Replacing the mechanical line shaft, which is necessary for the synchronous drive of the printing units of rotary offset printing presses, through an "electronic line shaft" consisting of highly precise speed- and angle-controlled AC motors a multi-variable control system is formed which offers much more degrees of freedom than before to control important process variables. Until now it was not possible to simultaeously control web tensile forces and register errors, especially the cut-off register error, which are coupled with each other. On the basis of an extended mathematical process model which has been derived it was possible to find algorithms for a non-interacting, stable and simultaneous control of these two variables in the same or in different paper sections using the angular velocities of non-printing nips as actuating variables. The new control scheme was implemented on the level of the electronic drive controls and tested with a commercial press for illustration printing. This results in a considerably faster, more accurate and wearless control, compared to the use of register rollers which are state of the art. Due to this the amount of waste paper is significantly reduced. Furthermore, the operation of the press is simplified because the tensile force profile can automatically be preselected dependent on the paper grade and converted to set point values for the machine control. These innovations increase the productivity of the plant.
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