. Long Answer First think about Torque, Speed and Tension. Torque is the twisting force exerted by the motor measured in Newton metres (Nm), importantly you can have torque without speed but if an axis is free to move, then by applying a constant torque the axis speed will in theory increase to infinity. Speed is measured in metres per second (m/s) or degrees per second (°/s), you can have speed without torque. Tension, in this application, is measured in kilograms per linear metre (Kg/m) or pounds per linear inch. How do Dancers work: With dancers the system operates the servo motors in Torque Mode. The torque of the motor is adjusted by the position of the dancer. The foil is tensioned by air pressure to the dancers and they will settle to balance the motor torque to the set pressure. With a large diameter reel of foil the motor torque needs to be higher than with a smaller diameter reel. The control is proportional only, the static position of the dancer changes with the varying diameter of the reels. The speed of the motor is not controlled, other than by imposing a speed limit. As the Press starts to draw foil over the blanket this upsets the balance between the dancer pressure and the motor torque causing the unwind dancer to call for less torque so foil unwinds easier and the rewind dancer to call for more torque to take up the slack until a new balance is achieved. How do Friction or Differential Shafts: With Friction Shafts (Differentials) the foil is tensioned by the internal pressure applied to the shaft friction material and not directly by the torque of the motor. In this case the speed of the shaft needs is controlled with respect to the core speed, over speeding the shaft with respect to the core, will generate excessive heat. The core should never lock to the shaft in normal running or should the motor drive backwards which unlocks the core from the shaft. Foil tension for a given pressure varies with the diameter of the foil reel. Normally ultrasonic sensors are used to measure the diameters and the air pressure adjusted accordingly. In differential mode the controller needs to measure the press speed and gear the unwind and rewind axes for the correct surface speeds. Known Limitations of Dancers The gain of the torque control loop (dancer position to motor torque) needs to kept low in order to eliminate oscillations, seen as the dancers kicking about. This leads to the dancers running out of the vertical position you would expect. The gain used is a compromise, between the dancers hitting the end limits for large diameter reels and them becoming unstable with small diameter reels. During Foilsave, the foil is stopped gradually. As the unwind slows the press still tries to pull foil and this pulls the dancer in towards the path rollers, it is normal for the dancer to hit the path rollers. Various techniques are used to limit this mechanical shock but these allow the foil to run on considerably. Coming out of Foilsave also presents a problem, as the unwind torque is at its maximum, which can start a wild oscillations. Some use their differential shafts in dancer control mode, with manual air pressure adjustment. Advice, stand well back! The New FoilControl© Package Scienta have been working on a new control system for the last year. One of the fundamental design criteria was to design a system which eliminated the problems associated with dancers, as this was the number one concern of our existing customers. We saw the simplest solution to this was the radical approach, to remove the dancers completely, the benefits of this just keep coming.
©Scienta Systems 2011 ------------------------------------------------------------------------ ¹ For a large format press a shock absorbing dancer may be needed. ² Twin Tower, twin web format is possible at extra cost. ©Scienta Systems 2011
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